Chemical composition of dew water at a suburban site in Nanjing, China, during the 2016-2017 winter

摘要

A dew condenser was used to collect dew and frost samples to investigate the chemical properties of dew water for comparison with frost, rain and fog in Nanjing, China during the 2016-2017 winter. Twenty-three separate ambient dew samples were collected at a grassland site in suburban Nanjing, China, from 1st November 2016 to 20th January 2017. Six frost samples, two fog water samples and 13 rainwater samples were also collected for comparative study. Cations (Na+, Ca2+, NH4+, K+, and Mg2+), anions (NO3-, NO2-, Cl-, SO42-, and F-) and pH were detected and measured in all wet depositional samples. In addition, seven kinds of water-soluble ions (WSIs) in atmospheric fine particulates (PM2.5) and atmospheric gas (SO2 and NH3) concentrations were investigated. The field study elucidated that dew water resembles frost water, with WSIs ranked in the order of Ca2+ NH4+ Na+ Mg2+ K+ for cations and SO42- NO3- Cl- NO2- F- for anions. Most dew and frost samples were alkaline and not as acidic as the rainwater and fog water samples. The mean concentrations of various ions and total ionic content (TIC) in the fog samples were the highest, while the ion concentration ranges in the dew, rain and frost samples were similar. The neutralization strength decreased in the order of NH4+ Ca2+ Mg2+ K+ for the fog samples and decreased in the order of Ca2+ NH4+ Mg2+ K+ for the dew, frost and rainwater samples. A high pairwise correlation between K+, Mg2+ and Ca2+ was observed, which means these cations shared a common crustal source. The SO42-, NO2- and Cl- ions were closely correlated, indicating that the ions were released simultaneously from anthropogenic sources. The low and negative correlations among the NO3-, SO42- and NH4+ ions in the dew water and PM2.5 may imply that fine-mode aerosols have little impact on the composition of dew, which means that the dew water component was dominated by coarse-mode aerosols. The NH3 in the atmosphere plays an important role in dew water due to the strong relationship between NH3 and NH4+.