Air quality forecast of PMsub10/sub in Beijing with Community Multi-scale Air Quality Modeling (CMAQ) system: emission and improvement

摘要

The MM5-SMOKE-CMAQ model system, which was developed by the United States Environmental Protection Agency (US EPA) as the MODELS-3 system, has been used for daily air quality forecasts in the Beijing Municipal Environmental Monitoring Center (Beijing MEMC), as a part of the Ensemble air quality Modeling forecast System for Beijing (EMS-Beijing) since the 2008 Olympic Games. According to the daily forecast results for the entire duration of 2010, the model shows good performance in the PM₁₀ forecast on most days but clearly underestimates PM₁₀ concentration during some air pollution episodes. A typical air pollution episode from 11–20 January 2010 was chosen, in which the observed air pollution index of particulate matter (PM₁₀-API) reached 180 while the forecast PM₁₀-API was about 100.In this study, three numerical methods are used for model improvement: first,by enhancing the inner domain with 3 km resolution grids, and expanding the coverage from only Beijing to an area including Beijing and its surroundingcities; second, by adding more regional point source emissions located at Baoding, Landfang and Tangshan, to the south and east of Beijing; third, by updating the area source emissions, including the regional area source emissions in Baoding and Tangshan and the local village/town-level area source emissions in Beijing.The last two methods are combined as the updated emissions method. Accordingto the model sensitivity testing results by the CMAQ model, the updatedemissionsmethod and expanded model domain method can both improve the modelperformance separately. But the expanded model domain method has betterability to capture the peak values of PM₁₀ than the updated emissionsmethod due to better reproduction of the pollution transport process in this episode.As a result, the hindcast results ("New(CMAQ)"), which are driven by theupdated emissions in the expanded model domain, show a much better modelperformance in the national standard station-averaged PM₁₀-API. Thedaily hindcast PM₁₀-API reaches 180 and is much closer to the observed value, and has a high correlation coefficient of 0.93. The correlation coefficient of the PM₁₀-API in all Beijing MEMC stations between the hindcast and observation is 0.82, clearly higher than the forecast 0.54. The FAC2 increases from 56% in the forecast to 84% in the hindcast, and theNMSE decreases from 0.886 to 0.196. The hindcast also has better modelperformance in PM₁₀ hourly concentrations during the typical airpollution episode. The updated emissions method accompanied by a suitable domain inthis study improved the model performance for the Beijing areasignificantly.