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). In megacities, abundant VOC and NO 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 (NO) as proxy data for VOC and NO emissions, respectively, with their ratio serving as an indicator of ozone sensitivity. Ground-level ozone (O) reanalysis from the Copernicus Atmosphere Monitoring is used to assess the O 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 × 10 mol cm year) and regionally varying NO (-0.22 to 0.07 × 10 mol cm year). O trends range from -0.31 to 0.70 ppb year, highlighting the relevance of precursor abundance on O levels. The strong correlation between precursor emissions and increasing temperature suggests that O will continue to rise as climate change persists.