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(2) Both a strong source (i.e. strong functionalization) and a strong sink of SOA (i.e. particle-phase photolysis) are needed to explain vertical organic aerosol (OA) profiles measured by aircraft during several field campaigns (DOE Green Ocean Amazon (GoAmazon2014/5), Atmospheric Tomography Mission (Atom) 2016, and the NASA Arctic Research of the composition of the Troposphere the Troposphere from Aircraft and Satellite (ARCTAS) 2008). While SOA treatments that do not include photolysis also agree with surface-based OA measurements e.g. with Aerosol Mass Spectrometer (AMS) and IMPROVE network at several locations, these treatments overpredict SOA at middle and upper troposphere. A strong sink of SOA, like photolysis, is needed to explain OA loadings at higher altitudes where wet removal sinks are small.
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(4) Differences in SOA treatments greatly affect the direct radiative forcing of aerosols ranging from -0.67 W m-2 (50% fragmentation and photolysis) to -2.1 W m-2 (50% fragmentation without photolysis). Notably, most SOA formulations predict similar global indirect forcing of SOA calculated as the difference in cloud forcing between present-day and pre-industrial simulations likely due to cancellations of errors in SOA formulations. However, the simple treatment is an anomaly and predicts 20% differences in global indirect forcing compared to explicit SOA formulations.
Compared to the default E3SMv1 model, our explicit SOA treatment with fragmentation and photolysis agrees much better with oxygenated organic aerosol (OOA) and IMPROVE OC measurements at multiple surface locations. In addition, our new formulation agrees much better with aircraft-based OA measurements in the middle and upper troposphere compared to E3SMv1, which likely overestimated OA concentrations at high altitudes.
Our results provide important insights about how different SOA processes likely impact aerosol distributions and radiative forcing.