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

Abstract

Over the past few decades, the tropospheric ozone precursor anthropogenic emissions - nitrogen oxides (NOx) and reactive carbon (RC) from northern mid-/high-latitude regions (e.g., North America, Europe) - have been decreasing, and those from (sub-)tropical regions (e.g., South Asia , the Middle East ) have been increasing, leading to an equatorward emission redistribution. In this study, we quantify the contributions of various sources of NOx and RC emissions to tropospheric ozone using a source attribution technique during the 2000-2018 period in a global chemistry transport model. We tag the ozone molecules with the source of their NOx or RC precursor emission in two separate simulations: NOx-tagged and RC-tagged. These tags include various natural (biogenic, biomass burning, lightning NOx and RC from methane oxidation) and regional anthropogenic precursor emission sources and influx from the stratosphere. We simulate similar to 336 TgO3 of tropospheric ozone burden (TOB) with an increasing trend of 0.91 TgO3yr-1 (0.28 %yr-1), largely contributed (and trend driven) by anthropogenic NOx emissions and methane oxidation. The ozone production efficiency of regional anthropogenic NOx emissions increases when emissions decrease (e.g., Europe, North America) and decreases when emissions increase (e.g., South Asia, Middle East, international shipping). Tropical regions, despite lower emissions, contribute more to TOB compared to emissions from higher latitudes, consistent with previous work, predominantly due to large convection (combined with intense sunlight and larger reaction rates) at the tropics, thereby lifting O3 and its precursor molecules into the free troposphere where ozone's lifetime is longer. We simulate a smaller relative contribution from tropical regions to the global mean surface ozone compared to their contribution to the TOB. The global population-weighted mean ozone is much larger compared to global mean surface ozone, 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 NOx emissions. The increasing trends in anthropogenic precursor emissions from these regions are the main drivers of increasing global population-weighted mean ozone.