Cloud And Fog Chemical Characteristics On Mt. Taishan, China

Cloud is a complex system with its chemical composition influenced by the interaction between gas, particle pollutants and the liquid water in troposphere. Acidification of cloud induces acid rain. Therefore, the investigation of cloud pollution process and mechanism can give us an overview of the transformation of acidic pollutant into cloud water in different weather and different oxidant and aerosol pollutant background. The research on cloud chemistry is highly significant to the investigation on acid rain formation and the acidic pollutant transformation in China.A total of 118 cloud/fog samples and 11 precipitation samples were collected during Mar. 2007- Nov. 2008 at the summit of Mt. Taishan in order to examine the impact of regional sources of pollution on cloud and fog water chemistry in the highly polluted North China Plain. The monitoring campaign was divisible into 3 periods in spring, summer and autumn-winter, respectively. All samples were analyzed for electric conductivity (EC), pH, F^-, Cl^-,NO₃^-, SO₄^2-, NH₄⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺ and organic acids. This paper mainly discussed the cloud and precipitation chemical composition, the influence factors on cloud chemistry and the interaction between aerosols and cloud droplets.The cloud/fog water pH ranged from 2.56 to 7.64 and the volume weighted mean (VWM) pH was 3.87. Some of the cloud water was severe acidified. More than 45% of the cloud samples can be identified as strongly acidified cloud. Cloud water collected in spring has polarized pH values (from 2.56 to 7.64) and the summer cloud water showed narrow pH range. There were only 3 cloud/fog events in autumn-winter campaign and all the samples in this campaign were strongly acidic cloud. The cloud and rain water on Mt. Taishan has a relatively high ion concentration compared with the monitoring results on some other rural high elevation sites on global. The cloud water ion concentration was 4.4 times of that in rain. The monitoring results illuminated that there are abundant acidic pollutants in the air on Huabei Plain and the precipitation was highly influenced by anthropogenic sources despite the monitoring site was on a high elevation above the boundary layer.The inorganic cloud/fog components are mainly from an anthropogenic source and are dominated by SO₄^2- (1062μeqL^-1), NO₃^- (405μeqL^-1), NH₄⁺ (1235μeqL^-1) and Ca²⁺ (260μeqL^-1), accounting for about 90% of the total ion concentration. Comparing with cloud water, rain has a higher sea source salt contribution, and cloud water tended to be influenced more by regional and anthropogenic sources. Cloud water in spring and autumn-winter showed similar ion proportion. The summer cloud has a ion proportion close to rain water. Ammonia played the major basic neutralizing role in cloud and rain water and Calcium can provide more basic input in spring because of the highly frequent sandstorms. About 99.5% of the sulfate in cloud water was nss-sulfate indicating a regional source. Sulfate to nitrate ratio of cloud water were 1.73, 4.40 and 1.79 for the three campaigns respectively, lower than the national average sulfate to nitrate ratio of rain water of 6.4, suggesting that the increase emission of NOx from vehicles in recent years also contributed to the acidification of cloud water. Organic acids in cloud water were obviously higher than rain. Formic acid was the most abundant organic aid in cloud water, and the most abundant organic acid in rain was acetate acid. Organic acid can contributed for 1.21%, 1.18%, 1.77% and 1.14% respectively for the three campaigns' cloud water and for all the monitored rain water.Cloud can scavenge the aerosols obviously. The calculated cloud droplets scavenging ratio of coarse particles (55.7%) was a little higher than that of the fine particles which was 52.0%. Higher scavenging ratios were observed in longer-lasting cloud or fog events. Nitrate (74.8%), potassium (69.7%) and ammonium (61.2%) tend to be more easily scavenged than calcium (40.2%) and sodium (32.1%) on PM2.5 which was inferred as the ion solubility difference for the ions. Sodium, ammonium and potassium tend to be enriched on fine particles while calcium and magnesium were more abundant on coarse particles. The simulation results showed that the close system was more similar with practical air condition, and H₂O₂ was the predominating S (â…£) oxidant. In the later stage of the simulation, O₃ can react with more S (â…£) than H₂O₂. After 4 hours simulating reaction under the proposed air condition, the percentage of the oxidizing amount of H₂O₂, O₃, and HCHO was 64.4%, 19.8%, and 15.8%, respectively.