Comparing the Effect of Anthropogenically Amplified Halogen Natural Emissions on Tropospheric Ozone Chemistry Between Pre-Industrial and Present-Day

Reactive halogens (X + XO, X = I, Br or Cl) catalytically destroy a fraction of tropospheric ozone under present-day (PD) conditions, however, their distribution and potential impact on tropospheric ozone under pre-industrial (PI) conditions remain largely unexplored. This study uses the Community Atmosphere Model with Chemistry (CAM-Chem) to investigate the effect of anthropogenically amplified natural emissions of halogenated species and their subsequent chemistry on tropospheric ozone under PI and PD atmospheric conditions. Model results show that the global tropospheric ozone depletion due to natural halogens is slightly more sensitive in PI than PD, with percentage changes in tropospheric ozone burden (TOB) of -14.1 +/- 0.6% for PI and -12.9 +/- 0.6% for PD. Individually, the role of iodine and chlorine in ozone depletion is equivalent in both periods (Delta TOBI: similar to-7% and Delta TOBCl: similar to-2.5%), while bromine plays a larger role in PI (Delta TOBBr: -5.5 +/- 0.6%) versus PD (Delta TOBBr: -4.3 +/- 0.7%). The increase in anthropogenic ozone precursor emissions from PI to PD has amplified the natural emission of inorganic halogens and led to a shift in the partitioning of inorganic halogens from reactive to reservoir species. Consequently, halogen-driven ozone depletion from the surface to the free troposphere is larger in PI than PD. In contrast, in the upper troposphere, the ozone depletion is larger in PD influenced mainly by stratospheric intrusion of reactive halogens from long-lived species. This study highlights the importance of including a complete chemical coupling of natural halogens and atmospheric pollutants in chemistry-climate models to adequately assess their effects on tropospheric ozone in a changing climate. Plain Language Summary Halogens (I, Br and Cl) emitted from natural sources catalytically destroy a fraction of tropospheric ozone, a trace gas that plays a key role in atmospheric chemistry, both in determining the oxidative capacity and as a component of photochemical smog, affecting air quality and public health. Previous studies have explored the effect of halogens on ozone in present and future time, while the role of halogens in the pre-industrial atmosphere is quite uncertain. Based on a state-of-the-art coupled chemistry-climate model, we explore the effect of both natural sources and chemistry of reactive halogens on tropospheric ozone under ambient conditions representative of the pre-industrial and present-day. Model results show that the increase in tropospheric pollutant emissions since pre-industrial times amplifies halogen natural emissions and affects the tropospheric role of halogens, resulting in a larger impact on near-surface ozone in less polluted conditions such as pre-industrial compared to present-day.