Oxidant Budgets of the Northern Hemisphere Troposphere Since 1950

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The hydroxyl radical (OH) is the dominant oxidizing agent in the troposphere, as such its concentration controls the abundances and lifetimes of most atmospheric pollutants, including the important greenhouse gas methane (CH4). Ozone (O3) is also an important oxidant and is itself a greenhouse gas. The concentrations of OH and O3 are interdependent, both being determined by a complex series of reactions involving CH4, carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NOX = NO + NO2). As emissions of these compounds have changed substantially since pre-industrial times, the tropospheric budgets of OH and O3 will also have changed. However, there are large uncertainties associated with current understanding of these past changes and consequently very large uncertainties in projected future changes and associated climate impacts.Most of this uncertainty in past trends comes from lack of observations to constrain studies. Whilst there are a few direct observational data sets which indicate how O3 concentrations changed through the 20th century, there are none for OH. Direct observational data sets of CH4, NMVOCs, CO and NOX, extend, at best, from the 1980s. These time series can be extended backward in time through the analysis of air trapped in firn (unconsolidated snow). Whilst such historic time series have been available for CH4 for some time, only recently have they become available for CO and for some NMVOCs, in particular alkanes. Furthermore, we have also recently determined, from firn analysis, historic time series of alkyl nitrates. Alkyl nitrates are products of the chemistry involving NOX and as such can be used as a diagnostic of the changes in NOX. These new (and in the case of the alkyl nitrates, unique), historic time series provide an exciting opportunity to investigate the changing OH and O3 budgets of the northern hemisphere troposphere since 1950 with observational constraints never available before. Very interestingly, the simple analyses carried out on these time series to date suggest that substantial changes in the atmospheric chemistry have occurred. To exploit the full value of these time series a detailed study is required with a comprehensive chemistry-climate model. Here we propose the first such study.The outcomes of this study will be: 1) a better understanding of the impact of changing anthropogenic emissions on the OH and O3 budgets of the northern hemisphere troposphere; 2) an improved modelling capability with which to project future changes and better inform climate policy.This proposal brings together experts in firn air data interpretation with experts in chemistry-climate modelling. Both groups also have considerable expertise in organic (including alkyl) nitrate chemistry. This proposal specifically builds on past NERC funded work on the trends of alkanes and alkyl nitrates in firm air using simply relationships and models.