Macroscale And Microscale Characteristics Of Fog And Haze And The Impact On Visibility In Winter Of Nanjing

Based on the data obtained by means of comprehensive observation in Nanjing university of information science&technology during the winter of2009and2010, the relationship between aerosol size distribution characteristics and meteorological factors, and its possible influence on visibility in urban Nanjing were analyzed. Besides, the related boundary layer structure, formation and dispersion mechanisms and microphysical structure for the fog event were also analyzed.It was found that the diurnal and interdiurnal variations of particle concentrations are obvious. Human activities and variation of atmospheric stability had great effect on daily variation of particle concentrations, while meteorological conditions such as precipitation, wind, relative humidity and so on had strong influence on interdiurnal variation. The aerosol size distribution was significantly affected by relative humidity. When RH was lower than54%, number concentration of aerosol particles less than1μm in diameter increased gradually as RH increased, and concentration of particles with diameter larger than1μm almost had no change. When RH was higher than54%, number concentration of aerosol particles ranging from0.01to0.2μm and from2.7to10μm decreased with the increase of RH, in contrast, concentration of aerosol particles between0.5and1.5μm in diameter increased. In addition, the particle number size distributions were different in rainy, foggy, sunny and haze weather conditions. Compared to sunny day, concentration of particles with different sizes all decreased in rainy day. In foggy weather, The number concentration of aerosol particles ranging from0.01to0.3μm and from2.7to10μm decreased, and aerosol particles between0.3and2.7μm increased in comparison with sunny day. Scavenging action of rain and fog to particles of different sizes from high to low was that coarse particles> nuclei mode particles> accumulation mode particles. In haze day, the peak of number concentration distribution moved toward a higher value. Compared to sunny day, the number concentration of aerosol particles ranging from0.03to0.1μm decreased, and aerosol particles between0.1and2.7μm increased in haze day. Based on Mie theory, the correlation between visibility and surface area concentration with different particle sizes indicated that particles between0.1and2μm in diameter shows a good correlation with visibility, which was the major contributor to visibility degradation in Nanjing. Using theobservation data obtained from17to18November2010, the related boundary layer structure, formation and dispersion mechanisms and microphysical structure for the fog event were analyzed. Results show that the fog formed under the inversion layer against a background of clear night, light wind, and radiative cooling. An endless supply of moisture advection resulted in such a dense fog.Different from the previous studies, the fog immediately entered the stage of dissipation after burst reinforcement, and the visibility increased to1kilometre within1hour. This might be due to an association with the disappearance of inversion layer and a faster temperature increase from solar radiation, and might also be due to gradient wind,which was also an important dynamic factor.