Isotopic imprints of aerosol ammonium over the north China plain.

Atmospheric PM poses a variety of health and environmental risks to urban environments. Ammonium is one of the main components of PM, and its role in PM pollution will likely increase in the coming years as NH emissions are still unregulated and rising in many cities worldwide. However, partitioning urban NH sources remains challenging. Although the N natural abundance (δN) analysis is a promising approach for this purpose, it has seldom been applied across multiple cities within a given region. This limits our understanding of the regional patterns and controls of NH sources in urban environments. Here, we collected PM samples using an active sampling technique during winter at six cities in the North China Plain to characterize the concentrations, δN and sources of NH in PM. We found substantial variations in both the concentrations and δN of NH among the sites. The mean NH concentrations across the six cities ranged from 3.6 to 12.1 μg m on polluted days and from 0.9 to 10.6 μg m on non-polluted days. The δN ranged from 6.5‰ to 13.9‰ on polluted days and from 8.7‰ to 13.5‰ on non-polluted days. The δN decreased with increasing NH concentrations at all six sites. We found that non-agricultural sources (vehicle exhaust, ammonia slip and urban wastes) contributed 72%-94% and 56%-86% of the NH on polluted and non-polluted days, respectively, and that during polluted days, combustion-related emissions (vehicle exhaust and ammonia slip) were positively associated with the proportion of urban area, population density and number of vehicles, highlighting the importance of local sources of particulate pollution. This study suggests that the analysis of N in aerosol NH is a promising approach for apportioning atmospheric NH sources over a large region, and this approach has potential for mapping rapidly and precisely the sources of NH emissions.