Direct evidence of local photochemical production driven ozone episode in Beijing: A case study

Highlights: • High OH radical concentration up to 1 × 10⁷ cm^-3 was measured in autumn Beijing, indicating active photochemistry. • Experimental determined integral P(O₃) reached 75 ppbv from 12:00 to 16:00. • Local photochemical production contributes ozone pollution in autumn Beijing. We present a comprehensive field campaign conducted in Beijing, September 2016, to elucidate the photochemical smog pollution, i.e. Ozone (O₃). The observed daily maximum hydroxyl radical (OH) and hydroperoxy radical (HO₂) concentrations were up to 1 × 10⁷ cm^-3 and 6 × 10⁸ cm^-3, respectively, indicating the active photochemistry in autumn Beijing. Photolysis of nitrous acid (HONO) and O₃ contributed 1-2 ppbv h^-1 to OH primary production during daytime. OH termination were dominated by the reaction with nitric oxide (NO) and nitrogen dioxide (NO₂), which were in general larger than primary production rates, indicating other primary radical sources maybe important. The measurement of radicals facilitates the direct determination of local ozone production rate P (O_x) (O_x = O₃ + NO₂). The integrated P(O_x) reached 75 ppbv in afternoon (for 4 h) when planetary boundary layer was well developed. At the same time period, the observed total oxidant concentrations O_x, increased significantly by 70 ppbv. In addition, the O_x measurement showed compact increase in 12 stations both temporally and spatially in Beijing, indicating that active photochemical production happened homogenously throughout the city. The back-trajectory analysis showed that Beijing was isolated from the other cities during the episode, which further proved that the fast ozone pollution was contributed by local photochemical production rather than regional advection.