Nitrogen isotopes suggest agricultural and non-agricultural sources contribute equally to NH and NH in urban Beijing during December 2018.

Agricultural and non-agricultural sources emission contribute to atmospheric ammonia (NH) and particulate ammonium (NH). However, our understanding on the sources of NH and NH in PM (particles smaller than 2.5 μm) during the winter period in the urban atmosphere is limited. Here, we measured the concentrations and stable nitrogen isotopic composition (δN) of NH and NH in parallel during December 2018 in urban Beijing to assess the non-agricultural and agricultural sources contributions to NH and NH in ambient air based on the Chemical Transport Model (CTM), a Bayesian isotope mixing model (SIMMR), and the δN signatures that we developed. Our study found weekly NH and NH concentrations were on average 2.5 ± 1.4 μg m and 3.8 ± 1.7 μg m, respectively during December 2018. Weekly concentration weighted δN(NH) values ranged from 4.5‰ to 13.7‰ with an average value of 8.2 ± 3.9‰ during December 2018. After accounting for nitrogen isotopic fractionation from NH gas to NH conversion, initial δN(NH) values ranged from -22.5‰ to -12.8‰ with an average value of -17.4 ± 3.5‰. Further, weekly measured δN(NH) values ranged from -22.2‰ to -10.2‰ (after correction) with an average value of -15.6 ± 5.3‰ during December 2018. Results from two different isotope-based method showed non-agricultural sources contributed 31.2%-63.1%, with an average value of 47.5 ± 14.6%, to NH and 32.3%-71.2%, with an average of 53.4 ± 16.1%, to ambient NH during December 2018 in Beijing. Results from CMAQ-ISAM suggest non-agricultural sources contributed on average 66.2 ± 1.9% to ambient NH and 66.4 ± 1.9% to ambient NH during December 2018. Results from this study suggest that agricultural and non-agricultural sources nearly equally contributed to NH and NH in urban Beijing during December 2018 with an uncertainty range of 13%-19% between isotope-based methods and CTM method.