Source apportionment of summertime ozone in China using a source-oriented chemical transport model

Contributions of industries, power plants, residential, transportation, open burning, and biogenic sectors to summertime ozone (O-3) in China in August 2013 are quantified using an improved O-3 source apportionment method implemented in a source-oriented Community Multiscale Air Quality (CMAQ) model. The new O-3 source apportionment method attributes in-situ O-3 formed in each time step to nitrogen oxides (NOx) and volatile organic compounds (VOCs) with a new attribution scheme that considers NOx-VOC-O-3 formation sensitivity in NOx-limited, transition, and VOC-limited regimes (three regimes, 3R). This represents an improvement from the traditional O-3 source apportionment models, in which O-3 formation sensitivity is divided into two-regimes (2R), either NOx-limited or VOC-limited. An expanded source-oriented gas phase chemical mechanism is developed to concurrently track NOx and primary VOCs and their oxidation products from different sources. The source-resolved NO2 photolysis rates and the NO to NO2 conversion rates by peroxy radicals from the source-oriented chemical mechanism are then used to apportion O-3 to NOx (O-3-NOx) and VOCs (O-3_VOC) from different emission sources. Industries, transportation, power plants and biogenic sources are the major emission sectors to O-3. High industrial O3NOx (8-15 ppb; 20-25% of total O-3) is mainly located in north and northeast China, the Yantze River Delta (YRD) and part of the Sichuan Basin (SCB). Transportation O3NOx accounts for 10-15% of total O-3 in most urban areas. Power plants' contributions to O-3_NOx (10-15%) are mostly located in Shanxi, Shaanxi, Ningxia and part of Shandong and Jiangsu. The contribution of soil NOx to O-3_NOx is highest in regions in west, central and south China with less direct anthropogenic emission influences. VOC-limited grid cells are mostly located at urban cores and large city clusters in the YRD and Pearl River Delta (PRD) regions. Biogenic and industrial sectors are the most important emission sectors that contribute to O-3_VOC, accounting for more than 85% of the O-3_VOC in China. In north and east provinces, more than 50% of the O-3 is non-background, with the provinces in north China having higher overall O-3. The Beijing-Tianjin-Hebei (BTH) region has an area-averaged 8-hr O-3 of 75 ppb in August 2013 and about half is non-background. Contributions of industries, transportation, power and biogenic sources to non-background O-3 are 37%, 22%, 13% and 23%, respectively. The relative contribution of biogenic emissions to non-background O-3 is lower on high 8-hr O-3 days. The 2R and 3R approaches show significant differences in assessing the relative importance of O-3_NOx and O3VOC, with the 2 regime approach predicting lower O3NOx but higher O3VOC. The new source allocation method for O-3_VOC can lead to up to 10% differences for industrial and biogenic sectors.