Observation and simulation of HOx radicals in an urban area in Shanghai, China

A continuous wintertime observation of ambient OH and HO2 radicals was first carried out in Shanghai, in 2019. This effort coincided with the second China International Import Expo (CIIE), during which strict emission controls were implemented in Shanghai, resulting in an average PM2.5 concentration of less than 35 μg/m3. The self-developed instrument based on the laser-induced fluorescence (LIF) technique reported that the average OH radical concentration at noontime (11:00-13:00) was 2.7 × 106 cm-3, while the HO2 concentration was 0.8 × 108 cm-3. A chemical box model utilizing the Regional Atmospheric Chemical Mechanism 2 (RACM2), which is used to simulate pollutant reactions and other processes in the troposphere and which incorporates the Leuven isoprene mechanism (LIM1), reproduced the OH concentrations on most days. The HO2 concentration was underestimated, and the observed-to-modelled ratio demonstrated poor performance by the model, especially during the elevated photochemistry period. Missing primary peroxy radical sources or unknown behaviors of RO2 for high-NOx regimes are possible reasons for the discrepancy. The daytime ROx production was controlled by various sources. HONO photolysis accounted for more than one half (0.83 ppb/h), and the contribution from formaldehyde, OVOCs and ozone photolysis was relatively similar. Active oxidation paths accelerated the rapid ozone increase in winter. The average ozone production rate was 15.1 ppb/h, which is comparable to that of a Beijing suburb (10 ppb/h for the 'BEST-ONE') but much lower than that of Beijing's center (39 ppb/h in 'PKU' and 71 ppb/h in 'APHH') in wintertime. Cumulative local ozone based on observed peroxy radicals was five times higher than the value simulated by the current model due to the underprediction of HO2 and RO2 under the high-NOx regime. This analysis provides crucial information for subsequent pollution control policies in Shanghai.