Convective transport over the central United States and its role in regional CO and ozone budgets

Abstract

We have constructed a regional budget for boundary layer carbon monoxide over the central United States (32.5°–50°N, 90°–105°W), emphasizing a detailed evaluation of deep convective vertical fluxes appropriate for the month of June. Deep convective venting of the boundary layer (upward) dominates other components of the CO budget, e.g., downward convective transport, loss of CO by oxidation, anthropogenic emissions, and CO produced from oxidation of methane, isoprene, and anthropogenic nonmethane hydrocarbons (NMHCs). Calculations of deep convective venting are based on the method of Pickering et al. [1992a] which uses a satellite-derived deep convective cloud climatology along with transport statistics from convective cloud model simulations of observed prototype squall line events. This study uses analyses of convective episodes in 1985 and 1989 and CO measurements taken during several midwestern field campaigns. Deep convective venting of the boundary layer over this moderately polluted region provides a net (upward minus downward) flux of 18.1×10⁸kg CO month⁻¹ to the free troposphere during early summer, assuming the June statistics are typical. Shallow cumulus and synoptic-scale weather systems together make a comparable contribution (total net flux 16.2×10⁸ kg CO month⁻¹). Boundary layer venting of CO with other O₃ precursors leads to efficient free tropospheric O₃ formation. We estimate that deep convective transport of CO and other precursors over the central United States in early summer leads to a gross production of 0.66–1.1 Gmol O₃ d⁻¹ in good agreement with estimates of O₃ production from boundary layer venting in a continental-scale model [Jacob et al., 1993a, b]. In this respect the central U.S. region acts as a “chimney” for the country, and presumably this O₃ contributes to high background levels of O₃ in the eastern United States and O₃ export to the North Atlantic.