Transport and scavenging of soluble gases in a deep convective cloud

Abstract

A one-dimensional entraining/detraining plume model is used to examine the transport and scavenging of soluble gases in tropical deep convection. The model is applied to a continental system observed over Brazil during the Trace and Atmospheric Chemistry Near the Equator-Atlantic (TRACE-A) TRACE-A aircraft campaign with outflows extending from 7 to 16 km altitude. Six gases are simulated: CO (inert tracer), CH3OOH, CH2O, H2O2, HNO3, and SO2. Observed (simulated) convective enhancement factors (CEF) at 7–12 km altitude, representing the ratios of postconvective to preconvective mixing ratios, are 2.4 (1.9) for CO, 11 (9.5) for CH3OOH, 2.9 (3.1) for CH2O, 1.9 (1.2) for H2O2, and 0.8 (0.4) for HNO3. Simulated scavenging efficiencies in the convective column are 5% for CH3OOH, 23% for CH2O, 66% for H2O2, 77% for HNO3, and 28% for SO2. The large CEF for CH3OOH reflects its low solubility and its boundary layer enrichment relative to the upper troposphere. The Henry's law constant for CH2O puts it at the threshold for efficient scavenging. Scavenging of SO2 is limited by the rate of aqueous phase reaction with H2O2, as H2O2 is itself efficiently scavenged by Henry's law equilibrium; efficient scavenging of SO2 requires unusually high cloud water pH (pH>6) to enable fast aqueous phase oxidation by O3. Both HNO3 and H2O2 are efficiently scavenged in the lower (warm) part of the cloud, but H2O2 is released as the cloud freezes due to low retention efficiency during riming. Significant scavenging of H2O2 still takes place by cocondensation with ice in the glaciated cloud but is less efficient than in the warm cloud. Inefficient scavenging of H2O2 in glaciated clouds may explain the observation, in TRACE-A and elsewhere, that H2O2 is enhanced in deep convective outflows while HNO3 is depleted. Model results indicate little direct transfer of air from the boundary layer to the cloud anvil in the convective plume, because of low-level detrainment in the warm cloud and high-level entrainment in the glaciated cloud. We find instead a convective ladder effect where midlevel outflow during the growing phase of the storm is reentrained into the convective plume as the storm matures.