Multiyear Measurements on 15N Natural Abundance of Precipitation Nitrate at a Rural Forested Site

Large anthropogenic NOx emissions increase atmospheric nitrate (NO3-) deposition. The China's Clean Air Action has been implemented to reduce anthropogenic NOx emissions since 2013. To examine if atmospheric NO3- deposition would decrease and its sources had changed as a result of this regulation, we collected the precipitation samples (n = 181) on a daily basis at a rural forested site in Northeast China from April 2014 to December 2017 and measured the concentrations and delta N-15 values of NO3-. We found annual NO3- deposition ranged from 6.5 to 7.3 kg N ha(-1) yr(-1) and gradually declined since 2015. The delta N-15-NO3- spanned a large range from -17.4 to 13.1 parts per thousand with a mass-weighted mean of 3.5 parts per thousand during the study period. The delta N-15-NO3- was higher by 10 parts per thousand in winter (3.3 +/- 4.2 parts per thousand) than in summer (7.4 +/- 3.9 parts per thousand), which was driven by the changes of both coal combustion and soil emission. Over the study period, NOx from coal combustion, vehicle exhaust, biomass burning and soil emission contributed 28.2 +/- 11.9%, 28.8 +/- 16.3%, 26.9 +/- 14.7%, and 16.1 +/- 7.2% to precipitation NO3-, respectively. We found that relative contribution of fossil fuel combustion did not decline with the China's Clean Air Action and considerable NOx reduction at the regional scale.