12 Mar 2024
 | 12 Mar 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

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

Aditya Nalam, Aura Lupascu, Tabish Ansari, and Timothy Butler

Abstract. Over the past few decades, the tropospheric ozone precursor anthropogenic emissions: nitrogen oxides (NOx) 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 NOx 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 NOx or RC precursor emission in two separate simulations, one for each of NOx 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 NOx 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 NOx 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 NOx emissions. The increasing trends in anthropogenic emissions from these regions are the main drivers of increasing global population-weighted mean ozone.

Aditya Nalam, Aura Lupascu, Tabish Ansari, and Timothy Butler

Status: open (until 23 Apr 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-432', Anonymous Referee #1, 08 Apr 2024 reply
  • RC2: 'Comment on egusphere-2024-432', Anonymous Referee #2, 12 Apr 2024 reply
Aditya Nalam, Aura Lupascu, Tabish Ansari, and Timothy Butler
Aditya Nalam, Aura Lupascu, Tabish Ansari, and Timothy Butler


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Short summary
Tropospheric O3 molecules are labelled with the identity of their precursor source in CAM-Chem to quantify the contribution from various emission sources to the tropospheric O3 burden (TOB) and its trends. With an equatorward shift, anthropogenic NOx emissions become significantly more efficient at producing O3 and play a major role in driving TOB trends. This is due to larger convection at the tropics effectively lifting O3 and its precursors to the free troposphere where O3 lifetime is longer.