Regional and sectoral contributions of NOx and reactive carbon emission sources to global trends in tropospheric ozone during the 2000–2018 period.

Over the past few decades, the tropospheric ozone precursor anthropogenic emissions: nitrogen oxides (NO_x) and reactive carbon (RC) from mid-latitude regions have been decreasing, and those from Asia and tropical regions have been increasing, leading to an equatorward emission redistribution. In this study, we quantify the contributions of various sources of NO_x and RC emissions to tropospheric ozone using a source attribution technique during the 2000–2018 period in a global chemistry transport model: CAM4-Chem. We tag the ozone molecules with the source of their NO_x or RC precursor emission in two separate simulations, one for each of NO_x and RC. These tags include various natural (biogenic, biomass burning, lightning and methane), and regional anthropogenic (North American, European, East Asian, South Asian etc.) precursor emission sources. We simulate ~336 Tg O₃ with an increasing trend of 0.91 Tg O₃/yr (0.28 %/yr), largely contributed (and trend driven) by anthropogenic NO_x emissions and methane. The ozone production efficiency of regional anthropogenic NOx emissions increases significantly when emissions decrease (Europe, North American and Russia-Belarus-Ukraine region's emissions) and decreases significantly when emissions increase (South Asian, Middle Eastern, International Shipping etc.). Tropical regions, despite smaller emissions, contribute more to tropospheric ozone burden compared to emissions from higher latitudes, consistent with previous work, due to large convection at the tropics thereby lifting O₃ and its precursor NO_x molecules into the free troposphere where ozone's lifetime is longer. We contrast the contribution to tropospheric ozone burden with that of the contribution to the global surface ozone. We simulate a smaller relative contribution from tropical regions to the global mean surface ozone compared to their contribution to the tropospheric ozone burden. The global population-weighted mean ozone (related to ozone exposure) is much larger compared to surface mean, mainly due to large anthropogenic emissions from densely populated regions: East Asia, South Asia, and other tropical regions, and a substantial contribution from international ship NO_x emissions. The increasing trends in anthropogenic emissions from these regions are the main drivers of increasing global population-weighted mean ozone. [ABSTRACT FROM AUTHOR]