Study On Characteristics Of Atmospheric Extinction Of Fog And Haze Event In Winter Of Nanjing

The simultaneous measurements of aerosol particle and fog droplet size distribution, visibility and relative humidity were used to analyze the persistent fog/haze event in Nanjing form December 15 to December 20 of 2007 in this thesis. Due to the heavy air pollution resulted from the Industrial Park located in the northeast of the observing site and the relatively stable boundary layer structure, fog and haze occurred alternately. The coexistence of fog and haze as well as the interaction between them made visibility maintain at less than 10 km. The key point of this paper was to analyze the fog droplets’ effect on aerosol particle size distribution and number concentration during fog, the extinction of aerosol particles and fog droplets in the process of transformation between fog and haze. we came to the following conclusions.Scavenging coefficients of aerosol particles for fog with the particle distribution was substantially "V" type distribution, and the scavenging effects of aerosol particles on particle size of <0.06 m and >0.5 m were significant. aerosol particle droplet size distribution, which appeared as bimodal type with the peak diameter being 0.025μm and 0.096μm before the fog tern to unimodal type with the peak at 0.096μm increasing and at 0.025μm disappearing. There was a time threshold, about 5 h after the formation of fog, for the removal of the aerosol particles, only when the duration of fog longer than the time threshold the significantly scavenging effect on the aerosol particles had observed, and the short duration of the fog did not has obvious effect on the removal of the aerosol particles. Even if the fog’ scavenging began to take effect, this effect was intermittent, not until the fog dissipated, which leaded to the aerosol particle number concentration during fog showed a "W" trend.On one hand, fog would reduce the number of aerosol particles, diminishing the extinction intensity of aerosol particles; on the other hand, fog would make aerosol particle size increase, enhancing the aerosol particle’s extinction intensity. However, the net effect of fog was increase the intensity of extinction of aerosol particles.The relationship of visibility in different conditions was fog<wet haze<haze ～mist. The low visibility in fog condition was caused by fog droplets and aerosol particles, the extinction of fog droplets and aerosol particles was similar in fog condition. The concentration of fog droplet, liquid water content and relative humidity in fog were higher than misty, haze and wet haze. Because of the stable boundary layer structure which was not conducive to the diffusion, the large number of aerosol particles with a large surface area concentration would produce a strong extinction. The extinction of droplets and aerosol particles were bigger than that in mist, wet haze and haze condition for 1～2 orders of magnitude. Although the average of width droplet spectrum was 40～50μm, droplets extinction, which was the dominant factor in deciding the change of visibility in fog, was highly variable while the extinction of aerosol particles remained relatively constant. Only in the mature stage of fog when visibility lower than 500 m the droplets extinction exceeded that of aerosol extinction and became the main factor of extinction.The low visibility in wet haze was mainly caused by the aerosol extinction. Although the number concentration in wet haze condition of aerosol was significantly less than that in fog, mist and haze, the relative humidity reached 88%, aerosol particles could grow to larger size. The low visibility in wet haze was primarily due to the extinction of a large number of wet aerosol particles distributed in the accumulation mode.The extinction in haze condition mainly caused by a large number of aerosol particles. aerosol particle had greater number concentration in haze than that in fog, mist, wet haze, but did not own large mean diameter. the nucleation mode particles consisting about 80% of the total number concentration was the main reason that caused the low visibility. The minimum value of relative humidity was in mist, which meant the weakest hygroscopic growth of aerosol particles in this condition that made the lower extinction of aerosol than fog, wet haze and haze, but the visibility was similar to that in haze, resulting from the droplets contributed by 10～ 15% to the extinction...