Tropospheric ozone formation depends on the emissions of volatile organic compounds (VOC) and nitrogen oxides (NO_x). In megacities, abundant VOC and NO_x sources cause relentlessly high ozone episodes, affecting a large share of the global population. This study uses data from the Ozone Monitoring Instrument for formaldehyde (HCHO) and nitrogen dioxide (NO₂) as proxy data for VOC and NO_x emissions, respectively, with their ratio serving as an indicator of ozone sensitivity. Ground-level ozone (O₃) reanalysis from the Copernicus Atmosphere Monitoring is used to assess the O₃ trends. We evaluate changes from 2005 to 2019 and their relationship with the warming environment in 41 megacities worldwide, applying seasonal Mann-Kendall, trend decomposition methods, and Pearson correlation analysis. We reveal significant increases in global HCHO (0.1 to 0.31 × 10¹⁵ mol cm⁻² year⁻¹) and regionally varying NO₂ (−0.22 to 0.07 × 10¹⁵ mol cm⁻² year⁻¹). O₃ trends range from −0.31 to 0.70 ppb year⁻¹, highlighting the relevance of precursor abundance on O₃ levels. The strong correlation between precursor emissions and increasing temperature suggests that O₃ will continue to rise as climate change persists.

Ozone (O₃) is a pollutant of concern in urban areas because of its effects on health, crops, ecosystems, and materials. Despite efforts to meet the Japanese air quality standard for O₃ in the Kanto region, the attainment percentage is close to zero. Considering that O₃ formation is sensitive to emissions of volatile organic compounds (VOC) and nitrogen oxides (NO_x), this study evaluated a range of reductions in the emissions of both precursors using a regional air quality model (ADMER-PRO) and estimated their benefits measured as the economic change due to O₃ concentration differences between scenarios. The simulation period was set during the 2016 O₃ season. The results showed that O₃ concentrations could be reduced using two approaches: significant reduction in VOC levels combined with minor NO_x level changes or significant NO_x emission reduction. Significant reduction in NO_x levels was the most effective strategy for a generalized decrease in the O₃ levels in the Kanto region, and the benefit analysis revealed that the most significant economic impacts could be achieved by adopting the latter approach.

Stabilised Criegee intermediates (sCIs), which are formed in the atmosphere through the ozonolysis of alkenes, are known precursors of sulphate aerosols (SO₄²⁻(p)).