Global patterns and trends in ground-level ozone chemical formation regimes from 1996 to 2022

Abstract. Ground-level ozone (O3) formation in urban areas is nonlinearly dependent on the relative availability of its precursors: oxides of nitrogen (NOx) and volatile organic compounds (VOCs). To mitigate O3 pollution, a crucial question is to identify the O3 formation regime (NOx-limited or VOC-limited). Here, we leverage ground-based O3 observations alongside space-based observations of O3 precursors, namely, nitrogen dioxide (NO2) and formaldehyde (HCHO), to study the long-term shifts in O3 chemical regimes across global source regions. We first derive the regime threshold values for the satellite-derived HCHO/NO2 ratio by examining its relationship with the O3 weekend effect. We find that a regime transition from VOC-limited to NOx-limited occurs around 3.1 [2.7–3.4] for HCHO/NO2 with slight regional variations. By integrating data from four satellite instruments, including Global Ozone Monitoring Experiment (GOME), SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY), Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI), we built a 27-year (1996–2022) satellite HCHO/NO2 record from which we assess the long-term trends in O3 production regimes. A discernible global trend towards NOx-limited regimes is evident, particularly in developed regions such as North America, Europe, and Japan, with emerging trends in developing countries like China and India over the past 2 decades. This shift is supported by both increasing HCHO/NO2 ratios and a diminishing O3 weekend effect. Yet, urban areas still hover in the VOC-limited and transitional regime on the basis of annual averages. Our findings stress the importance of adaptive emission control strategies to mitigate O3 pollution.