Faraday Discussion 130: Atmospheric Chemistry

Modelling NOx sources using satellite observations

Satellite observations have been used to provide important new information about emissions of nitrogen oxides.

Nitrogen oxides (NOx) are significant in atmospheric chemistry, having a role in ozone air pollution, acid deposition and climate change.  We know that human activities have led to a three- to six-fold increase in NOx emissions since pre-industrial times, and that there are three main surface sources of NOx: fuel combustion, large-scale fires, and microbial soil processes.  How each of these sources contributes to the total NOx emissions is subject to some doubt, however.

The problem is that current NOx emission inventories rely on 'bottom-up' approaches, compiling large quantities of statistical information from diverse sources such as fuel and land use, agricultural data, and estimates of burned areas.  This results in inherently large uncertainties. 

To overcome this, Lyatt Jaeglé and colleagues from the University of Washington, USA, used new satellite observations from the Global Ozone Monitoring Experiment (GOME) instrument. As the spatial and seasonal distribution of each of the sources of NOx can be clearly mapped from space, the team could provide independent 'top-down' constraints on the individual strengths of NOx sources, and thus help resolve discrepancies in existing inventories.

Jaeglé's analysis of the satellite observations, presented at the recent Faraday Discussion on "Atmospheric Chemistry", shows that fuel combustion dominates emissions at northern mid-latitudes, while fires are a significant source in the Tropics. Additionally, she discovered a larger than expected role for soil emissions, especially over agricultural regions with heavy fertilizer use.

Turning to space to model atmospheric changes opens up unprecedented possibilities for improving our understanding of the factors controlling the variability in soil and fire emissions, allowing scientists to begin to predict the shifts in these sources in the face of increasing land-use and climate change. Jaeglé is sure of where future challenges lie, commenting, "Over the last few years, new satellite instruments probing the composition of our lower atmosphere have been revolutionizing atmospheric chemistry.  The challenge is to use these new global observations to understand and predict the effects of human activities on atmospheric composition, and thus on human health and ecosystems".
 

1. Could you explain the significance of your article to the non-specialist?

Many hundreds of different organic compounds are emitted into the atmosphere from human-influenced and natural sources. Their oxidation potentially has harmful impacts on human health and the environment, through formation of secondary pollutants such as ozone and oxidised organic particles. The oxidation mechanisms involve the formation of thousands of intermediate oxidised organic compounds. In this work, a highly detailed representation of the oxidation chemistry has been used to predict the structures and distributions of the oxidised products during the heatwave period in August 2003, and to test their significance in influencing the rates of oxidation and the formation of ozone. 

2. What has motivated you to conduct this work? 

The chemical mechanism used in this work (the Master Chemical Mechanism, MCM) aims to represent current understanding of the elementary processes involved in atmospheric organic oxidation, based on the results of laboratory studies of reaction kinetics and mechanisms. The MCM is the most comprehensive description of atmospheric organic oxidation available. It can therefore assist in the prediction of the identities and likely ambient concentrations of a large number of organic species. This, in turn, can assist in the development of ambient measurement techniques and the identification of detected, but unknown species. 

3. Where do you see this work developing in the future? 

The mechanism is being extended to include a description of the transfer of less volatile organic oxygenated products from the gas phase to airborne particles, usually referred to as secondary organic aerosol (SOA). Future work will therefore focus on the role of emitted organic compounds in the formation of SOA and will consider how this depends on which organic compounds are emitted, on the relative amounts of natural and human emissions, and on other ambient conditions. 

4. Are there any particular challenges facing future research in this area? 

The are still uncertainties in the understanding of some areas of organic oxidation mechanisms, and in the mechanisms of SOA formation. It is essential that the underlying, fundamental research of elementary processes in this area continues.