Testing radical assay by nitrite chemical entrapment (TRANCE)

Radicals play a pivotal role in the atmosphere even though their concentrations are extremely small. The hydroxyl radical (OH) controls the oxidation of most atmospheric pollutants, including many important greenhouse gases, whilst the nature and fate of peroxy radicals (RO2) has a major impact on tropospheric ozone chemistry. It is therefore extremely important that global chemistry models accurately simulate the OH and peroxy radical concentrations. Current radical measurement methods, whilst sensitive, fast and well proven, are large, expensive and complicated and therefore observations of radicals are very limited in space and time. A consequence of this is that OH estimations in global models are often tested against long-lived tracers (e.g. methyl chloroform) even though the emissions of these tracers themselves are not necessarily accurately known. Further, the existing techniques for measuring peroxy radicals are unable to provide a measure of individual organic RO2 with which to validate the models. Therefore it would be of immense benefit if there were extensive, long term measurements of radicals with which to assess and improve models, but this would need small, easily-deployable systems which are not currently available. Hydroxyl and peroxy radicals almost certainly play a key role in other important processes such as photochemically induced emissions from snowpacks which have a great influence on the chemistry of the polar troposphere. Detailed radical measurements within the snowpack would be hugely beneficial in understanding this chemistry, but are not currently possible. Currently available kinetic data suggests that small organic peroxy radicals can be converted rapidly and quantitatively into stable organic nitrites by reaction with high concentrations of NO. This project will examine the feasibility of quantifying organic peroxy radicals by this conversion into distinct organic nitrite compounds which can be measured by highly sensitive negative ion gas chromatography-mass spectrometry (NI-GCMS). Additionally, if OH is allowed to react with a hydrocarbon to produce a distinct peroxy radical, then the nitrite conversion method offers the opportunity to measure OH as well. This novel method offers more information on individual organic peroxy radicals than current technology affords, should be able to measure OH and organic peroxy radicals concurrently, and has the potential to also measure Cl and NO3 radicals. The radical conversion and sampling is distinct from the analysis instrument which should allow the future development of a small, low-cost sampling device suitable for the large scale deployments required to test global chemistry models, as well as a sampling system well suited to measuring radicals in snowpack experiments.