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.

In this study, we estimated the future emission inventory of primary air pollutants in Japan in 2050 after introducing low-carbon technology based on the results of the socio-economic model provided by the Japanese government. The results suggested that introducing net-zero carbon technology would contribute to a 50-60 % decrease in primary NO_x, SO₂, and CO emissions and a ~30 % decrease in primary emissions of volatile organic compounds (VOCs) and PM_2.5. The estimated emission inventory and future meteorological conditions in 2050 were applied as inputs to a chemical transport model. A scenario involving the application of future reduction strategies with relatively moderate global warming (RCP4.5) was evaluated. The results showed that the concentration of tropospheric ozone (O₃) was highly reduced compared with that in 2015 after applying net-zero carbon reduction strategies. On the other hand, the fine particulate matter (PM_2.5) concentration under the 2050 scenario was expected to be equal or higher because of the growth in secondary aerosol formation caused by the increase in short-wave radiation. Finally, the premature mortality change from 2015 to 2050 was analyzed, and the change in air quality contributed by net-zero carbon technology will contribute to a ~4000 decrease in premature deaths in Japan.

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.

Ethylene oxide (EtO) is a volatile organic compound that poses both acute and chronic risks to human health, prompting governments worldwide to implement measures aimed at reducing ambient EtO concentrations. In this study, regional chemical transport model calculations were performed using the EtO emission inventory provided by the Ministry of Economy, Trade and Industry and the Ministry of the Environment, Japan. The target area and period were the Greater Tokyo Area (GTA) in 2017. The results indicated that the modeled atmospheric EtO concentrations were lower than the observed concentration at all monitoring sites in the GTA, with the discrepancy reaching up to one order of magnitude. Long-range transport of EtO from the Asian continent to Japan was not identified as the primary cause of this discrepancy. Instead, the significant discrepancy might be attributed to global background concentrations of EtO, resulting from its long tropospheric lifetime. To improve the accuracy of atmospheric EtO assessments using chemical transport models, it is essential to account for global emissions. Graphical Abstract