Characterizing large nitrous oxide N2O emissions from urban areas

Nitrous oxide (N₂O) is a strong greenhouse gas (GHG) that is also one of the most important stratospheric ozone depleting substances (ODS). With concentrations rapidly rising in the atmosphere, understanding the sources of N₂O is important to implementing policies that reduce N₂O emissions. In existing inventories, most N₂O is related to agricultural emissions related to fertilizer application. However, urban processes can also be large sources of N₂O, as a side effect of attempting to reduce emissions of air pollutants such as nitrogen oxides (NOx). Both the nitrification and denitrification processes of wastewater treatment can produce large concentrations of N₂O, and traffic emissions are also expected. The New York City Metro Area (pop. 20M) is the most populous urban area in the United States of America (USA) and the largest urban source of CO₂ and methane (CH₄) in the US. The city maintains 14 wastewater treatment plants for the 8 million residents of the five city boroughs of NYC. Our urban core observatory in Manhattan regularly measures N₂O enhancements of +100 ppbv. Our mobile lab sampling around the city allowed us to study the source characteristics of this highly variable N₂O concentrations in the urban atmosphere. We identified large fluxes of N₂O from some, but not all, wastewater treatment plants. We also characterized N₂O emissions from “clean air” buses with urea-based catalysts, which are much more common in Europe than in the USA.