Inferring vertical variability and diurnal evolution of O formation sensitivity based on the vertical distribution of summertime HCHO and NO in Guangzhou, China.

The vertical distributions of formaldehyde (HCHO) and nitrogen dioxide (NO) and their indicative roles in ozone (O) sensitivity are important for designing O mitigation strategies. Using hyperspectral remote sensing observations, tropospheric vertical profiles of HCHO, NO, and aerosol extinction were investigated in Guangzhou, China from July to September 2019. On both O non-exceedance and polluted days, the HCHO and aerosol vertical profiles exhibited similar Gaussian shapes, but the NO profile exhibited an exponential decreasing shape. HCHO and aerosol were especially sensitive to O pollution, with higher values generally occurring at approximately noon and late afternoon at higher altitudes. We attempted to study the diurnal evolution of O sensitivity at different altitudes based on the HCHO to NO ratio (FNR) vertical profile. The FNR thresholds marking the transition regime (2.5 < FNR < 4.0) were derived from the relationship between the increase in O (∆O) and FNR. Our results showed that O sensitivity tends to be VOC-limited both at lower (below approximately 0.4 km) and higher (above approximately 1.8 km) altitudes throughout the daytime. In the middle altitudes, the photochemical formation of O was mainly in the transition/NO-limited regime in the morning and afternoon but in the VOC-limited regime at noontime. The relationship between TROPOMI column FNR and near-surface O sensitivity was further investigated. Compared with the MAX-DOAS near-surface FNR, slightly higher values of column FNR would increase the number of days classified as transition regimes, which was mainly caused by the inhomogeneous vertical distribution of HCHO and NO in the lower troposphere. This study provides an improved understanding of vertical variability and diurnal evolution of O formation sensitivity.