| During two sea-fog field observation programs on Donghai Island in Guangdong province of China from March to April2010and from February to March2011, meteorological elements, structures of the atmospheric boundary layer, visibility, fog water, and spectrum of rain droplets, fog droplets and aerosols were obtained. The variations of synoptic systems, meteorological elements and sea surface temperature (SST) were analyzed. We studied the evolution and relationship of rain droplets, fog droplets and aerosols during the sea fog cases, as well as the dominant mechanisms of fog formation and development. The analysis on chemical compositions of sea fog and the comparative study were carried out to display the properties of fog-water. Finally, we discussed the effect of different factors on sea fog water ion concentrations based on the analysis of sea fog water chemical composions and different macro-and microphysical elements. The main conclusions are as follows:(1)The average fog days in Leizhou Penisular were20-30d. When the temperature was16～22℃, the fog days were relatively high. Most sea fog cases were affected by depression and cold front, with virtual temperature inversion. The average wind speed was1～5m s-1; the dominate wind direction was from east and clockwise rotation with height. The SST was about18～21℃, the wind from south and air temperature higher than SST was propitious to fog as well.(2) Based on the analysis of microphysical characteristics of fog droplets, aerosols and rain droplets, we found that:The number concentration of sea fog was lower than that of continent fog with relatively low liquid water content (LWC), due to the low number concentrations of large fog droplets caused by turbulence deposition and some droplets exceeding the measuring range. Activation and condensation growth played an important role on fog droplets formation and growth. Compared with continent fog and mountain fog, the influences of collision-coalescence, turbulence and advection factors were more significant in sea fog. The spectra of fog droplets could be expressed by formula of Gamma and spectra width broadened by the strength of collision-coalescence. The distribution of aerosol was unimodal. The number concentration of aerosol was one order of magnitude lower than that in other regions, and the aerosol number concentration decreased significantly during the fog cases. The ratio of number concentration between fog droplets and aerosol (Nf/Na) increased with the growth of LWC. The precipitation fog occurred with stable atmospheric stratification, and the temperature near the surface was low. The width of fog droplets spectrum was narrow.(3)The average pH and electrical conductivity (EC) value in2010was5.20and1884μS cm-1; while the average values of pH and EC in2011were3.34and505μS cm-1, respectively, and the pH value had a negative correlation with EC. The fog water became more acidity in2011might owing to the proportion of acidity and alkalinity matter. Cl-and Na+mainly originated from oceanic aerosols; NO3-and SO42-could be influenced by the same source, such as anthropogenic pollutions. The variation pattern of ion concentration was in form of "U", related to the ion loading and LWC. The average total ion concentration in2010and2011were38260μeq L-1and5600μeq L-1, respectively, but the ion loadings were almost in the same magnitude. When the pollutant substances emission changed little, the decline of TIC was owing to the growth of LWC caused by the decline of turbulence with the rise of altitude.(4)The chemical composition of fog water was depended upon the species and size of aerosols. Affected by one large dust storm originated from the north of China, the number concentration of large aerosol particles (with a diameter larger than1μm) increased, indicating that the concentrations of Ca2+and Mg2+in fog water increased. During the precipitation fog, the extent of variation was slight, with the lowest standard deviation. Cold front and depression promoted the decline of ion loading. The air mass from a short distance away was mainly affected by pollutants from coastal areas and had the highest ion loading, while the airflow from the sea had the lowest ion concentration and ion loading. The collection efficiency decreased with the increase of wind speed. The ranges of wind speed, wind direction and temperature corresponding to the maximum total ion concentration (TIC) were3.5～4m s-1,78.75～90°and21～22℃, respectively. In view of low correlation coefficients between microphysical characteristic quantities and TIC, a new parameter of Lr=1/(LWC×r) was proposed as a predictive parameter for TIC, where1/r and LWC represented scavenging efficiency and dilution effect. The correlation coefficient between Lr and TIC was0.74. |
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