Future changes in isoprene-epoxydiol-derived secondary organic aerosol (IEPOX SOA) under the Shared Socioeconomic Pathways: the importance of physicochemical dependency
Authors: Jo Duseong S. , Hodzic Alma , Emmons Louisa K. , Tilmes Simone , Schwantes Rebecca H. , Mills Michael J. , Campuzano-Jost Pedro , Hu Weiwei , Zaveri Rahul A. , Easter Richard C. , Singh Balwinder , Lu Zheng , Schulz Christiane , Schneider Johannes , Shilling John E. , Wisthaler Armin , Jimenez Jose L. 
Published: March 2021

Abstract. Secondary organic aerosol (SOA) is a dominant contributor of fine
particulate matter in the atmosphere, but the complexity of SOA formation
chemistry hinders the accurate representation of SOA in models.
Volatility-based SOA parameterizations have been adopted in many recent
chemistry modeling studies and have shown a reasonable performance compared
to observations. However, assumptions made in these empirical
parameterizations can lead to substantial errors when applied to future
climatic conditions as they do not include the mechanistic understanding of
processes but are rather fitted to laboratory studies of SOA formation. This
is particularly the case for SOA derived from isoprene epoxydiols
(IEPOX SOA), for which we have a higher level of understanding of the
fundamental processes than is currently parameterized in most models. We
predict future SOA concentrations using an explicit mechanism and compare
the predictions with the empirical parameterization based on the volatility
basis set (VBS) approach. We then use the Community Earth System Model 2
(CESM2.1.0) with detailed isoprene chemistry and reactive uptake processes
for the middle and end of the 21st century under four Shared Socioeconomic
Pathways (SSPs): SSP1–2.6, SSP2–4.5, SSP3–7.0, and SSP5–8.5. With the
explicit chemical mechanism, we find that IEPOX SOA is predicted to increase
on average under all future SSP scenarios but with some variability in
the results depending on regions and the scenario chosen. Isoprene emissions
are the main driver of IEPOX SOA changes in the future climate, but the IEPOX SOA
yield from isoprene emissions also changes by up to 50 % depending on the
SSP scenario, in particular due to different sulfur emissions. We conduct
sensitivity simulations with and without CO2 inhibition of isoprene
emissions that is highly uncertain, which results in factor of 2
differences in the predicted IEPOX SOA global burden, especially for the
high-CO2 scenarios (SSP3–7.0 and SSP5–8.5). Aerosol pH also plays a
critical role in the IEPOX SOA formation rate, requiring accurate
calculation of aerosol pH in chemistry models. On the other hand, isoprene
SOA calculated with the VBS scheme predicts a nearly constant SOA yield from
isoprene emissions across all SSP scenarios; as a result, it mostly follows
isoprene emissions regardless of region and scenario. This is because the
VBS scheme does not consider heterogeneous chemistry; in other words, there
is no dependency on aerosol properties. The discrepancy between the explicit
mechanism and VBS parameterization in this study is likely to occur for
other SOA components as well, which may also have dependencies that cannot
be captured by VBS parameterizations. This study highlights the need for
more explicit chemistry or for parameterizations that capture the
dependence on key physicochemical drivers when predicting SOA
concentrations for climate studies.