Isotopic comparison of ammonium between two summertime field campaigns in 2013 and 2021 at a background site of North China.

Ammonia (NH) 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 NH in urban areas. However, few studies have quantified the contributions of different sources to regional NH. 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 (δN), and sources of ammonium (NH). The results indicate that NH (NH = NH + NH) concentration has increased in 2021, but the f (NH/ NH) has decreased significantly. The δN-NH 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 resulting from the reduction of atmospheric acid gases and alterations in the sources of NH. Bayesian simulation analysis reveals substantial variations in NH sources between 2013 and 2021 observations. Non-agricultural sources have significantly increased their contribution to NH concentration, with vehicle exhaust and NH slip experiencing growth rates of 187 % and 104 %, respectively. Our results confirm the dominate contribution of non-agricultural sources to regional NH at the present stage and propose relevant mitigation strategies, which would provide essential insights for reducing NH emissions in North China.