The effect of Natural Halogens on Tropospheric Ozone Chemistry in the Pre-Industrial vs Present-Day

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 as a greenhouse gas 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-day (PD) and future time, while the role of halogens in pre-industrial (PI) ambient conditions is quite uncertain. We use the Community Atmospheric Model with Chemistry (CAM˗Chem) to explore the effect of both natural sources and chemistry of halogens on tropospheric ozone in the PI and PD. The model results show that natural halogens have comparatively a larger impact on ozone under less polluted atmospheric conditions, with percentage changes in tropospheric ozone burden (TOB) of ˗14% for PI and ˗13% for PD. Individually, the role of iodine in ozone destruction is equivalent in both periods (ΔTOB^I: ˗7%). Bromine plays a larger role in PI (ΔTOB^Br: ˗5%) vs. PD (ΔTOB^Br: ˗4%), while chlorine plays a larger role in PD (ΔTOB^Cl: ˗2.5% vs. ˗2%). The increase in anthropogenic ozone precursor emissions from PI to PD drives an enhancement in the inorganic halogen budget, as well as a change in the partitioning of inorganic halogens, shifting from reactive (X+XO, X= I, Br or Cl) to reservoirs (e.g. HOX and XONO₂) species. Consequently, model results show that the halogen-mediated ozone depletion in the global lower troposphere is higher in PI than in PD. This study highlights the importance of including a complete chemical representation of natural halogens in chemistry-climate models to adequately assess their effects on tropospheric ozone in a changing climate