Characteristics Of Aerosol Hygroscocpicity And Its Influence On Aerosol Optical Properties In The Pearl River Delta,China

This paper described in detail the research and developement of aerosol hygroscopicity measurement equipments.The characteristics of aerosol hygroscopcity,and the relationship among particle number size distribution,water uptake,and chemical composition were analyzed in the Pearl River Delta region(PRD).The influence of aerosol hygroscopicity,relative humidity,and aerosol mixing state on aerosol optical properties was also investigated.The main conclusions in this study are as follows:A multifunctional Hygroscopicity-Tandem Differential Mobility Analyzer(H-TDMA)and a Parallel Nephelometers(PNEPs)were designed and custom-built.Owing to some technologies such as automatic feedback control,temperature and relative humidity stabilization,and sheath flow closed loop applied in the H-TDMA system,RH stability as well as accuracy of particle size and growth factor measurement reaches advanced level of similar products.The H-TDMA system is capable of measuring aerosol number size distribution(PNSD),particle hygroscopic growth factor(HGF),volatile shrinkage factor(VSF)simultaneously,providing an important means for monitoring of aerosol hygroscopicity,volatility and aerosol mixing state.The difference of mean hygroscopic growth factor(gmean)for 40-200nm particle was not obvious between winter and summer.gmean increased with particle size,while the number fraction of non(less)hygroscopic mode particles(NFNH,LH)decreased with the particle size.During the daytime(evening and early morning),particles tend to be quasi-internal(external)mixing.Daily variation of gmean is influenced by primary and secondary emission and evolution of mixing layer height.NFLH was correlated to the atmospheric oxidation which can be presented by OC:EC.Such conditions may lead to more formation of less hygroscopic particles in winter,resulting in higher NFLH.The gmean and mixing state of aerosol particles can also be affected by air mass origin.The ZSR mixing rule combined with chemical composition data from AMS can make accurate predictions of gmean(R2=0.41).However,the assumptions of hygroscopic parameter for organics as well as the proportion of water soluble organics will affect the result of Gf closure.The PNSD showed multi peak distribution in the PRD,while the Aitken mode and accumulation mode particles dominated.The accumulation mode particles with peak size around 300nm accounted for 70%of total volume concentration.The total number concentration in the PRD was significantly lower than that observed in other mega-cities in china.The diurnal variation of Aitken mode particles is related to traffic emissions and the growth of nucleation mode particles,while the diurnal variation of nucleation mode particles mainly affected by new particle formation(NPF)events.Low relative humidity,high wind speed,strong UV radiation and low condensation sink are favorable for the occurrence of NPF events.Intense photochemical and biological activities may be the source of condensable vapor for particle growth,even during winter in the PRD.The effects of aerosol hygroscopic growth on the aerosol liquid water content(ALWC)and visibility were studied.The results are as follows,the ALWC calculated by Gf and PNSD were consistent with those modelled by ISORROPIA based on chemical composition measurement.The possible cause of the deviation is that the organic matter is not considered in the model.The influence of RH,PNSD and aerosol hygroscopicity on ALWC decreased in turn,and the high PM and high RH were sufficient conditions for high ALWC.The accumulation mode particles contributed most ALWC.Adverse weather conditions are the causes of haze weather in the PRD.When RH is low,visibility degradation is caused by the accumulation of PM2.5.When RH is high,low visibility is mainly affected by aerosol hygroscopic growth effect.The best-fitted parameterization of f(RH),which was established by PNEPs,was applied to visibility closure study.The calculated visibility was consistent with measured ones without precipitation.ALWC can reach up to several times of PM mass concentration in high relative humidity environment.To investigate the BC mixing state,two optical closures studies under dry and wet conditions were carried out respectively.The results are as follows,under dry condition,whether or not the completely volatile particles exist alone,has little effect on the efficiency and the contribution of light(except for light absorption).BC was mostly in the combination of partly external mixed and partly "core shell" mixed.The diurnal variation of BC mixing state was related to the emission,atmospheric aging process and the evolution of mixing layer height.Under wet condition,the calculated f(80)sp were in good agreement with the measured values,and BC was also in the combination of partly external mixed and partly "core shell”mixed.The f(RH)of light scattering,extinction and backscattering with three above mixing assumptions increased with the increasing RH.However,f(RH)absp with core-shell mixing assumption decreased when RH was larger than 75%,due to the decrease of the light entering into the BC core.