By disrupting normal societal activities such as driving, COVID-19 lockdowns afforded a unique opportunity to study their impacts on the environment. Researchers now report in ACS’ Environmental Science & Technology Letters that satellite data from before and during the spring 2020 lockdown in Los Angeles shows that vehicles, rather than agriculture, are the main source of urban airborne ammonia (NH 3 ), which forms small particles that contribute to air pollution and harm human health. When emitted into the atmosphere, NH 3 is converted into tiny particles of inorganic compounds, including ammonium sulfate and ammonium nitrate. On a national or global scale, most NH 3 pollution comes from agricultural sources, such as livestock manure. But vehicles also contribute to the problem because their catalytic converters or selective catalytic reduction systems—which are designed to reduce emissions of nitrogen oxide (NO x ) pollutants including NO 2 —have the undesirable side-effect […]