Oxidant Budgets of the Northern Hemisphere Troposphere Since 1950

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.

The long term beneficiary of this project is the global society through better international management of climate change. The direct outcomes of this proposed project are some way off achieving this goal but they are an important step in this process. For governments to sensibly manage climate change they need accurate projections of future change and these are based on forecast runs of chemistry-climate models, or components of these embedded in Earth system models. Being able to model and understand past changes in atmospheric chemical composition is vital to being able to make accurate model projections of future climate. This proposed project uses recently obtained observational data in a novel approach to evaluate and improve our ability to model these past changes. The potential beneficiaries, in order of the timescales at which they will benefit, are: chemistry-climate modellers, Earth system modellers, government policy makers, global society.

The model that will be used in this project is the UM-UKCA, which is the UK's community chemistry-climate model and a component of the UK's Earth System Model (UK-ESM1). Improved understanding of the model behaviour and model improvements that come from this project will therefore feed directly into the UK's Earth system modelling effort, which is jointly being carried out by the National Centre for Atmospheric Science (NCAS) and the UK Met Office (UKMO).

The UM-UKCA model (as a core component of ESM-1) will be used to perform runs for future IPCC (Intergovernmental Panel on Climate Change) assessments along with other models which are part of the international Chemistry-Climate Model Initiative (CCMI). By: 1) further developing the UM-UKCA and 2) feeding the results into CCMI, this proposed project will help to improve the model results used by IPCC to assess the past impact, and to project the future impact, of human activity on atmospheric composition and climate. It is the conclusions of the IPCC that inform international policy under the United Nations Framework Convention on Climate Change (UNFCCC).

This proposal therefore feeds into international policy-making related to the management of climate change and has the potential to benefit global society.