Semivolatile POA and parameterized total combustion SOA in CMAQv5.2: impacts on source strength and partitioning
Benjamin N. Murphy1, Matthew C. Woody1, Jose L. Jimenez2,3, Ann Marie G. Carlton4, Patrick L. Hayes5, Shang Liu2, Nga L. Ng6,7, Lynn M. Russell8, Ari Setyan9, Lu Xu6, Jeff Young1, Rahul A. Zaveri10, Qi Zhang11, and Havala O. T. Pye11National Exposure Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC, USA 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA 3Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA 4Department of Chemistry, University of California, Irvine, CA 92697, USA 5Department of Chemistry, Université de Montréal, Montréal, QC, Canada 6School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, USA 7School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA 8Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA 9EMPA, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland 10Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, Richland, WA, USA 11Department of Environmental Toxicology, University of California, Davis, California, USA
Received: 02 Mar 2017 – Accepted for review: 13 Mar 2017 – Discussion started: 15 Mar 2017
Abstract. Mounting evidence from field and laboratory observations coupled with atmospheric model analyses show that primary combustion emissions of organic compounds dynamically partition between the vapor and particulate phases, especially as near-source emissions dilute and cool to ambient conditions. The most recent version of the Community Multiscale Air Quality (CMAQ) Model v5.2 accounts for the semivolatile partitioning and gas-phase aging of these primary organic aerosol (POA) compounds consistent with experimentally derived parameterizations. We also include a new surrogate species, potential secondary organic aerosol from combustion emissions (pcSOA), which provides a representation of the SOA from anthropogenic combustion sources that could be missing from current chemical transport model predictions. The reasons for this missing mass likely include the following: 1) unspeciated semivolatile and intermediate volatility organic compound (SVOC and IVOC, respectively) emissions missing from current inventories, 2) multigenerational aging of organic vapor products from known SOA precursors (e.g. toluene, alkanes, etc), 3) underestimation of SOA yields due to vapor wall losses in smog chamber experiments, and 4) reversible organic-water interactions and/or aqueous-phase processing of known organic vapor emissions. CMAQ predicts the spatially-averaged contribution of pcSOA to OA surface concentrations in the continental United States to be 38.6 % and 23.6 % in the 2011 winter and summer, respectively.
Whereas many past modeling studies focused on a particular measurement campaign, season, location, or model configuration, we endeavor to evaluate the model and important uncertain parameters with a comprehensive set of United States-based model runs using multiple horizontal scales (4 km and 12 km), gas-phase chemical mechanisms, seasons and years. The model with representation of semivolatile POA improves predictions of hourly OA observations over the traditional nonvolatile model at sites during field campaigns in southern California (CalNex, May–June 2010), northern California (CARES, June 2010), the southeast US (SOAS, June 2013; SEARCH, January and July, 2011). Model improvements manifest better correlations (e.g. correlation coefficient at Pasadena at night increases from 0.38 to 0.62) and reductions in underprediction during the photochemically active afternoon period (e.g. bias at Pasadena from −5.62 to −2.42 μg m−3). Daily-averaged predictions of observations at routine monitoring networks from simulations over the continental U.S. (CONUS) in 2011 show modest improvement during winter with mean biases reducing from 1.14 to 0.73 μg m−3, but less change in the summer when the decreases from POA evaporation were similar to the magnitude of added SOA mass. Because the model-performance improvement realized by including the relatively simple pcSOA approach is similar to that of more-complicated parameterizations of OA formation and aging, we recommend caution when applying these more-complicated approaches as they currently rely on numerous uncertain parameters.
The pcSOA parameters optimized for performance at the southern and northern California sites lead to higher OA formation than is observed in the CONUS evaluation. This may be due to any of the following: variations in real pcSOA in different regions or time periods, too high concentrations of other OA sources in the model that are important over the larger domain, or other model issues such as loss processes. This discrepancy is likely regionally and temporally dependent and driven by interferences from factors like varying emissions and chemical regimes.
Murphy, B. N., Woody, M. C., Jimenez, J. L., Carlton, A. M. G., Hayes, P. L., Liu, S., Ng, N. L., Russell, L. M., Setyan, A., Xu, L., Young, J., Zaveri, R. A., Zhang, Q., and Pye, H. O. T.: Semivolatile POA and parameterized total combustion SOA in CMAQv5.2: impacts on source strength and partitioning, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-193, in review, 2017.