Role of convection in redistributing formaldehyde to the upper troposphere over North America and the North Atlantic during the summer 2004 INTEX campaign

Abstract

Measurements of CHMultiple tracers were used to arrive at a set of UT CH 2 O background and perturbed air mass periods, and 46% of the TDLAS measurements fell within the latter category. At least 66% to 73% of these elevated UT observations were caused by enhanced production from CH 2 O precursors rather than direct transport of CH 2 O from the boundary layer. This distinction is important, since the effects from the former can last for over a week or more compared to one day or less in the case of convective transport of CH 2 O itself.In general, production of CH 2 O from CH 4 was found to be the dominant source term, even in perturbed air masses. This was followed by production from MHP, methanol, PAN type compounds, and ketones, in descending order of their contribution. In the presence of elevated NO from lightning and potentially from the stratosphere, there was a definite trend in the CH 2 O discrepancy, which for the highest NO mixing ratios produced a median CH 2 O measurement/model ratio of 3.9 in the 10-12-km range. Discrepancies in CH 2 O and HO 2 in the UT with NO were highly correlated and this provided further information as to the possible mechanism(s) responsible. These discrepancies with NO are consistent with additional production sources of both gases involving CH 3 O 2 + NO reactions, most likely caused by unmeasured hydrocarbons.3

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

“…Thus, the lower than unity values for outflow : inflow ratios for HCHO and H 2 O 2 of 0.54 (0.55 ± 0.09) and 0.59 (0.61 ± 0.08), respectively, are most likely due to partial rain-out of these soluble species. Other studies (Bertram et al, 2007;Fried et al, 2016) derived much lower ratios between outflow and boundary layer inflow, indicating significant entrainment. To estimate the potential role of entrainment for our measurements we apply a two-box model (Cohan et al, 1999) to calculate outflow (OF) mixing ratios from the inflow (IN) and the entrainment (EN) according to OF = x EN + (1 − x)IN.…”

“…While insoluble species are efficiently transported to the UT via convection, soluble species are scavenged by cloud and rain droplets, with subsequent removal by precipitation (Wang and Crutzen, 1995;Crutzen and Lawrence, 2000). Nevertheless, recent observations have shown that highly soluble species can also reach the UT via deep convection, most likely due to incomplete removal of these species during pre-cipitation events (Crutzen and Lawrence, 2000;Marie et al, 2000, Barth et al, 2001Yin et al, 2002;Borbon et al, 2012;Bela et al, 2016;Fried et al, 2016). Considerable uncertainty remains with respect to the processes that control the concentrations of soluble species in the outflow of deep convection.…”

“…As shown by Fried et al (2008), the temporal evolution of the HCHO mixing ratio depends on the concentration of HCHO at the cloud top, the concentrations of HCHO precursors and radicals, and the processing time. Within the first few hours it is unlikely that the HCHO concentration will reach steady state.…”

“…Based on a 1-D-model study Marie et al (2000) show that incomplete scavenging of H 2 O 2 and other species can lead to significant enhancement in the outflow of deep convection. Besides H 2 O 2, formaldehyde (HCHO) enhancement was also observed in the outflow of deep convection (Prather and Jacob, 1997;Cohan et al, 1999;Stickler et al, 2006;Fried et al, 2008;Borbon et al, 2012). This may be due to transport from the source region (boundary layer) or to secondary production from convectively transported HCHO precursors like methanol (CH 3 OH), acetone (CH 3 COCH 3 ), acetaldehyde (CH 3 CHO) and methylhydroperoxide (CH 3 OOH) (Prather and Jacob, 1997;Fried et al, 2008), which are generally less soluble than HCHO.…”

Impacts of formaldehyde photolysis rates on tropospheric chemistry

“…Over the past decade it has emerged that measured HO x in the UT exceeds that predicted by models and that a range of HO x precursors may contribute to this discrepancy. Such precursors include formaldehyde, alkyl peroxides, acetone and isoprene derivatives that are thought to be transported from lower altitudes (Brune, 1998;Collins et al, 1999;Crawford et al, 1999;Colomb et al, 2006;Fried et al, 2008;Jaeglé et al, 2001). These studies have indicated the importance of deep tropical convection and convection from the marine boundary layer on the UT HO x budget.…”