Cloud pumping in a one‐dimensional photochemical model

Abstract

Cloud pumping is included in a one‐dimensional steady state eddy diffusive photochemical model of the troposphere. The cloud pumping is based on updraft statistics for the tropical maritime strip from 5° to 25°N latitude. Chemical production and destruction terms in the species continuity‐transport equations are supplemented with source and sink terms, which are constructed from transition probability densities (TPDs). The TPDs describe both the updrafts and the subsidences that conserve atmospheric mass. Eddy diffusion is retained for smaller‐scale turbulence, such as the turbulence in the planetary boundary layer. For water soluble species the cloud pumping is artificially reduced, as a means of parameterizing the effects of cloud chemistry and precipitation. The amount of reduction depends on the species and is based on a photochemical study for maritime clouds. Sensitivity studies were performed as follows: mixing ratio boundary conditions were used at the surface and flux boundary conditions at an altitude of 10 km (the upper boundary in the model), the model was run with and without cloud pumping, and the species concentrations were compared at 10 km. For three species, ClNO, Cl2O, and CH3SH, which previous calculations showed to have negligible concentrations in the upper troposphere, cloud pumping increased the upper level concentrations by 5 orders of magnitude or more. For four species, (CH3)2S, BrONO2, Br, and HBr, the increase was 2–3 orders of magnitude. For nine species, H2S, SO, I, CH3I, HI, SO3, HSO3, H2SO4, and SO2, the increase was 1–2 orders of magnitude. For five species, COCl2, NH3, CH3NO3, NH2, and ClONO2, the increase was by a factor of 2–10. For 16 species, N2O5, CH3NO2, HNO2, HO2NO2, NO3, H, HNO3, H2O2, NOx, CO, C2Cl4, CH2Cl2, CH3NO4, CHCl3, Cl, and CH3O, the increase was 11–100%. For 10 species, HO2, OH, H2CO, CH3Cl, CH3Br, Cl2, HOCl, CH3OOH, CS2, and CH3CCl3, the increase was 2–10%. For eight species, HCN, CH4, COS, Br2, HOBr, CCl3, NOCl, and N2H4, the increase was 1% or less. For seven species, CH3O2, ClO, ClOO, CHCl2, CH2Cl, CH2Br, and N2H3, cloud pumping decreased the upper level concentrations by factors of 0.9–0.2. Vertical profiles were plotted for the concentrations of nine species, (CH3)2S, SO2, H2SO4, NH3, HNO3, NOx, H2O2, CO, and OH. Included on these plots were a profile computed with the average cloud pumping on the tropical maritime strip, a profile without pumping, and three profiles representing various amounts of cloud pumping found on the strip. These sensitivity studies indicate that regions with weaker convection, such as the mid‐latitudes, may also experience substantial effects from cloud pumping. A test run for CO was made to determine whether its sensitivity resulted directly from the cloud pumping of CO itself, or indirectly from its chemical reactions with other cloud‐pumped species. The indirect (chemical) effects were found to decrease the upper level concentration of CO; hence the direct effects of cloud pumping on CO were actually larger than the sensitivity studies had suggested. Similar test runs were made for 14 other species, and the effects of cloud pumping on each species were listed as direct, indirect, or mixed. Fairly good agreement with available measurements was found for the profile of (CH3)2S computed with cloud pumping; however, the upper level concentration of SO2 was found to be smaller than measured values by a factor of 2–4. Further research is needed to optimize the combination of cloud pumping, eddy diffusion, and heterogeneous chemistry for the purpose of obtaining more accurate results.