Ammonia (NH₃) is the primary atmospheric alkaline gas, playing a crucial role in the atmospheric chemistry. Recently, non-agricultural emissions have been identified as the dominant sources of NH₃ in urban areas. However, few studies have quantified the contributions of different sources to regional NH₃. This study conducted two summertime field observations in 2013 and 2021 at a background site of North China to comprehensively explore the regional variations in concentration, nitrogen isotope composition (δ¹⁵N), and sources of ammonium (NH₄⁺). The results indicate that NH_x (NH_x = NH₃ + NH₄⁺) concentration has increased in 2021, but the f_NH4+ (NH₄⁺/ NH_x) has decreased significantly. The δ¹⁵N-NH₄⁺ values show a significant increase, ranging from −4.7 ± 8.1‰ to +12.0 ± 2.4‰. The increase can be attributed to two primary factors: changes in f_NH4+ resulting from the reduction of atmospheric acid gases and alterations in the sources of NH₃. Bayesian simulation analysis reveals substantial variations in NH₃ sources between 2013 and 2021 observations. Non-agricultural sources have significantly increased their contribution to NH_x concentration, with vehicle exhaust and NH₃ slip experiencing growth rates of 187% and 104%, respectively. Our results confirm the dominate contribution of non-agricultural sources to regional NH₃ at the present stage and propose relevant mitigation strategies, which would provide essential insights for reducing NH₃ emissions in North China.

Ammonia; Non-agricultural sources; δ15N; Ammonium; North China