Condensable aerosols contribute to inconsistent reporting

Condensable primary organic aerosol emissions, described from here on as condensables, are organic compounds that occur in the gas (vapour) phase at the chimney stack, but as the stack air is cooled and diluted, they can undergo both condensation and evaporation processes. If we measure them incorrectly, we might under- or overestimate the amount of particulate matter (PM) or gas that enters the atmosphere. This has led to considerable debate in recent years as there are no clear definitions in inventories such as EMEP that explain how such condensable organics should be counted.

The emission factors (EF) are clearly dependent on filter types, dilution, and sampling conditions of the emissions. Studies have shown that these discrepancies will impact the consistency of emissions if treated differently by different nations. Most of the focus has been on harmonising the emission factors for residential wood-burning but it can also apply to other contexts such as road transport. Ongoing efforts to harmonise data have been made in 2020–2022. One problem has been that there are no fixed guidelines that describe how to choose appropriate emission factors in some inventories.

Closer examination has revealed a wide variation in implied biomass emissions that was too large to be explained by the inclusion of condensables and the variation in technology and abatement levels alone. There is thus a strong need for consistent sets of emission data from residential wood-burning. New emission factors have been established for the most important sources of primary PM from residential combustion, based on the available literature, consistently taking condensable organics into account. But the emission factors were found to vary by a factor of up to 10 for the same installations due to variations in measurements and local conditions. Another important factor is how to weigh in “bad combustion” of moist wood and loading, which can impact emission numbers and are currently not accounted for, despite being common issues among inexperienced users.

Given all the uncertainties described above, three scenarios were defined (ideal, typical and high EF scenario) to illustrate the range of uncertainty, and to support the modelling exercise in this project. These are the “typical” case, which is as described in the preceding paragraph, an alternative “ideal” case that excludes the impact of “bad combustion”, and a “high EF” scenario in which higher emission factors are assumed than in the typical scenario. The modelling indicates that including condensables in a consistent way for all countries gives model results (concentrations, trends and bias) that are in better agreement with observations for OC and PM_2.5 than EMEP emissions, which use inconsistent data for condensables from different countries.

The report was produced in collaboration between five European research institutes and can be found here: https://pub.norden.org/temanord2022-540/#108663