Biogenic secondary organic aerosol sensitivity to organic aerosol simulation schemes in climate projections

Organic aerosol (OA) can have important impacts on air quality and human health because of its large contribution to atmospheric fine aerosol and its chemical composition, including many toxic compounds. Simulation of this type of aerosol is difficult, since there are many unknowns in its nature and mechanism and processes involved in its formation. These uncertainties become even more important in the context of a changing climate because different mechanisms, and their representation in atmospheric models, imply different sensitivities to changes in climate variables. In this work, the effects caused by using different schemes to simulate OA are explored. Three schemes are used in this work: (1) a molecular scheme; (2) a standard volatility basis set (VBS) scheme with anthropogenic aging; and (3) a modified VBS scheme containing functionalization, fragmentation and formation of nonvolatile secondary organic aerosol (SOA) for all semi-volatile organic compounds (SVOCs). Five years of historic and five years of future simulations were performed using the RCP8.5 climatic scenario. The years were chosen in a way to maximize the differences between future and historic simulations. The study focuses on biogenic SOA (BSOA), since the contribution of this fraction of BSOA among OA is major in both historic and future scenarios (40 % to 78 % for different schemes in historic simulations). Simulated OA and BSOA concentrations with different schemes are different, with the molecular scheme showing the highest concentrations among the three schemes. The comparisons show that for the European area, the modified VBS scheme shows the highest relative change between future and historic simulations, while the molecular scheme shows the lowest (a factor of 2 lower). These changes are largest over the summer period for BSOA because the higher temperatures increase terpene and isoprene emissions, the major precursors of BSOA. This increase is partially offset by a temperature-induced shift of SVOCs to the gas phase. This shift is indeed scheme dependent, and it is shown that it is the least pronounced for the modified VBS scheme including a full suite of aerosol aging processes, comprising also formation of nonvolatile aerosol. For the Mediterranean Sea, without BVOC emissions, the OA changes are less pronounced and, at least on an annual average, more similar between different schemes. Our results warrant further developments in organic aerosol schemes used for air quality modeling to reduce their uncertainty, including sensitivity to climate variables (temperature).