Articles | Volume 18, issue 16
Research article
16 Aug 2018
Research article |  | 16 Aug 2018

Nitrogen isotope fractionation during gas-to-particle conversion of NOx to NO3 in the atmosphere – implications for isotope-based NOx source apportionment

Yunhua Chang, Yanlin Zhang, Chongguo Tian, Shichun Zhang, Xiaoyan Ma, Fang Cao, Xiaoyan Liu, Wenqi Zhang, Thomas Kuhn, and Moritz F. Lehmann

Abstract. Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO3), the end product of the oxidation of NOx gases (NO + NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO3 to constrain NOx source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the δ15N values of fresh pNO3 (δ15N–pNO3) in PM2.5 at a rural site in northern China, where atmospheric pNO3 can be attributed exclusively to biomass burning. The observed δ15N–pNO3 (12.17±1.55 ‰; n = 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04±4.13 ‰). The large difference between δ15N–pNO3 and δ15N–NOx (Δ(δ15N)) can be reconciled by the net N isotope effect (εN) associated with the gas–particle conversion from NOx to NO3. For the biomass burning site, a mean εN( ≈ Δ(δ15N)) of 10.99±0.74 ‰ was assessed through a newly developed computational quantum chemistry (CQC) module. εN depends on the relative importance of the two dominant N isotope exchange reactions involved (NO2 reaction with OH versus hydrolysis of dinitrogen pentoxide (N2O5) with H2O) and varies between regions and on a diurnal basis. A second, slightly higher CQC-based mean value for εN (15.33±4.90 ‰) was estimated for an urban site with intense traffic in eastern China and integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates for NOx at this site. Based on the δ15N values (10.93±3.32 ‰; n = 43) of ambient pNO3 determined for the urban site, and considering the location-specific estimate for εN, our results reveal that the relative contribution of coal combustion and road traffic to urban NOx is 32 % ± 11 % and 68 %± 11 %, respectively. This finding agrees well with a regional bottom-up emission inventory of NOx. Moreover, the variation pattern of OH contribution to ambient pNO3 formation calculated by the CQC module is consistent with that simulated by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), further confirming the robustness of our estimates. Our investigations also show that, without the consideration of the N isotope effect during pNO3 formation, the observed δ15N–pNO3 at the study site would erroneously imply that NOx is derived almost entirely from coal combustion. Similarly, reanalysis of reported δ15N–NO3 data throughout China and its neighboring areas suggests that NOx emissions from coal combustion may be substantively overestimated (by  > 30 %) when the N isotope fractionation during atmospheric pNO3 formation is neglected.

Short summary
We demonstrate that it is imperative that future studies, making use of isotope mixing models to gain conclusive constraints on the source partitioning of atmospheric NOx, consider this N isotope fractionation. Future assessments of NOx emissions in China (and elsewhere) should involve simultaneous δ15N and δ18O measurements of atmospheric nitrate and NOx at high spatiotemporal resolution, allowing former N-isotope-based NOx source partitioning estimates to be reevaluated more quantitatively.
Final-revised paper