Mobile MAX-DOAS measurements and source analysis of NO2, HCHO, and HONO during the Chengdu 2023 FISU world university games

To maintain air quality during the 31st World University Games, Chengdu employed a range of monitoring and control strategies in 2023. High-resolution regional pollutant distributions were reconstructed by integrating the vertical column densities (VCDs) from mobile MAX-DOAS measurements with Gaussian process regression analysis. The correlation between the spatial distribution derived from MAX-DOAS and that of GEMS and TROPOMI satellite data exceeded 0.6. This paper explores the impact of air quality improvements during the games on the sources of HCHO and the formation process of HONO. During the control period, primary emissions and secondary formations of HCHO contributed 50.85 % ± 24.24 % and 42.81 % ± 7.57 % to the total atmospheric HCHO, respectively. The study indicates that with improved air quality, HCHO primary emissions decrease while secondary emissions and atmospheric radiation transmission intensities rise. It is found that HONO always appears under the condition of high aerosol optical depth (AOD) and NO₂, but high NO₂ concentration and AOD are not necessarily accompanied by high concentrations of HONO. To assess the influence of temperature and humidity on the formation of HONO from NO₂, we calculated the emission rates $\Delta \text{HONO}/\Delta N{O}_2$ to quantify the impact of primary sources on total HONO concentrations. The analysis results show that the turning point of relative humidity is 65 % (60 %–70 %) and the turning point of temperature is 31 °C (30–32 °C). Lower temperatures and higher humidity levels were found to decrease the rate of secondary HONO formation from NO₂.