Size Distribution Characteristics Of Atmospheric Aerosol And Its Influence On Visibility In Xi’an

Atmospheric aerosols are one of the main pollutants affecting the ambient air quality,having important effect and harm on climate change,atmospheric visibility and human health.The behavior of aerosols in the atmosphere depends on its size distribution and composition.In order to identify the characteristics of aerosol size distribution and its influence on visibility in Xi’an,the electronic low pressure impactor plus（ELPI+）and the forward scattering visibility instrument were used to observe the atmospheric aerosol concentration（6 nm～10μm）and the atmospheric visibility for two years in this thesis.Seasonal variations in aerosol concentration and particle size distribution were analyzed together with the meteorological elements measured simultaneously during the same period.The evolution characteristics of aerosol number concentration spectrum under heavy pollution,dust and precipitation were discussed,and the relationship between meteorological factors and aerosol number concentration was further analyzed.The influence of aerosol surface area concentration and relative humidity（RH）on visibility has been studied based on Mie scattering theory.The main conclusions are as follows:（1）During the observation period,the average concentrations of aerosol number,surface area and volume were 38476.96±10585.79 cm^-3,1365.31±508.16μm²·cm^-3 and 365.78±142.92μm³·cm^-3,respectively.The aerosol with diameter between 0.006 and 0.017μm contributed the most to the total aerosol number concentration,accounting for 64.58%,and the aerosol having0.170～1.600μm and 6.800～10.000μm diameter aerosol,accounting for 68.45%and 44.78%of the total aerosol surface area concentration and volume concentration,respectively.（2）The aerosol number concentration in Xi’an has the properties of seasonal variation,i.e.high level in summer and winter and low in winter and spring and autumn.Aerosol particle size distribution has different modes,nucleation mode（D_p<20 nm）and Aitken mode（20 nm<D_p<100 nm）number concentration were the highest in summer.The aerosol number concentration of accumulation mode（100 nm<D_p<1μm）and coarse mode（1μm<D_p<10μm）showed higher values in winter,corresponding to heavy pollution weather.（3）During periods of severe air pollution,the total aerosol number concentration increased by 26.10%,and the coarse mode aerosol number concentration increased the most.During the precipitation period,the removal rate of coarse modal particles caused by precipitation was56.76%.,the number concentration of nucleation mode particles and Aitken mode particles would rapidly increase following precipitation.The dusty weather increased the number concentration of coarse mode aerosol by 81.98%.（4）The number concentration of ultrafine particle（D_p<100 nm）increased with rising temperature,and the number concentration of particles exceeding 100 nm firstly increased and then decreased with rising temperature.The number concentration of aerosol increased first and then decreased with the increase of RH,but the"inflection point"of particle number concentration distribution in different modes was different.The“inflection point”of nucleation mode and Aitken mode particles occurred at 40%of RH,and the“inflection point”of accumulation mode and coarse mode particles would take place at RH of 90%.（5）During the observation period,the atmospheric visibility in Xi’an was 8.81±5.98 km,with the highest visibility in summer（14.02±5.88 km）and the lowest visibility in winter（6.01±3.07 km）.The aerosol surface area concentration was negatively correlated with atmospheric visibility at different RHs.The aerosol in the particle size range between 0.400 and1.000μm have a strong influence on attenuating visible light.