Some of the earliest concerns about aviation environmental impacts emerged in relation to the effect of aircraft on air quality (ICAO 2007; Lee and Raper 2003, 77; Lee 2004, 2; see Chapter 1). Air quality is degraded by air pollution, the condition in which substances emitted by anthropogenic activities occur at elevated concentrations and produce a measurable effect on humans, animals, vegetation or materials (Seinfeld and Pandis 2006, 21). As explained in Chapter 2, aviation gives rise to a range of emitted species, most of which cause air pollution (Price and Probert 1995). In the past, the effect of aircraft and airports on air quality has been regarded a localised issue that is significant only in the immediate vicinity of airports and beneath their arrival and departure flight paths. However, recently, scientists have acknowledged that a continuum of air quality exists across the surface of the Earth; whilst human activities – including air transport – create ‘hotspots’ in which air pollution is concentrated, some pollutants may also have effects at considerable distances from the source of the emissions (Seinfeld and Pandis 2006; UK Air Quality Archive 2008). This is the case with air transport, which produces highly localised air pollution in the vicinity of airports as well as more widespread effects elsewhere (Hume and Watson 2003). Whilst many aviation emissions degrade air quality, the substances of greatest concern for air quality management are nitrogen oxides (NOx) and particles. Emission levels of NOx and particles may be critical during the landing and take-off (LTO) cycle, because those substances may cause air quality standards to be exceeded and may in turn constrain airport growth. Surface activities at airports – including the transport of employees, passengers, baggage and freight to, from and within airport sites – may also create substantial air pollution; at some airports, the localised air pollution due to aviation-related surface transport may be more significant than that generated by aircraft. Overall, aviation-related pollution is expected to increase alongside the projected rapid growth of air transport because the growth of demand for air travel is far outpacing

the rate of technological and operational improvements in airframe and engine performance (see Chapter 2). Yet air quality standards reflect public concerns about the effects of pollution and those standards tend to become more stringent with time. Therefore, the impact of air transport on air quality is likely to remain an important issue on the decadal timescale, at least at the local level.