Isotopic imprints of aerosol ammonium over the north China plain.

Atmospheric PM 2.5 poses a variety of health and environmental risks to urban environments. Ammonium is one of the main components of PM 2.5 , and its role in PM 2.5 pollution will likely increase in the coming years as NH 3 emissions are still unregulated and rising in many cities worldwide. However, partitioning urban NH 4 ⁺ sources remains challenging. Although the ¹⁵N natural abundance (δ¹⁵N) 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 4 ⁺ sources in urban environments. Here, we collected PM 2.5 samples using an active sampling technique during winter at six cities in the North China Plain to characterize the concentrations, δ¹⁵N and sources of NH 4 ⁺ in PM 2.5. We found substantial variations in both the concentrations and δ¹⁵N of NH 4 ⁺ among the sites. The mean NH 4 ⁺ concentrations across the six cities ranged from 3.6 to 12.1 μg m⁻³ on polluted days and from 0.9 to 10.6 μg m⁻³ on non-polluted days. The δ¹⁵N ranged from 6.5‰ to 13.9‰ on polluted days and from 8.7‰ to 13.5‰ on non-polluted days. The δ¹⁵N decreased with increasing NH 4 ⁺ 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 4 ⁺ 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 ¹⁵N in aerosol NH 4 ⁺ is a promising approach for apportioning atmospheric NH 3 sources over a large region, and this approach has potential for mapping rapidly and precisely the sources of NH 3 emissions. [Display omitted] • Non-agricultural and agricultural sources contributed 82% and 18% to NH 4 ⁺ in PM 2.5. • Aerosol NH 4 ⁺ on polluted winter days in six cities was mainly of local origin. • Combustion-derived NH 4 ⁺ positively related to urban area size and vehicle numbers. • Agricultural contribution to NH 4 ⁺ increased with increasing agricultural coverage. [ABSTRACT FROM AUTHOR]