Reply on RC2

Abstract

The ambient OH and HO2 concentrations were measured continuously during the STORM (STudy of the Ozone foRmation Mechanism) campaign at the Shenzhen site, located in Pearl River Delta in China, in autumn 2018. The diurnal maximum OH and HO2 concentrations, measured by laser-induced fluorescence, were 4.5 × 106 cm−3 and 4.5 × 108 cm−3, respectively. The state-of-the-art radical chemical mechanism underestimated the observed OH concentration, similar to the other warm-season campaigns in China. The OH underestimation was attributed to the missing OH sources, which can be explained by the X mechanism. Good agreement between the observed and modeled OH concentrations was achieved when an additional numerical X equivalent to 0.1 ppb NO concentrations was added to the base model. The modeled HO2 could reproduce the observed HO2, indicating the HO2 heterogeneous uptake on HO2 chemistry was negligible. Photolysis reactions dominated the ROx primary production rate. The HONO, O3, HCHO, and carbonyls photolysis accounted for 29 %, 16 %, 16 %, and 11 % during the daytime, respectively. The ROx termination rate was dominated by the reaction of OH + NO2 in the morning, and thereafter the radical self-combination gradually became the major sink of ROx in the afternoon. The atmospheric oxidation capacity was evaluated, with a peak of 0.75 × 108 molecules cm−3 s−1 around noontime. A strong positive correlation between O3 formation rate and atmospheric oxidation capacity was achieved, illustrating the atmospheric oxidation capacity was the potential tracer to indicate the secondary pollution.