| The mixing state and hygroscopic properties of aerosol are the two factors that introduce great uncertainties in estimating aerosol direct climatic effect and environmental effect.In-situ ground based measurements of aerosol properties were carried out from June to August,2011 at the top(30.13°N,118.15°E,1840 m above sea level(asl))and bottom(30.06°N,118.170E,461 m asl)of Mt.Huang(the Yellow Mountain),and from September to October,2012 at the bottom(30.12°N,118.19°E,910 m asl)of Mt.Huang.Aerosol optical properties,mixing state,and the hygroscopic properties of optical properties were investigated as follows based on the measurements.1.The variation of aerosol optical properties at different altitudes.Aerosols at both the top and bottom of Mt.Huang were dominated by the non-light-absorbing component in 2011.Aerosol total scattering and absorption coefficients(?sp,neph,550 and(?ap,550)at the top were lower than that at the bottom with the averages(standard deviations)of 62.6(49.2)Mm-1,5.5(3.7)Mm-1,0.89(0.04),and 0.13(0.02)for?sp neph,550,?ap,550,?550(single scattering albedo),and ?neph,550(hemispheric backscattering fraction),respectively,at the top.The respective average values of ?sp,neph,550,?ap,550 and ?550 at the bottom(Zhaixi)were 107.7(75.2),14.4(5.9),and 0.83(0.12).rsp.neph.550 and ?ap,550 at both the two sites were lower compared to the other site in China.Aerosol at the top of Mt.Huang could represent the regional average state,however,aerosol at the bottom were mainly contributed from local and regional sources.In 2012,the average values(standard deviations)of ?sp.neph,550,?hbsp,neph,550(hemispheric backscattering coefficient),and ?ap.550 at the bottom(Yungu Resort)of Mt.Huang were 71.7(35.8)Mm-1,9.8(4.2)Mm-1,and 13.0(4.8)Mm-1,respectively.And the averages of aneph,450-700(Angstrom index),?550,and/?neph,550 were 2.17(0.22),0.84(0.05).and 0.14(0.02).The diurnal patterns of ?sp,neph550 and ?ap,550 at three sites were all closely related to the development and decay of PBL,and diurnal variation of the mountain and valley breezes.The photochemical reactions,which led to the enhancement of?sp,neph,550,and the aging process,which resulted in the enhancement of ?ap,550,contributed to the differences in the diurnal patterns of ?sp,neph,550 and ?ap,550.2.The optical closure study and aerosol mixing state based on the observation.An optical closure study with Mie model using the high size-segregated BC volume fraction averaged over the study period was enough to reconstruct aerosol total scattering and absorption coefficients,with correlation coefficients of above 0.98,conforming that the Mie model was appropriate to model aerosol optical properties at Mt.Huang,and the accuracy and reliability of the measured physicochemical properties of aerosol.Two scenarios of aerosol mixing state(r1 and r2),which considered the existence of the externally mixed non-light-absorbing component or not,were retrieved based on the optical closure study.r1 and r2 showed that about 70%and 50%of total BC were externally mixed with the non-light-absorbing components,respectively.The discrepancy of r1 and r2 was mainly attributed to the existence of the externally mixed non-light-absorbing components in the relatively larger particles.r1 and r2 changed periodically due to the difference of vertical transport and air sources under different synoptic systems and their diurnal variation were influenced by the development and decay of PBL and the mountain and valley breezes,as well as the emissions from around and regional areas.3.The changes of aerosol optical properties with relative humidity.The hygroscopic growth factors of aerosol diameter were used in Mie model to obtain the hygroscopic growth factors of aerosol optical properties with the assumptions of external,core-shell,and retrieved r1 and r2,based on k-Kohler theory.The results showed that the hygroscopic growth factors of the total scattering f(?sp,550),hemispheric backscatteringf(?hbsp,550),and absorption coefficients f(?ap,550)as well as single scattering albedo f(?550)at relative humidity(RH)of 80%were 1.69?1.75,1.29?1.39,1.00?1.07,and 1.07?1.12,respectively.And the respective values at RH of 90%were 2.70-2.90,1.71?1.94,1.00-1.08,and 1.10?1.21.f(?sp,550)and f(?hbsp,550)increased with increasing RH under different mixing state.However,f(?ap,550)have different trends because of the different ratio of "core" and "shell" in different mixing state.The changes of ?extinction,550(aerosol extinction coefficient)with RH and aerosol concentration calculated based on the parameterization of the hygroscopic growth factors of aerosol optical properties and RH showed that ?extinction,550 increased with increasing aerosol concentration and RH.?extinction,550 became more sensitive to RH with increasing aerosol concentration,which revealed that the increasing RH enhanced the aerosol extinction so that the visibility decreased obviously,when aerosol concentration got higher.This study has an insight into the inherent relationship between aerosol optical properties and physicochemical characteristics at a background site in eastern China,and provided a numerical basis to further investigate the specific mixing state and hygroscopic properties of aerosol,so that the climatic and environmental effects can be evaluate more accurately. |
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