Field measurements of OH and other small radicals/intermediates in the atmosphere and comparison with models

The OH radical removes the majority of trace gases in the atmosphere emitted either naturally or via human activities, initiating the formation of a wide range of secondary products, many of which are implicated in poor air quality, for example ozone and organic aerosols. However, in situ measurements of OH and other radical species in the atmosphere are extremely challenging, and laser-induced fluorescence (LIF) spectroscopy offers a very sensitive method for their quantitative detection. The presentation will describe LIF detection of atmospheric OH, HO2 and RO2 radicals (with some limited speciation) at low pressure using the so called FAGE (fluorescence assay by gas expansion) technique from ground and airborne platforms. Laser-based measurements will also be described of formaldehyde, glyoxal and OH reactivity, the latter being a kinetic parameter representing the total removal rate of OH via reaction with its atmospheric sinks. The specific detection of CH3O2 radicals will also be described via a new method. Comparison of field measurements of radicals and other short-lived species with numerical model calculations constitutes one of the best ways to evaluate chemical mechanisms within models. The detailed Master Chemical Mechanism is used which contains up to ca. 6,700 chemical species and 17,000 reactions. The presentation will show examples of measurements and model comparisons from field campaigns in a range of environments, including the remote marine boundary layer, a tropical rainforest and the mega-cities of London and Beijing. The impact of some new laboratory kinetic and photochemical data on atmospheric composition will also be discussed