Atmospheric chemistry – air quality and climate

Air quality has improved dramatically over the past 50 years. Professor Mike Piling, Head of Physical Chemistry at the University of Leeds, will reveal the latest about air quality research at the Festival of Science today.

The Clean Air Act was passed in 1956 in response to the devastating London smog of 1952 and led to substantial improvements. Since then road traffic has become a major contributor to air pollution and much of current legislation is designed to minimise the effects of traffic and other emissions on human health. How are we doing? Are we meeting current air quality objectives and will we meet those that have been set for the future? Are there any new threats to our achievement of those objectives?

The Government’s air quality objectives

Several pollutants, such as nitrogen dioxide, particulate matter and ozone have deleterious effects on human health, especially for those with respiratory problems. Short term exposure to particles, for example, brings forward several thousand deaths and hospital admissions every year. Long term exposure is likely to be even more damaging. In order protect human health, the Government’s Air Quality Strategy sets limits, so-called objectives, on future atmospheric concentrations of a range of pollutants. These objectives have to be achieved in the near future (typically in 2005 and 2010) and are closely linked to similar limits (known as limit values) set by the EU.

Will we meet the objectives?

Recent work conducted by the Air Quality Expert Group, set up to advise Defra on air pollution, shows that there are several areas in the UK, particularly in London, where we shall fail to meet the objectives. Substantial improvements in emissions from road vehicles have led to impressive decreases in emissions, but it is clear that they are not sufficient to reduce atmospheric concentrations of particulate matter and of nitrogen dioxide sufficiently, and that people will continue to be exposed to levels of pollution that can cause ill health. These effects can be worsened by winter weather episodes that trap the air close to the ground, leading to even higher concentrations of pollutants. Such events occurred in London in 1991 and in Manchester and Glasgow in 2001. The frequency of such episodes in the future is not clear.

Photochemical smog – August 2003.

Not all pollutants are emitted directly (primary pollutants). Some are formed by chemical reactions in the atmosphere (secondary pollutants). Nitrogen Dioxide and particulate matter are both primary and secondary, while ozone is purely a secondary pollutant. It is formed photochemically (by the action of sunlight) on nitrogen oxides and volatile organic compounds (VOCs – e.g. butane and benzene). The process of ozone formation takes hours to days, so that ozone control is a trans-boundary issue and we have to work closely with other European countries to control ozone. The highest concentrations occur in summer under anticyclonic conditions, when we experience still sunny conditions, and when air is transported to the UK from northern Europe. August 2003 saw a particularly severe ozone (or photochemical smog) episode, when concentrations in SE England rose to very high values during the heatwave. About 2000 deaths in England have been attributed to the heatwave and it is estimated that 25-40% of these resulted from poor air quality – elevated concentrations of both ozone and particulate matter.

Global contributions to UK air pollution

A further concern is that background concentration of ozone – that found in clean environments, such as the west of Ireland under westerly airflows – is steadily increasing. This observation demonstrates that some aspects of air quality are beginning to be limited by processes occurring on a global scale. Pollution is transported between continents and the export of pollution and precursors of pollution from the US affects Europe while export from E Asia affects the US. The increasing ozone background will significantly limit the local / regional pollutant concentrations that we can add to that background if we are to meet our air quality objectives.

The need for a holistic approach to air quality legislation

Current legislation aims to control pollutants individually – we have separate objectives for particulate matter, ozone and nitrogen dioxide, for example. Yet it is clear that there are close interactions between these pollutants and their precursors.

The main precursor of nitrogen dioxide is nitric oxide, which is emitted directly from combustion processes, including petrol and diesel vehicles. Nitric oxide reacts with ozone to form nitrogen dioxide and so measures taken to reduce nitrogen oxides in cities can lead to increases in ozone.

Some particulate traps fitted on diesel vehicles, while reducing emissions of particles, have led to increases in primary nitrogen dioxide and hence to increases in the concentrations of this pollutant close to roads.

Some of the same chemical reactions lead to production of both ozone and secondary particulate matter.

It would clearly be better to take a holistic approach and to develop an air quality strategy that accommodates these interactions. Such an approach is, of course, much more difficult and requires an improved fundamental understanding of the processes – chemical and physical – occurring in the atmosphere.

Lab experiments and field campaigns – the development of a fundamental approach to predicting air quality

This understanding is distilled into models that can make predictions of pollutant concentrations and their dependence on emissions and on meteorology. They include a quantitative description of the chemical reactions occurring in the atmosphere that give rise to secondary pollutants. It is essential that these models are based on a sound understanding of these reactions, that can only come from laboratory measurements. An appreciation of the way in which these reactions interact to affect pollutant concentrations requires detailed measurements in the atmosphere, usually in the form of short intensive field campaigns. It is also possible to carry out experiments in simulated atmospheres, using large outdoor chambers, in which mixtures of gases are exposed to sunlight and the products of the reactions analysed. Examples will be given of all of these types of experiment and of the way they are brought together into models of the atmosphere. In particular the following recent field campaigns will be discussed:

1. A large field campaign took place in Writtle in Essex in August 2003. It included measurements of a wide range of gas phase chemical species, particles and meteorological parameters. It coincided with the extreme photochemical episode that took place in early August, which affected much of SE England. Very high ozone concentrations were observed and it was found that concentrations of isoprene were also high during the episode. Isoprene is a hydrocarbon that is emitted from vegetation – it is biogenic in origin. Its rate of emission depends on light intensity and on temperature; the temperature during this period was unusually high (>35 0C), thus accounting for these high concentrations. Model results show that the isoprene, combined with anthropogenic nitrogen oxide emissions and sunlight, contributed substantially to the rate of ozone formation. Biogenic emissions of hydrocarbons are of great importance in the formation of ground level ozone in hot, heavily forested areas, which are subject to man – made emissions of nitrogen oxides, such as SE USA. They have not previously been considered to be of major importance in the UK and this observed contribution derives from the unusually high temperatures. Extreme events of this sort are expected to become more important as climate change develops – a factor of ten increase in incidence is predicted over the next 75 years.

2. An airborne campaign was conducted in July 2004 using the new NERC/Met Office aircraft. The aircraft is a BAe 146 that has been heavily instrumented over the last couple of years. This was its first campaign. It flew from the Azores and linked with aircraft from France, Germany and the USA to examine the transport of pollutants from N. America to Europe. Several experiments were conducted in which different aircraft intercepted and examined the same air mass on different days as it was transported across the Atlantic. Only preliminary results are available, but they show clearly, for example, plumes of pollutants resulting from biomass burning in Alaska. The campaign should aid considerably our understanding of the incidence of inter-continental transport of pollution and its impact, for example, on background ozone.

Air quality and Climate Change

Current time horizons for predictions of future air quality are of the order of 10 – 20 years. These predictions rely on an analysis of likely changes in technology and of, for example, road usage. Even during that time horizon, but increasingly beyond it, we need to take account of the global changes that are occurring, partly as a result of climate change, but also because of the increasing importance of inter-continental transport on, for example, background ozone. These effects add another dimension to the interactions that need to be considered in developing air quality strategies, for we should also include the effects of the global environment on local and regional air quality and of UK emissions on climate forcing and on the ability of the atmosphere to process emissions – its so-called oxidising capacity. The atmosphere is complex and non-linear and it is evolving, so that we cannot rely on an empiricism. We need a fundamental approach, using lab and measurements and models, to predict the future composition of our atmosphere on local, regional and global scales.