The influence of deep convection on HCHO and H<sub>2</sub>O<sub>2</sub> in the upper troposphere over Europe

Abstract

Abstract. Deep convection is an efficient mechanism for vertical trace gas transport from Earth's surface to the upper troposphere (UT). The convective redistribution of shortlived 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 (H 2 O 2 ), which are important HO x precursors (HO x = OH + HO 2 radicals). Due to the solubility of HCHO and H 2 O 2 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 CH 4 are 0.94 ± 0.04 and 0.99 ± 0.01, respectively. For the soluble species HCHO and H 2 O 2 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 H 2 O 2 cannot explain their enhancement in the UT. A plausible explanation, in particular for the enhancement of the highly soluble H 2 O 2 , is degassing from cloud droplets during freezing, which reduces the retention coefficient.

Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

“…Based on observed OH levels and photolysis frequencies during OMO the H 2 O 2 lifetime in the upper troposphere is of the order of several days, sufficiently long for the excess H 2 O 2 to reach the western parts of the AMA, producing the observed longitudinal H 2 O 2 gradient observed in both observations and EMAC simulations ( Figure 16). The total amount of H 2 O 2 injected into the UT by convective outflow depends on the scavenging efficiency (Mari et al, 2000;Barth et al, 2016;Bozem et al, 2017).…”

“…The steady-state calculations are based exclusively on observed concentrations of HO 2 and OH radicals and thus yield only the net photochemical production, while the EMAC simulations and the observations will also account for vertical and horizontal advection from up-wind source regions. Bozem et al (2017) troposphere. With a photochemical lifetime of several days, this excess in H 2 O 2 reaches the western AMA, giving rise to the observed and model simulated longitudinal gradients.…”

Impact of the South Asian monsoon outflow on atmospheric hydroperoxides in the upper troposphere

“…Additional uncertainties related to the molecular parameters are process-specific. For H 2 O 2 in particular, recommended absorption cross sections and their temperature dependencies were applied, and unity quantum yields were assumed (Burkholder et al, 2015). However, the recommended H 2 O 2 absorption cross sections are confined to a wavelength range below 350 nm, which is insufficient to capture atmospheric photolysis completely.…”

“…For MHP the temperature dependence of the absorption cross sections is unknown. Therefore the recommended room temperature data were used under all conditions as well as unity quantum yields (Burkholder et al, 2015). Combined total uncertainties of 15 % and 25 % are estimated for H 2 O 2 and MHP photolysis frequencies, respectively.…”

H2O2and CH3OOH (MHP) in the Remote Atmosphere: 2. Physical and Chemical Controls

“…H 2 O 2 and MHP have similar mixing ratios in the boundary layer (H 2 O 2 /MHP ∼1–3), but H 2 O 2 is preferentially removed by cloud water (high scavenging efficiency) and precipitation that forms during convection while MHP is lofted with minimal loss (low scavenging efficiency) (Barth et al., 2016; Bela et al., 2018; Cuchiara et al., 2020; Heikes et al., 1996; O’Sullivan et al., 1999). The scavenging efficiency of these hydroperoxides depends upon the interactions of these species with the environment as they are lofted: interactions with the freezing and/or evaporation of cloud particles may lead to less efficient scavenging of H 2 O 2 and/or more efficient scavenging of MHP (Bela et al., 2016; Bozem et al., 2017; Y. Li et al., 2019). Following convection, MHP in the UTLS may be enhanced by 3–6 times background levels (Cohan et al., 1999; Jaeglé et al., 2000; Ravetta et al., 2001).…”