Ozone trends and their sensitivity in global megacities under the warming climate

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.