Influence of fair‐weather cumulus clouds on isoprene chemistry

Abstract

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 NOx 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.