New SOA Treatments Within the Energy Exascale Earth System Model (E3SM): Strong Production and Sinks Govern Atmospheric SOA Distributions and Radiative Forcing

Abstract

Secondary organic aerosols (SOA) are large contributors to fine particle mass loading and number concentration and interact with clouds and radiation. Several processes affect the formation, chemical transformation, and removal of SOA in the atmosphere. For computational efficiency, global models use simplified SOA treatments, which often do not capture the dynamics of SOA formation. Here we test more complex SOA treatments within the global Energy Exascale Earth System Model (E3SM) to investigate how simulated SOA spatial distributions respond to some of the important but uncertain processes affecting SOA formation, removal, and lifetime. We evaluate model predictions with a suite of surface, aircraft, and satellite observations that span the globe and the full troposphere. Simulations indicate that both a strong production (achieved here by multigenerational aging of SOA precursors that includes moderate functionalization) and a strong sink of SOA (especially in the middle upper troposphere, achieved here by adding particle-phase photolysis) are needed to reproduce the vertical distribution of organic aerosol (OA) measured during several aircraft field campaigns; without this sink, the simulated middle upper tropospheric OA is too large. Our results show that variations in SOA chemistry formulations change SOA wet removal lifetime by a factor of 3 due to changes in horizontal and vertical distributions of SOA. In all the SOA chemistry formulations tested here, an efficient chemical sink, that is, particle-phase photolysis, was needed to reproduce the aircraft measurements of OA at high altitudes. Globally, SOA removal rates by photolysis are equal to the wet removal sink, and photolysis decreases SOA lifetimes from 10 to~3 days. A recent review of multiple field studies found no increase in net OA formation over and downwind biomass burning regions, so we also tested an alternative, empirical SOA treatment that increases primary organic aerosol (POA) emissions near source region and converts POA to SOA with an aging time scale of 1 day. Although this empirical treatment performs surprisingly well in simulating OA loadings near the surface, it overestimates OA loadings in the middle and upper troposphere compared to aircraft measurements, likely due to strong convective transport to high altitudes where wet removal is weak. The default improved model formulation (multigenerational aging with moderate fragmentation and photolysis) performs much better than the empirical treatment in these regions. Differences in SOA treatments greatly affect the SOA direct radiative effect, which ranges

The NASA Atmospheric Tomography (ATom) Mission: Imaging the Chemistry of the Global Atmosphere

“…For the cloud condensation nuclei (CCN) size range in the remote troposphere (100-500 nm; Brock et al 2019) ATom showed that OA was nearly ubiquitous, and a major fraction of the remote aerosol, mostly of secondary origin and highly aged (Hodzic et al 2020). A detailed comparison with stateof-the-art current CCMs by the same authors showed that while model skill has improved over time, this improvement is partially due to cancelling errors in both the source and loss terms (Hodzic et al 2016(Hodzic et al , 2020Lou et al 2020). The inorganic fraction of very remote CCN, on the other hand, was highly acidic (typically pH < 0) and consisted mostly of sulfate, with important implications for the particle phase state, hygroscopicity, radiative and chemical properties and, by implication, for the representation of ammonia source in CCMs (Nault et al 2021).…”

Wildfire Smoke Demonstrates Significant and Predictable Black Carbon Light Absorption Enhancements

“…Our empirical function of BC absorption enhancement versus coating thickness implies strong absorption of solar radiation by ambient BC aerosol and allows for realistic representation of this effect in climate models as coatings develop with age. Most current treatments assume unrealistic BC morphology (Bauer et al., 2010; Jacobson, 2000; Koch et al., 2009; Lou et al., 2020) which can lead to very high estimates of BC absorption cross‐section (Bond et al., 2006) and overestimate the direct radiative effect. When the more realistic core‐shell morphology is assumed for BC particles it is common to assume homogeneity in BC core size and coating thickness (Chung et al., 2012; Jacobson, 2012; Yu, 2011) which we show will also lead to overestimates of BC absorption.…”

Evaluation of Secondary Organic Aerosol (SOA) Simulations for Seoul, Korea

“…However, all models fail to capture this rapid change in SOA composition, regardless of the chemical aging process, indicating possible missing sources of SOA from biomass burning sources. Several studies reported that the GFED biomass burning emissions underestimate POA emissions (Lou et al., 2020) and also found that a simple empirical treatment that accounts for the aging of POA from biomass burning sources to form SOA shows good performance in simulating surface OA loadings (Lou et al., 2020). These implications call for additional sensitivity tests for the KORUS‐AQ period but are beyond the scope of this paper.…”