1.5 Transport of Soluble Trace Gases in Thunderstorms

Mary Barth (ACOM/MMM), Megan Bela (U. Colorado), Alan Fried (U. Colorado)

Convective transport is a major pathway for rapidly moving chemical constituents and water from the boundary layer to the upper troposphere and in some cases to the lower stratosphere. These plumes of convective outflow in the upper troposphere are often rich in ozone precursors. Nitrogen oxides are formed from lightning, while volatile organic compounds, peroxides, and formaldehyde are transported from the boundary layer. However, the key precursors, hydrogen peroxide, methyl hydrogen peroxide, and formaldehyde, are soluble and can be partially removed from the atmosphere via dissolution into cloud drops that grow into precipitation. ACOM and university scientists estimated the fraction of these peroxides and formaldehyde removed by thunderstorms that were observed during the Deep Convective Clouds and Chemistry (DC3) field campaign, which took place over the central U.S. in May and June 2012.

The DC3 aircraft sampled the composition of the air in both the low-level inflow region and the upper-level outflow region (where the visible anvil is). Vertical profiles by the aircraft measured trace gas concentrations in clear sky outside the storms. Using these data, investigators determined the entrainment rate of clear air into the storm with trace gases that are not soluble or chemically changing over a several hour period. The scavenging efficiency of peroxides and formaldehyde could be determined by comparing the measured outflow concentrations to the estimated concentrations if the soluble trace gas were only transported, using the entrainment rate model. Fried et al. (2016) found formaldehyde scavenging efficiencies were 41-58% for storms sampled in Oklahoma, Colorado, and Arkansas. Barth et al. (2016) estimated that the more soluble hydrogen peroxide scavenging efficiencies were > 77% while the less soluble methyl hydrogen peroxide scavenging efficiencies were 12-84%. The high scavenging efficiencies found for methyl hydrogen peroxide are surprising and may be caused by entrainment of low concentrations of the species into the storm and/or cloud physical processes. Simulations performed by WRF-Chem reveal that methyl hydrogen peroxide scavenging efficiencies vary depending on the fraction of the trace gas retained in frozen cloud particles as cloud drops freeze (Bela et al., 2016). The low concentrations of methyl hydrogen peroxide in upper-level, convective outflow regions is crucial to understand, as previous studies have suggested that methyl hydrogen peroxide is the primary precursor producing ozone in these regions.

Sample template image

Figure 1. a) Air motions associated with deep convection in an environment with high vertical wind shear. The schematic is annotated with locations of the measured trace gas mixing ratios in the boundary layer inflow, free troposphere background, anvil outflow, and storm core top. b) Correspondence of the scavenging efficiency of methyl hydrogen peroxide and entrainment for 6 DC3 storms. Values are derived from observations. c) Correspondence between methyl hydrogen peroxide scavenging efficiency and the retention factor of methyl hydrogen peroxide in drops that are freezing. Values are from WRF-Chem simulations of three different thunderstorms.