Distribution, Source And Environmental Significance Of Iodine And Bromine Species In Atmospheric Aerosol

Aerosol plays an important role in global environment. Its climate effect and environment significance has attracted more and more attentions. Iodine and bromine in the atmosphere have direct influences on many kinds of photochemical reactions in troposphere, which lead to ozone depletion. Besides, iodine in the atmosphere also accelerates the generation of Cloud Condensation Nuclei and aerosol particles. The research on their source and the mechanism of atmospheric bromine/iodine chemistry is becoming a hotspot. Current researches mainly focus on the model calculation of atmospheric chemistry, but the extraction and measurement of bromine and iodine inside aerosol is still not mature which makes it hard to achieve the field measurements. There are also major divergences between the reported field measurements and calculation results.Based on the research of Libing et al., extraction and determination of bromine and iodine species in aerosols was systematically compared and discussed:(1) An IC-ICPMS approach was developed to analyze IO₃^-, I^-, BrO₃^- and Br^- in atmospheric aerosol. Ion exchange column ICS-A23 was employed to separate IO₃^-, I^-, BrO₃^- and Br^- with (NH4)2CO3 as the mobile phase. The method sensitivity was improved using Shield Torch technique of Agilent 7500a ICPMS and Model injection valve with inject volume 1.0 ml.(2) Experiment shows that bromine and iodine species standard solutions is stable in the 0.01% KOH medium, which provides technical support for sampling and holding of bromine and iodine species in aerosol. As for the pretreatment of bromine and iodine species in aerosol, the efficiency of the extraction and the stability of bromine and iodine was compared under different extraction conditions, which shows that the optimum condition is ultra pure water ultra sonic-assisted extraction for 5 min. The recovery experiment on CF and GF blank under different extraction conditions proves GF has less interference with the stability of bromine and iodine species than CF. Based on the extraction results IC-ICPMS approach was developed to analyze total bromine/iodine, total soluble bromine/iodine, in-soluble bromine/iodine, bromate, bromide, iodate and iodide in aerosol collected during the 2^nd and 3^rd China Arctic expedition, offshore areas of China and Hefei. Combined with air mass back trajectories, meteorological/earthquake data, iodine species data that are collected during the 22^nd Antarctic expedition and bromine/iodine species data of other areas referenced in previous researches, discussion is carried on the distribution/source/environmental significance and concludes:(1) In spite of the differences of bromine concentration in aerosol among different areas and different seasons, the speciation distribution is quite stable. Bromide is the dominant species in aerosol from Arctic Ocean, offshore of northwest pacific and China, and urban areas of Hefei. The concentration tendencies of total bromine, total soluble bromine and bromide indicates that it's lower in polar areas than mid-latitude and tropical areas, and different source is the main contributing factor; the relative contribution of in-soluble bromine to total bromine was 10%³1% with a higher concentration in mid-latitude and tropical areas than polar areas; the relative contribution of soluble organic bromine to total bromine was 2%²3% with a higher concentration in urban areas than marine area, mid-latitude and tropical areas than polar areas; bromate was undetectable in any samples which may be caused by the lack of conditions to generate bromate aerosol with significant concentration; for samples from urban areas of Hefei, bromide enriches in coarse (diameter > 10μm) aerosol fractions particles while insoluble bromine enriches in PM2.5 particles.(2) Distributions of iodine species differ among aerosol from different areas, and there're even significant differences between different years within the same area. On the whole iodide and insoluble iodine have the widest global distribution. In polar areas, northwest pacific, offshore of China, marine area of Southeast Asia, the East Indian Ocean, the Ocean between Australia and Antarctic, Mace Head Ireland and urban areas of Hefei, the dominant iodine species is soluble iodine, EF reduces with a lower latitude mainly because of the differentiate of source. The significant distinguishes among IO₃^- / I^- samples form different areas verifies the model in atmospheric iodine chemistry, while meantime it uncovers some deficiencies. Insoluble iodine and soluble organic iodine are the most global widespread iodine species with 24%⁸0% relativity to total iodine, but the molecular structure is still unknown, the mechanism of which is not widely studied yet. For samples from urban areas of Hefei, insoluble iodine enriches in PM2.5 particles.(3) In the Arctic Ocean, offshore of northwest pacific and China, marine area of Southeast Asia, the East Indian Ocean, and marine boundary layer of the ocean between Australia and Antarctic, bromine concentration is dominated by the strength of sea-salt aerosol, while iodine concentration has more connections with the melting of sea ice in polar region and the gas phase oxidation of marine biologically produced volatile iodine species. The concentration of bromine/iodine inside coarse(diameter > 2.5μm) particles from aerosol of urban areas of Hefei are affected by coal burning, while the origin of concentration for fine particles is still not clear; the insoluble bromine possibly originates from the activity of human beings and crustal movement; soluble organic bromine has a possible connection with the coal burning.(4) It shows seasonal variations in iodine and its dominant species inside TSP samples of the Arctic Ocean and different size fractions aerosol from urban areas of Hefei. The tendency between the two different areas diverges in that terrestrial area has a bigger concentration during autumn/winter than spring/summer, while the peak concentration possibly arrives in spring in the Arctic Ocean.(5) Typhoon and crustal movement have a tight connection with the concentration and species distribution of bromine/iodine. Bromine/iodine inside aerosol may get a significant depletion during long-term sampling. Long-term samples from different areas were adopted and analyzed to achieve bromine/iodine species from bulk firsthand materials, among which the data from the Arctic region is an initial opening around the world. All these field measurements build a solid groundwork for further researches on atmospheric chemistry of halogen.