Diagnosis of Ozone Formation Sensitivities in Different Height Layers via MAX-DOAS Observations in Guangzhou

Diagnosing the vertical distributions of ozone formation sensitivities within the boundary layers is the prerequisite for formulating proper ozone control strategies. From 1 January 2020, to 20 August 2021, a multi-function MAX-DOAS instrument was set up in Guangzhou to detect the NO2 and HCHO vertical profiles. The ozone formation sensitivities in different height layers were diagnosed using the FNRsec, which was defined as the ratio of secondary HCHO to NO2 VMRs (HCHOsec/NO2). The secondary sources in ambient HCHO participate in the photochemical reactions directly, and the FNRsec can indicate the ozone formation sensitivities more accurately. Therefore, the secondary source was separated from ambient HCHO via a multi-linear regression method, and its relative contribution was approximately 66.9 +/- 6.3%. The sensitivity thresholds of ozone formation were determined by the slope of the linear regression analysis for O-3 versus the normalized secondary HCHO or NO2 mixing ratios for different FNRsec ranges, which were 0.22 and 0.36. For the ground layer, the VOC-limited, transitional, and NOx-limited regimes accounted for 40.66%, 32.42%, and 26.92%, respectively. Diurnally, the ground layer was usually in the VOC-limited regime in morning, but in the transitional regime in noon and afternoon. Vertically, the ozone formation sensitivity changed with altitude from VOC-limited (0.02-0.22 km) to transitional (0.22-0.42 km) to NOx-limited (0.42-2.02 km). These findings can help formulate proper ozone control strategies in time according to the varying local conditions.