Search

Nitrate (NO 3 - ) has become recognized as the most important water-soluble ion in fine particulate (PM 2.5 ), and has been proposed as a driving factor for regional haze formation. However, nitrate formation mechanisms are still poorly understood. In this study, PM 2.5 samples were collected from September 2017 to August 2018 in Shijiazhuang, a city located on the North China Plain, and NO 3 - concentration, δ 18 O-NO 3 - and δ 15 N-NO 3 - values in PM 2.5 were analyzed. NO 3 - concentrations increased as PM 2.5 levels increased during both polluted and non-polluted days over the entire year. δ 18 O-NO 3 - values during cold months (63.5-103‰) were higher than those during warm months (50.3-85.4‰), these results suggested that the nitrate formation pathways shifted from the NO 2  + OH (P OH ) in warm months to the N 2 O 5  + H 2 O (P N2O5 ) and NO 3  + VOCs (P NO3 ) pathways in cold months. Especially during cold months, δ 18 O-NO 3 - values increased from 65.2-79.9‰ to 80.7-96.2‰ when PM 2.5 increased from ∼25 to >100 μg/m 3 , but when PM 2.5 > 100 μg/m 3 , there were relatively small variations in δ 18 O-NO 3 - . These results suggested that nitrate formation pathways changed from P OH to P N2O5 and P NO3 pathways when PM 2.5 < 100 μg/m 3 , but that P N2O5 and P NO3 dominated nitrate production when PM 2.5 > 100 μg/m 3 . Higher δ 15 N-NO 3 - values in warm months (-11.8-13.8‰) than in cold months (-0.7-22.6‰) may be attributed to differences in NO x emission sources and nitrogen isotopic fractionation among NO x and NO 3 - . These results provide information on the dual isotopic compositions of nitrate to understand nitrate formation pathways under different PM 2.5 levels., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)