LAUSR

Abstract The Community Multiscale Air Quality (CMAQ) model with a source-oriented SAPRC-11 photochemical mechanism is developed in this study to quantify the source region contributions to surface O3 in Beijing and Shanghai in August 2013. Non-background O3 attributed to NOx (O3_NOx) and volatile organic compounds (O3_VOC) emitted from different source regions is determined using a three-regime approach that correctly attributes O3 to NOx and VOC precursors throughout the entire range of NOx-VOC-O3 formation sensitivity. Averaged over the entire month and all grid cells, local emissions (51%) and emissions from Hebei (31%) are the two major contributors to non-background daily maximum 8-h (DM8H) O3 in Beijing. In Shanghai, local, Zhejiang and Jiangsu emissions account for 53%, 19% and 14% of the non-background DM8H O3. Significant variations in local emission contributions are predicted among different model grid cells for both cities (Beijing, 6–80%; Shanghai, 3–76%). On high O3 days in Beijing, the wind is persistently from the south with high wind speed (~5 m s−1) in the evening and night. This leads to significant regional contributions of O3 from Hebei, along with regional transport of VOCs and NOx. In Shanghai, high O3 days are associated with southwesterly/westerly wind in the morning, rotating to southeast in the early afternoon in a counter-clockwise direction. The surface wind then gradually turns back to southwest in the afternoon until the next morning, along with reduced wind speed. In Shanghai, daytime O3 at the urban center is almost entirely due to local emissions. Low wind speed in the evening and night time allows local NOx emissions to efficiently titrate regional O3. In both cities, NOx emissions are not transported regionally as efficiently as VOCs. Source region contribution analysis of the concentration weighted maximum incremental reactivity (CWMIR) shows that VOCs from other regions are less reactive than locally emitted VOCs. HCHO and acetaldehyde (CCHO) generated from the oxidation of other VOCs are important contributors to regionally transported reactive VOCs. In both regions, the overall CWMIR in both cities is quite similar (~4 mol O3 per mole of VOC).