Local and inter-regional contributions to PM2.5 nitrate and sulfate in China

Emissions of NOx, SO2 and primary nitrate and sulfate from seven regions in China (North, Northeast, East, Central, South, Southwest and Northwest) were separately tracked in a source-oriented Community Multiscale Air Quality (CMAQ) model to quantify the local and inter-regional contributions to PM2.5 nitrate and sulfate concentrations in different cities and provinces. In January, high concentrations of nitrate (∼30 μg m−3) occurred in the North China Plain (NCP) and the Middle and Lower Yangtze Plain (MLYP), as well as the Sichuan Basin. NOx emissions from North, Central and East China were transported over long distances to form rather uniform concentration of nitrate in the NCP and MLYP regions and significantly impacted nitrate concentrations in downwind regions as far as the Pearl River Delta (PRD) region in South China. Wintertime sulfate concentrations demonstrated a more significant inter-regional southward transport pattern and wider spatial distributions than nitrate. The top-five ranked provinces with combined nitrate and sulfate concentrations (Chongqing, Sichuan, Guizhou, Anhui and Hunan) were all affected by emissions from the North, Central and East China, in addition to their local region emissions. In August, slower northerly and northeasterly wind reduced inter-regional transport. Nitrate and sulfate concentrations peaked in North China as dilution was more severely restricted by mountain ranges further north. Three of the top five high concentration provinces (Tianjin, Hebei and Beijing) were located in North China and had more than 55% of local region contributions. Emissions from North China accounted for ∼20% in the remaining two provinces (Shandong in East China and Henan in Central China). In addition to emissions from North China, ∼30% or more of the nitrate and sulfate concentrations in four of the top five high concentration provinces (Tianjin, Henan, Hebei and Shandong) were due to emissions from East China. Time series of daily regional contributions in three megacities (Beijing, Shanghai and Chongqing) and a large city cluster (PRD) also indicated stronger and more frequent inter-regional transport in winter than in summer. Although the cities were mostly affected by local region emissions, influences of long range transport were especially obvious for sulfate concentration in the PRD region in winter, where emissions from North, East and Central China often accounted for more than 80% of the total concentrations. The results from this study suggest that provincial or region-level emission controls alone may not be sufficient to effectively reduce nitrate and sulfate concentrations in many areas and coordinated local and inter-regional emission control strategies are necessary for the country.