Nitrogen isotope fractionation during gas-to-particle conversion of NO x to NO3' in the atmosphere - Implications for isotope-based NO x source apportionment

Abstract

Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate ( pNO3'), the end product of the oxidation of NO x gases (NO+NO2) in the upper troposphere. The application of stable nitrogen (N) (and oxygen) isotope analyses of pNO3' to constrain NO x source partitioning in the atmosphere requires knowledge of the isotope fractionation during the reactions leading to nitrate formation. Here we determined the 15N values of fresh pNO3' ( 15N- pNO3') in PM2.5 at a rural site in northern China, where atmospheric pNO3' can be attributed exclusively to biomass burning. The observed 15N- pNO3' (12.17±1.55‰; n Combining double low line 8) was much higher than the N isotopic source signature of NO x from biomass burning (1.04±4.13‰). The large difference between 15N- pNO3' and 15N-NO x ("( 15N)) can be reconciled by the net N isotope effect ( μN) associated with the gas-particle conversion from NO x to NO3'. For the biomass burning site, a mean μN( ‰ "( 15N)) of 10.99±0.74‰ was assessed through a newly developed computational quantum chemistry (CQC) module. μN 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 μN (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 NO x at this site. Based on the 15N values (10.93±3.32‰; n Combining double low line 43) of ambient pNO3' determined for the urban site, and considering the location-specific estimate for μN, our results reveal that the relative contribution of coal combustion and road traffic to urban NO x is 32%±11% and 68%±11%, respectively. This finding agrees well with a regional bottom-up emission inventory of NO x. 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 15N- pNO3' at the study site would erroneously imply that NO x is derived almost entirely from coal combustion. Similarly, reanalysis of reported 15N-NO3' data throughout China and its neighboring areas suggests that NO x emissions from coal combustion may be substantively overestimated (by > 30%) when the N isotope fractionation during atmospheric pNO3' formation is neglected.