Process-Based Isolation of Pyrogenic Ammonia in Urban Atmosphere and Implications for Ammonium Nitrate Control

Here, an explicit framework based on the atmospheric process analysis of urban–rural NH3 was proposed to estimate the relative contributions of biogenic and pyrogenic ammonia (NH3) to the atmosphere in Shanghai before and during the COVID-19 lockdown. Under similar meteorological conditions, a much higher ambient NH3 reduction rate was observed during the pandemic at the urban site than at the rural site. Corroborated by atmospheric transport modeling, this difference was mainly attributed to the changes in emissions and gas-to-particle conversion rather than the changes in transport and wet deposition (the significance of dry deposition cannot be completely disregarded). The convergent evidence in this work suggests that biogenic sources dominated urban NH3 emissions, while pyrogenic NH3 emissions (mainly traffic) also represented an important player (∼30%) before the city shutdown. We found that in urban areas, the incorporation of NH3 into fine particulate ammonium nitrate might be significantly enhanced by pyrogenic NH3 emissions, which are practically controllable as exemplified by China’s ongoing clean air actions in vehicle and power plant sectors.