Vertical measurements of stable nitrogen and oxygen isotope composition of fine particulate nitrate aerosol in Guangzhou city: Source apportionment and oxidation pathway

Abstract

Nowadays, the emission source and formation mechanism of fine particulate nitrate (pNO3−) in China are mired in controversy. In this study, the stable nitrogen isotope (δ15N-NO3−) and triple oxygen isotope (Δ17O-NO3−) were determined for the pNO3− samples collected at three heights under different atmospheric oxidation capacity (AOC) (Ox = O3 + NO2: 107 ± 29 μg m−3 at ground, 102 ± 28 μg m−3 at 118 m, 122 ± 23 μg m−3 at 488 m) conditions during the sampling period based on the Canton Tower, Guangzhou, China. The Bayesian mixing model showed that coal combustion was the largest contributor to pNO3− in this city, followed by biomass burning, vehicle exhaust, and soil emission. Interestingly, we found that vertical NOx and pNO3− concentrations displayed an opposite pattern owing to the different formation mechanisms among heights. The average contributions of oxidation pathways for (NO2 + OH, P1), (NO3 + DMS/HC, P2), and (N2O5 + H2O, P3) were 61 %, 12 %, and 27 % at the ground, respectively, and these values would vary greatly among heights. These results implied that both AOC and NOx loading played an important role in pNO3− production. The pNO3− displayed a positive correlation with NOx (r = 0.95) with an enhanced contribution of the P1 pathway under the relatively high AOC condition. However, pNO3− has a negative correlation with NOx (r = −0.99) with a rise of heterogeneous reaction (P2 and P3) under the relatively low AOC condition. Therefore, the current emission control strategy for air pollution in China needs to consider the AOC conditions among regions to effectively mitigate particulate air pollution.