Underestimated ammonia vehicular emissions in metropolitan city revealed by on-road mobile measurement

Atmospheric ammonia (NH3) plays a significant role in the nitrogen cycle, and can have impacts on air quality, ecological balance and climate change. While NH3 associated with natural and agricultural processes has long been considered the primary source, the contribution of combustion-related NH3, particularly from vehicular emissions, keeps on the rise. We found that high on-road NH3 concentrations occurred in a metropolitan city based on mobile measurement, and inferred that urban vehicular NH3 emission was likely underestimated in the past. NH3 emission factors (EFs) were derived from ring roads and tunnels, showing levels 74% and 20% higher than the latest standard proposed by Euro VII, respectively. To quantify the underestimation, two methods based on car ownership and traffic flow were used to estimate the annual vehicular NH3 emission in Shanghai as 2.59 and 1.76 Gg, respectively, substantially surpassing the predicted results by the Dynamic Projection model for Emissions in China. Given these discoveries, we recommend that it is urgent and imperative to establish relevant national standards and limits aiming at regulation on vehicular NH3 emissions. And more representative EFs measurements should be adopted to improve the accuracy of inventory estimation.