Dr David Rowley
Chemistry of the Earth's Atmosphere
Changes to the chemical composition of the Earth's atmosphere can have profound environmental consequences. Such changes result from direct atmospheric pollution but also the chemical and photochemical transformation of atmospheric pollutants, and lead to issues such as poor air quality and smog formation; atmospheric ozone depletion and global warming. A full understanding of the causes of such environmental issues requires a full knowledge of the nature and rapidity of the transformation of pollutants, and research in our group is focussed on understanding, through laboratory studies, the fundamental gas phase kinetics and mechanisms of reactions at the heart of atmospheric change.
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.
Figure 1: Time and wavelength resolved UV absorbance recorded following photolysis of a HO2 radical precursor gas mixture at 298K. Photolysis is initiated at t = 0 s.
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 (Figure 2).
Figure 2: Changes in the modelled abundance of atmospheric trace species as a result of inclusion of new kinetic data recorded for the HO2 + HO2 reaction in this work. Changes are expressed as ratios of abundances to a model run with the previously recommended HO2 self-reaction rate coefficients. Model run courtesy of Dr M.J. Evans, School of Environmental science, University of Leeds.
In ongoing work we are investigating the effects of humidity on other important atmospheric reactions involving the HO2 free radical. The rate enhancement of the self-reaction of HO2 is attributed to the formation and differential reactivity of a HO2.H2O complex, and evidently this complex might also behave differently in other reactions of HO2. New spectroscopic methods are therefore being investigated to probe the HO2.H2O complex directly.
Stratospheric Ozone Chemistry
Ozone (O3) in the upper atmosphere (the stratosphere) plays a vital role in absorbing harmful solar ultraviolet radiation, particularly in the biologically damaging UV-B part of the spectrum. As a result, depletion of stratospheric ozone following anthropogenic pollution is an environmental concern and model simulations of ozone loss attempt to replicate and predict ozone abundances. Such models also help elucidate the role played by stratospheric composition in affecting the Earth's climate, and it is essential that models therefore contain an accurate description of stratospheric gas phase chemistry under appropriate conditions. In our work, we carry out laboratory studies of gas phase halogen chemistry of relevance to stratospheric ozone loss. Halogens, principally chlorine, are released into the atmosphere in long lived anthropogenic pollutants such as the CFC's (chlorofluorocarbons) and the degradation chemistry following CFC release leads to the polar ozone 'holes' observed routinely in Springtime.
We have recently studied the principal gas phase reaction resulting in Springtime polar ozone depletion events, the ClO association reaction. This reaction produces a ClO dimer species, which may then be photolysed by sunlight to regenerate chlorine atoms which destroy further O3:
ClO + ClO + M → Cl2O2 + M
Cl2O2 + hν → Cl + ClOO
ClOO + M → Cl + O2
2 × (Cl + O3 → ClO + O2)
net: 2 O3 → 3 O2
Our study of the ClO radical association reaction monitored ClO radicals using an ensemble of vibronic bands rather than at a single wavelength, wherein other absorbing species could interefere. This work characterized the reaction kinetics of ClO association as a function of temperature and pressure relevant to the upper atmosphere. Our results show that the reaction is more rapid than some previously reported work indicates, with potentially more efficient ozone loss expected to result from the reaction sequence illustrated above. More recently, we have studied the thermal stability of the ClO dimer species through analysis of the equilibrium between ClO radicals and Cl2O2 at near ambient temperatures.
In related work, we have been investigating the self and cross- reactions of other halogen monoxide free radicals which play roles in a variety of regions in the atmosphere. Our research also encompasses the absorption spectroscopy of laser generated weakly - bound gaseous molecules which may act as active species 'reservoirs' in the atmosphere, and ab initio studies of the mechanisms of selected atmospherically important reactions.