Regional air pollution and its radiative forcing: Studies with a single-column chemical and radiation transport model

This study focuses on the effects of subgrid (on general circulation model (GCM) scales) convective venting of the planetary boundary layer to the free troposphere and on the interactive effects of aerosols, ozone, UV actinic flux, and radiative forcing of climate. We developed a single-column chemical transport model (SCCTM) consistent with the global Goddard Earth Observing System (GEOS) GCM and Chemical Transport Model (CTM). The SCCTM includes vertical transport by convection, turbulent mixing, a flexible photochemical scheme, and interactive calculations of radiative fluxes and photolysis rates. The model is designed as a chemical and physical core to be used in a completely interactive GCM. At this time it is driven by data from the GEOS Data Assimilation System archived by the Data Assimilation Office at NASA Goddard. We simulated an ozone/aerosol pollution episode in the Baltimore-Washington region and a convective event in the central United States. These physically distinct case studies provide a thorough test for the chemical scheme and physical parameterizations employed in the SCCTM. The ozone episode simulation showed strong sensitivity to aerosol optical depth, with increased average PBL ozone due to the effects of aerosols on photolysis rates. Observed aerosols produced a surface cooling of up to 100 W m^-2 and stabilized the atmosphere suppressing convection. In the convective event over Oklahoma a squall line carried pollutants into the free troposphere increasing O₃ by up to 35 ppbv and peroxyacetylnitrate by up to 400 pptv. The maximum instantaneous radiative forcing due to this ozone reached 0.75 W m^-2 at the tropopause level. Accurate representation of the interaction among particles, trace gases, radiation, and convection is essential for global climate models.