Study On The Physical And Chemical Characteristics Of Three-stage Fog Water In New-type Urbanization Area In Northern Jiangsu

To investigate the relationships among the chemical compositions of fog droplets at different scales,fog microphysical characteristics,and atmospheric pollutants,the research group conducted comprehensive observation tests on four fog processes in Donghai County,a new-type urbanization area in northern Jiangsu Province in the winter of 2020.The active The three-stage Caltech active strand cloud collector was used to collect fog water samples with diameters of 4-16μm,16-22μm and>22μm.FM120 fog droplet spectrum was used to measure fog microphysical quantities synchronously,combined with UAV vertical observation data and ground meteorological elements data,etc.We investigated the chemical characteristics of fog droplets with different particle sizes,the reasons for the difference in fog water ion concentration,and the evolution characteristics and main physical processes of fog microphysical quantities during the explosive growth phase of fog.The results show that:（1）The pH value,electrical conductivity（EC）,total ion concentration（TIC）and each ion component of the fog water are dependent on the size of the fog water.The small droplets of4-16μm are the most acidic,and the electrical conductivity and total ion concentration are the highest.High concentrations of non-sea salt calcium ions(nssCa²⁺)and NH₄⁺completely neutralize the acidic substances in the fog water,resulting in an alkaline pH.The ion concentration level of fog water is relatively high,and the maximum ion concentration of small droplets was 463527.9μeq/L.The main ions of fog water are SO₄^2-,NO₃^-,NH₄⁺and Ca²⁺,which are enriched in small droplets.The high concentration and proportion of NO₂^-in fog water is mainly due to the heterogeneous chemical reaction between NO₂and droplets to generate HONO,which dissolves into the fog droplets with the pH value.（2）The ion concentration of fog water measured on December 12,2020 was 1-2 orders of magnitude higher than that on December 11.Compared to the first fog case,atmospheric particulate matter and gaseous pollutants are enriched near the surface during the second fog process due to stronger near-surface inversion,increased local emission and air transport,which increases the local atmospheric aerosol load,thus resulting in lower supersaturation in this fog process and generating fewer droplets with smaller particle sizes.Small droplets has faster liquid phase oxidation reaction rate.S/LWC parameters comprehensively reflect the scavenging ability and dilution effect of fog droplets.In the second fog process,the particle size of fog droplets is smaller,and the conversion efficiency of gas-liquid and particle-liquid is higher,which leads to higher concentration of fog water ions collected in the second fog.（3）In a typical explosive growth fog process on December 28,2020,due to the low supersaturation of fog in the local environment,most coarse aerosol particles acted as giant condensation nodules and activated to form droplets under lower supersaturation conditions.At this moment,the limited water vapor inhibited the condensation growth of fog droplets.As the water vapor reaches supersaturation,a large number of condensation nodules increase the number of fog droplets explodes through activation,resulting in a significant increase in the water content of fog droplets.In the mature phase,large droplets are formed due to condensation growth and collision of droplets,which contribute more to the water content of droplets.Finally,due to the influence of temperature rise and turbulence near the ground,the fog dissipates.