A model study of the impact of well-mixed Greenhouse Gases (GHGs) on tropospheric chemistry from the preindustrial era to the present day

 

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. Here, 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 CH_4, an important tropospheric ozone precursor, cause globally uniform ozone increases in the troposphere with global average surface ozone concentrations increasing by ~4 ppbv from preindustrial to present day. CH₄ increases also cause lower stratospheric ozone increases possibly resulting from reduced ozone loss due to cooling induced by water vapor increases. CH₄ increases can further enhance stratospheric ozone by converting active chlorine to hydrochloric acid (HCl); changes in HCl will be explored to explain CH₄-induced stratospheric ozone increases. CO₂ increases produce noticeable tropospheric ozone decreases in the tropics, particularly in the upper troposphere, and minimal increases in the extratropics from CO₂-induced changes in atmospheric circulation, while stratospheric ozone increases uniformly resulting from the CO₂-induced cooling. 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.