Study On Three-stage Fog Water Chemistry In Lushan Mountain In Winter

Exploring the chemical characteristics of droplets at different scales is an important part of deepening the research of fog microphysical and chemistry.From December 2016 to January2017,the comprehensive observation experiment of fog was carried out in Lushan,ground meteorological observations and microphysical data of three fog processes were obtained.The Caltech Active Strand Cloud Collector?CASCC 3_stage?was used for size resolved fog sampling.Fifty percent droplet size cuts of 22,16,4?m diameter are featured in stage one,two and three,respectively.A total of 73 samples of three fog events were collected.The pH,conductivity?EC?and 9 kinds of water-soluble inorganic ion(Na⁺?NH₄⁺?K⁺?Mg²⁺?Ca²⁺?Cl^-?NO₂^-?NO₃^-?SO₄^2-)concentrations??eq/L?of the three-stage fog water were quantitatively obtained by experimental analysis.The results show:?1?The first and second fog processes belong to advection fog,and the third fog process belongs to advection radiation fog.The enhancement and maintenance of the fog process is closely related to the activities of the weather system?high-altitude trough,cold high pressure,shear line,southwest jet,low vortex circulation and western Pacific subtropical high,etc.?.The spatial distribution of the internal structure of the fog is uneven and has a mass-like feature.?2?The fog water of Lushan was seriously acidified in winter,with a pH of 3.96-5.82 and the sample with pH<5.6 accounts 98.6%.Moreover,the heavy acid sample?pH<4.5?of 4-16?m small droplets accounts for up to 54.2%,and the small droplets is more acidic and conductive.NH₄⁺?Ca²⁺?NO₃^-?SO₄^2-are the main ionic components of fog water,[NH₄⁺+Ca²⁺+NO₃^-+SO₄^2-]accounted for 83.8%?88.0%and 88.7%of the total ion concentration?TIC?in the three-stage fog water respectively.Combined with three fog processes,TIC?NH₄⁺?K⁺?NO₃^-and SO₄^2-are enriched in small droplets of 4-16?m,showing size-dependent characteristics.?3?The fog water is a mixed acidification of sulfuric acid and nitric acid.The main reason for acidification is the imbalance between the alkaline buffer material and the acidic component and the possible organic acid contribution.The fractional acidity of the three-stage fog water is7.0%?5.6%and 5.1%,respectively.SO₄^2-is the main acid-producing factor and NH₄⁺is the main neutralizing factor.?4?According to the results of historical research,four experiments on fog water chemistry conducted in 1987-2017 showed that NH₄⁺,Ca₂⁺,NO₃^-and SO₄^2-are always the main ionic components in Lushan fog water.The proportions of NO₃^-and NH₄⁺in anions and cations increased,and the proportions of SO₄^2-and Ca²⁺in anions and cations decreased.In the past 30years,[SO₄^2-]/[NO₃^-]of fog water has been decreasing.The fog water of Lushan has been transformed from acidification of sulfuric acid type in 1987 and 1993 to mixed acidification,and there is a tendency to continue to acidify to nitric acid.?5?There are some differences in the chemical characteristics of different fog processes at the same observation point.In the second event?12.25.14:00-26.21:00 BT?,the concentrations of TIC?NH₄⁺?Ca²⁺?NO₃^-and SO₄^2-show a U-shaped trend of decrease first and then increas with the duration of fog,and there are an enrichment in the droplets of 16-22?m.This phenomenon may be the result of a combination of lower aerosol concentration,temperature drop,pollutant gas and aerosol transport in the fog region.