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

摘要

The Community Multiscale Air Quality (CMAQ) model version 4.6 has beenrevised with regard to the representation of chlorine (HCl, ClNO) andsulfur (dimethylsulfide, or DMS, and HS), and evaluated againstobservations and earlier published models. Chemistry parameterizations werebased on published reaction kinetic data and a recently developed cloudchemistry model that includes heterogeneous reactions of organic sulfurcompounds. Evaluation of the revised model was conducted using a recentlyenhanced data base of natural emissions that includes ocean and continentalsources of DMS, HS, chlorinated gases and lightning NO for thecontinental United States and surrounding regions. Results using 2002meteorology and emissions indicated that most simulated "natural" (plus background) chemical and aerosolspecies exhibit the expected seasonal variations at the surface. Ozoneexhibits a winter and early spring maximum consistent with ozone data andan earlier published model. Ozone distributions reflect the influences ofatmospheric dynamics and pollutant background levels imposed on the CMAQsimulation by boundary conditions derived from a global model. A series ofmodel experiments reveals that the consideration of gas-phase organic sulfurchemistry leads to sulfate aerosol increases over most of the continentalUnited States. Cloud chemistry parameterization changes result in widespreaddecreases in SO across the modeling domain and both increases anddecreases in sulfate. Most cloud-mediated sulfate increases occurred mainlyover the Pacific Ocean (up to about 0.1 μg m) but also over anddownwind from the Gulf of Mexico (including parts of the eastern US).Geographic variations in simulated SO and sulfate are due to the linkbetween DMS/HS and their byproduct SO, the heterogeneity ofcloud cover and precipitation (precipitating clouds act as net sinks forSO and sulfate), and the persistence of cloud cover (the largestrelative sulfate increases occurred over the persistently cloudy Gulf ofMexico and western Atlantic Ocean). Overall, the addition of organic sulfurchemistry increased hourly surface sulfate levels by up to 1–2 μg m but reduced sulfate levels in the vicinity of high SOemissions (e.g., wildfires). Simulated surface levels of DMS comparereasonably well with observations in the marine boundary layer where DMSoxidation product levels are lower than observed. This implies either a lowbias in model oxidation rates of organic sulfur species or a low bias in theboundary conditions for DMS oxidation products. This revised version of CMAQprovides a tool for realistically simulating the influence of naturalemissions on air quality.