Ground-Based MAX-DOAS Observations of Tropospheric Ozone and Its Precursors for Diagnosing Ozone Formation Sensitivity

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Diagnosing ozone (O3) formation sensitivity using tropospheric observations of O3 and its precursors is important for formulating O3 pollution control strategies. Photochemical reactions producing O3 occur at the earth’s surface and in the elevated layers, indicating the importance of diagnosing O3 formation sensitivity at different layers. Synchronous measurements of tropospheric O3 and its precursors nitrogen dioxide (NO2) and formaldehyde (HCHO) were performed in urban Hefei to diagnose O3 formation sensitivity at different atmospheric layers using multi-axis differential optical absorption spectroscopy observations. The retrieved surface NO2 and O3 were validated with in situ measurements (correlation coefficients (R) = 0.81 and 0.80), and the retrieved NO2 and HCHO vertical column densities (VCDs) were consistent with TROPOMI results (R = 0.81 and 0.77). The regime transitions of O3 formation sensitivity at different layers were derived using HCHO/NO2 ratios and O3 profiles, with contributions of VOC-limited, VOC-NOx-limited, and NOx-limited regimes of 74.19%, 7.33%, and 18.48%, respectively. In addition, the surface O3 formation sensitivity between HCHO/NO2 ratios and O3 (or increased O3, ΔO3) had similar regime transitions of 2.21–2.46 and 2.39–2.71, respectively. Moreover, the O3 formation sensitivity of the lower planetary boundary layer on polluted and non-polluted days was analyzed. On non-polluted days, the contributions of the VOC-limited regime were predominant in the lower planetary boundary layer, whereas those of the NOx-limited regime were predominant in the elevated layers during polluted days. These results will help us understand the evolution of O3 formation sensitivity and formulate O3 mitigation strategies in the Yangtze River Delta region.