Formation Mechanism And Source Of Water-Soluble Compositions Of Aerosol And Cloud Water In Southern China

The term“aerosol-cloud interactions”describes both the effects of aerosol particles on clouds and the cloud effects on aerosol particles.Aerosol particles affect clouds by acting as condensation nuclei and impacting their properties,such as chemical composition,size distribution,and lifetime.At the same time,cloud processes can also promote the formation of aerosol chemical composition.The water-soluble compositions of clouds and aerosols are mainly composed of sulfate(SO₄^2-),nitrate?NO₃^-?and water-soluble organic carbon?WSOC?.These components have a significant impact on human health,visibility and global climate.Over the past decades,the research of cloud-aerosol interaction has increased dramatically.Although the understanding of cloud-aerosol interaction has been improved,it still contributes to the largest uncertainties in the estimation and interpretation of the Earth's changing energy budget,according to the fifth IPCC report.The physical and chemical processes,the formation mechanisms,and source analysis of water-soluble components in clouds and aerosols based on the field observation are discussed,which will be helpful to understand the formation mechanisms of aerosols and the aerosol-cloud interaction.At present,the knowledge of cloud chemistry and aerosol-cloud interaction in South China is limited.In this study,we collected size-segregated?<0.49,0.49-0.95,0.95-1.5,1.5-3.0,3.0-7.2,7.2-10?m?aerosol samples from March 2013 to April 2014in Guangzhou and cloud water for one month?May to June,2018?at Mountain Nanling site.We discussed the characteristics,formation mechanisms and source of water-soluble compositions in size-segregated aerosol and cloud water,and we discussed the physical and chemical characteristics of cloud processes.The main conclusions are as follows:1)Characteristics and formation mechanisms of sulfate and nitrate in size-segregated atmospheric particles.SO₄^2-and NO₃^-accounted for more than 60%of the total water-soluble inorganic compounds in PM₃.The polluted days favored the formation of SO₄^2-in summer and autumn,and the formation of NO₃^-in winter and spring.Due to the rainfall,sulfur oxidation ratios?SOR?and relative humidity?RH?showed a negative correlation in rainy season.However,in dry season,SOR and RH showed a positive correlation.It suggests that liquid phase oxidation plays an important role in the formation of sulfate.The correlation of size-segregated atmospheric particles showed that sulfate was mainly formed by gas phase photochemical oxidation in<0.49um.Nitrates are mainly formed by heterogeneous reactions in the 1.5-3.0 um,which was in forms of Ca?NO₃?₂ and Mg?NO₃?₂ in this size bin.In winter,nitrate may be mainly formed by homogeneous reaction in the<0.49 um.2)Characteristics and chemical compositions of cloud,rain and PM_2.5 at Mountain Nanling site.The average pH value of cloud water samples was 4.4.The acidity of cloud water samples was lower than the previous studies,which suggests that the problem of acid rain in South China is effectively controlled.Sulfate,nitrate and ammonium were the most abundant total water-soluble ion?TWSI?components in cloud water,accounting for 89%of the TWSI mass concentration.Their values were244.9?eq L^-1,150.08?eq L^-1 and 327.4?eq L^-1,respectively.The main heavy metals in cloud water were Zn,Al and Fe,accounting for 78.2%of the heavy metal concentration,which were 76.24?eq L^-1,58.44?eq L^-1 and 47.77?eq L^-1,respectively.The average pH of rainwater was 5.5.The concentrations of sulfate,nitrate,ammonium and calcium were 8.5 mg L^-1,5.9 mg L^-1,6.3 mg L^-1 and 18.5 mg L^-1,respectively.The concentration of PM_2.5 in Mountain Nanling site was 10-30?g m^-3,which indicated that atmospheric particulate pollution was limited here.The concentrations of sulfate,nitrate and ammonium in PM_2.5 were 2.72?g m^-3,1.18?g m^-3 and 0.69?g m^-3,respectively.3)Physical and chemical processing of cloud.The average mass scavenging ratio?SRm?of aerosol for each cloud event was 54.48%,which was similar to SRm?52.0%?in Mount Taishan.SRm increased rapidly with the increasing of LWC,LWC<0.06 g m^-3.SRm increased slowly with the increasing of LWC,0.06 g m^-3<LWC<0.08 g m^-3.SRm was difficult to change with the increasing of LWC,maintaining in 65%,0.08g m^-3<LWC.The whole cloud processes was divided into formation stage,maintenance stage and dissipation stage.The concentrations of heavy metals,sulfate and ammonium in formation stage were higher than dissipation stage,least at maintenance stage.However,the concentrations of nitrate and WSOC were higher in maintenance stage than formation stage.NO₂ concentration in local emission was about5 times of SO₂ concentration,and it indicated that heterogeneous reactions of NO₂account for high part of nitrate in Mountain Nanling site.The proportion and concentration of nitrate in cloud water increased significantly,comparing clean cloud water samples to polluted cloud water samples.4)The secondary formation of WSOC was more significant factor,in which 68.5%of WSOC was associated with secondary formation.WSOC was associated with role of transition metals with water soluble organic carbon in the secondary formation.Biomass burning contributed to 19.4%of WSOC.In the first cloud event,WSOC in the formation and dissipation stage was mainly affected by biomass combustion.In the mid-term of field observation,transition metals from metal smelting and crustal dust contributed to the formation of WSOC.At the later stage of field observation,WSOC mainly originated from secondary formation.