| Polycyclic aromatic hydrocarbons (PAHs), an important fraction of semi-volatile organic compounds (SVOCs), are formed as byproducts of any incomplete combus-tion from traffic exhausts, industrial activities, domestic heating, forest fires and bio-mass burnings. PAHs are hydrophobic, stable, and sparingly soluble in water. The tropospheric removal of PAHs involves wet and dry deposition, and degradation through photolysis and reactions with various atmospheric oxidants such as OH, NOX and O3. The degradation of PAHs not only influences their atmospheric distribution, but also may lead to more toxic degradation productions such as nitro-PAHs.Chlorinated polycyclic aromatic hydrocarbons (ClPAHs) are PAH derivatives, which are ubiquitous contaminants found in urban air, snow, automotive exhaust, tap water, and sediments. Their main sources include municipal incineration, chlorine disinfection of water, and atmospheric reactions of PAHs. Due to the structural simi-larities with polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofu-rans (PCDFs) and polychlorinated biphenyls (PCBs), some of ClPAHs have been shown enhanced toxicities, mutagenicities and aryl hydrocarbon receptor activities compared with corresponding parent PAHs. In general, ClPAHs are primarily present in the gaseous phase in the atmosphere and only approximate 19% of total ClPAHs are associated with atmospheric particulates. On account of their widespread occur-rence and dioxin-like toxicities, the atmospheric fate of ClPAHs has become a serious environmental concern as well as a public health priority.Polychlorinated biphenyls (PCBs) are a group of persistent organic pollutants (POPs) which have been commercially produced and used as coolants, insulating flu-ids and stabilizing additives since 1929 in USA. The high toxicity and persistence of PCBs were confirmed in the late 1970s, and the commercial production of PCBs is now prohibited in many countries. However, PCBs can still emit into the atmosphere through vaporization and incineration of PCBs containing materials. Hence, PCBs can continuously contaminate the environment by long-range atmospheric transport, and the atmospheric fate of PCBs deserves more attention.1.Atmospheric Oxidation of PAHs(1) OH radical-initiated atmospheric oxidation of fluoranthene (Flu) and ben-zo[a]pyrene (BaP)The products of OH-initiated atmospheric oxidation of Flu contains fluoranthols, fluoranthones, fluoranthenequinones, dialdehydes and epoxides. The reaction of BaP with OH produces benzo[a]pyren-ols, benzo[a]pyrene-epoxides, nitro-BaPs, benzo[a]pyrene-7,10-dione, alkyl substituted benzanthraldehydes. Water va-pour plays an important role in the formation of nitro-benzo[a]pyrene. At 298 K and 1 atm, the calculated overall rate constant for the reations of OH with Flu and BaP are 1.72×10-11 cm3 molecule-1 s-1 and 2.29×10-10 cm3 molecule-1 s-1, respectively. The atmospheric lifetime of Flu by OH radicals is about 0.69 days.(2) O3 initiated atmospheric oxidation of acenaphthylene (Ary)The reaction of the unsaturated cyclo-pentafused ring with O3 is the dominant pathway leading to the formation of the primary ozonides. The secondary ozonide formed from isomerization of the thermalized Criegee intermediate is unstable and can decompose to yield ring-opening products. The products formed from the gas-phase ozonolysis reaction of Ary include secondary ozonide, naphtha-lene-1,8-dicarbaldehyde,1,8-naphthalic anhydride, oxaacenaphthylen-2-one, 1-naphthaldehyde,2-hydroxy-l-naphthaldehyde, and a-hydroxyhydroperoxide etc. The calculated overall rate constant matches well with the available experimental value. The atmospheric lifetime of Ary determined by O3 is about 0.75 h.(3) Cl atom-initiated atmospheric oxidation of anthracene (Ant) and pyrene (Pyr)All of the Cl addition pathways are highly exothermic with no potential barriers, and they can readily occur under general atmospheric conditions. The products of the Cl atom-initiated atmospheric oxidations of Ant and Pyr include monochloro-Ants, monochloro-Pyrs, dichloro-Ants, dichloro-Pyrs,2-chloroanthracen-l-one, 1-chloro-2-hydroperoxyanthracene, epoxides, dialdehydes,9-nitroanthracene, 1-nitroanthracene and nitropyrenes, etc. Particularly, water plays a crucial role in the gas-phase formation of 9-nitroanthracene, which can be easier formed by the gas-phase reaction of Ant with Cl atoms than that of Ant with OH radicals. The cal-culated overall rate constants of Ant and Pyr (ka and kp) are 5.87×10-12 and 2.81×10-12 cm3 molecule-1 s-1 at 298 K and 1 atm, respectively. In the marine boundary layer, the atmospheric lifetimes of Ant and Pyr determined by Cl atoms are calculated to be 4.93 and 7.27 days, respectively.(4) Relationship of PAHs with Oxy(Quinone) and Nitro detrvatives during air mass transportThe concentrations of PAH, quinones and nitro derivatives have been measured at three sites along the coast of Saudi Arabia to the north of the city of Jeddah. It shows that PAH reduce in concentrations from northwest to southeast, due to the emission from a petrochemical works. The concentrations of quinones had no much variation between the sampling sites indicating its secondary pollutants nature formed from PAH oxidation. The nitro-PAH reveal a concentration gradient which is similar to but smaller than that for the PAH, a balance was formed between atmospheric for-mation and removal by photolysis. The 2-nitrofluoranthene:1-nitropyrene ratio in-creases from north to south, and it is consistent with atmospheric oxidation formation, while the ratio of 2-nitrofluoranthene:2-nitropyrene is consistent with hydroxyl radi-cal. During air mass transport along the Red Sea coast, only for the day with the highest concentrations, the changes in PAH congener ratios agree with reaction with a relatively low concentration of hydroxyl radical. It is concluded that chemical reaction, emissions from other sources along the air mass trajectory are also probably leading to changes in PAH ratios.2.Atmospheric Oxidation of CIPAHs(1) OH radical-initiated atmospheric oxidation of 9,10-dichlorophenanthrene (9,10-Cl2Phe)The products for OH-initiated atmospheric oxidation of 9,10-Cl2Phe include chlorophenanthrols,9,10-dichlorophenanthrene-3,4-dione, dialdehydes, chlorophe-nanthrenequinones, nitro-9,10-Cl2Phe and epoxides etc. The overall rate constant of the OH addition to 9,10-Cl2Phe is 2.35×10-12 cm3 molecule-1 s-1 at 298 K and 1 atm. The atmospheric lifetime of 9,10-Cl2Phe by OH radicals is about 5.05 days.(2) NO3 radical-initiated atmospheric oxidation of 9-chloroanthracene (9-ClAnt)Compared with H abstractions by NO3 radicals, the NO3 additions are the ener-getically more favorable reaction pathways for the reaction of 9-ClAnt with NO3 rad-icals. The NO3-initiated atmospheric oxidation of 9-ClAnt generates a class of anth-racene derivatives containing 9-chloroanthracen-yl nitrates, 9-chloroanthracenesdiones, epoxides, dialdehydes,9-chloroanthracene-1,4-dione, anthracene-9,10-dione,9-chloroanthracen-1-one,10-chloroanthracen-1-one, 10-chloroanthracen-9-ol and 10-chloro-l-nitroanthracene, etc. The overall rate con-stant for the NO3 additions to 9-ClAnt is 9.11×10-13 cm3 molecule-1 s-1 at 298 K. The atmospheric lifetime of 9-ClAnt determined by NO3 radicals is about 0.61 hr.3. Atmospheric Oxidation of Other Organic Pollutants(1) OH radical-initiated atmospheric oxidation of PCB126Compared to the H abstractions and Cl abstractions by OH radical, the reactions of OH addition to PCB126 are the dominant processes. The OH addition to the carbon atoms C2, C6, C8, C11 and C12 are easier due to the lower potential barriers. The OH-initiated atmospheric oxidation of PCB126 generates a class of ring-retaining and ring-opening products containing 3,3’,4,4’,5-pentachlorobiphenyl-ol congeners, chlo-rophenols,2,3,4,7,8-pentachlorodibenzofuran,2,3,4,6,7-pentachlorodibenzofuran, di-aldehydes,3,3’,4,4’,5-pentachlorobiphenyl-2-one, dichlorobenzene radicals, etc. The formation of polychlorinated dibenzofurans (PCDFs) from the atmospheric oxidation of PCBs is determined for the first time. The overall rate constant of the OH addition to PCB126 is calculated to be 2.52×10-13cm3 molecule-1 s-1 at 298 K and 1 atm. The atmospheric lifetime of PCB126 determined by OH radical is about 47.08 days.(2) OH radical-initiated atmospheric oxidation of phorateOH addition to the P=S bond in phorate is strongly exothermic process without a barrier and can occur readily under the general atmospheric conditions. The further reactions of OH-phorate adducts (IM1, IM2 and IM3) with O2 are favored over their unimolecular decomposition processes. Phorate oxon and SOOH are the dominant products. For H abstractions from phorate by OH radicals, H atoms in the-CH2-por-tion of the-CH2CH3 group are more activated than H atoms in the-CH3 portion. Typically two possible reaction routes were identified for the intermediates generated from H abstraction, including the unimolecule decomposition and the oxidation of the intermediates.(3) OH radical-initiated atmospheric oxidation of N-methyl perfluorobutane sul-fonamidoethanol(C4F9SO2N(CH3)CH2CH2OH)A detailed reaction mechanism is considered, including the H abstraction, OH radical addition and subsequent reactions. For the OH radical addition pathways, OH addition to the S=O bond in NMeFBSE is strongly exothermic process. Cleavage of the S-C bond is favored over that of the S-N bond. C4F9 and HSO3N(CH3)CH2CH2OH are the dominant products. Subsequent reactions of the re-sulting radicals from OH addition and H abstraction yield some important products,* such as PFCAs (C3F7COOH, C2F5COOH, CF3COOH), NMeFBSA (C4F9SO2NH(CH3)), C4F9SO2N(CH3)CH2CHO and C4F9SO2N(CH3)CH2COOH, which may contribute to the burden of perfluorinated contamination in remote loca-tions. Using the atmospheric fate of NMeFBSE as a guide, it seems that N-methyl Perfluorooctane Sulfonamidoethanol (NMeFOSE) contributes to the ubiquity of per-4 fluoroalkyl sulfonate and carboxylate compounds in the environment. |
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