How Much Do Vehicles Contribute to Urban NH3 Pollution?

Author: ChemistryViews
Published On: December 11, 2021
Copyright: Wiley-VCH GmbH

Ammonia that is emitted into the atmosphere can be converted to inorganic compounds, e.g., ammonium sulfate and ammonium nitrate. These compounds are solid and contribute to fine particulate matter pollution. A large part of NH₃ pollution generally comes from agricultural sources, such as livestock.

However, vehicles with internal combustion engines also contribute to the problem. Catalytic converters or selective catalytic reduction (SCR) systems used in the vehicles reduce emissions of NO_x pollutants such as NO₂, but they have the undesirable side-effect of producing ammonia. It is challenging to tell how much agriculture and traffic, respectively, contribute to NH₃ emissions in urban environments.

Daven K. Henze, University of Colorado, Boulder, USA, and colleagues have, for the first time, used satellite data to answer this question. The researchers focused on western Los Angeles, CA, USA. The team analyzed satellite readings of NH₃ and NO₂ levels before and during the COVID-19 lockdown in the spring of 2020. Changes in NO₂ concentrations were mainly linked to changes in traffic during the lockdown. Thus, the levels of NO₂ can serve as a proxy for traffic volume and as an indicator of vehicular as opposed to agricultural ammonia emissions. The team correlated concentrations of the two pollutants and took meteorological effects into account to calculate the part of ammonia emissions that can be traced to vehicles.

They found that vehicles accounted for 60–84 % of total NH₃ emissions at the studied location, which is substantially higher than the 13–22 % share previously estimated by U.S. government agencies. According to the researchers, their findings suggest the health impacts of vehicle-related ammonia may have been severely underestimated.