Airborne Measurements of Ozone Precursor Fluxes - Proof of Concept

The vast majority of air pollutants are emitted directly into the atmosphere from activities occurring at the Earth's surface. These activities may be anthropogenic in origin or may be natural (biogenic) processes. Of particular relevance to air quality are the emissions of oxides of nitrogen (NOx) and volatile organic compounds (VOCs). NOx or the sum of nitric oxide, NO and nitrogen dioxide, NO2, is emitted by vehicles, power stations and many other industrial activities during the combustion of fossil fuels. Volatile organic compounds (VOCs) may be emitted into the atmosphere as unburnt or partially burnt fossil fuels, by the evaporation of solvents and other industrial chemicals and may also be emitted by plants as a biogenic process. In combination NOx and VOC combine through photochemical reactions in the atmosphere leading to the formation of two other extremely important pollutants - ozone and particulate matter. Species such as NOx, certain VOCs, Ozone and particulate matter are all regulated by EU Air Quality directives.

Whilst the chemical reactions and atmospheric processing of NOx and VOcs is reasonably well understood, and can be modelled with some skill, large uncertainties arise in models from uncertainty associated with the initial rate of emissions. Defra is responsible for a highly detailed emissions inventory for the UK (the National Atmospheric Emissions Inventory NAEI), which is the starting point for most atmospheric simulations of air quality. The NAEI is constructed to give spatially resolved emissions, calculated from national activity datasets and emissions factors which are then spatially disaggregated to 1x1km grids. The framework under which the NAEI operates is itself tightly constrained by regulated procedures for reporting.

In recent years it has become clear that measured trends in certain pollutants, for example NO2, have not followed trends predicted by inventories. In parallel, other studies have shown that species such as biogenic isoprene are also not currently well reproduced by the NAEI. Continued exceedences of certain air pollution targets is of significant concern to the responsible Government department Defra, who have identified reducing this uncertainty associated with emissions as a key evidence need.

Emissions inventories are essentially paper-based calculations of likely emissions, and it is not straightforward to challenge these with real-world emissions measurements, on large spatial scales. In this project we will apply and demonstrate a novel "top down" experimental method for measuring pollution fluxes at the city-wide or landscape scale. We will fit fast-response sensors for NOx and VOCs on NERC's light research aircraft, a Dornier 228, and fly the instruments over Greater London and over rural East Anglia as low and as slow as possible. This aircraft is already fitted with an instrument to measure atmospheric turbulence, and by combining the data from the turbulence probe with that from the chemical sensors (using data analysis techniques known as eddy covariance and virtual disjunct eddy covariance) we aim to demonstrate that we can make top down flux measurements for NOx and VOCs at these scales. We will then compare the experimentally derived data with that produced by the National Atmospheric Emissions Inventory, as a starting point for understanding where and why difference occur.

The technique, if successful, would be of considerable strategic importance to Defra, who are co-funding this project. There are however very important applications outside of the urban domain. Understanding emission fluxes of gases in remote and pristine environments is a key element of understanding the earth system. The techniques to be developed here have potential transfers in to fields such as forest and marine biogeochemical gas exchange.

This project has been designed in close collaboration with an end user of the research, Defra, and as such the pathways to achieve impact in the policy domain are clear and already established. Defra are the UK Government Department with responsibility for air quality in the country. Defra are providing substantial cash sponsorship of this project (£120,000) and are therefore fully invested its objectives and in achieving a policy impact outcome. There is an identified evidence requirement from Defra to improve knowledge of atmospheric emissions of certain key air pollutants, and to compare the NAEI estimates of emissions with real-world emissions. The NAEI is uses established international protocols, and as such is a reflection of best available practice both in the UK and beyond.

We have designed a research-led experimental programme using state of the art instrumentation, which we believe will provide a proof of principle for an independent measure of NOx and VOC emissions on scales that may be directly compared to the NAEI. This approach uses new numerical techniques in combination with the most advanced chemical sensors available for this purpose. Our objective is to compare paper estimates with observations over distance scales typical of a city and a rural landscape. We will work in close collaboration with Defra in the initial design of the experiment, and Defra will provide us with advice on the optimal testing regime for this study. We will work closely also with AEA Technology, who are the Defra subcontractor responsible for the maintenance of the NAEI, in this process.

On completion of the proof of concept flights we will work with the Atmosphere and Local Environment (ALE) team at Defra and with the NAEI to produce initial comparisons of observations vs model estimates. We will also work with Defra to determine the likely precision of the technique and provide guidance for how it may be developed for operational emissions validation in the future. We will produce a technical report and regular briefings for Defra, and materials suitable for other agencies such as the EA.

It is our intention and that of Defra to submit this research project as a contributing activity in the Living with Environmental Change programme, the major £1bn cross Government initiative. This will provide a ready route for dissemination of findings to other organisations with responsibilities in air quality, including the devolved administrations, and local Government. Local government is a potentially key user of this research since air quality responsibilities are passed typically to the local city-level. The Department for Communities and Local Government (DCLG) and the Local Government Association are partners in LWEC (along with NERC and Defra) and we will use LWEC as a key means to reach these stakeholders.

Whilst we will test the proof of concept of airborne flux measurements for these reactive pollutants in an air quality context, there are applications outside of this domain for the technique and the technologies. The estimation of biogeochemical of fluxes is a key research requirement in Earth system science, and the techniques to be developed here are directly applicable. In additional to the usual peer-reviewed publications, we will produce materials to disseminate our findings through IGBP publications and meetings. Of particular relevance are the programmes iLEAPs, IGAC and SOLAS, all of which have research foci associated with determining terrestrial and oceanic trace gas emissions.