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

摘要

Atmospheric fine-particle (PM2.5) pollution is frequently associated with the formation of particulate nitrate (pNO(3)(-)), 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 pNO(3)(-) 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 delta N-15 values of fresh pNO(3)(-) (delta N-15-pNO(3)(-)) in PM2.5 at a rural site in northern China, where atmospheric pNO(3)(-) can be attributed exclusively to biomass burning. The observed delta N-15-pNO(3)(-) (12.17 +/- 1.55 parts per thousand; n = 8) was much higher than the N isotopic source signature of NOx from biomass burning (1.04 +/- 4.13 parts per thousand). The large difference between delta N-15-pNO(3)(-) and delta N-15-NOx (Delta(delta N-15)) can be reconciled by the net N isotope effect (epsilon(N)) associated with the gasparticle conversion from NOx to NO3-. For the biomass burning site, a mean epsilon(N)(approximate to Delta(delta N-15)) of 10.99 +/- 0.74 parts per thousand was assessed through a newly developed computational quantum chemistry (CQC) module. epsilon(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 epsilon(N) (15.33 +/- 4.90 parts per thousand) 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 delta N-15 values (10.93 +/- 3.32 parts per thousand; n = 43) of ambient pNO(3)(-) determined for the urban site, and considering the location-specific estimate for epsilon(N), our results reveal that the