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

Nowadays, the emission source and formation mechanism of fine particulate nitrate (pNO) in China are mired in controversy. In this study, the stable nitrogen isotope (δN-NO) and triple oxygen isotope (ΔO-NO) were determined for the pNO samples collected at three heights under different atmospheric oxidation capacity (AOC) (O = O + NO: 107 ± 29 μg m at ground, 102 ± 28 μg m at 118 m, 122 ± 23 μg m 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 pNO in this city, followed by biomass burning, vehicle exhaust, and soil emission. Interestingly, we found that vertical NO and pNO concentrations displayed an opposite pattern owing to the different formation mechanisms among heights. The average contributions of oxidation pathways for (NO + OH, P1), (NO + DMS/HC, P2), and (NO + HO, 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 NO loading played an important role in pNO production. The pNO displayed a positive correlation with NO (r = 0.95) with an enhanced contribution of the P1 pathway under the relatively high AOC condition. However, pNO has a negative correlation with NO (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.