Abstract

Fog deposition is an important sink for atmospheric aerosols, in recent years nitrate has become an important component of fog water, clarifying the source of nitrate that removed from fog water and the formation mechanism of nitrate becomes more important. In this study, we collected fog water samples from a winter fog process at Mount Lu in January 2016 using a three-stage fog water sampler and analyzed their water-soluble ion composition and nitrate with nitrogen and oxygen isotope (δ15N, Δ17O). The analytical results showed that nitrate was the key ionic component (27.8%±3.1%) of the fog water at Mount Lu, and it accounted for a similar proportion of the fog droplets in different particle sizes. The oxygen isotope anomalies of nitrate (Δ17 O-NO3–) in fog water of different grain sizes (S1: ≥22 μm; S2: 16-22 μm; S3: 4-16 μm) were 27.3‰±1.4‰, 27.8‰±1.2‰, and 27.7‰±1.5‰, respectively, and its nitrogen isotope values (δ 15N-NO3–) were 1.2‰±2.3‰, -2.5‰±2.2‰, and -2.8‰±2.8‰, respectively. By combining nitrogen and oxygen isotopes and Bayesian mixing modeling, the sources and formation mechanisms of nitrate in Mount Lu fog water were quantitatively analyzed. Model calculations reveal that in the fog process, the main generation mechanism of nitrate in Mount Lushan fog water is N 2O5 hydrolysis reaction (contribution of 42%-46%), and the main source of nitrate was vehicle emission (contribution of 56%±6%); moreover, no significant differences were found in the source composition and generation pathways of nitrate in fog droplets of different particle sizes (P˃0.05). During fog dissipation, fog water nitrate Δ17O and δ15N showed low values (δ15N-NO3–:-25.9‰, Δ17 O-NO3–:24.9‰), suggesting that microbial nitrogen cycling was the main source of nitrate in the fog water (44.5%), and that the share of NO2+·OH pathway increased (17% in the fog rose to 30% in the fog dissipation phase), and the backward air mass trajectories suggest that the change in the nitrate source and generation mechanism is a result of a shift in particulate matter transport from long-range transport (~80%) to local generation dominance (60%).