Impact of long-lived Greenhouse Gases (GHGs) on tropospheric chemistry

Concentrations of well-mixed greenhouse gases (WMGG), including ozone depleting substance (ODSs), have increased dramatically since the preindustrial times. Increases in WMGGs affect significantly the chemical and dynamical structure of both the stratosphere and troposphere. We apply single-forcing sensitivity simulations of the GFDL coupled chemistry-climate model (CM3) to investigate the impact of preindustrial to present day changes in carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ODSs individually on atmospheric composition, particularly the tropospheric composition, and climate. We contrast results at present day (mean 1996-2005) and preindustrial (mean 1860-1869) to assess the influence from changes in each of these gases. Increases in CH₄ and CO₂ result in increases in total ozone column (TOC) while N₂O and ODS increases cause TOC to decrease in the present day relative to preindustrial.  Increases in CH₄ cause globally uniform ozone increases in the troposphere from preindustrial to present day. CO₂ increases produce noticeable tropospheric ozone decreases in the tropics and minimal increases in the extratropics from CO₂-induced changes in atmospheric circulation. Increases in ODSs not only lead to the well-known strong stratospheric ozone loss, but also decreases in the tropospheric ozone possibly resulting from reduced ozone transport from the stratosphere. Finally, N₂O increases lead to stratospheric ozone loss albeit with a smaller signal compared to that from increase in ODSs and produce negligible changes in tropospheric ozone. We will further analyze the role of changes in chemistry and atmospheric circulation in driving changes in atmospheric ozone concentration from past changes in GHGs and ODSs.