Nitrogen isotope fractionation during gas-to-particle conversion of NO<sub><i>x</i></sub> to NO<sub>3</sub><sup>−</sup> in the atmosphere – implications for isotope-based NO<sub><i>x</i></sub> source apportionment

Abstract

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.

Differentiation Between Nitrate Aerosol Formation Pathways in a Southeast Chinese City by Dual Isotope and Modeling Studies

“…The MixSIAR allows the user to create a mixing model, based on their data structure and research questions, via options for fixed/random effects, source data types, priors, and error terms (Stock & Semmens, 2017 In the first model, we considered two main pathways (HNO 3 (1) and HNO 3 (2)) in the formation of NO 3 − . The contribution ratio (γ) of HNO 3 (1) can be calculated via the MixSIAR model, as follows (Chang et al, 2018;Zong et al, 2017):…”

Characterization of organic aerosols from a Chinese megacity during winter: predominance of fossil fuel combustion

“…Our results suggest that NO 2 influences the organic molecular compositions in urban aerosols. It is noteworthy that the major source of NO 2 is vehicular exhaust (Kendrick et al, 2015); however, coal combustion can also emit NO x into the atmosphere (Chang et al, 2018). Previous studies have also reported that NO x could affect SOA formation (Kanakidou et al, 2005;Mochizuki et al, 2015).…”

“…These results indicate that fossil fuel combustion is an important contributor to Nanjing aerosol during winter. Wang and Kawamura (2005) reported that fossil fuel combustion (52 %) was the largest contributor to total organics followed by biomass burning (14 %) and SOA (10 %) from Nanjing, whereas coal combustion was found to contribute 39.5 % to PM 2.1 in Nanjing in 2010 (Chen et al, 2015). Gao et al (2013) pointed out that coal combustion was the dominant (58 %) contributor to PM 2.5 followed by biomass burning (31 %) and vehicular emissions (11 %) in the Pearl River Delta region (two urban, two suburban and two rural sites) for samples collected in 2009.…”

Enhanced Primary Production in the Oligotrophic South China Sea Related to Southeast Asian Forest Fires

“…They found that HNO 3 formed in the experimental chamber when burning pine has a high δ 15 N value of +11.2‰. Chang et al (2018) observed high δ 15 N-NO 3 − values of +12.2‰…”