Modeling Fine Particulate Matter and Ozone in the Sichuan Basin, Southwestern China: Status, Source Contributions, and Emission Reduction Targets

Sichuan Basin (SCB) is located in southwestern China and has a population of 108.1 million from 18 prefecture cities, including the two largest cities in western China (Chengdu and Chongqing). The basin is one of the areas that are most affected by air pollution in China. PM_2.5 (i.e., particulate matter with an aerodynamic diameter <2.5 μm) and ozone (O₃) are the major air pollutants. In order to help efficiently reduce air pollution, we need to understand (1) the characteristics of PM_2.5 and O₃ pollution, (2) source contributions to the two pollutants, and (3) how much emissions need to be reduced to meet relevant national and international standards. Therefore, we used the weather research forecasting and the community multiscale air quality (WRF/CMAQ) modeling system and source-oriented CMAQ models for the year of 2015 to understand the aforementioned three questions. PM_2.5 pollution is greatest in winter and it increases from the rims to the inner areas of SCB, having two hotspots (150-200 μg m^-3) near the urban centers of Chongqing and Chengdu in winter. Although PM_2.5 concentration has great spatio-temporal variations within the SCB, residential and industrial activities are the largest contributors in the basin and inter-city transport of PM_2.5 and its precursors are evident. Emissions outside SCB have large fractional contributions (>30%) to PM_2.5 only for some SCB areas, where PM_2.5 concentrations are relatively lower in the basin. 8-h O₃ concentrations are higher in some rural and sub-urban areas in summer (65-80 ppb), including those in western and southwestern rims of SCB and those downwind the urban center of Chongqing. In these areas, anthropogenic emissions can contribute up to 50% of daytime O₃ concentrations. PM_2.5 and O₃ were modeled under 10%-90% SCB emission reductions, and we found that approximately 50% of SCB emissions should be reduced to meet China's PM_2.5 standard for all the 18 urban centers in winter. However, to make the entire basin meet the WHO guidelines, emissions within and outside SCB should be both controlled greatly.