Source apportionment and mechanistic analysis of PM_2.5 nitrate: interplay of OH and N₂O₅ in spatial and temporal...

Particulate nitrate, a substantial component of PM_2.5 during pollution episodes, requires precise source apportionment for effective air quality management. This study investigated nitrate formation mechanisms through dual-isotope analysis at an urban ZongMing (ZM) and suburban TungHai (TH) site in Taichung City, Taiwan, with PM_2.5 samples collected during August and October 2020, including diurnal sampling at TH. Analysis of nitrate isotopic compositions (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) revealed spatial heterogeneity between the sampling sites, with TH exhibiting elevated mean δ¹⁵N-NO₃^- values (2.6 ± 2.4 ‰) and distinct diurnal patterns in its 12-h measurements, while daily integrated samples at ZM showed mean values of 0.6 ± 2.1 ‰. Similar δ¹⁸O-NO₃^- patterns at both sites indicated comparable oxidation mechanisms. OH-mediated oxidation and its products maintained substantial contributions to PM_2.5 nitrate during both day and night periods, dominating in summer (81-83 %) before declining in autumn (77 ± 13 %) as the N₂O₅ pathway increased. The N₂O₅ pathway showed enhanced effectiveness under elevated nocturnal humidity, correlating strongly with PM_2.5 concentrations (R² = 0.52). Source apportionment using MixSIAR identified biomass burning and coal combustion as primary NO_x sources, with additional contributions from vehicular and biogenic soil emissions. These findings emphasize the importance of considering both diurnal and seasonal variations in formation mechanisms for developing targeted air quality control strategies.