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

Atmospheric fine-particle (PM 2 . 5 ) pollution is fre- quently associated with the formation of particulate nitrate ( p NO − 3 ), the end product of the oxidation of NO x gases (NO + NO 2 ) in the upper troposphere. The application of sta- ble nitrogen (N) (and oxygen) isotope analyses of p NO − 3 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 δ 15 N values of fresh p NO − 3 ( δ 15 N- p NO − 3 ) in PM 2 . 5 at a rural site in northern China, where atmospheric p NO − 3 can be at- tributed exclusively to biomass burning. The observed δ 15 N- p NO − 3 (12 . 17 ± 1 . 55 ‰; n = 8) was much higher than the N isotopic source signature of NO x from biomass burning (1 . 04 ± 4 . 13 ‰). The large difference between δ 15 N- p NO − 3 and δ 15 N-NO x (