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Vehicles are the Main Source of Urban Airborne Ammonia Pollution

COVID-19 lockdowns offered a special opportunity to study the impacts of normal societal activities, like driving, on the environment by briefly disrupting them.

At present, scientists report that satellite data obtained before and during the spring 2020 lockdown in Los Angeles indicated that vehicles, instead of agriculture, are considered to be the primary source of urban airborne ammonia (NH3). This develops small particles that add up to air pollution and cause damage to human health.

The study was published in Environmental Science & Technology Letters from the American Chemical Society (ACS).

When released into the air, NH3 gets converted into small particles of inorganic compounds, such as ammonium nitrate and ammonium sulfate. On a global or national scale, the majority of the NH3 pollution comes from agricultural sources, like livestock manure.

However, vehicles also add up to the issue since their catalytic converters or selective catalytic reduction systems—which are developed to decrease emissions of nitrogen oxide (NOx) pollutants such as NO2—cause the unwanted side effect of generating ammonia emissions.

As far as cities are concerned, it has been difficult to confirm whether traffic or agriculture releases more NH3, and the default presumption is that agriculture is the bigger culprit, although there are a few time-consuming measurement studies that recommended otherwise in a few cities.

Daven K. Henze and collaborators wished to see whether satellite data could be utilized to answer this question for the first time from space, since such a method, in principle, could be employed more widely in urban areas across the world.

The scientists concentrated on western Los Angeles, where earlier on-the-ground measurements discovered that vehicle emissions of NH3 were being undervalued. The satellite readings of NH3, as well as NO2, were examined by the researchers. Since the primary source of NO2 in the region is on-road transportation, the compound can act as a proxy for variations in traffic volume and a pointer of vehicular instead of agricultural ammonia emissions.

The researchers correlated concentrations of the two pollutants, and also considered meteorological effects, to evaluate the amount of ammonia emissions that can be traced to vehicles. They discovered that vehicles accounted for 60%–84% of total NH3 emissions at this urban location. This was consistent with estimates offered by modeling but considerably greater compared to the 13%–22% share estimated by government agencies.

The scientists state that their findings propose the health impact of vehicle-related ammonia may rival that of NOx, yet it has been mostly under-recognized and uncontrolled. The researchers acknowledge financial support from NASA.

Journal Reference:

Cao, H., et al. (2021) COVID-19 Lockdowns Afford the First Satellite-Based Confirmation That Vehicles Are an Under-recognized Source of Urban NH3 Pollution in Los Angeles. Environmental Science & Technology Letters.


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