While a major source of air pollution in the vicinity of airports originates from aircraft operations, air quality is also impacted by ground support equipment, surface access road transport and airport on-site energy generation. Aircraft engines produce similar emissions to other sources of fossil fuel combustion with the most significant being nitrogen oxides (NOX), particulate matter (PM), volatile organic compounds (VOCs), sulphur dioxide (SO2), carbon monoxide (CO) and unburnt hydrocarbons (HC).
Nitrogen oxides (NOX)
Aircraft engine NOX emissions (nitric oxide – NO and nitrogen dioxide – NO2) are primarily produced at high pressure and temperature combustor conditions during take-off, and are of a concern in terms of health impacts linked to impaired immune and respiratory functions as well as increased response to allergens.
In the presence of heat and sunlight, surface NOX reacts with VOCs, HC and CO to form ground level ozone or smog that cause health impacts
[11]
[12]
.
In addition, NOX are precursors to other oxidised nitrogen compounds that contributes to the formation of secondary particulate matter18. Thus, NOX have both a direct and an indirect impact on air quality and the environment.
Particulate matter (PM)
Particulate matter is a general term used to describe non-volatile (nvPM) or volatile particles (vPM) in various sizes and composition. At the engine exhaust, particulate emissions mainly consist of nvPM containing soot, or black carbon
[13]
. Health impacts from PM emissions include effects on the cardiovascular and respiratory systems, with ultrafine particles able to enter the bloodstream and act as carriers for toxic substances. EU air quality limits exist for PM
10 and PM
2.5 concentrations, but not for ultrafine particle sizes. However, PM
2.5 mass is considered a good indicator of general risk associated with exposure to particulate matter. Increased surface PM
2.5 concentrations have been shown to lead to a rise in annual premature mortalities due to increases in cases of cardiopulmonary disease and lung cancer
[14]
[15]
.
Sulphur dioxide (SO2)
SO2 emissions result from the combustion of fuels containing sulphur. Most of the sulphur is emitted as gaseous sulphur dioxide (SO2), but about 5% is fully oxidised to form gaseous sulphuric acid (H2SO4), which can subsequently condense on the surfaces of other ambient or soot particles in the atmosphere. While the current limit for sulphur in aviation fuel is 3,000 ppmv (parts per million by volume), actual levels are believed to be averaging ~600 – 800 ppmv although data availability is poor
[13]
.
Ozone (O3)
Ozone is a secondary pollutant, not emitted directly by aircraft engines, but produced when carbon monoxide (CO) or volatile organic compounds (VOCs) are oxidized in the presence of nitrogen oxides (NOX) and sunlight. Low altitude ground level ozone is a major component of smog, which can cause several health issues, respiratory problems, and damage to crops and ecosystems
[16]
.
18Secondary particulate matter is formed from chemical reactions in the atmosphere from the gases ammonia (NH3), sulphur dioxide (SO2) and nitrogen oxides (NOX), and from organic compounds.
Impact of COVID-19 on airport air quality
The measures introduced by most European countries in response to COVID-19 led to significant reductions in emissions of air pollutants, particularly in the transportation sector. Zurich airport analysed air and road traffic, emissions and measured concentrations of various pollutants for a pre-COVID period in 2019 compared to a period in 2020
[21]
. The study found that the 91% drop in air traffic led to an 87% reduction in NO
X emissions at the airport, but only a 44% reduction in NO
2. Reductions in ultrafine nvPM number were of a similar magnitude to NO
X. These results are also driven by influences from other-than- aircraft sources, such as regional road traffic, that also contribute to the emissions concentrations at the airport. It was also noted that the effects of emissions from airport sources significantly decrease over short distances from the airport, and that a decrease in concentrations at the airport is not carried over into the region level at the same rate. This confirmed previous modelling that shows a rapidly decreasing influence of the airport outside its perimeter.
These findings are consistent with those from additional studies performed on this topic
[22]
[23]
. While the effects of lockdown will be temporary, these findings provide insight into the potential for mitigating public health risk by reducing ’business as usual’ air pollutant emissions from economic activities.
