Investigation Of Chloride Depletion For Sea Spray Aerosols Based On High Performance Single Particle Aerosol Mass Spectrometer

As one of the largest natural sources of aerosols,sea spray aerosol(SSA)particles can act as cloud condensation nuclei(CCN)or ice nuclei(IN)to participate in cloud formation,and then affect atmospheric radiative forcing and rainfall,which has a profound significance for climate change.The heterogeneous reactions involving SSA are essential to drive the circulation of nitrogen,sulfur and halogen species in the atmosphere.The displacement reaction of chloride has a significant impact on the physicochemical properties of SSA.In previous studies,many laboratory and field measurement results have improved our understanding of the physicochemical properties of SSA and the mechanism of chloride depletion.However,the mechanism of organic species and mixing state on the chloride depletion of SSA remains ambiguous.In particular,the interaction between ubiquitous organic acids and SSA is rarely reported.In this study,we employed a high-performance single particle aerosol mass spectrometer(HP-SPAMS)to investigate the chemical composition,particle size distribution and mixed state,and calculate the degree of chloride depletion,in the inland region(Nanling national background station,Guangdong)and a coastal region(Qingdao).In addition,the effect of cloud and fog process on the chloride depletion of SSA during the inland transport was also discussed by combining GCVI and HPSPAMS.Based on field observations,we investigated the physicochemical characteristics and aging degree of SSA in coastal areas and after long-range inland transport,providing an additional basis for improving the chlorine circulation model,and contributing to a more comprehensive understanding of the atmospheric evolution processes,environmental behaviors and fate of SSA.The major results are followed as below:(1)Physicochemical characteristics and degree of chloride depletion of SSA in an inland region: The air masses originated from the South China Sea arrived at the sampling site through the Pearl River Delta(PRD)region during the outbreak of the East Asian summer monsoon.According to the chemical composition,we have obtained 3 particle types of SSA-Aged,SSA-Bio and SSA-Ca.SSA-Aged was regarded as the aged Na-rich SSA particles.In comparison to the SSA-Aged,SSABio involved some extra organic species associated with biological origin(i.e.,organic nitrogen and phosphate).Each type occupies for approximately 50% of total detected SSA particles.Besides,SSA-Ca may relate to organic shell of Na-rich SSA particles,which is negligible(~3%).Tight correlation between Na and diverse organic acids was exhibited for the SSA-Aged(r2 = 0.52,p < 0.01)and SSA-Bio(r2 = 0.61,p < 0.01),reflecting a significant impact of organic acids to the chloride depletion.The chloride depletion occupied by organic acids is estimated to be up to 34%.A distinctly different degree of chloride depletion was observed between SSAAged(1.8%)and SSA-Bio(5.4%),while the hourly mean peak area of Cl/Na was used to evaluate the degree of chloride depletion.(2)Physicochemical characteristics and degree of chloride depletion of SSA in a coastal region: Due to the influence of Siberian high,Qingdao area was affected not only by marine air masses,but also by inland pollution air mass originated from the north sector.2 particle types of SSA were obtained under the potential interaction between these two air masses.SSA-aged,which is similar to the characteristics of long-range inland transport SSA-Aged,accounting for 62%.SSA-mixed not only comprised of the sodium-related characteristics,but also externally mixed with black carbon and organic carbon,accounting for 38%.The contributions of different kinds of acids to the chloride depletion of SSA in inland region or coastal region are as follows: nitric acids > organic acids > sulfuric acids.The contribution ratio of acids is approximately 85: 10: 5,respectively.For the coastal SSA,up to 28% of the chloride depletion could be attributed to the organic acids.The chloride depletion of SSA was evaluated by hourly mean relative peak area of Cl/Na.A distinctly different degree of chloride depletion was observed between SSA-aged(9.6%)and SSA-mixed(22.6%).(3)Effect of mixing state on chloride depletion of SSA: 1)A distinctly different degree of chloride depletion between the SSA-Aged and SSA-Bio during inland transport may be attributed to the organic coating,which inhibits the reactive activity of SSA chloride depletion.Organic coating majorly comprised of phosphate and organic nitrogen,which presented a function with hourly mean peak area of Cl/Na.The increased phosphate and organic nitrogen varied with decreasing chloride depletion.In comparison to SSA-Aged(0.22),a relative high number fraction of phosphate(0.60)was observed in SSA-Bio.2)In Qingdao,the chloride depletion in difference between SSA-aged and SSA-mixed may be attributed to the external mixing of SSA with black carbon and organic carbon.The increased black carbon and organic carbon represented the reduced chloride depletion.In comparison to SSA-aged,a higher number fractions of black carbon(0.70 vs 0.03,respectively)and organic carbon(0.85 vs 0.10,respectively)were observed in SSA-mixed.Black carbon and organic carbon exhibit strong hydrophobicity.Since SSA externally mixed with them,the reactivity of chloride displacement reactions could be inhibited by the weakened aqueous reactions,which is affected by the reduced hygrocopicities and moisture content of aerosols.(4)The effects of cloud process,trajectories of air masses,diurnal variations,particle size(0.2-2?m),relative acidity and relative humidity on SSA chloride depletion were also discussed in this study.1)Cloud process,air mass trajectories and diurnal variations had little impacts on the chloride depletion of SSA.2)During the longrange transport,a more severe degree of chloride depletion was observed in supermicrometer(> 1 ?m)relative to sub-micrometer(< 1 ?m)SSA.In coastal areas,particle size barely had little effect on the chloride depletion of SSA.3)The increase of relative acidity(Ra)and relative humidity(RH)could promote the chloride displacement reactions of SSA.