Nitrogen isotope fractionation during gas-to-particle conversion of NO_<i>x</i> to NO₃⁻ in the atmosphere – implications for isotope-based NO_<i>x</i> source apportionment

• Main Authors: Y. Chang, Y. Zhang, C. Tian, S. Zhang, X. Ma, F. Cao, X. Liu, W. Zhang, T. Kuhn, M. F. Lehmann
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-08-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/11647/2018/acp-18-11647-2018.pdf

<p>Atmospheric fine-particle (PM_2.5) pollution is frequently associated with the formation of particulate nitrate (<i>p</i>NO₃⁻), the end product of the oxidation of NO_<i>x</i> gases (NO&thinsp;+&thinsp;NO₂) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of <i>p</i>NO₃⁻ to constrain NO_<i>x</i> source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the <i>δ</i>¹⁵N values of fresh <i>p</i>NO₃⁻ (<i>δ</i>¹⁵N–<i>p</i>NO₃⁻) in PM_2.5 at a rural site in northern China, where atmospheric <i>p</i>NO₃⁻ can be attributed exclusively to biomass burning. The observed <i>δ</i>¹⁵N–<i>p</i>NO₃⁻ (12.17±1.55&thinsp;‰; <i>n</i> = 8) was much higher than the N isotopic source signature of NO_<i>x</i> from biomass burning (1.04±4.13&thinsp;‰). The large difference between <i>δ</i>¹⁵N–<i>p</i>NO₃⁻ and <i>δ</i>¹⁵N–NO_<i>x</i> (Δ(<i>δ</i>¹⁵N)) can be reconciled by the net N isotope effect (<i>ε</i>_N) associated with the gas–particle conversion from NO_<i>x</i> to NO₃⁻. For the biomass burning site, a mean <i>ε</i>_N( ≈ Δ(<i>δ</i>¹⁵N)) of 10.99±0.74&thinsp;‰ was assessed through a newly developed computational quantum chemistry (CQC) module. <i>ε</i>_N 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 <i>ε</i>_N (15.33±4.90&thinsp;‰) 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 NO_<i>x</i> at this site. Based on the <i>δ</i>¹⁵N values (10.93±3.32&thinsp;‰; <i>n</i> = 43) of ambient <i>p</i>NO₃⁻ determined for the urban site, and considering the location-specific estimate for <i>ε</i>_N, our results reveal that the relative contribution of coal combustion and road traffic to urban NO_<i>x</i> is 32&thinsp;%&thinsp;±&thinsp;11&thinsp;% and 68&thinsp;%±&thinsp;11&thinsp;%, respectively. This finding agrees well with a regional bottom-up emission inventory of NO_<i>x</i>. Moreover, the variation pattern of OH contribution to ambient <i>p</i>NO₃⁻ 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 <i>p</i>NO₃⁻ formation, the observed <i>δ</i>¹⁵N–<i>p</i>NO₃⁻ at the study site would erroneously imply that NO_<i>x</i> is derived almost entirely from coal combustion. Similarly, reanalysis of reported <i>δ</i>¹⁵N–NO₃⁻ data throughout China and its neighboring areas suggests that NO_<i>x</i> emissions from coal combustion may be substantively overestimated (by  &gt; 30&thinsp;%) when the N isotope fractionation during atmospheric <i>p</i>NO₃⁻ formation is neglected.</p>