The impact of clouds on radical concentrations: Observations of OH and HO2 during HCCT-2010

Clouds play a crucial role in the chemistry of the atmosphere, occupying, on average, ~ 15 % of the volume of the lower atmosphere (Lelieveld & Crutzen, 1990). Modelling studies have shown that aqueous phase chemistry in clouds can influence gas phase radical chemistry and in turn can cause significant reductions in the oxidative capacity (e.g. Lelieveld & Crutzen, 1991; Kreidenweis et al., 2003) and, hence, removal rate of VOCs. A number of aircraft projects have identified significantly reduced HO2 concentrations when flying through clouds that exceeds the depletion expected due to the reduction in radiation alone (Olson et al., 2004; Commane et al., 2010). These experimental observations are relatively sparse, however, and until recently a comprehensive study to validate model predictions has been lacking. Here we report preliminary measurements of OH and HO2 radicals made during the HCCT (Hill Cap Cloud Thuringia) project that took place at Mt. Schmücke, Thuringia in Germany during September/October 2010. The University of Leeds Fluorescence Assay by Gas Expansion (FAGE) instrument was located at the summit of Mt. Schmücke and made near-continuous measurements of the radicals at the top of a 22 m tower. The site was regularly influenced by clouds throughout the measurement period and co-located measurements of liquid water content were made at the site enabling the influence of this microphysical parameter on the radical budget to be determined. On average, the photolysis rate of O3 to form O(1D), the primary daytime source of HOx radicals, was ~ 65 % lower in-cloud relative to the out of cloud observations. The HO2 concentrations were significantly depleted in cloud, with concentrations ~ 90 % lower relative to the out of cloud observations; an OH signal above the noise of the instrument was not observed during cloud events. These results suggest that heterogeneous processes in clouds do perturb the gas-phase radical chemistry. Further investigations into the dependency of the uptake efficiency on the different microphysical and chemical conditions encountered during the different cloud events will also be presented. Lelieveld, J. and Crutzen P. J., 1990, Influences of Cloud Photochemical Processes on Tropospheric Ozone, Nature 343 (6255), 227-233. Lelieveld, J. and Crutzen P. J., 1991, The Role of Cloud in Tropospheric Photochemistry, Journal of Atmospheric Chemistry, 12, 229-267. Kreidenweis, S. M. et al., 2003, Modification of aerosol mass and size distribution due to aqueous phase SO2 oxidation in clouds: Comparison of several models. Journal of Geophysical Research-Atmospheres, 108. Olson, J. R. et al., 2004, Testing fast photochemical theory during TRACE-P based on measurements of OH, HO2 and CH2O. Journal of Geophysical Research-Atmospheres, 109. Commane, R. et al., 2010, Observations of OH and HO2 over West Africa. Atmospheric, chemistry and Physics, 10, 8783-8801.