New measurements from Milan’s city airport reveal how everyday airport operations, not just jet exhausts, quietly release industrial chemicals into the air we breathe, reshaping how aviation pollution is understood.
Study: Tracking the source: First evidence of Benzothiazoles in outdoor airport aerosol. Image credit: Jaromir Chalabala/Shutterstock.com
Particulate matter (PM) with a diameter of 10 micrometers (PM10) in the atmosphere is a significant carrier of benzothiazoles (BTH), contributing to air pollution. These compounds were investigated in relation to airport activities in a recent study published in the journal Environmental Chemistry and Ecotoxicology.
Aircraft tire wear is an overlooled pollution source
Flying is a significant part of modern life, and its role in transportation is only set to grow. However, it is also recognized as a contributor to environmental harm through air pollution. While exhaust-related pollution has been studied in the past, including the emission of carbon dioxide and nitrogen oxides, little is known about non-exhaust pollutants, such as Tire Wear Particles (TWPs).
TWPs are tiny particles constantly shed from the tires of land vehicles and aircraft as they run across the hard pavement. They end up as microplastic pollution in water, soil, and air, with 83 tons being released from Frankfurt Airport alone in 2019, solely due to aircraft tire wear.
TWPs have been studied for their formation, destiny, inorganic components, form and shape, and other chemical properties. Their organic composition remains unclear, even though they contain not only rubber polymer but also additives that can be hazardous to the environment.
For instance, TWPs can release BTH, organic compounds used to produce antifreeze, de-icing solutions, pesticides, and in other major industries, including steel, paper, and textiles. They are irritant, corrosive, and acutely toxic to living organisms.
BTH levels have been used to trace non-exhaust road traffic emissions; however, they have not been used to trace TWPs from flying aircraft. This literature gap motivated the current study.
Airport activity linked to benzothiazoles in PM10 air
This study is the first of its kind. The study was conducted at Milano Linate Airport, based on outdoor PM10 concentrations measured in February and March 2023. The PM10 concentration in these months ranged from 11 to 81 μg m−3, with a median of 31 μg m−3. This is significantly lower than the levels reported in some heavily polluted international airports, including one in China. The PM10 levels at the airport and four other sites in the city of Milano were similar.
In addition to measuring eight types of BTH in the airport aerosol, the investigators also measured major ions and organic acids found in PM10 to understand the trends of each molecule type. They also accounted for wind strength and direction to separately assess how airports contributed to BTH in contrast to nearby roads or parking lots.
The findings indicate that the Linate airport area serves as a significant local source of benzothiazole components in outdoor atmospheric PM10. These include BTH, 2-amino-benzothiazole (BTH-NH2), 2-methyl-benzothiazole (BTH-Me), 2-methylthio-benzothiazole (BTH-MeS), 2-mercapto-benzothiazole (BTH-SH), and 2-benzothiazole-sulfonic acid (BTH-SO3H). In composition, the mix strongly resembled that found in cities with dense traffic.
Compared to February, the levels of BTH-MeS rose by 37 %, BTH-NH2 by 54 %, and BTH-SO3H by 161 %. However, PM10 levels fell. This could be because the airport became busier in March relative to February, although TWPs are too small to increase the mass of PM10 significantly.
Sulfate and ammonium ions made up nearly 74 % of the total ions. Very high nitrate levels were observed, mainly from the conversion of nitrogen oxides emitted from the airplane engines to nitrates, coupled with nitrate-polluted air in the city environment. Such secondary reactions appeared to be common in the airport area.
The measurements of these chemicals varied regularly over the week, peaking on Friday and Saturday and dropping on Tuesday. This suggests a relationship with airport activity rather than the actual flight activity, as arrivals and departures do not follow this trend. In particular, BTH-SO3H and BTH-NH2 levels were closely correlated, indicating their common source, probably the airport.
Another group of BTHs all peaked on the same two days, indicating that not all BTHs arise from the same source. Some, like BTH-MeS, may be a biodegradation product of BTH-SH.
Airborne sodium, chloride, and magnesium levels were relatively abundant, possibly derived from antifreeze or de-icing products, since Linate airport is 130 km from the nearest seashore.
Using hierarchical cluster analysis (HCA), a chemometric method, the scientists found that the measurements fell into three clusters of BTH, each associated with distinct activities. One cluster was linked to airport activity, specifically the number of vehicles or airplanes in operation. Another cluster was related to de-icing or anti-icing procedures at airports. The third cluster was associated with medium-range transport. This emphasizes that exhaust emissions are not the primary source of BTHs. The wind analysis also supported this hypothesis.
However, the risk of occupational exposure to BTHs at this airport was low.
New understandings of airport air pollution
This unique study examined eight chemicals related to tire wear in outdoor PM10 air at an airport. It showed that Milano Linate Airport acts as an important local contributor to BTHs in the air, similar in composition to the aerosol in cities with heavy traffic. Multiple source-related clusters were identified, two of which were related to airport activity. However, the risk of hazardous exposure to BTHs at this airport was low.
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