Influence of fair‐weather cumulus clouds on isoprene chemistry

Abstract

[1] Fair-weather cumulus clouds are not resolved in regional-and global-scale atmospheric chemistry models because their horizontal extent is less than the horizontal resolution of the model. A Large-Eddy Simulation (LES) model, with finer grid resolution, can resolve the energy containing turbulent eddies and fair-weather cumulus clouds. Isoprene, which is mainly emitted from deciduous forests and plays a significant role in producing ozone, has a chemical lifetime similar to the boundary layer turbulence turnover time, indicating that turbulent transport, cloud processes, and chemistry are all potentially important for the prediction of ambient isoprene concentrations. The LES model coupled with chemistry developed in this study is an ideal tool to examine the influence of fair-weather cumulus clouds on isoprene chemistry. With a LES model that includes a moderately complex gas-phase chemical mechanism of isoprene oxidation, we find enhancement of isoprene, methacrolein, and methylvinyl ketone in the cloud layer while changes in these chemical species' mixing ratios in the subcloud layer relative to the cloud-free case vary depending on the chemical lifetimes. We demonstrate that nitrogen oxides put into the system can modulate the chemical lifetimes of isoprene and related chemical species, which in turn changes the vertical distribution of the chemical species. For high NO x conditions, ozone in the subcloud layer for the cloudy case is $5 ppbv lower than that for the cloud-free case, suggesting potential positive ozone bias in large-scale models that do not include fair-weather cumulus cloud processes.

A new WRF-Chem treatment for studying regional-scale impacts of cloud processes on aerosol and trace gases in parameterized cumuli

“…The fractional difference in the BC between the DeepShallow and Control simulations approaches 50 % as the convective clouds transport BC from below the cloud into the cloud layer. The net effect of the non-precipitating cumuli is to mix BC over the sub-cloud and cloud layers, which is consistent with the findings of others (e.g., Vilà-Guerau de Arellano et al, 2005;Kim et al, 2012).…”

LES study of the impact of moist thermals on the oxidative capacity of the atmosphere in southern West Africa

“…Formaldehyde mixing ratios are close to 2 ppbv, which explains that this is not a important sink as isoprene, but remains not negligible in the OH loss. The OH production in the biogenic case is similar to Kim et al (2012) However, the mechanism used to represent atmospheric chemistry in our simulation has a fast OH cycling through the reaction of peroxy radicals RO 2 with HO 2 . An uncertainty comes from this reaction yield that is higher than laboratory studies (Hasson et al, 2004;Jenkin et al, 2007;Groß et al, 2014;Winiberg et al, 2016).…”

“…the height at which a passive bottom-up scalar emitted the third day is equal to 0.5% of its surface value. Kim et al (2012) mentioned that this definition could be valuable when studying the boundary layer deepening due to clouds. This diagnostic captures the growth of the boundary layer height due to the development of cumulus clouds (Fig.…”

“…When updrafts lead to cloud formation, Vilà-Guerau de Arellano et al (2005) found a decrease of 10 to 50% of tracer mixing ratios High resolution simulations which explicitly resolve the turbulent and convective advection terms were conducted (Vilà-Guerau de Arellano and Cuijpers, 2000; Vilà-Guerau de Arellano et al, 2005;Kim et al, 2012Kim et al, , 2016 The goal of this work is to evaluate the role of thermals on OH reactivity in the framework of a convective boundary layer with contrasted chemical environments in southern West Africa. This region is characterized by high occurrence of low-level stratus and stratus deck poorly represented in climate models, leading to errors in radiative forcing (Schrage et al, 2007;Knippertz et al, 2011;Schrage and Fink, 2012).…”

“…This is higher than the segregation computed at the surface in the biogenic case of the present study but when considering the complete mixing volume, the segregation computed with boundary layer averaged mixing ratios is negative and its maximum value is -16% at 1300 UTC. Kim et al (2012) Conditions from Column and Vertically Resolved Observations Relevant to Air Quality) campaign. Isoprene was a dominant sink for OH, about 25-30% of the OH reactivity was linked to BVOC reactions at 0.3 km during midday.…”