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Observation Study of Relative Humidity Effects on Aerosol Light Scattering at a Regional Backgound Site in the Yangtze Delta Region

Author: ZhangZuo
Tutor: SunJunYing
School: Chinese Academy of Meteorological Sciences
Course: Meteorology
Keywords: relative humidity aerosol scattering coefficient scattering enhancement factor chemical composition
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Type: Master's thesis
Year: 2014
Downloads: 5
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Abstract


Aerosol hygroscopic property plays a vital role in aerosol optical properties, visibilitydegradation and haze occurrence. Meanwhile, the Yangtze delta region, as one of the mostpopulated and developed region of China, has become a significant source of gas andparticulate pollutants and secondary aerosol production. To achieve a better understanding ofthe effect of aerosol hygroscopic growth on scattering properties and radiative forcing in thebackground region, a field campaign has been carried out at LinAn reginal background stationfrom Jan to Sept2013.During the observation, the mean and standard deviation of scattering coefficient at450nm,550nm and700nm was213.6±174.1Mm-1,168.0±117.6Mm-1and117.6±104.2Mm-1,respectively; the mean and standard deviation of backscattering coefficeint at450nm,550nmand700nm was24.3±18.7Mm-1,20.0±15.5Mm-1and17.5±13.8Mm-1, respectively. Theaverage hemispheric backscatter fraction was0.128±0.019,the average ngstr m exponentwas1.46±0.25,PM10mass scattering efficiency was3.42±1.00m2/g. Enhancement factors at85%were available from Jan to May,2013. Enhancement factors at85%RH for scatteringcoefficient f(85%), backscattering coefficient fb(85%) and hemispheric backscatter fractionf (85%) were1.56±0.09,1.24±0.07and0.80±0.04, respectively. The humidograms duringMarch were well fitted into empirical equation f(RH)=1+a(RH/100)b, and index η was definedbased on the fitting curve to quantitively evaluate the curvature of humidogram. The larger ηwas, the greater the curvature of humidograms would be. A relationship between η and themass percentage of in fine particles was found, namely the more, the smaller ηwas; once the mass percentage of exceeded18%, η was~1.1, humidogram was nearlystraight, i.e. with the increase of RH, the scattering coefficient increased with a nearly constantspeed. At85%RH, aerosol direct radiative forcing increased by46%compared with that in dryconditions, namely RH contributed32%to light scattering at85%RH.

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