Global Transformation and Fate of Secondary Organic Aerosols: Implications of Low Volatility SOA and Gas-Phase Fragmentation Reactions

Tuesday, 16 December 2014
ManishKumar Baban Shrivastava1, Richard C Easter1, Xiaohong Liu2, Alla Zelenyuk1, Balwinder Singh1, Kai Zhang3, Po-Lun Ma1, Duli Chand1, Steven John Ghan1, Jose L Jimenez4, Qi Zhang5, Jerome D Fast6, Philip J Rasch3 and Petri Tiitta7, (1)Pacific Northwest National Laboratory, Richland, WA, United States, (2)University of Wyoming, Laramie, WY, United States, (3)Pacific Northwest National Lab, Richland, WA, United States, (4)University of Colorado at Boulder, Boulder, CO, United States, (5)University of California Davis, Davis, CA, United States, (6)Pacific Northwest Natl Lab, Richland, WA, United States, (7)University of Eastern Finland, Kuopio, Finland
Secondary organic aerosols (SOA) are often represented crudely in global models. We have implemented three new detailed SOA treatments within the Community Atmosphere Model version 5 (CAM5) that allow us to compare the semi-volatile versus non-volatile SOA treatments (based on some of the latest experimental findings) and also investigate the effects of gas-phase fragmentation reactions. For semi-volatile SOA treatments, fragmentation reactions decrease simulated SOA burden from 7.5 Tg to 1.8 Tg. For the non-volatile SOA treatment (with fragmentation), the burden is 3.1 Tg. Larger differences between non-volatile and semi-volatile SOA (upto a factor of 5) correspond to continental outflow over the oceans. Compared to a global dataset of surface Aerosol Mass Spectrometer measurements and the US IMPROVE network measurements, the non-volatile SOA with fragmentation treatment (FragNVSOA) agrees best at rural locations. Urban SOA is under-predicted but this may be due to the coarse model resolution. Our revised treatments show much better agreement with aircraft measurements of organic aerosols (OA) over the N. American Arctic and sub-Arctic in spring and summer, compared to the standard CAM5 formulation. This is due to treating SOA precursor gases from biomass burning, and long-range transport of biomass burning OA at elevated levels (also supported by satellite data), which undergoes less wet removal compared to the surface OA sources in the standard CAM5. Although the total simulated OA from biomass burning agrees better with aircraft measurements, recent field observations typically report lower SOA formation, suggesting that constraining the POA-SOA split from biomass burning should be the focus of future studies. The non-volatile and semi-volatile configurations predict the direct radiative forcing of SOA as -0.5 W m-2 and -0.26 W m-2 respectively, at top of the atmosphere, which are higher than previously estimated by most models, but in reasonable agreement with a recent constrained modeling study.