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Tropospheric Oxidation Processes
Most anthropogenic pollutants enter and leave in the lower the atmosphere (the troposphere) and an important natural pollutant removal process in this region is atmospheric oxidation. This process, which is initiated following the photolysis of naturally occurring ozone (O3) gas, and the reaction of excited oxygen atoms produced in this process with water vapour, relies upon the production of the reactive OH free radical species. OH reacts directly with many atmospheric pollutants, initiating their chemical degradation and ultimately removal, and therefore plays a key role in preserving atmospheric composition. OH also enters into a rapid equilibrium with another reactive free radical species in the air, namely HO2 (hydroperoxy). Consequently, processes which remove OH or HO2 from the air are environmentally important. Typically, the ratio of abundances of HO2 to OH (collectively HOx) in the lower atmosphere is of the order of 100, reflecting the different reactivity of these species. As a result, many of the important processes removing HOx are those involving hydroperoxy. One such reaction is the self reaction of HO2, producing highly soluble hydrogen peroxide, which can be removed from air in precipitation: HO2 + HO2 → H2O2 + O2 This reaction, which is especially important in 'clean' or background air (air in the absence of short lived pollutants such as nitrogen oxides) is effective throughout the troposphere, and thus inclusion of this reaction in numerical models for simulation of atmospheric composition requires characterization of the gas phase kinetics under all relevant environmental conditions. It transpires that the kinetics of the HO2 self-reaction are far from straightforward. The reaction displays both bimolecular and termolecular channels, and hence a complex pressure dependence. Both of these reaction channels also exhibit different negative temperature dependencies, and the reaction is found to be catalysed in the presence of water vapour (ubiquitous in the troposphere) or other polar molecules (which are often used as precursors to generate HO2 in the laboratory.) In recent work in our group, we have carried out laboratory studies of the gas phase HO2 + HO2 reaction under a wide range of conditions (T, p, humidity) relevant to the tropospshere. Such studies were carried out using laser photolysis of suitable gaseous precursor mixtures, and rapid time resolved ultraviolet absorption spectroscopy of the reacting gas mixture. Uniquely, our set up allows the monitoring of a broad spectral window as a function of time, allowing the entire HO2 absorption band to be used to quantify the free radical concentration as a function of reaction time (as shown in Figure 1). Furthermore , the product H2O2 could also be monitored, allowing excellent constraint on the kinetics studies.Our studies of the HO2 + HO2 reaction have been carried out to lower temperatures than previously reported work, and indicate a strong negative temperature dependence, making the reaction particularly efficient at low temperatures such as those encountered at the tropical tropopause. A parameterization of the HO2 self-reaction rate coefficient as a function of the principal atmospheric variables was produced in this work, and inclusion of this in an atmospheric model of chemical composition (in collaboration with Dr. Mat Evans, School of Environmental Science, University of Leeds) showed enhanced H2O2 abundances compared to a model incorporating the previously recommended values for the HO2 self-reaction rate coefficient In ongoing work we are investigating the effects of humidity on other important atmospheric reactions involving the HO2 free radical.
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