Modeling natural emissions in the Community Multiscale Air Quality (CMAQ) model – Part 2: Modifications for simulating natural emissions

Abstract. The Community Multiscale Air Quality (CMAQ) model version 4.6 has been revised with regard to the representation of chlorine (HCl, ClNO₂) and sulfur (dimethylsulfide, or DMS, and H₂S), and evaluated against observations and earlier published models. Chemistry parameterizations were based on published reaction kinetic data and a recently developed cloud chemistry model that includes heterogeneous reactions of organic sulfur compounds. Evaluation 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 NO_x for the continental United States and surrounding regions. Results using 2002 meteorology and emissions indicated that most simulated "natural" (plus background) chemical and aerosol species exhibit the expected seasonal variations at the surface. Ozone exhibits a winter and early spring maximum consistent with ozone data and an earlier published model. Ozone distributions reflect 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 model experiments reveals that the consideration of gas-phase organic sulfur chemistry leads to sulfate aerosol increases over most of the continental United States. Cloud chemistry parameterization changes result in widespread decreases in SO₂ across the modeling domain and both increases and decreases in sulfate. Most cloud-mediated sulfate increases occurred mainly over the Pacific Ocean (up to about 0.1 μg m⁻³) but also over and downwind from the Gulf of Mexico (including parts of the eastern US). Geographic variations in simulated SO₂ and sulfate 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 hourly surface sulfate levels by up to 1–2 μg m⁻³ but reduced sulfate levels in the vicinity of high SO₂ emissions (e.g., wildfires). Simulated surface levels of DMS compare reasonably well with observations in the marine boundary layer where DMS oxidation product levels are lower than observed. This implies either a low bias in model oxidation rates of organic sulfur species or a low bias in the boundary conditions for DMS oxidation products. This revised version of CMAQ provides a tool for realistically simulating the influence of natural emissions on air quality.