Study On Chemical Characteristics Of Air Condensation Water

Great attention has been paid on ecological environment and economic social losses caused by acid deposition. Most of the atmospheric water system is kept in the form of water vapor which can shape into various hydrometeors (such as rain, snow, fog, dew, and rime) at high altitudes or on the ground surface due to physical-chemical reactions under different ambient conditions. Wet deposition is composed of the hydrometeors suspended in the atmosphere. The chemical compositions of the hydrometeors have ever been changed in the process of the hydrometeors forming by absorbing all kinds of pollutants. A synthetical study on chemical characteristics of various hydrometeors is useful to investigate the air pollutions and the differences between various hydrometeors. It can also provide convictive theory support for establishing feasible policies and regulations.Current research, both at home and abroad, mostly focuses on the macro-meteorological phenomena such as rain and snow. However, little research involves micro-meteorological phenomena such as fog, dew, and rime has been found, especially for dew and rime. Most of the atmospheric chemical processes are inseperable from water vapor participation. Discussion of water vapor chemistry is conducive to explore the mechanisms of liquid chemical processes in the atmosphere. From that point, this paper proposes the concept of air condensation water (ACW) and has carried out several research works as follows:(1) Chemical characteristics in different hydrometeors. Compared with the results over the most common atmospheric hydrometeors, the decreasing order of total water-soluble ions (TWSI) is fog>dew>rime>rain>ACW>snow, while the increasing order of pH is fog<rime<rain<snow<ACW<dew. Judging by the absolute ionic contents, the first group (fog, dew, and rime) are one or two orders of magnitude higher than those often found in the second group (rain, snow, and ACW). Rain usually shows a higher ionic content because of its huge settlement and constantly absorbing air pollutants during its falling.(2) Chemical characteristics in ACW. The volume-weighted mean (VWM) of pH is6.5±0.2, which shows weakly acidic (near neutral). The pH found in ACW is comparable to those in dew water, while considerably higher than those in fog or rain water. The value of electrical conductivity (EC) is one or two orders of magnitude lower than those found in other hydrometeors, accounted only for1.9%of fog water during the same period and at the same site. NHt4+and SO42-are the dominant ions in ACW and SO42-/No3-is13.1which is nearly10times more than other hydrometeors. NO3-content is lower than NO2-which has never been found in other hydrometeor chemistry. The contents of Ca2+and Mg2+account only for1.2%of fog water,2.3%of dew water,15.8%of rain water, and20.1%of rime water. The amount of water-soluble organic acids (CH3COOH, MSA, HCOOH and H2C2O4) in ACW is4.1%then in the fog and rain water during the same period and at the same site are0.9%and0.8%respectively with TWSI. Some negative correlation is determined between pH and total organic acids both in ACW and Mt. Huang fog water. Organic acids make an important contribution to the total free acidity (TFA) with the contribution rates are4.3%in ACW and0.2%in Mt. Huang fog water respectively. Higher organic acid is a specific character of ACW and a marked difference compared with other hydrometeors.(3) Typical chemical processes in ACW chemistry. The correlation coefficient between SO42-and NO3-is0.3in ACW then0.9in Mt. Huang fog water. There are significant correlations between SO42-, NO3-and metal ions (K+, Ca2+, and Mg2+) in Mt. Huang fog water, but the correlations is not evident in ACW (correlation coefficient is minus). The correlation coefficients among metal ions are very high in ACW. It is implied, in ACW, the levels of alkali cations is low and the predominant alkali cations is not metal ions. The facts that correlation coefficient between NH4+and SO42-is0.9while between NH4+and NO3-is0.2indicate ammonia is mostly in the form of sulfate in Shanghai atmosphere. The correlation coefficients between MSA-and HCOO-, CH3COO-, C2O42-are0.7,0.4, and0.5, besides the correlation coefficient between MSA-and SO42-achieves0.4. Therefore, organic acids in ACW possibly come from a series of photooxidation processes of Dimethyl Sulfide (DMS) in the atmosphere. The diurnal variations of TWSI in ACW exhibite great differences and show a bimodal shape with the peaks at13:00and21:00～23:00pm everyday. The bimodal variation perhaps caused by the photochemical reactions and spring inversion respectively.