Role of Cloud Physics in Thunderstorms on Ozone Production

Convective outflow plumes in the upper troposphere are a region where ozone (O₃) production can be prolific. Modeling studies from the 1990s estimated 4-15 ppbv O₃ produced per day for midlatitude convection, while analysis of Deep Convective Clouds and Chemistry (DC3) storms showed agreement with this estimate for a severe convection case, but even more O₃ production (20 ppbv over a 10-hour period) in the outflow of a mesoscale convective system. The ozone production depends on nitrogen oxides (NO_x), hydrogen oxides (HO_x) and their precursors (primarily formaldehyde, CH₂O, and peroxides H₂O₂ and CH₃OOH). Analysis of DC3 storms and SEAC⁴RS storms have revealed that formaldehyde and peroxides, which are soluble trace gases susceptible to wet scavenging, are affected by cloud physics in the storm, particularly the riming process where snow or graupel collect cloud drops that freeze. When the cloud drops freeze, some fraction (rf) of the dissolved trace gas is retained in the frozen particles and precipitated by the storm, and thereby reducing the amount of soluble trace gas transported to the upper troposphere. Here, we use modeling tools (both box modeling and WRF-Chem) to explore the effect of the chosen rf on the amount of O₃ produced in the convective outflow. Preliminary box modeling of the outflow conditions of a multicell storm observed on 2 September 2013 during SEAC⁴RS reveal, as expected, that more O₃ is produced when more CH₂O, H₂O₂, and CH₃OOH is transported to the upper troposphere. When the retention fraction is set to zero (the trace gases are released back to the gas phase), 4 ppbv of O₃ is produced over a 3-hour period. This contrasts to when prescribed rf >0.3 where the O₃ production rate is ~3 ppbv over the 3-hour period. This presentation will expand upon this preliminary analysis, examining ozone production in more severe storms as well as investigating the contribution of various precursors to the ozone production.