Simulating emission and chemical evolution of coarse sea-salt particles in the Community Multiscale Air Quality (CMAQ) model

摘要

Chemical processing of sea-salt particles in coastal environmentssignificantly impacts concentrations of particle components and gas-phasespecies and has implications for human exposure to particulate matter andnitrogen deposition to sensitive ecosystems. Emission of sea-salt particlesfrom the coastal surf zone is known to be elevated compared to that from theopen ocean. Despite the importance of sea-salt emissions and chemicalprocessing, the US EPA's Community Multiscale Air Quality (CMAQ) model hastraditionally treated coarse sea-salt particles as chemically inert and hasnot accounted for enhanced surf-zone emissions. In this article, updates toCMAQ are described that enhance sea-salt emissions from the coastal surfzone and allow dynamic transfer of HNO₃, H₂SO₄, HCl, andNH₃ between coarse particles and the gas phase. Predictions of updatedCMAQ models and the previous release version, CMAQv4.6, are evaluated usingobservations from three coastal sites during the Bay Regional AtmosphericChemistry Experiment (BRACE) in Tampa, FL in May 2002. Model updates improvepredictions of NO₃−, SO₄2−, NH₄+, Na+, and Cl−concentrations at these sites with only a 8% increase in run time. Inparticular, the chemically interactive coarse particle mode dramaticallyimproves predictions of nitrate concentration and size distributions as wellas the fraction of total nitrate in the particle phase. Also, the surf-zoneemission parameterization improves predictions of total sodium and chlorideconcentration. Results of a separate study indicate that the model updatesreduce the mean absolute error of nitrate predictions at coastal CASTNET andSEARCH sites in the eastern US. Although the new model features improveperformance relative to CMAQv4.6, some persistent differences exist betweenobservations and predictions. Modeled sodium concentration is biased low andcauses under-prediction of coarse particle nitrate. Also, CMAQ over-predictsgeometric mean diameter and standard deviation of particle modes at theBRACE sites. These over-predictions may cause too rapid particle drydeposition and partially explain the low bias in sodium predictions. Despitethese shortcomings, the updates to CMAQ enable more realistic simulations ofchemical processes in environments where marine air mixes with urbanpollution. The model updates described in this article are included in thepublic release of CMAQv4.7 (href="http://www.cmaq-model.org" target="_blank">http://www.cmaq-model.org).