Nitrogen isotopes suggest agricultural and non-agricultural sources contribute equally to NH3 and NH4+ in urban Beijing during December 2018

Agricultural and non-agricultural sources emission contribute to atmospheric ammonia (NH3) and particulate ammonium (NH4+). However, our understanding on the sources of NH3 and NH4+ in PM2.5 (particles smaller than 2.5 mu m) during the winter period in the urban atmosphere is limited. Here, we measured the concentrations and stable nitrogen isotopic composition (delta N-15) of NH3 and NH4+ in parallel during December 2018 in urban Beijing to assess the non-agricultural and agricultural sources contributions to NH3 and NH4+ in ambient air based on the Chemical Transport Model (CTM), a Bayesian isotope mixing model (SIMMR), and the delta N-15 signatures that we developed. Our study found weekly NH4+ and NH3 concentrations were on average 2.5 +/- 1.4 mu g m(-3) and 3.8 +/- 1.7 mu g m(-3), respectively during December 2018. Weekly concentration weighted delta N-15(NH4+) values ranged from 4.5%o to 13.7%o with an average value of 8.2 +/- 3.9%o during December 2018. After accounting for nitrogen isotopic fractionation from NH3 gas to NH4+ conversion, initial delta N-15(NH3) values ranged from 22.5%o to 12.8%o with an average value of 17.4 +/- 3.5%o. Further, weekly measured delta N-15(NH3) values ranged from 22.2%o to 10.2%o (after correction) with an average value of 15.6 +/- 5.3%o 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 NH4+ and 32.3%-71.2%, with an average of 53.4 +/- 16.1%, to ambient NH3 during December 2018 in Beijing. Results from CMAQ-ISAM suggest non-agricultural sources contributed on average 66.2 +/- 1.9% to ambient NH4+ and 66.4 +/- 1.9% to ambient NH3 during December 2018. Results from this study suggest that agricultural and non-agricultural sources nearly equally contributed to NH3 and NH4+ in urban Beijing during December 2018 with an uncertainty range of 13%-19% between isotope-based methods and CTM method.

Automated summary

By measuring the concentrations and stable nitrogen isotopic composition of NH3 and NH4+ in urban Beijing during December 2018, this study found that agricultural and non-agricultural sources nearly equally contributed to NH3 and NH4+. However, there is uncertainty in these results.