Determination of uptake coefficients for heterogeneous loss of HO2 radicals to atmospheric aerosols

The free-radicals OH, HO2 and RO2 mediate virtually all of the oxidative chemistry in the lower atmosphere, being responsible for the transformation of primary emissions into secondary pollutants such as NO2, O3 and particulates. Free-radicals control the lifetime of climate gases (e.g. CH4), the budget of O3, and the production of acidic species. Understanding the behaviour of free-radicals in the atmosphere is of paramount importance in understanding the lifetime and spatial scales of pollutant transport. Predictive models for future air quality and climate change contain complex chemical schemes, and comparison with measurements of free-radicals in the present atmosphere constitutes the best validation of these schemes. Some field campaigns report significant discrepancies between models and in situ observations of OH and HO2 radicals (HOx), for example in marine and urban environments. Heterogeneous losses of HO2 to aerosol have been invoked to explain these differences. Reactions of HO2 on cloud droplets and cirrus particles have also been invoked to explain model discrepancies for HOx in the upper troposphere. However, the heterogeneous and multiphase chemistry of HOx is relatively understudied, and there is considerable variation in the literature for HO2 uptake coefficients, determined in the laboratory under limited conditions of temperature/relative humidity and aerosol type. In addition, these experiments were performed at relatively high concentrations of HO2 where second order loss via the HO2 self-reaction was dominant over the first order heterogeneous loss, complicating the extraction of accurate values. The primary aim of this project is to use the FAGE (Fluorescence Assay by Gas Expansion) technique to measure HO2 with exquisite sensitivity (0.04 pptv detection limit) so that its concentration can be kept extremely low such that the rate of the gas phase self-reaction is negligible compared with the rate of heterogeneous loss, which will be measured in an aerosol flow-tube. Measurements of HO2 uptake coefficients using aerosols with a range of diameters and composition relevant to the atmosphere (H2SO4, (NH4)2SO4, sea salt, oleic acid and soot) will be made, and data recorded under different temperatures, relative humidities and pH will enable parameterisations to be obtained. The results will be exploited through incorporation into a box model to re-evaluate the role of heterogeneous loss for two previous campaigns held in marine and urban environments.