Aqueous Phase Reaction Processes And Influence Mechanisms Of Heavy Metals In The Atmosphere During Urban Haze

With the large-scale urbanization and rapid economic development,major cities in China,especially those in the north and middle east,have been suffering from severe haze pollution hazards in recent decades.Fine particulate matter(PM_2.5)is the primary factor of urban haze pollution,and the high atmospheric water content promotes the secondary aerosol reaction and leads to the explosive generation of atmospheric particulate matter.During this process,heavy metals carried by PM_2.5 can enter the aqueous phase through water-particle partitioning,increasing the health hazards of heavy metals during the haze period.Previous studies on atmospheric heavy metals mainly focused on the particulate and gaseous phases,but less paid attention to the aqueous phase.Therefore,it has important theoretical and practical significance to study the aqueous phase reaction and its impact mechanism of atmospheric heavy metals during the haze period.Funded by the National Natural Science Foundation of China（NSFC）（41877476）,we collected atmospheric particle phase and aqueous phase samples from Jinan and Shanghai city in winter and spring haze period,and analyzed the concentration characteristics,water-particle partitioning,aqueous phase reactions,occurrence forms,and source analysis of heavy metals in PM_2.5 and atmospheric water.In addition,the reaction processes and influence mechanism of Hg in atmospheric gaseous phase,particulate phase and aqueous phase was systematically studied.The details conclusions are as follows:（1）The variation characteristics of the same heavy metals were relatively consistent between PM_2.5 and atmospheric water in urban atmosphere in winter and spring.The concentrations of particle-bound heavy metals（PHMs）and dissolved heavy metals in atmospheric water（DHMs）during haze period were significantly higher than those during non-haze period in Shanghai and Jinan.The concentrations of heavy metals in PM_2.5 and atmospheric water in Jinan were significantly higher than those in Shanghai.Comparied with previous studies,it was found that the pollution levels of heavy metals in PM_2.5 in Jinan and Shanghai have been decreased significantly in the last eight years.With the aggravation of haze pollution,the concentrations of Hg in all of the gasous phase,aqueous phase and particulate phase of the atmosphere in Shanghai increased 2 times of those in non-haze period,approximately.（2）During the cleaning period in Shanghai,PM_2.5 exhibited competitive adsorption and uptake of Hg in the gasous and aqueous phases.However,with the increase of pollution level and atmospheric water concentration,the deliquescence of aerosol particles in high relative humidity（RH）environment triggered the transformation process from PHg to DHg.In addition,atmospheric water absorbs highly water-soluble reactive gaseous mercury（RGM）and other species.At the same time,the high RH environment formed a reducing environment,which led Hg²⁺in PM_2.5 and atmospheric water be reduced to Hg⁰,and then released in the form of GHg.This led to the increase of DHg and GHg during the haze period in Shanghai.（3）The water-particle partitioning coefficients(K_p-wp)of heavy metals showed a significant decrease with the increase of haze pollution.The main reasons for the significant decrease of K_p-wp in the atmosphere of Jinan and Shanghai included the particle dilution effect triggered by the rapid growth of PM_2.5,the release of heavy metals in PM_2.5 due to high temperature,the deliquescence of particulate matter under high RH conditions,the absorption of heavy metals in the gasous and particulate phases by atmospheric water,and the nature of some highly water-soluble heavy metals.It is also an important reason for the increase of heavy metal concentrations in the aqueous phase of the atmosphere during the winter haze period in northern and southern cities of China.Water-soluble inorganic ions,such as SO₄^2-and NO₃^-formed complexes with heavy metals in PM_2.5,and Ca²⁺formed insoluble particles through co-precipitation with Cd and Pb to influence the presence of heavy metals in PM_2.5.（4）X-ray absorption near edge structure（XANES）analysis results showed that the components of As（III）and As（V）in PM_2.5 during the haze period changed significantly,and the high-intensity emission of As（V）species caused by coal-burning heating and other activities during the haze period led to a significant decrease in the percentage of As（III）in PM_2.5 in Jinan with the increase of pollution.The proportion of As（III）in PM_2.5 in Shanghai gradually increased with the increase of haze pollution.Large amount of As（III）emissions and reduction reactions triggered by high RH environment during the haze period was the main reasons for the increase of As（III）proportion.Pb in PM_2.5 in Jinan and Shanghai mainly existed as 2Pb CO₃·Pb（OH）₂,Pb（NO₃）₂,Pb C₂O₄ and Pb SO₄ compounds,and the concentrations of Pb（NO₃）₂ in PM_2.5increased significantly with the aggravation of haze pollution,indicating that secondary aerosol reactions profoundly influenced the existing forms of heavy metals in the atmosphere.Sulphur in PM_2.5 in Jinan and Shanghai existed in the forms of both Ca SO₄·2H₂O and（NH₄）₂SO₄,and（NH₄）₂SO₄ accumulated rapidly with the aggravation of haze pollution,indicating a stronger secondary aerosol reaction in the urban atmosphere during the haze period.（5）The results of HYSPLIT model and PSCF and CWT analysis showed that Jinan and Shanghai were mainly influenced by air masses from polar continent in winter,and received mainly short-and medium-scale air mass inputs during the haze period.Jiangsu Province was the main potential contributing source area for PHg and GHg in Shanghai.In addition,the release of marine Hg⁰ might also be a major source of atmospheric GHg in Shanghai.The exogenous input of DHg was not significant,mainly produced through the conversion of Hg in the particulate and gaseous phases.Northern Anhui,northeastern Henan,southern Hebei and western Shandong were the main potential source areas for As,Sb,Cd and Pb in PM_2.5 and atmospheric water in Jinan.Each source area contributed significantly to atmospheric heavy metal pollution in Jinan.Jiangsu,eastern Anhui and northern Zhejiang were the main potential source areas for heavy metals in atmospheric particulate and aqueous phases in Shanghai.In addition,heavy metal emissions from marine shipping activities were also an important contributor to atmospheric heavy metal pollution in Shanghai.The wet and dry deposition also had an effect on the exogenous input and accumulation of heavy metals in the atmosphere of Jinan and Shanghai.（6）The rapid increase of GHg concentration in Shanghai atmosphere mainly originated from coal combustion and vehicle emissions,while biomass combustion was also an important source of GHg and PHg in Shanghai.Coal combustion,biomass combustion,industrial production,transportation and natural source emissions were considered to be the main sources of inorganic ion and other heavy metals in PM_2.5 and atmospheric water in Jinan and Shanghai.The contribution of coal combustion to atmospheric PM_2.5 in Jinan and Shanghai was 26.1%and 21.3%,respectively.Coal combustion activities were the primary source of inorganic ions and heavy metals in atmospheric water in Jinan,while biomass combustion was the most significant source among all sources in Shanghai.As,Sb,Cd and Pb emitted from coal combustion and industrial activities were the direct sources of atmospheric heavy metal pollution in two cities.In addition,the deliquescence of aerosol particles and the absorption of heavy metals in other phases by atmospheric water were the important cause of heavy metal accumulation in atmospheric water.