Observation And Model Study Of Relative Humidity Effects On Aerosol Light Scattering At A Regional Backgound Site In The Yangtze Delta Region

Accurate description of aerosol hygroscopic property plays a vital role in estimating and modeling aerosol direct radiative forcing and visibility degradation.A long-term field campaign has been carried out at LinAn reginal background station from August 2014 to July 2015.During the observation,the mean and standard deviation of scattering coefficient ?sp,backscattering coefficient ?bsp and obsorption coefficient ?ap at 550 nm was 197±147 Mm^-1?26±17 Mm^-1 and 20±15 Mm^-1,respectively.Single scattering albedo,backscatter fraction and ?ngstr?m exponent was 0.89±0.04,0.14±0.02 and 1.63±0.28,respectively.Scattering enhancement factor f?80%?,backscattering enhancement factor fb?80%?and backscatter fraction enhancement factor fb?80%?was 1.66±0.146,1.30±0.082 and 0.81±0.036,respectively.Upscatter fraction ?? decreased from 0.271±0.016 in the dry condition to 0.236±0.016 under 80% relative humidity?RH?.Particles in spring and winter are more hygroscopic than in summer and autumn.The humidograms at Lin'an can be well fitted into empirical equation f?RH?= 1+aRHb and the results were given under various circumastances.Tested the feasibility of using three methods calculating hygroscopic growth factor?a??-K?hler equation from Petters and Kreidenweis?2007?,?b?empirical equation from Tang?1996?and?c?ISSOROPIA II to model f?RH?,which provides useful information for climate modeling,radiative forcing and analyzing remote sensing data.Sensitivity study shows that hygroscopic growth factor determined by chemical composition was the largest influencing factor of f?RH?,then is the mode diameter and sigma of particle number size distribution,both the real part and imaginary part of refractive index under dry conditions shows the least influence.Haze easily occurred to Lin'an when RH>60%,once the ?sp was larger than 100 Mm^-1,there was a large probability that visibility <10 km.Heavy haze easily occurred to Lin'an when RH>80%,once the ?sp was larger than 100 Mm^-1,there was a large probability that visibility <5 km.The influence of aerosol water uptake to visivility and radiative forcing were estimated.The annual RH was ⁷8.9% and the contribution of aerosol water uptake to aerosol extinction coefficients was ⁴1%.The ratio of radiative forcing at 80% RH to that under dry conditions was 1.63.The monthly ratio ranges from 1.3 to 1.9,and the contribution of aerosol water uptake to aerosol radiative forcing was 23-47%.