Testing radical assay by nitrite chemical entrapment (TRANCE)

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences

Abstract

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.

Publications


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Description Our main achievement has been the development of an analytical system which can be used for the measurement of C1 to C4 alkyl nitrites using highly sensitive gas chromatography-negative ion mass spectrometry. This has involved the development of calibration methods, sample drying techniques and the optimization of sample trapping and injection conditions for these compounds.
However, the development of an analytical method for the measurement of low concentrations of alkyl nitrites itself is not of great significance to the scientific community since the alkyl nitrites are at extremely low concentrations in the atmosphere (one of the main reasons for choosing to convert RO2 radicals into nitrites in the first place).
The primary objective of this research, that is the demonstration of the conversion of peroxy radicals to the corresponding nitrites, has not yet been achieved.
The method of converting radicals to nitrites involves the use of high concentrations of NO in air, which inevitably leads to the rapid production of NO2. Unfortunately it appears that the presence of high NO2 concentrations in the sampled gas results in the destruction of any alkyl nitrites present in the sample during collection and/or analysis. A considerable number of methods for NO2 removal prior to sample trapping were subsequently tried, with no great success, some because the NO2 scrubber destroyed/adsorbed the alkyl nitrites and some because they appeared not to remove NO2 efficiently despite reports of their success in the literature.
It was also discovered, somewhat surprisingly, that NO was being concentrated in our system and subsequently being oxidised to NO2 in situ. This resulted in exceptionally high concentrations of NO2 being introduced into the analytical system, negating the effects of any upstream NO2 scrubbers. Additionally these high levels of NO2 caused irreversible changes to the trapping materials and to the GC column which subsequently rendered them unsuitable for alkyl nitrite analysis. After this discovery, there was insufficient time to replace the columns and investigate solutions to the problems further.
It is possible that if we use a weaker adsorbent in our system, one that does not trap NO, then the issue with NO2 affecting the alkyl nitrite analysis will be much reduced and may even be removed altogether with the use of mild NO2 scrubbers. This would then re-open the door to the conversion of radicals to nitrites and their subsequent measurement.
Exploitation Route Some of the contamination/interferent problems described above would have to be overcome before attempting to take this project further.
Sectors Environment