| In order to understand the chemical characteristics of fog water in Nanjing, three-stage Caltech Active Strand Cloud Collector (CASCC) was used to collect fog droplets in three separate droplet size ranges (from December 7th to December 9th. 2013). Fifty percent droplet size cuts of 22,16, and 4μm diameter are featured in stages one, two and three, respectively. Twenty-three fog samples were collected in two fog events and divided into eight periods. Concentrations of cations and anions in each sample were detected by ion chromatography. Composition of the fog water, chemical composition distribution characteristics of fog water in each stage, the correlation of ions in each stages and the relationship between ion concentrations and gaseous pollutants or microphysics were considered together during the analysis. The conclusions were as follows:Analysis of the three-stage fog water in Nanjing, the results showed that the pH was range from 3.24 to 6.51, with a mean of 5.2, mostly resided in acidic range, which is similar to the previous research in Nanjing. The CASCC data showed significant size-dependence for the pH. The pH value increases with the increase of particle diameter, small droplets of acidity than big droplets, the pH average is 5.68, 5.27,4.58 from the first to the third stage, respectively; From the perspective of the pH value of the fog water in Nanjing, the change of the whole process of the fog of large and small drops of mist pH change trend.The electrical conductivity and the changing trends of total ion concentration (TIC) of the fog water on the outskirts of Nanjing are consistency; At the same time in accordance with the basic quality of PM10 concentration in the air. The conductivity of the first stage fog water is markedly lower than the third stage, but the second, forth and the fifth sample period, we found that the conductivity with the distribution of particle diameter, present a "U" type distribution.The main ions of the winter fog water in Nanjing are NH/, Ca2+and NO3", SO42- mainly. From the three-stage fog water, ion concentration of small droplets is obviously higher than that of large droplets, little drops of the total ion concentration is 2.34 and 2.24 times than the middle and the large drops. The ions concentration showed that it is higher at night than during the day for the fog water. In the process of the two fog periods, the ions concentration distribution (Cl-、NO3-、SO42-、K+ Ca2+、Na+、Mg2+,) of the first stage is bimodal type. In the second stage, the ions concentration (K+、Mg2+、Ca2+、Cl-) of the fog water is bimodal type. The distribution of ions concentration of Na+ showed three peak distributions. The ions concentration (NO3-、S042-、NH4+) showed unimodal distribution. In the third stage, except for the Na+ shows three peak distribution, the rest of the ion distribution type are unimodal distribution. The CASCC data showed significant size-dependence for the ions concentration in the fog water. The concentration of Cl-、NO3-、SO42-、NH4+ and K+in the small drops are obviously higher than the large drops. However the distribution of the Ca2+、Na+、Mg2+is more complex, showed a "U" and "L" type.Statistical analysis showed that the main cause is the first step on the acid composition is given priority to with H2SO4, the second and the third stage are mainly composed of HNO3, H2SO4. Cl- and K+, Ca2+, Mg2+in the third stage was mainly affected by local source.The concentration of SO42- and NO3- in the fog water samples and the pollution gases in air SO2 and NO2 concentration had the same change trend, But the ion concentration slightly lags behind the gas pollution. The ion concentrations in the fog water samples have great relationship with microphysical characteristics. Analysis, the ion concentration and the fog microphysical parameters, such as LWC has a certain relationship. |
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