Sensitivity of Tropospheric Ozone Over the Southeast USA to Dry Deposition.

Dry deposition (DD) is a major loss process for tropospheric ozone and some reactive nitrogen and carbon precursors. We investigate the response of summertime ozone and its production chemistry over the Southeast United States (USA) to variability in this sink. Turning off DD of oxidized nitrogen, ozone, or all species over the United States in the Geophysical Fluid Dynamics Laboratory AM3 model increases regional mean surface ozone by 5, 18, or 25 ppb, respectively. Additional sensitivity simulations demonstrate that, assuming linearity, surface ozone has a similar sensitivity to ozone DD as to NOx emissions. Trends in ozone production efficiency derived from observed relationships between ozone and precursor oxidation products may not solely reflect precursor emission changes if ozone DD varies (e.g., with meteorology). We conclude that DD variability merits consideration when interpreting observed ozone trends. Quantifying the impact of changes in sinks versus sources will require long‐term DD measurements across the region of interest. Plain Language Summary: Ozone is an air pollutant near Earth's surface and a greenhouse gas higher in the atmosphere. While many studies consider which ozone sources should be controlled to reduce air pollution, we focus here on a poorly understood removal pathway, "dry deposition" (DD), to the Earth's surface. We use a model to quantify how ozone responds to changes in the DD of ozone and related gases (oxidized nitrogen and carbon gases). We find that a 50% decrease in DD changes ozone concentrations in the Southeast United States by at least as much as recent changes in national emissions of nitrogen oxides (which form ozone in the presence of sunlight and reactive carbon gases). Our model suggests that DD variability, potentially driven by changes in meteorology or land use, could affect surface ozone trends. However, we lack the observations that would allow an assessment of how DD changes over space and time. Key Points: A 50% decrease in ozone dry deposition has twice the effect on surface ozone over the Southeast USA as a 25% increase in NOx emissionsTwo different metrics for ozone production efficiency show opposite‐signed responses to shutting off ozone dry depositionBounding the influence of ozone dry deposition on tropospheric composition requires long‐term, regional observations of its variability [ABSTRACT FROM AUTHOR]