The influence of deep convection on HCHO and HO in the upper troposphere over Europe

• Main Authors: H. Bozem, A. Pozzer, H. Harder, M. Martinez, J. Williams, J. Lelieveld, H. Fischer
• Format: Article
• Language: English
• Published: Copernicus Publications 2017-10-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/17/11835/2017/acp-17-11835-2017.pdf

Deep convection is an efficient mechanism for vertical trace gas transport from Earth's surface to the upper troposphere (UT). The convective redistribution of short-lived trace gases emitted at the surface typically results in a C-shaped profile. This redistribution mechanism can impact photochemical processes, e.g. ozone and radical production in the UT on a large scale due to the generally longer lifetimes of species like formaldehyde (HCHO) and hydrogen peroxide (H₂O₂), which are important HO_<i>x</i> precursors (HO_<i>x</i> =  OH + HO₂ radicals). Due to the solubility of HCHO and H₂O₂ their transport may be suppressed as they are efficiently removed by wet deposition. Here we present a case study of deep convection over Germany in the summer of 2007 within the framework of the HOOVER II project. Airborne in situ measurements within the in- and outflow regions of an isolated thunderstorm provide a unique data set to study the influence of deep convection on the transport efficiency of soluble and insoluble trace gases. Comparing the in- and outflow indicates an almost undiluted transport of insoluble trace gases from the boundary layer to the UT. The ratios of out : inflow of CO and CH₄ are 0.94 ± 0.04 and 0.99 ± 0.01, respectively. For the soluble species HCHO and H₂O₂ these ratios are 0.55 ± 0.09 and 0.61 ± 0.08, respectively, indicating partial scavenging and washout. Chemical box model simulations show that post-convection secondary formation of HCHO and H₂O₂ cannot explain their enhancement in the UT. A plausible explanation, in particular for the enhancement of the highly soluble H₂O₂, is degassing from cloud droplets during freezing, which reduces the retention coefficient.