Modeling natural emissions in the Community Multiscale Air Quality (CMAQ) model – Part 2: Modifications for simulating natural emissions
Authors: Mueller S. F. , Mao Q. , Mallard J. W. 
Published: June 2010

Abstract. A recent version (4.6) of the Community Multiscale Air Quality (CMAQ) model was used as the basis for testing model revisions for including reactions involving chlorine (HCl, ClNO2) and reduced sulfur (dimethylsulfide, or DMS, and H2S) species not normally treated in the CB05 gas chemical mechanism and cloud chemistry module. Model chemistry revisions were based on published reaction kinetic data and a recent cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Testing of the revised model was conducted using a recently enhanced data base of natural emissions that includes ocean and continental sources of DMS, H2S, chlorinated gases and lightning NOx for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated chemical and aerosol species exhibit the expected seasonal variations in grid-average surface concentrations. Ozone exhibits a winter and early spring maximum – reasonably consistent with ozone data and model results produced by others – in a pattern that reflects the influences of atmospheric dynamics and pollutant background levels imposed on the CMAQ simulation by boundary conditions derived from a global model. A series of experimental model simulations reveals that the addition of gas phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Modifications to the cloud chemistry module result in widespread decreases in SO2 across the modeling domain and a mix of sulfate increases and decreases. Most cloud-mediated sulfate increases occurred over the Pacific Ocean (up to about 0.1 μg m-3) and at slightly lesser amounts over and downwind from the Gulf of Mexico (including portions of the Eastern US). Variations in the chemical response are due to the link between DMS/H2S and their byproduct SO2, the heterogeneity of cloud cover and precipitation (precipitating clouds act as net sinks for SO2 and sulfate), and the persistence of cloud cover (the largest relative sulfate increases occurred over the persistently cloudy Gulf of Mexico and western Atlantic Ocean). Overall, the addition of organic sulfur chemistry increased surface hourly sulfate levels by as much as 1–2 μg m-3 in selected grid cells. The added chemistry produced significantly less sulfate in the vicinity of high SO2 emissions (e.g., wildfires), perhaps in response to lower OH from competing reactions with DMS and its derivatives. Simulated surface levels of DMS compare favorably with published observations made in the marine boundary layer. However, DMS derivatives are lower than observed implying either less chemical reactivity in the model or a low bias in the boundary conditions for DMS derivatives such as dimethylsulfoxide. The sensitivity of sulfate to cloud cover and the aqueous sulfate radical is also explored. This revised version of CMAQ provides a tool for more realistically evaluating the influence of natural emissions on air quality.