19 May 2015

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

Summary

Secondary organic aerosols (SOA) are often the dominant components of fine aerosols at many locations globally, but are also the least understood. SOA particles are created by complex multiscale interactions between human activities (fossil-fuel burning), biomass burning, terrestrial biosphere and marine biogenic emissions that are linked by physical and chemical atmospheric processes. Although SOAs are large contributors to fine particle amounts and radiative forcing, they are often represented crudely in global models. For the first time, research led by the U.S. Department of Energy researchers at Pacific Northwest National Laboratory replaced the previous crude SOA treatments with much more advanced treatments in a global climate model.  The new treatments account for chemical reactions in the atmosphere that are both sources and sinks of SOA precursor gases (multigenerational aging), low “effective volatility” of SOA particles due to aging processes in the particle-phase, and “missing” semi-volatile/intermediate volatility precursors from global biomass burning and fossil-fuel sources. The new treatments caused large increases in simulated aerosol amounts, lifetimes, and direct radiative forcing compared to previous global modeling treatments and dramatically improved agreement with a suite of surface-based, aircraft, and satellite organic aerosol measurements, especially in regions affected by biomass burning emissions. The ratio of their revised non-volatile SOA to previous semi-volatile SOA burden varied by a factor of 2-5. Their new model treatments also largely increased loadings and lifetimes of SOA particles corresponding to continental outflow over marine environments, where cloud reflectivity (albedo) is highly sensitive to cloud seed (cloud condensation nuclei or CCN) concentrations. Their work shows that new and advanced aerosol model treatments are expected to change the radiative forcing of aerosols simulated by current generation global climate models. This will have large potential impacts on our understanding of aerosol-cloud-radiative forcing interactions.

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Publications
2015.  "Global Transformation and Fate of SOA: Implications of Low Volatility SOA and Gas-Phase Fragmentation Reactions."  Journal of Geophysical Research-Atmospheres, doi:10.1002/2014JD022563.
Acknowledgments

This was work was supported by the U.S. Department of Energy, Office of Science, Biological and Environmental Research's Applying Computationally Efficient Schemes for Biogeochemical Cycles (ACES4BGC) project under Scientific Discovery through Advanced Computing (SciDAC-3), and by the U.S. Department of Energy, Office of Science, Biological and Environmental Research's Atmospheric Systems Research (ASR) Program. Computational resources for the simulations were provided by the PNNL Institutional Computing (PIC) facility, EMSL (a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research located at Pacific Northwest National Laboratory), and NERSC (the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under contract DE-AC02-05CH11231). The surface and AMS OA data sets are obtained via https://sites.google.com/site/amsglobaldatabase/. The IMPROVE data are obtained via http://vista.cira.colostate.edu/improve/data/improve/improve_data.htm. The CAM5 model results for the four different treatments will be made available upon request. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-76RL01830. The authors thank Joel Thornton and Shantanu Jathar for helpful discussions.