Condensable primary organic aerosol emissions are a class of organic compounds that arevapour phase at stack conditions, but which undergo both condensation and evaporation processes as the stack air is cooled and diluted upon discharge into ambient air. Emission factorsmeasured in or close to the high-temperature high-concentration exhaust stack or pipe maymisrepresent, and even miss, the amount of PM or gas that actually enters the atmosphere,depending on the filters, dilution and sampling conditions of the emission measurement. Inthe current emission reporting to EMEP/CLRTAP there is no clear definition of whether condensable organics are included or not, and, if included, to what extent.

In March 2020 MSC-W hosted an expert workshop on condensable organic aerosol emissions (funded by the Nordic Council of Ministers), which brought together experts in emissions, measurements, emission inventories, atmospheric chemistry, air quality models andpolicy from Europe and North America, and to create a much better understanding of theissues and possible approaches for dealing with this important class of compounds.

More than 30 experts took part in the (zoom) meeting, including EMEP Chairs (EMEPSteering Body, TFIAM, TFMM, TFEIP, TFTEI), EMEP Centres (MSC-W, CEIP, CIAM),inventory developers (TNO, CIAM, COPERT, and national experts from UBA - Germany,SINTEF - Norway, IVL, ACES, Swedish EPA - Sweden, CITEPA, INERIS - France, ECCC- Canada, Univ. Patras - Greece), measurement experts (PSI - Switzerland, INERIS - France,Univ. York - England, NC State University - USA), industry (Concawe), the US EPA and theEuropean Commission. The workshop discussed a number of approaches for dealing with thisimportant class of compounds. This executive summary presents some of the key messagesfrom the workshop. Further background, addressing the technical matters in more detail canbe found in the main body of the report.

The main idea of the workshop was to promote discussion among different communitiesand Task Forces that have different expertise and needs with regard to condensable organics and PM emission inventories. In order to aid these discussions, a number of importantquestions were identified:

1. For which source categories are condensable organics important?

2. What is included in official national and other emission inventories?

3. Do we expect emissions of condensable organics to be missing in these inventories?

4. Can we reliably predict the contribution of condensable vapours from major sources toambient PM using data from a smaller number of representative cases?

5. Can we recommend a practical approach for inclusion (or exclusion) of condensables in (a) emission inventories, and (b) chemical transport models?

One of the ongoing major tasks for the Air Convention (CLRTAP) is the revision of the socalled ‘Gothenburg’ Protocol 1, with a final report of the review to be completed during 2022.It is important to note that for this review process, the consideration of condensables facesseveral (competing) challenges:

(a) The need for emission data as soon as possible that are consistent across countries inorder to get a fair ‘optimised’ distribution of emission abatement efforts aimed at improving health and ecosystems protection targets;

(b) The difficulties to change existing practices of some countries;

(c) The wish of scientists to start multi-year work programs for the best possible way todefine and quantify condensable emissions and/or secondary PM formation in the atmosphere.

(d) The need to assess the available options for short and longer term actions in terms ofe.g. time frame (feas