| This dissertation studies the statistical and optical properties of aerosols/cloud condensation nuclei (CCN), and explores the macro and micro structures of Nanjing fog and haze events in winter as well as the evolutionary properties of aerosols/CCN by analyzing such macro and micro measurements of fog and haze events collected between Dec.2012 and Jan.2014 in Nanjing as aerosol particle size distribution, CCN distribution, and observations obtained by laser and radar equipment. On that basis, it investigates the effects of thermodynamic factors and dynamic factors upon fog/haze, and haze-fog transformation. The investigation has produced some illuminating findings, which are presented as follows:Aerosol/CCN observation indicates:(1) The peak diameter was 1μm,0.3μm and 1μm in haze, haze-fog transformation, and fog respectively. Aerosol concentrations in haze ranged between 348.2-15489.2cm-3 in the accumulation mode and between 534.4-4634.1cm-3 in the nucleus mode. In haze-fog transformation, number concentrations in both modes decreased slightly, indicating that with the increase of relative humidity (RH) in the transformation stage, aerosols under both conditions experienced noticeable activation. Aerosol concentrations in fog ranged between 153.4-152459.8cm-3, which can be attributed to the increase of turbulent exchange in such weather, as uneven turbulent motion facilitated the hygroscopic capability of aerosols.(2) Aerosol particle size distribution was unimodal across the three different stages. Aerosol spectrum remained quite stable over time while number concentrations of aerosols decreased in the transformation and haze stages because aerosol particles experienced activation, condensed and increased in diameter as fog condensation nuclei. Meanwhile, as growing fog droplets could absorb aerosol particles and had hygroscopic properties, significant differences were observed in aerosol spectrum in the later two stages.(3) CCN fit spectrum parameter C≥2200cm3, k<1, which indicates the observation site was a continental polluted area.Investigation in to the optical properties of aerosols shows:(1) Backscattering coefficient (σbsc) was 0.027±0.44km-1 sr-1,0.024±0.22 km-1 sr-1, and 0.021±0.10 km-1 sr-1 respectively in haze, haze-fog transformation and fog stages. Extinction coefficient (σext) averaged (standard deviations) 0.37±0.17, 0.377±0.20, and 0.303±1.08 respectively in the three stages. Corresponding aerosol optical depth (AOD) was 0.37±0.17,0.377±0.20, and 0.303±1.08 respectively. Radar detection was space-based. Aerosol optical properties differed at different heights. Besides, in haze weather, aerosols exhibit greater variations in aerosol variation, uneven spatial distribution, and bigger fluctuations in terms of optical properties, hence relatively larger standard deviations in statistical results.(2) Aerosol extinction coefficient measured at a height less than 1.0 km decreased exponentially. The fog stage was found to have the largest aerosol extinction coefficient, followed by the transformation stage. The coefficient was the smallest in haze.(3) Aerosol optical depth (AOD) in both haze and haze-fog transformation stages was larger in the morning and evening and smaller at noon. This was because atmospheric humidity was relatively high in the morning and evening and that hygroscopic particles expanded while RH dropped at noon, hence the decreasing of AOD. In the fog stage, AOD was larger at noon and smaller in the morning and evening, with the peak value occurring around 11:00. This was the result of upward transportation of aerosols, which in turn resulted from the fact that atmospheric stratification grew increasingly unstable as the ground was heated by the sun over time.Investigation into the effects of thermodynamic factors upon fog and haze events shows:(1) The smaller the dew point depression of near surface temperatures, i.e., low saturations in near surface atmosphere, the larger K index and A index, i.e., the understratum of troposphere being less stable, the more serious the haze weather.(2) The haze stage had higher surface temperatures, followed by the haze-fog transformation stage, which was in turn followed by the fog stage. Relatively low surface temperatures can help vapor condense and result in moisture absorption, activation and increase in particle size. Temperature difference in the inversion layer was less than 4℃ in the haze stage and the difference increased to 6℃ in haze-fog transformation because of the increase in inversion intensity. The fog stage and transformation stage were similar in inversion intensity. The inversion layer increased in depth while the boundary layer remained stable, which could facilitate the accumulation of vapor.(3) Downward long-wave radiation was between 220-290 W m-2 on sunny days, between 270-310 W m-2 in the haze stage, and between 280-350 W m-2 in the fog stage. Downward long-wave radiation increased in the transformation stage, and gradually approximated upward long-wave radiation in intensity with the development of fog. This is because dry particles floating in low altitudes were much weaker than big droplets in terms of their ability to absorb long waves whereas compared with sunny weather, aerosol particles in haze weather would increase downward long-wave radiation.Investigation into the effects of dynamic factors upon fog and haze events shows:(1) Within the fog/haze region, surface wind speed could exert impact upon fog and haze events via horizontal transportation. When surface wind speed was relatively high, transportation of fog/haze to other regions would increase, which was detrimental to the maintenance and development of fog/haze and led to higher visibility. Vertical shear of horizontal winds between 500hPa and 850hPa was closely related to vertical mixing in the middle and lower strata of the troposphere. When vertical shear of horizontal winds increased, vertical mixing in the middle and lower strata of the troposphere above the polluted region strengthened, which could facilitate the dissipation of fog/haze, reduce the accumulation of fog/haze in near surface layers, and lead to the increase of visibility.(2) Weak northerly wind made aerosol particles and their precursors accumulate at the observation site. Emissions from some areas and particles transported to the polluted area from the west were the major sources of haze. Changes in wind field brought transportation of vapor from different directions, which was another way that wind might affect fog/haze weather.(3) Turbulent motion was found to have threshold effect in fog/haze development. As turbulent intensity changed, average aerosol radius and activation rate showed the tendency to increase at first and decrease later. When the intensity was smaller then the threshold, turbulent motion would facilitate the transformation of haze to fog, given appropriate amounts of vapor. When the intensity was larger then the threshold, atmospheric stratification was unstable, preventing haze from being transformed to fog. |
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