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  • 101. Haapanala, Paivi
    et al.
    Raisanen, Petri
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Sensitivity of the shortwave radiative effect of dust on particle shape: Comparison of spheres and spheroids2012Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 117, artikel-id D08201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The sensitivity of direct shortwave radiative effects of dust (DRE) to assumed particle shape is investigated. Radiative transfer simulations are conducted using optical properties of either spheres, mass-equivalent spheroids (mass-conserving case), or (mass-equivalent) spheroids whose number concentration is modified so that they have the same midvisible optical thickness (tau(545 nm)) as spheres (tau-conserving case). The impact of particle shape on DRE is investigated for different dust particle effective radii, optical thickness of the dust cloud, solar zenith angle, and spectral surface albedo (ocean, grass, and desert). It is found that the influence of particle shape on the DRE is strongest over ocean. It also depends very strongly on the shape distribution of spheroids used, to a degree that the results for two distributions of spheroids may deviate more from each other than from those for spheres. Finally, the effects of nonsphericity largely depend on whether the mass- or tau-conserving case is considered. For example, when using a shape distribution of spheroids recommended in a recent study for approximating the single-scattering properties of dust, the DRE at the surface differs at most 5% from that from spherical particles in the mass-conserving case. This stems from compensating nonsphericity effects on optical thickness, asymmetry parameter, and single-scattering albedo. However, in the tau-conserving case, the negative DRE at the surface can be up to 15% weaker for spheroids than spheres.

  • 102. Haapanala, Paivi
    et al.
    Raisanen, Petri
    McFarquhar, Greg M.
    Tiira, Jussi
    Macke, Andreas
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    DeVore, John
    Nousiainen, Timo
    Disk and circumsolar radiances in the presence of ice clouds2017Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, nr 11, s. 6865-6882Artikel i tidskrift (Refereegranskat)
  • 103. Han, Z.
    et al.
    Sakurai, T.
    Ueda, H.
    Carmichael, G. R.
    Streets, D.
    Hayami, H.
    Wang, Z.
    Holloway, T.
    Engardt, Magnuz
    SMHI, Forskningsavdelningen, Luftmiljö.
    Hozumi, Y.
    Park, S. U.
    Kajino, M.
    Sartelet, K.
    Fung, C.
    Bennet, Cecilia
    SMHI, Forskningsavdelningen, Luftmiljö.
    Thongboonchoo, N.
    Tang, Y.
    Chang, A.
    Matsuda, K.
    Amann, M.
    MICS-Asia II: Model intercomparison and evaluation of ozone and relevant species2008Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 42, nr 15, s. 3491-3509Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Eight regional Eulerian chemical transport models (CTMs) are compared with each other and with an extensive set of observations including ground-level concentrations from EANET, ozone soundings from JMA and vertical profiles from the TRACE-P experiment to evaluate the models' abilities in simulating O(3) and relevant species (SO(2), NO, NO(2), HNO(3) and PAN) in the troposphere of East Asia and to look for similarities and differences among model performances. Statistical analysis is conducted to help estimate the consistency and discrepancy between model simulation and observation in terms of various species, seasons, locations, as well as attitude ranges. In general, all models show a good skill of simulating SO(2) for both ground level and the lower troposphere, although two of the eight models systematically overpredict SO(2) concentration. The model skills for O(3) vary largely with region and season. For ground-level O(3), model results are best correlated with observations in July 2001. Comparing with O(3) soundings measured in the afternoon reveals the best consistency among models in March 2001 and the largest disparity in O(3) magnitude in July 2001, although most models produce the best correlation in July as well. In terms of the statistics for the four flights of TRACE-P experiment, most models appear to be able to accurately capture the variability in the lower troposphere. The model performances for NO(x) are relatively poor, with lower correlation and with almost all models tending to underpredict NO(x) levels, due to larger uncertainties in either emission estimates or complex chemical mechanism represented. All models exhibit larger RMSE at altitudes < 2 km than 2-5.5 kin, mainly due to a consistent tendency of these models towards underprediction of the magnitude of intense plumes that often originate from near surface. Relatively lower correlation at altitudes 2-5.5 km may be attributed to the models' limitation in representing convection or potential chemical processes. Most of the key features in species distribution have been consistently reproduced by the participating models, such as the O(3) enhancement in the western Pacific Ocean in March and in northeast Asia in July, respectively, although the absolute model values may differ considerably from each other. Large differences are found among models in the southern parts of the domain for all the four periods, including southern China and northern parts of some Southeast Asia countries where the behaviors of chemical components and the ability of these models are still not clearly known because of a lack of observational databases. (C) 2007 Elsevier Ltd. All rights reserved.

  • 104. Hayami, Hiroshi
    et al.
    Sakurai, T.
    Han, Z.
    Ueda, H.
    Carmichael, G. R.
    Streets, D.
    Holloway, T.
    Wang, Z.
    Thongboonchoo, N.
    Engardt, Magnuz
    SMHI, Forskningsavdelningen, Luftmiljö.
    Bennet, Cecilia
    SMHI, Forskningsavdelningen, Luftmiljö.
    Fung, C.
    Chang, A.
    Park, S. U.
    Kajino, M.
    Sartelet, K.
    Matsuda, K.
    Amann, M.
    MICS-Asia II: Model intercomparison and evaluation of particulate sulfate, nitrate and ammonium2008Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 42, nr 15, s. 3510-3527Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Eight chemical transport models participate in a model intercomparison study for East Asia, MICS-Asia II. This paper analyzes calculated results for particulate matter of sulfate, nitrate and ammonium through comparisons with each other and with monthly measurements at EANET (the acid deposition monitoring network in East Asia) and daily measurements at Fukue, Japan. To the EANET measurements, model ensemble means better agree with model individual results for sulfate and total ammonium, although total nitrate is consistently and considerably underestimated. To measurements at Fukue, the models show better agreement than for the EANET measurements. This is likely because Fukue is centered in many of the model domains, whereas the EANET stations are mostly in Southeast Asia and Russia. Moreover, it would be important that Fukue is in Northeast Asia, where the emission inventory is more reliable than Southeast Asia. The model-model comparisons are made in view of the total amount in the atmosphere, vertical profile, coefficient of variation in surface concentrations, and transformation changes with distance. All the models show reasonable tendencies in vertical profiles and composition ratios. However, total amounts in the atmosphere are discrepant among the models. The consistency of the total amount in the atmosphere would influence source-receptor analysis. It seems that model results would be consistent, if the models take into account the primitive processes like emission, advection/diffusion, chemical transformation and dry/wet deposition, no matter the processes are modeled simply or comprehensively. Through the comparison study, we learned that it would be difficult to find any problems from one comparison (model-observation comparison with one data or many but at one station or in a short period). Modelers tend to examine model performances only from model-observation comparisons. However, taking budget in a certain or whole model domain would be important, before the models are applied to source-receptor analysis. (C) 2007 Elsevier Ltd. All rights reserved.

  • 105. Hedberg, E
    et al.
    Gidhagen, Lars
    SMHI, Forskningsavdelningen, Luftmiljö.
    Johansson, C
    Source contributions to PM10 and arsenic concentrations in Central Chile using positive matrix factorization2005Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 39, nr 3, s. 549-561Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Sampling of particles (PM10) was conducted during a one-year period at two rural sites in Central Chile, Quillota and Linares. The samples were analyzed for elemental composition. The data sets have undergone source-recepior analyses in order to estimate the sources and their abundance's in the PM10 size fraction. by using the factor analytical method positive matrix factorization (PMF). The analysis showed that PM10 was dominated by soil resuspension at both sites during the summer months, while during winter traffic dominated the particle mass at Quillota and local wood burning dominated the particle mass at Linares. Two copper smelters impacted the Quillota station, and contributed to 10% and 16% of PM10 as an average during summer and winter. respectively. One smelter impacted Linares by 8% and 19% of PM10 in the summer and winter, respectively. For arsenic the two smelters accounted for 87% of the monitored arsenic levels at Quillota and at Linares one smelter contributed with 72% of the measured mass. In comparison with PMF, the use of a dispersion model tended to overestimate the smelter contribution to arsenic levels at both sites. The robustness of the PMF model was tested by using randomly reduced data sets, where 85%, 70%, 50% and 33% of the samples were included. In this way the ability of the model to reconstruct the sources initially found by the original data set could be tested. On average for all sources the relative standard deviation increased from 7% to 25% for the variables identifying the sources, when decreasing the data set from 85% to 33% of the samples, indicating that the solution initially found was very stable to begin with. But it was also noted that sources due to industrial or combustion processes were more sensitive for the size of the data set, compared to the natural sources as local soil and sea spray sources. (C) 2004 Elsevier Ltd. All rights reserved.

  • 106. Hiemstra, J.A.
    et al.
    Saaroni, H.
    Amorim, Jorge Humberto
    SMHI, Forskningsavdelningen, Luftmiljö.
    The Urban Heat Island: Thermal Comfort and the Role of Urban Greening2017Ingår i: The Urban Forest - Cultivating Green Infrastructure for People and the Environment / [ed] D. Pearlmutter, C. Calfapietra, R. Samson, L. O'Brien, S.K. Ostoić, G. Sanesi, R.A. del Amo, Springer International Publishing , 2017, s. 7-19Kapitel i bok, del av antologi (Refereegranskat)
  • 107. Hillring, B
    et al.
    Krieg, Roland
    SMHI, Forskningsavdelningen, Luftmiljö.
    Wind energy potential in southern Sweden - Example of planning methodology1998Ingår i: Renewable energy, ISSN 0960-1481, E-ISSN 1879-0682, Vol. 13, nr 4, s. 471-479Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study presents a planning model for Swedish activities in the field of wind power. Models and results of calculations of the land-based wind energy potential with the Wind Atlas Analysis and Application Programme, WA(s)P, are described and analysed in a geographical information systems (GIS) called the ArcView(R) GIS system. One county in southern Sweden was chosen as a case study to present the methods used. The results from that case study indicate a great wind energy potential but there are nevertheless many factors limiting that potential. The study calls for a further development of planning tools in the field. Important areas for the future are the development of knowledge in market issues, wind power technology, environmental issues, and public opinion on constructing wind turbines. (C) 1998 Elsevier Science Ltd. All rights reserved.

  • 108. Hole, Lars
    et al.
    Engardt, Magnuz
    SMHI, Forskningsavdelningen, Luftmiljö.
    Climate change impact on atmospheric nitrogen deposition in northwestern Europe: A model study2008Ingår i: Ambio, ISSN 0044-7447, E-ISSN 1654-7209, Vol. 37, nr 1, s. 9-17Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A high-resolution chemical transport model, driven by meteorology representing current and future climate, was used to investigate the effects of possible future changes in climate on nitrogen deposition in northwestern Europe. The model system was able to resolve the climatology of precipitation and chemical properties observed in northern Europe during the 1980s, albeit with some underestimation of the temporal and spatial variability of meteorological parameters and chemical components. The results point toward a substantial increase (30% or more) in nitrogen deposition over western Norway as a consequence of increasing precipitation but more moderate changes for other areas. Deposition of oxidized nitrogen will increase more than the deposition of reduced nitrogen. Over Sweden, oxidized nitrogen will increase only marginally and reduced nitrogen will decrease, although annual precipitation is expected to increase here as well. This is probably because more reduced nitrogen will be removed further west in Scandinavia because of the strong increase in precipitation along the Norwegian coast. The total deposition of oxidized nitrogen over Norway is expected to increase from 96 Gg N y(-1) during the current climate to 107 Gg N y(-1) by 2100 due only to changes in climate. The corresponding values for Sweden are more modest, from 137 Gg N y(-1) to 139 Gg N y(-1).

  • 109. Holloway, Tracey
    et al.
    Sakurai, Tatsuya
    Han, Zhiwei
    Ehlers, Susanna
    Spak, Scott N.
    Horowitz, Larry W.
    Carmichael, Gregory R.
    Streets, David G.
    Hozumi, Y.
    Ueda, Hiromasa
    Park, S. U.
    Fung, Christopher
    Kajino, M.
    Thongboonchoo, Narisara
    Engardt, Magnuz
    SMHI, Forskningsavdelningen, Luftmiljö.
    Bennet, Cecilia
    SMHI, Forskningsavdelningen, Luftmiljö.
    Hayami, Hiroshi
    Sartelet, Karine
    Wang, Zifa
    Matsuda, K.
    Amann, Markus
    MICS-Asia II: Impact of global emissions on regional air quality in Asia2008Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 42, nr 15, s. 3543-3561Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This study quantifies the seasonality and geographic variability of global pollutant inflow to Asia. Asia is often looked to as a major source of intercontinental air pollution transport with rising emissions and efficient pollutant export processes. However, the degree to which foreign emissions have been imported to Asia has not been thoroughly examined. The Model Inter-Comparison Study for Asia (MICS-Asia) is an international collaboration to study air pollution transport and chemistry in Asia. Using the global atmospheric chemistry Model of Ozone and Related Tracers (MOZART v. 2.4), and comparing results with a suite of regional models participating in MICS-Asia, we find that imported O-3 contributes significantly throughout Asia. The choice of upper boundary condition is found to be particularly important for O-3, even for surface concentrations. Both North America and Europe contribute to ground-level O-3 concentrations throughout the region, though the seasonality of these two sources varies. North American contributions peak at over 10% of monthly mean O-3 during winter months in East Asia, compared to Europe's spring- and autumn-maxima (5-8%). In comparison to observed data from the Acid Deposition Monitoring Network in East Asia (EANET), MOZART concentrations for 03 generally fall within the range of the MICS models, but MOZART is unable to capture the fine spatial variability of shorter-lived species as well as the regional models. (C) 2007 Elsevier Ltd. All rights reserved.

  • 110.
    Holmin Fridell, Sofi
    et al.
    SMHI, Samhälle och säkerhet.
    Jones, Jörgen
    SMHI, Affärsverksamhet.
    Bennet, Cecilia
    SMHI, Forskningsavdelningen, Luftmiljö.
    Södergren, Helena
    SMHI, Affärsverksamhet.
    Kindell, Sven
    SMHI, Affärsverksamhet.
    Andersson, Stefan
    SMHI, Forskningsavdelningen, Luftmiljö.
    Jakobsson, Mattias
    SMHI, Affärsverksamhet.
    Luftkvaliteten i Sverige år 20302013Rapport (Övrigt vetenskapligt)
    Abstract [sv]

    Sveriges luftkvalitet påverkas av lokala och nationella utsläppskällor såväl som av emissioner från Europa. Utsläppen av många luftföroreningar har minskat under de senaste årtiondena tack vare kontinuerligt skärpta emissionskrav. Trots kraftiga minskningar av utsläppen både i Sverige och övriga Europa har inte luftkvaliteten i våra städer, med avseende på kvävedioxid (NO2), ozon (O3) och partiklar (PM10), förbättrats avsevärt det senaste decenniet. Inte heller har t.ex. surheten i Sveriges skogsmarker minskat sedan 1980-talet (Naturvårdsverket, a).SMHI genomförde under 2011 och 2012 en kartläggning av luftmiljö och deposition fram till år 2020 (Andersson et al, 2011 och Omstedt et al, 2012a). I detta fortsättningsprojekt har kartläggningen gjorts ytterligare 10 år framåt i tiden, till år 2030. Studien behandlar både lokal luftkvalitet och beräkningar av bakgrundshalter och deposition.Lokala beräkningar har utförts för 46 gator/vägar i eller i nära anslutning till tätortsmiljö. Beräkningarna omfattar halter av kvävedioxid och partiklar (PM10 och PM2.5). Deposition redovisas för svavel- och kväveföreningar uppdelat på total-, våt- samt torrdeposition.Lufthalter inklusive AOT40 redovisas för ozon.Lokalt uppvisar PM10 flest överskridandena av miljökvalitetsnormerna och miljökvalitetsmålet Frisk luft. Årsmedelvärdet varierar mellan de studerade gatumiljöerna från knappt 10 till 37 μg m–3, och 90-percentilen från knappt 17 till 80 μg m–3. Miljökvalitetsmålet för PM10 beräknas överskridas i 42 av de 46 studerade trafikmiljöerna.PM2.5-halterna ligger väl under miljökvalitetsnormen för samtliga studerade trafikmiljöer. Miljökvalitetsmålet överskrids i åtta av de 42 studerade trafikmiljöerna. Årsmedelvärdet varierar mellan4 och 12 μg m–3.För NO2 varierar årsmedelvärdet i de studerade städerna mellan 6 och 25 μg m–3, 98-percentilen av dygnsmedelvärden mellan 12 och 46 μg m–3 och 98-percentilen av timmedelvärdet mellan 16 och 67 μg m–3. Miljökvalitetsnormerna beräknas inte överskridas i någon av de studerade trafikmiljöerna. Miljökvalitetsmålet Frisk luft för NO2 avseende 98-percentil timmedelvärden överskrids i 4 av de 46 studerade miljöerna. För årsmedelvärden noteras inget överskridande av målet.För fyra gator har en känslighetsanalys genomförts där trafikökningen har hållits oförändrad jämfört med år 2008. Uteblivna trafikökningar till år 2030 jämfört med 2008 leder till minskade haltnivåer mellan 3 och 11 % för PM10 och NO2. PM2.5 påverkas endast marginellt av förändrade trafikflöden.Som följd av minskade emissioner kommer deposition av svavel och oxiderat kväve att minska till år 2030. Depositionen kommer vara fortsatt störst i södra Sverige. Depositionen av reducerat kväve kommer på de flesta platser vara oförändrad.Luftkvaliteten med avseende på marknära ozon kommer att förbättras i Sverige fram till år 2030. Halterna av ozon kommer fortsatt att ligga under miljökvalitetsmålet för ozons påverkan på grödor och skog.De största osäkerheterna i denna studie antas finnas i emissionsdata, trafikökningar på enskilda gator, fordonssammansättningen (t.ex. andelen dieselbilar) och andelen bilar med dubbdäck. I studien används meteorologin för år 2008 vilket gör att erhållna resultat inte inkluderarvariabilitet i meteorologin.

  • 111. Huijnen, V.
    et al.
    Eskes, H. J.
    Poupkou, A.
    Elbern, H.
    Boersma, K. F.
    Foret, G.
    Sofiev, M.
    Valdebenito, A.
    Flemming, J.
    Stein, O.
    Gross, A.
    Robertson, Lennart
    SMHI, Forskningsavdelningen, Luftmiljö.
    D'Isidoro, M.
    Kioutsioukis, I.
    Friese, E.
    Amstrup, B.
    Bergström, Robert
    SMHI, Forskningsavdelningen, Luftmiljö.
    Strunk, A.
    Vira, J.
    Zyryanov, D.
    Maurizi, A.
    Melas, D.
    Peuch, V-H
    Zerefos, C.
    Comparison of OMI NO2 tropospheric columns with an ensemble of global and European regional air quality models2010Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, nr 7, s. 3273-3296Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a comparison of tropospheric NO2 from OMI measurements to the median of an ensemble of Regional Air Quality (RAQ) models, and an intercomparison of the contributing RAQ models and two global models for the period July 2008 - June 2009 over Europe. The model forecasts were produced routinely on a daily basis in the context of the European GEMS ("Global and regional Earth-system (atmosphere) Monitoring using Satellite and in-situ data") project. The tropospheric vertical column of the RAQ ensemble median shows a spatial distribution which agrees well with the OMI NO2 observations, with a correlation r=0.8. This is higher than the correlations from any one of the individual RAQ models, which supports the use of a model ensemble approach for regional air pollution forecasting. The global models show high correlations compared to OMI, but with significantly less spatial detail, due to their coarser resolution. Deviations in the tropospheric NO2 columns of individual RAQ models from the mean were in the range of 20-34% in winter and 40-62% in summer, suggesting that the RAQ ensemble prediction is relatively more uncertain in the summer months. The ensemble median shows a stronger seasonal cycle of NO2 columns than OMI, and the ensemble is on average 50% below the OMI observations in summer, whereas in winter the bias is small. On the other hand the ensemble median shows a somewhat weaker seasonal cycle than NO2 surface observations from the Dutch Air Quality Network, and on average a negative bias of 14%. Full profile information was available for two RAQ models and for the global models. For these models the retrieval averaging kernel was applied. Minor differences are found for area-averaged model columns with and without applying the kernel, which shows that the impact of replacing the a priori profiles by the RAQ model profiles is on average small. However, the contrast between major hotspots and rural areas is stronger for the direct modeled vertical columns than the columns where the averaging kernels are applied, related to a larger relative contribution of the free troposphere and the coarse horizontal resolution in the a priori profiles compared to the RAQ models. In line with validation results reported in the literature, summertime concentrations in the lowermost boundary layer in the a priori profiles from the DOMINO product are significantly larger than the RAQ model concentrations and surface observations over the Netherlands. This affects the profile shape, and contributes to a high bias in OMI tropospheric columns over polluted regions. The global models indicate that the upper troposphere may contribute significantly to the total column and it is important to account for this in comparisons with RAQ models. A combination of upper troposphere model biases, the a priori profile effects and DOMINO product retrieval issues could explain the discrepancy observed between the OMI observations and the ensemble median in summer.

  • 112.
    Häggkvist, Kenneth
    et al.
    SMHI, Affärsverksamhet.
    Persson, Christer
    SMHI, Forskningsavdelningen, Luftmiljö.
    Robertson, Lennart
    SMHI, Forskningsavdelningen, Luftmiljö.
    Spridningsberäkningar rörande gasutsläpp från ett antal källor inom SSAB Luleå-verken1986Rapport (Övrigt vetenskapligt)
  • 113. Im, Ulas
    et al.
    Christensen, Jesper H.
    Nielsen, Ole-Kenneth
    Sand, Maria
    Makkonen, Risto
    Geels, Camilla
    Andersson, Camilla
    SMHI, Forskningsavdelningen, Luftmiljö.
    Kukkonen, Jaakko
    Lopez-Aparicio, Susana
    Brandt, Jorgen
    Contributions of Nordic anthropogenic emissions on air pollution and premature mortality over the Nordic region and the Arctic2019Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 19, nr 20, s. 12975-12992Artikel i tidskrift (Refereegranskat)
  • 114.
    Jansson, Anna
    et al.
    SMHI, Samhälle och säkerhet.
    Persson, Christer
    SMHI, Forskningsavdelningen, Luftmiljö.
    Strandberg, Gustav
    SMHI, Forskningsavdelningen, Klimatforskning - Rossby Centre.
    2D meso-scale re-analysis of precipitation, temperature and wind over Europe - ERAMESAN: Time period 1980-20042007Rapport (Övrigt vetenskapligt)
    Abstract [en]

    The need for long time series of gridded meteorological data with a fine spatial and temporal resolution has increased in recent years. The requirements for this type of gridded meteorological data fields arise from many different areas of the society, in connection to atmospheric environment studies of air quality and deposition and trends in these parameters, regional climate change, wind energy, hydrological studies etc. The aim of the present project is to investigate the possibility of producing historical, high quality and time consistent, meso-scale re analyses for the whole of Europe regarding precipitation, 2 m temperature and wind for at least 25 years back in time.The MESAN analysis system (Häggmark et al., 2000) at SMHI was chosen as a basis for the reanalysis and the system was adjusted to cover the whole of Europe. In order to find the most appropriate first guess fields to be used in the MESAN system, a pilot study was performed. ERA- 40 data from ECMWF was selected as best possible first guess fields for the re analysis. The performed re-analysis, which is denoted ERAMESAN, includes gridded data covering all Europe with a time resolution of 6 h and a spatial resolution of 0.1º (11 km) in a rotated latitude longitude coordinate system for the time-period 1980-2004. All analyses are archived in GRIB-format and stored on disc at SMHI. The dataset is also available within the EUMETNET optional programme Showcase EUROGRID.A partial validation for the years 1998-2000, using a cross validation procedure with independent observations (5.5% of the total amount of stations), shows an improvement in ERAMESAN compared to the ERA-40 data for all studied parameters with regard to root mean square deviation, mean absolute deviation and mean bias deviation for all seasons. The deviations are roughly of the order of 15% smaller compared to what is obtained from ERA-40. The frequency distribution of large precipitation amounts per day and high wind speeds are substantially better described in ERAMESAN compared to ERA-40. However, the tendency to underestimate the frequency of very large precipitation amounts or high wind speeds, compared to observations, can be seen also for ERAMESAN. It is important to be aware of this limitation when using ERAMESAN data for practical applications concerning evaluation of risks for extreme wind speeds or very large precipitation amounts or in e.g. wind energy studies.

  • 115. Jenkin, M. E.
    et al.
    Khan, M. A. H.
    Shallcross, D. E.
    Bergström, Robert
    SMHI, Forskningsavdelningen, Luftmiljö.
    Simpson, D.
    Murphy, K. L. C.
    Rickard, A. R.
    The CRI v2.2 reduced degradation scheme for isoprene2019Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 212, s. 172-182Artikel i tidskrift (Refereegranskat)
  • 116. Johansson, Christer
    et al.
    Norman, Michael
    Gidhagen, Lars
    SMHI, Forskningsavdelningen, Luftmiljö.
    Spatial & temporal variations of PM10 and particle number concentrations in urban air2007Ingår i: Environmental Monitoring & Assessment, ISSN 0167-6369, E-ISSN 1573-2959, Vol. 127, nr 1-3, s. 477-487Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The size of particles in urban air varies over four orders of magnitude (from 0.001 mu m to 10 mu m in diameter). In many cities only particle mass concentrations (PM10, i.e. particles < 10 mu m diameter) is measured. In this paper we analyze how differences in emissions, background concentrations and meteorology affect the temporal and spatial distribution of PM10 and total particle number concentrations (PNC) based on measurements and dispersion modeling in Stockholm, Sweden. PNC at densely trafficked kerbside locations are dominated by ultrafine particles (< 0.1 mu m diameter) due to vehicle exhaust emissions as verified by high correlation with NOx. But PNC contribute only marginally to PM10, due to the small size of exhaust particles. Instead wear of the road surface is an important factor for the highest PM10 concentrations observed. In Stockholm, road wear increases drastically due to the use of studded tires and traction sand on streets during winter; up to 90% of the locally emitted PM10 may be due to road abrasion. PM10 emissions and concentrations, but not PNC, at kerbside are controlled by road moisture. Annual mean urban background PM10 levels are relatively uniformly distributed over the city, due to the importance of long range transport. For PNC local sources often dominate the concentrations resulting in large temporal and spatial gradients in the concentrations. Despite these differences in the origin of PM10 and PNC, the spatial gradients of annual mean concentrations due to local sources are of equal magnitude due to the common source, namely traffic. Thus, people in different areas experiencing a factor of 2 different annual PM10 exposure due to local sources will also experience a factor of 2 different exposure in terms of PNC. This implies that health impact studies based solely on spatial differences in annual exposure to PM10 may not separate differences in health effects due to ultrafine and coarse particles. On the other hand, health effect assessments based on time series exposure analysis of PM10 and PNC, should be able to observe differences in health effects of ultrafine particles versus coarse particles.

  • 117. Jonsson, Oskar
    et al.
    Andersson, Camilla
    SMHI, Forskningsavdelningen, Luftmiljö.
    Forsberg, Bertil
    Johansson, Christer
    Air pollution episodes in Stockholm regional background air due to sources in Europe and their effects on human population2013Ingår i: Boreal environment research, ISSN 1239-6095, E-ISSN 1797-2469, Vol. 18, nr 3-4, s. 280-302Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using air quality measurements, we categorized air pollution according to source sectors in a rural background environment in southern Sweden based on hourly air-mass backward trajectories during 1997-2010. Concentrations of fine (PM2.5) and sum of fine and coarse particulate matter (PM10), accumulation mode particle number, black carbon and surface ozone were 4.0, 3.9, 4.5, 6.8 and 1.3 times higher, respectively, in air masses from the southeast as compared with those in air masses from the cleanest sector in the northwest, consistent with air-mass transport over areas with relatively high emissions of primary particulate matter (PM) and secondary PM precursors. The highest ultrafine particle numbers were associated with clean air from the northwest. We estimate that almost 7.8% and 0.6% higher premature human mortality is caused by PM2.5 and ozone exposure, respectively, when air originates from the southeast as compared with that when air originates from the northwest. Reductions of emissions in eastern Europe would reduce the highest air pollution concentrations and associated health risks. However, since air masses from the southwest are more frequent, emissions in the western part of Europe are more important for annual mean premature mortality.

  • 118. Jonsson, P
    et al.
    Bennet, Cecilia
    SMHI, Forskningsavdelningen, Luftmiljö.
    Eliasson, I
    Lindgren, E S
    Suspended particulate matter and its relations to the urban climate in Dar es Salaam, Tanzania2004Ingår i: Atmospheric Environment, ISSN 1352-2310, E-ISSN 1873-2844, Vol. 38, nr 25, s. 4175-4181Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Relationships between sources and levels of particulate matter and climatic parameters (urban heat island intensity, wind speed, temperature and relative humidity) were investigated in the coastal city of Dar es Salaam, Tanzania's largest city. Measurements were made during the wet and dry seasons of 2001 at an urban and a rural site. Five elements were used to represent different sources: K in fine particles (biomass), Zn in fine particles (industry), Cl in coarse particles (sea spray), Ti in coarse particles (soil) and Pb in fine particles (traffic). The concentrations of these elements varied considerably between the urban and rural site during both the wet and dry season, with the urban site in the dry season having the highest concentrations. Diurnal differences are also apparent, although not as straightforward. In an attempt to explain these differences, correlations between all elements and the climatic parameters were investigated. The results show that the nocturnal urban heat island intensity was positively correlated and wind speed negatively correlated with particulate levels, presumably due to the increased atmospheric stability. (C) 2004 Elsevier Ltd. All rights reserved.

  • 119.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Boundary symmetries in linear differential and integral equation problems applied to the self-consistent Green's function formalism of acoustic and electromagnetic scattering2006Ingår i: Optics Communications, ISSN 0030-4018, E-ISSN 1873-0310, Vol. 265, nr 2, s. 383-393Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Explicit symmetry relations for the Green's function subject to homogeneous boundary conditions are derived for arbitrary linear differential or integral equation problems in which the boundary surface has a set of symmetry elements. For corresponding homogeneous problems subject to inhomogeneous boundary conditions implicit symmetry relations involving the Green's function are obtained. The usefulness of these symmetry relations is illustrated by means of a recently developed self-consistent Green's function formalism of electromagnetic and acoustic scattering problems applied to the exterior scattering problem. One obtains explicit symmetry relations for the volume Green's function, the surface Green's function, and the interaction operator, and the respective symmetry relations are shown to be equivalent. This allows us to treat boundary symmetries of volume-integral equation methods, boundary-integral equation methods, and the T matrix formulation of acoustic and electromagnetic scattering under a common theoretical framework. By specifying a specific expansion basis the coordinate-free symmetry relations of, e.g., the surface Green's function can be brought into the form of explicit symmetry relations of its expansion coefficient matrix. For the specific choice of radiating spherical wave functions the approach is illustrated by deriving unitary reducible representations of non-cubic finite point groups in this basis, and by deriving the corresponding explicit symmetry relations of the coefficient matrix., The reducible representations can be reduced by group-theoretical techniques, thus bringing the coefficient matrix into block-diagonal form, which can greatly reduce ill-conditioning problems in numerical applications. (c) 2006 Elsevier B.V. All rights reserved.

  • 120.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Electromagnetic scattering by nonspherical particles: Recent advances2010Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 111, nr 11, s. 1788-1790Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    This note gives a short introduction to the reprint of the article "Numerical methods in electromagnetic scattering theory" by Kahnert, M (JQSRT 2003:79-80:775-824). Some of the most important developments in the field since the publication of this article are briefly reviewed. A list of typos that have been identified in the original article is given in the appendix. (C) 2009 Elsevier Ltd. All rights reserved.

  • 121.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Information constraints in variational data assimilation2018Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 144, nr 716, s. 2230-2244Artikel i tidskrift (Refereegranskat)
  • 122.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Irreducible representations of finite groups in the T-matrix formulation of the electromagnetic scattering problem2005Ingår i: Optical Society of America. Journal A: Optics, Image Science, and Vision, ISSN 1084-7529, E-ISSN 1520-8532, Vol. 22, nr 6, s. 1187-1199Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    For particles with discrete geometrical symmetries, a group-theoretical method is presented for transforming the matrix quantities in the T-matrix description of the electromagnetic scattering problem from the reducible basis of vector spherical wave functions into a new basis in which all matrix quantities become block diagonal. The notorious ill-conditioning problems in the inversion of the Q matrix are thus considerably alleviated, and the matrix inversion becomes numerically more expedient. The method can be applied to any point group. For the specific example of the D-6h group, it is demonstrated that computations in the new basis are faster by a factor of 3.6 as compared with computations that use the reducible basis. Most importantly, the method is capable of extending the range of size parameters for which convergent results can be obtained by 50%. (c) 2005 Optical Society of America

  • 123.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Light scattering by particles with boundary symmetries2008Ingår i: Light Scattering Reviews 3: Light Scattering and Reflection / [ed] Kokhanovsky, Alexander, Berlin: Springer , 2008, s. 69-107Kapitel i bok, del av antologi (Refereegranskat)
  • 124.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Modelling radiometric properties of inhomogeneous mineral dust particles: Applicability and limitations of effective medium theories2015Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 152, s. 16-27Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect of inhomogeneous mineralogical composition on the optical properties of mineral dust particles is investigated. More specifically, spheres composed of a non-absorbing mineral with multiple spherical hematite inclusions are considered. The size of the particles, the number of inclusions, and the hematite volume fraction are varied, and the differential and integral optical properties are compared to those computed for homogeneous spheres. The effective refractive index of the homogeneous spheres is obtained (i) by use of four conventional effective medium approximations; and (ii) by freely varying the real and imaginary parts of the refractive index until a best-fit of the scattering matrix elements is achieved for all scattering angles and particle sizes. Among the integral radiometric observables, the single scattering albedo is most sensitive to particle inhomogeneity, while the extinction and scattering efficiency and the asymmetry parameter are rather insensitive. The phase function, the degree of linear polarisation, the linear depolarisation, and, indeed, all elements of the scattering matrix are strongly modulated by particle inhomogeneity. None of the effective medium approaches, not even the best-fit method, are able to reproduce the single scattering albedo and the scattering matrix elements over the entire range of particle sizes. (C) 2014 Elsevier Ltd. All rights reserved.

  • 125.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Modelling the optical and radiative properties of freshly emitted light absorbing carbon within an atmospheric chemical transport model2010Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, nr 3, s. 1403-1416Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Light absorbing carbon (LAC) aerosols have a complex, fractal-like aggregate structure. Their optical and radiative properties are notoriously difficult to model, and approximate methods may introduce large errors both in the interpretation of aerosol remote sensing observations, and in quantifying the direct radiative forcing effect of LAC. In this paper a numerically exact method for solving Maxwell's equations is employed for computing the optical properties of freshly emitted, externally mixed LAC aggregates. The computations are performed at wavelengths of 440 nm and 870 nm, and they cover the entire size range relevant for modelling these kinds of aerosols. The method for solving the electromagnetic scattering and absorption problem for aggregates proves to be sufficiently stable and fast to make accurate multiple-band computations of LAC optical properties feasible. The results from the electromagnetic computations are processed such that they can readily be integrated into a chemical transport model (CTM), which is a prerequisite for constructing robust observation operators for chemical data assimilation of aerosol optical observations. A case study is performed, in which results obtained with the coupled optics/CTM model are employed as input to detailed radiative transfer computations at a polluted European location. It is found that the still popular homogeneous sphere approximation significantly underestimates the radiative forcing at top of atmosphere as compared to the results obtained with the aggregate model. Notably, the LAC forcing effect predicted with the aggregate model is less than that one obtains by assuming a prescribed mass absorption cross section for LAC.

  • 126.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Numerical solutions of the macroscopic Maxwell equations for scattering by non-spherical particles: A tutorial review2016Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 178, s. 22-37Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Numerical solution methods for electromagnetic scattering by non-spherical particles comprise a variety of different techniques, which can be traced back to different assumptions and solution strategies applied to the macroscopic Maxwell equations. One can distinguish between time- and frequency-domain methods; further, one can divide numerical techniques into finite-difference methods (which are based on approximating the differential operators), separation-of-variables methods (which are based on expanding the solution in a complete set of functions, thus approximating the fields), and volume integral-equation methods (which are usually solved by discretisation of the target volume and invoking the long-wave approximation in each volume cell). While existing reviews of the topic often tend to have a target audience of program developers and expert users, this tutorial review is intended to accommodate the needs of practitioners as well as novices to the field. The required conciseness is achieved by limiting the presentation to a selection of illustrative methods, and by omitting many technical details that are not essential at a first exposure to the subject. On the other hand, the theoretical basis of numerical methods is explained with little compromises in mathematical rigour; the rationale is that a good grasp of numerical light scattering methods is best achieved by understanding their foundation in Maxwell's theory. (C) 2015 Elsevier Ltd. All rights reserved.

  • 127.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Numerically exact computation of the optical properties of light absorbing carbon aggregates for wavelength of 200 nm-12.2 mu m2010Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, nr 17, s. 8319-8329Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The optical properties of externally mixed light absorbing carbon (LAC) aggregates are computed over the spectral range from 200 nm-12.2 mu m by use of the numerically exact superposition T-matrix method. The spectral computations are tailored to the 14-band radiation model employed in the Integrated Forecasting System operated at the European Centre for Medium Range Weather Forecast. The size- and wavelength dependence of the optical properties obtained with the fractal aggregate model differs significantly from corresponding results based on the homogeneous sphere approximation, which is still commonly employed in climate models. The computational results are integrated into the chemical transport model MATCH (Multiple-scale Atmospheric Transport and CHemistry modelling system) to compute 3-D fields of size-averaged aerosol optical properties. Computational results obtained with MATCH are coupled to a radiative transfer model to compute the shortwave radiative impact of LAC. It is found that the fractal aggregate model gives a shortwave forcing estimate that is twice as high as that obtained with the homogeneous sphere approximation. Thus previous estimates based on the homogeneous sphere model may have substantially underestimated the shortwave radiative impact of freshly emitted LAC.

  • 128.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    On the Discrepancy between Modeled and Measured Mass Absorption Cross Sections of Light Absorbing Carbon Aerosols2010Ingår i: Aerosol Science and Technology, ISSN 0278-6826, E-ISSN 1521-7388, Vol. 44, nr 6, s. 453-460Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent modeling studies based on the Rayleigh-Debye-Gans (RDG) approximation have revealed a discrepancy between modeled and measured mass absorption cross sections (MAC) for atmospheric light absorbing carbon (LAC) aerosols. One plausible explanation is that this discrepancy is due to errors introduced by neglecting electromagnetic interactions among monomers in LAC aggregates within the RDG approximation. Here we compute MAC by use of numerically exact solutions to Maxwell's equations and investigate the sensitivity of the results to a variation in the aggregates' physical properties and refractive index. The results do confirm that approximate methods can introduce large errors in the results for the optical properties. However, these errors alone cannot explain the discrepancy between measured and modeled values of MAC. An agreement between observations and theoretical results can only be attained when assuming a fairly high value of the real and imaginary parts of the refractive index along the void-fraction curve and a mass density not exceeding 1.5-1.7 g/cm3.

  • 129.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    On the observability of chemical and physical aerosol properties by optical observations: Inverse modelling with variational data assimilation2009Ingår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 61, nr 5, s. 747-755Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Determining size-resolved chemical composition of aerosols is important for modelling the aerosols' direct and indirect climate impact, for source-receptor modelling, and for understanding adverse health effects of particulate pollutants. Obtaining this kind of information from optical remote sensing observations is an ill-posed inverse problem. It can be solved by variational data assimilation in conjunction with an aerosol transport model. One important question is how much information about the particles' physical and chemical properties is contained in the observations. We perform a numerical experiment to test the observability of size-dependent aerosol composition by remote sensing observations. An aerosol transport model is employed to produce a reference and a perturbed aerosol field. The perturbed field is taken as a proxy for a background estimate subject to uncertainties. The reference result represents the 'true' state of the system. Optical properties are computed from the reference results and are assimilated into the perturbed model. The assimilation results reveal that inverse modelling of optical observations significantly improves the background estimate. However, the optical observations alone do not contain sufficient information for producing a faithful retrieval of the size-resolved aerosol composition. The total mass mixing ratios, on the other hand, are retrieved with remarkable accuracy.

  • 130.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Optical properties of black carbon aerosols encapsulated in a shell of sulfate: comparison of the closed cell model with a coated aggregate model2017Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 25, nr 20, s. 24579-24593Artikel i tidskrift (Refereegranskat)
  • 131.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    The T-matrix code Tsym for homogeneous dielectric particles with finite symmetries2013Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 123, s. 62-78Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A T-matrix code tailored to non-axisymmetric particles with finite symmetries is described. The code exploits geometric symmetries of particles by use of group theoretical methods. Commutation relations of the T-matrix are implemented for reducing CPU-time requirements. Irreducible representations of finite groups are employed for alleviating ill-conditioning problems in numerical computations. Further, an iterative T-matrix method for particles with small-scale surface perturbations is implemented. The code can compute both differential and integrated optical properties of particles in,either fixed or random orientation. Methods for testing the convergence and correctness of the computational results are discussed. The package also includes a database of pre-computed group-character tables, as well as an interface to the GAP programming language for computational group theory. The code can be downloaded at http://www.rss.chalmers.se/similar to kahnert/Tsym.html. (C) 2013 Elsevier Ltd. All rights reserved.

  • 132.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    T-matrix computations for particles with high-order finite symmetries2013Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 123, s. 79-91Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The use of group theoretical methods can substantially reduce numerical ill-conditioning problems in T-matrix computations. There are specific problems related to obtaining the irreducible characters of high-order symmetry groups and to the construction of a transformation from the basis of vector spherical wave functions to the irreducible basis of high-order symmetry groups. These problems are addressed, and numerical solutions are discussed and tested. An important application of the method is non-convex particles perturbed with high-order polynomials. Such morphologies can serve as models for particles with small-scale surface roughness, such as mineral aerosols, atmospheric ice particles with rimed surfaces, and various types of cosmic dust particles. The method is tested for high-order 3D-Chebyshev particles, and the performance of the method is gauged by comparing the results to computations based on iteratively solving a Lippmann-Schwinger T-matrix equation. The latter method trades ill-conditioning problems for potential slow-convergence problems, and it is rather specific, as it is tailored to particles with small-scale surface roughness. The group theoretical method is general and not plagued by slow-convergence problems. The comparison of results shows that both methods achieve a comparable numerical stability. This suggests that for particles with high-order symmetries the group-theoretical approach is able to overcome the illconditioning problems. Remaining numerical limitations are likely to be associated with loss-of-precision problems in the numerical evaluation of the surface integrals. (C) 2012 Elsevier Ltd. All rights reserved.

  • 133.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Variational data analysis of aerosol species in a regional CTM: background error covariance constraint and aerosol optical observation operators2008Ingår i: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 60, nr 5, s. 753-770Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A multivariate variational data assimilation scheme for the Multiple-scale Atmospheric Transport and CHemistry (MATCH) model is presented and tested. A spectral, non-separable approach is chosen for modelling the background error constraints. Three different methods are employed for estimating background error covariances, and their analysis performances are compared. Observation operators for aerosol optical parameters are presented for externally mixed particles. The assimilation algorithm is tested in conjunction with different background error covariance matrices by analysing lidar observations of aerosol backscattering coefficient. The assimilation algorithm is shown to produce analysis increments that are consistent with the applied background error statistics. Secondary aerosol species show no signs of chemical relaxation processes in sequential assimilation of lidar observations, thus indicating that the data analysis result is well balanced. However, both primary and secondary aerosol species display emission- and advection-induced relaxations.

  • 134.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Andersson, Emma
    How much information do extinction and backscattering measurements contain about the chemical composition of atmospheric aerosol?2017Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, nr 5, s. 3423-3444Artikel i tidskrift (Refereegranskat)
  • 135.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, T
    Uncertainties in measured and modelled asymmetry parameters of mineral dust aerosols2006Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 100, nr 1-3, s. 173-178Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The error caused by the uncertainty in the refractive index in the determination of the asymmetry parameter g is studied for a variety of mineral dust aerosol samples at two different optical wavelengths. Lorenz-Mie computations for spherical model particles are compared with results based on laboratory-measured phase functions in conjunction with a commonly used extrapolation method. The difference between the g-value based on measurements and the g-value based on Lorenz-Mie simulations is generally on the same order of magnitude as the error caused by the uncertainty in the refractive index m. For larger effective radii the error in g related to the use of spherical model particles is even larger than that related to the uncertainty in in. This indicates that the use of spherical model particles can be among the major error sources in the determination of the asymmetry parameter of dust aerosols. (c) 2005 Elsevier Ltd. All rights reserved.

  • 136.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, T.
    Raisanen, P.
    Mie simulations as an error source in mineral aerosol radiative forcing calculations2007Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 133, nr 623, s. 299-307Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The role of aerosols remains a major uncertainty for climate and climate change. For the direct radiative forcing by mineral aerosols, the uncertainty in the refractive index in has been regarded as the most important error source, while the impact of aerosol non-sphericity has been considered a minor issue and is neglected in climate models. Here, the errors caused by the spherical particle approximation (SPA) are evaluated by comparing radiative fluxes based on (i) Mie simulations and (ii) laboratory measurements of aerosol optical properties. Furthermore, they are contrasted with the errors related to the uncertainty in the refractive index. These two error sources are found to be of comparable magnitude, although they are strongly dependent on optical depth, surface albedo, and particle size. Thus, our results provide evidence that, contrary to common beliefs, the use of spherical model particles in radiative transfer simulations is probably among the major sources of error in quantifying the climate forcing effect of mineral aerosols. This stems from misrepresentation of the scattering phase function and the asymmetry parameter. Aerosol single-scattering computations based on non-spherical model particles are expected to reduce the shape-related errors and thus significantly improve the accuracy of radiative forcing simulations. Copyright (c) 2007 Royal Meteorological Society.

  • 137.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, T
    Veihelmann, B
    Spherical and spheroidal model particles as an error source in aerosol climate forcing and radiance computations: A case study for feldspar aerosols2005Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 110, nr D18, artikel-id D18S13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A case study for feldspar aerosols is conducted to assess the errors introduced by simple model particles in radiance and flux simulations. The spectral radiance field and net flux are computed for a realistic phase function of feldspar aerosols measured in the laboratory at 633 nm. Results are compared to computations with spherical and spheroidal model particles. It is found that the use of spherical model particles introduces large spectral radiance errors at top of atmosphere (TOA) between -6 and 31%. Using a new shape parameterization of spheroids reduces the error range to -1 to 6%. Spherical model particles yield an absolute TOA spectral net flux error of -6.1 mW m(-2) nm(-1). An equiprobable shape distribution of spheroids results in only minor improvements, but the new shape parameterization yields an error of only -0.8 mW m(-2) nm(-1). A variation of the refractive index m reveals that the resulting changes in the TOA spectral net flux are slightly smaller than the error caused by assuming the particles to be spherical. However, the uncertainty of m is commonly considered the major error source in aerosol radiative forcing simulations, whereas the use of spherical model particles is often not seriously questioned. This study implies that this notion needs to be reconsidered. Should the relative spectral net flux errors be representative for the entire spectrum, then the use of spherical model particles may be among the major error sources in broadband flux simulations. The new spheroidal shape parameterization can, however, substantially improve the results.

  • 138.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Variational data-analysis method for combining laboratory-measured light-scattering phase functions and forward-scattering computations2007Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 103, nr 1, s. 27-42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A method is developed based on the variational data-analysis formalism to combine laboratory-measured scattering phase functions with forward-scattering phase function computations based on independent size distribution (SD) measurements. The algorithm yields an optimal estimate of the true phase function of the system that is not only based on the measurements and the computational results but also on all available information of the error variances and, if applicable, error covariances of the measured and computed phase functions. The high flexibility of the method is demonstrated by applying it to phase functions of feldspar and fly ash aerosols. Further, the algorithm is employed to determine the asymmetry parameter g of nine different mineral aerosol samples at two different optical wavelengths, and to assess the relative importance of different error sources in the determination of g. It is found that the use of spherical model particles in simulations of g can result in errors on the same order of magnitude as the uncertainty of the refractive index. The use of spherical model particles in computations of forward scattering, however, is found to be only a minor error source. (c) 2006 Elsevier Ltd. All rights reserved.

  • 139.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Lindqvist, Hannakaisa
    Models for integrated and differential scattering optical properties of encapsulated light absorbing carbon aggregates2013Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 21, nr 7, s. 7974-7993Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Optical properties of light absorbing carbon (LAC) aggregates encapsulated in a shell of sulfate are computed for realistic model geometries based on field measurements. Computations are performed for wavelengths from the UV-C to the mid-IR. Both climate- and remote sensing-relevant optical properties are considered. The results are compared to commonly used simplified model geometries, none of which gives a realistic representation of the distribution of the LAC mass within the host material and, as a consequence, fail to predict the optical properties accurately. A new core-gray shell model is introduced, which accurately reproduces the size- and wavelength dependence of the integrated and differential optical properties. (C) 2013 Optical Society of America

  • 140.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Lindqvist, Hannakaisa
    Review: Model particles in atmospheric optics2014Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 146, s. 41-58Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    This review paper provides an overview over model geometries for computing light scattering by small particles. The emphasis is on atmospheric optics, although much of this review will also be relevant to neighbouring fields, in particular to astronomy. Various morphological particle properties are discussed, such as overall nonsphericity, pristine shapes, aggregation, and different forms of inhomogeneity, e.g. porous and compact inhomogeneous morphologies, as well as encapsulated aggregates. Models employed to reproduce the optical properties of complex particles range from strongly simplified to highly realistic and morphologically sophisticated model geometries. Besides reviewing the most recent literature, we discuss the idea behind models of varying degree of complexity with regard to the intended use of the models. Applications range from fundamental studies of light scattering processes to routine applications of particle optics look-up tables in operational modelling systems. (C) 2014 Elsevier Ltd. All rights reserved.

  • 141.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Lindqvist, Hannakaisa
    Ebert, Martin
    Optical properties of light absorbing carbon aggregates mixed with sulfate: assessment of different model geometries for climate forcing calculations2012Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 20, nr 9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Light scattering by light absorbing carbon (LAC) aggregates encapsulated into sulfate shells is computed by use of the discrete dipole method. Computations are performed for a UV, visible, and IR wavelength, different particle sizes, and volume fractions. Reference computations are compared to three classes of simplified model particles that have been proposed for climate modeling purposes. Neither model matches the reference results sufficiently well. Remarkably, more realistic core-shell geometries fall behind homogeneous mixture models. An extended model based on a core-shell-shell geometry is proposed and tested. Good agreement is found for total optical cross sections and the asymmetry parameter. (C) 2012 Optical Society of America

  • 142.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Markkanen, Johannes
    Morphological models for inhomogeneous particles: Light scattering by aerosols, cometary dust, and living cells2016Ingår i: Light Scattering Reviews, Volume 11: Light Scattering and Radiative Transfer / [ed] Kokhanovsky, Alexander, Berlin: Springer , 2016, 2, s. 299-337Kapitel i bok, del av antologi (Refereegranskat)
  • 143.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Mauno, Paivi
    On the impact of non-sphericity and small-scale surface roughness on the optical properties of hematite aerosols2011Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 112, nr 11, s. 1815-1824Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We perform a comparative modelling study to investigate how different morphological features influence the optical properties of hematite aerosols. We consider high-order Chebyshev particles as a proxy for aerosol with a small-scale surface roughness, and spheroids as a model for nonspherical aerosols with a smooth boundary surface. The modelling results are compared to those obtained for homogeneous spherical particles. It is found that for hematite particles with an absorption efficiency of order unity the difference in optical properties between spheres and spheroids disappears. For optically softer particles, such as ice particles at far-infrared wavelengths, this effect can be observed for absorption efficiencies lower than unity. The convergence of the optical properties of spheres and spheroids is caused by absorption and quenching of internal resonances inside the particles, which depend both on the imaginary part of the refractive index and on the size parameter, and to some extent on the real part of the refractive index. By contrast, small-scale surface roughness becomes the dominant morphological feature for large particles. This effect is likely to depend on the amplitude of the surface roughness, the relative significance of internal resonances, and possibly on the real part of the refractive index. The extinction cross section is rather insensitive to surface roughness, while the single-scattering albedo, asymmetry parameter, and the Mueller matrix are strongly influenced. Small-scale surface roughness reduces the backscattering cross section by up to a factor of 2-3 as compared to size-equivalent particles with a smooth boundary surface. This can have important implications for the interpretation of lidar backscattering observations. (C) 2011 Elsevier Ltd. All rights reserved.

  • 144.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Mauno, Paivi
    On the impact of non-sphericity and small-scale surface roughness on the optical properties of hematite aerosols (vol 112, pg 1815, 2011)2012Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 113, nr 1, s. 117-117Artikel i tidskrift (Refereegranskat)
  • 145.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Nousiainen, Timo
    Thomas, Manu Anna
    SMHI, Forskningsavdelningen, Luftmiljö.
    Tyynela, Jani
    Light scattering by particles with small-scale surface roughness: Comparison of four classes of model geometries2012Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 113, nr 18, s. 86-97Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We compare four different model geometries for particles with small-scale surface roughness. The geometries are based on regular and stochastic surface perturbations, as well as on 2D- and 3D-roughness models. We further compare T-matrix and discrete dipole computations. Particle size parameters of 5 and 50 are considered, as well as refractive indices of 1.6+0.0005i and 3+0.1i. The effect of small-scale surface roughness on the intensity and polarisation of the scattered light strongly depends on the size parameter and refractive index. In general, 2D surface roughness models predict stronger effects than 3D models. Stochastic surface roughness models tend to predict the strongest depolarising effects, while regular surface roughness models can have a stronger effect on the angular distribution of the scattered intensity. Computations with the discrete dipole approximation only cover a limited range of size parameters. T-matrix computations allow us to significantly extend that range, but at the price of restricting the model particles to symmetric surface perturbations with small amplitudes. (C) 2012 Elsevier Ltd. All rights reserved.

  • 146.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Rother, Tom
    Electromagnetic Wave Scattering on Nonspherical Particles: Basic Methodology and Simulations2014 (uppl. 2)Bok (Övrigt vetenskapligt)
  • 147.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Rother, Tom
    Modeling optical properties of particles with small-scale surface roughness: combination of group theory with a perturbation approach2011Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 19, nr 12, s. 11138-11151Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A T-matrix method for scattering by particles with small-scale surface roughness is presented. The method combines group theory with a perturbation expansion approach. Group theory is found to reduce CPU-time by 4-6 orders of magnitude. The perturbation expansion extends the range of size parameters by a factor of 5 compared to non-perturbative methods. An application to optically hard particles shows that small-scale surface roughness changes scattering in side-and backscattering directions, and it impacts the single-scattering albedo. This can have important implications for interpreting remote sensing observations, and for the climate impact of mineral aerosols. (C) 2011 Optical Society of America

  • 148.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Sandvik, Anne Dagrun
    Biryulina, Marina
    Stamnes, Jakob J.
    Stamnes, Knut
    Impact of ice particle shape on short-wave radiative forcing: A case study for an arctic ice cloud2008Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 109, nr 7, s. 1196-1218Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We used four different non-spherical particle models to compute optical properties of an arctic ice cloud and to simulate corresponding cloud radiative forcings and fluxes. One important finding is that differences in cloud forcing, downward flux at the surface, and absorbed flux in the atmosphere resulting from the use of the four different ice cloud particle models are comparable to differences in these quantities resulting from changing the surface albedo from 0.4 to 0.8, or by varying the ice water content (IWC) by a factor of 2. These findings show that the use of a suitable non-spherical ice cloud particle model is very important for a realistic assessment of the radiative impact of arctic ice clouds. The differences in radiative broadband fluxes predicted by the four different particle models were found to be caused mainly by differences in the optical depth and the asymmetry parameter. These two parameters were found to have nearly the same impact on the predicted cloud forcing. Computations were performed first by assuming a given vertical profile of the particle number density, then by assuming a given profile of the IWC. In both cases, the differences between the cloud radiative forcings computed with the four different non-spherical particle models were found to be of comparable magnitude. This finding shows that precise knowledge of ice particle number density or particle mass is not sufficient for accurate prediction of ice cloud radiative forcing. It is equally important to employ a non-spherical shape model that accurately reproduces the ice particle's dimension-to-volume ratio and its asymmetry parameter. The hexagonal column/plate model with air-bubble inclusions seems to offer the highest degree of flexibility. (c) 2007 Elsevier Ltd. All rights reserved.

  • 149.
    Kahnert, Michael
    et al.
    SMHI, Forskningsavdelningen, Luftmiljö.
    Scheirer, Ronald
    SMHI, Forskningsavdelningen, Atmosfärisk fjärranalys.
    Multiple scattering by aerosols as seen from CALIPSO - a Monte-Carlo modelling study2019Ingår i: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 27, nr 23, s. 33684-33700Artikel i tidskrift (Refereegranskat)
  • 150. Kanngiesser, Franz
    et al.
    Kahnert, Michael
    SMHI, Forskningsavdelningen, Luftmiljö.
    Calculation of optical properties of light-absorbing carbon with weakly absorbing coating: A model with tunable transition from film-coating to spherical-shell coating2018Ingår i: Journal of Quantitative Spectroscopy and Radiative Transfer, ISSN 0022-4073, E-ISSN 1879-1352, Vol. 216, s. 17-36Artikel i tidskrift (Refereegranskat)
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