Atmospheric Fate Considerations

Volatility (as measured by vapor pressure) affects environmental fate in two ways: by controlling the rate of partitioning between the vapor and the particle phases, and by controlling, together with water solubility (expressed as Henry's law constant), the rate of partitioning between the vapor phase in the atmosphere and the dissolved phase in water. Dioxins have a wide range of volatilities according to their degree of chlorination. In general, the higher-chlorinated dioxin congeners are less volatile than the lower-chlorinated congeners. PCDDs and PCDFs are included among compounds called semivolatile organic compounds (SVOCs); these substances have vapor pressures approximately between 10~4 and 10~n atm (101-10-6 Pa) at ambient temperatures.

There are relatively few studies focusing on the vapor/particle partitioning of PCDD and PCDFs. In general, the hepta- and octachlorinated congeners are thought to be almost exclusively associated with atmospheric aerosols under ambient conditions. A measurable proportion of the tetra- and pentachlorinated congeners are present in the vapor phase. Partitioning between the

Congeners

Dioxins

2,3,7,8-tetraCDD

1,2,3,7,8-pentaCDD

1.2.3.7.8.9-hexaCDD 1,2,3,4,6,7,8-heptaCDD OctaCDD

General structure

Furans

2,3,7,8-tetraCDF 1,2,3,7,8-pentaCDF 2,3,4,7,8-pentaCDF 1,2,3,4,7,8-hexaCDF

1.2.3.4.7.8.9-heptaCDF OctaCDD

PCB Congeners and IUPAC No.

Non-ortho-PCBs

3,3',4,4'-tetrachlorobiphenyl PCB#77 3,4,4',5-tetrachlorobiphenyl PCB#81 3,3',4,4',5-pentachlorobiphenyl PCB#126 3,3',4,4',5,5'-hexachlorobiphenyl PCB#169

Mono-ortho-PCBs

2,3,3',4,4'-pentachlorobiphenyl PCB#105 2,3,3,4,4',5-pentachlorobiphenyl PCB#114 2,3',4,4',5-pentachlorobiphenyl PCB#118 2',3,4,4',5-pentachlorobiphenyl PCB#123 2,3,3',4,4',5-hexachlorobiphenyl PCB#156 2,3,3',4,4',5'-hexachlorobiphenyl PCB#157 2,3,4,4,5,5,-hexachlorobiphenyl PCB#167 2,3,3',4,4',5,5'-heptachlorobiphenyl PCB#189

General structure

Figure 1 PCDDs (dioxins), PCDFs (furans), and polychlorinated biphenyls (PCBs) with 'dioxin-like' toxicity.

Table 1 Physical and chemical properties of dioxins and dioxin-like compounds

Property

PCDDa

PCDFa

PCBb

Octanol/water partition Coefficient (log Kow) Water solubility (mg/L) Vapor pressure (mmHg) Henry's law constant (atmm3mol-1) BAF or BCF

1.6 x 10-5 to 1.0 x 10-4 130000

9.21 x 10-7 1.48 x 10-5 61 000

7.7 x 10-5 to 4.1 x 10-3 5.2 x 10-4 to 2.0 x 10-3 60 000-270 000c

aUSEPA (2002) Dose-Response Assessment from Published Research of the Toxicity of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Related Compounds to Aquatic Wildlife - Laboratory Studies. Cincinnati, OH: National Center for Environmental Assessment, Office of Research and Development (EPA/600/R-02/095).

bUSEPA (2003) Non-dioxin-like PCBs: Effects and consideration in ecological risk assessment. Experimental Toxicology Division National Health and Environmental Effects Research Laboratory office of Research and Development. (NCEA-C-1340. ERASC-003. June). For selected Aroclors; water solubility, vapor pressure, and Henry's law measured at 25 °C. cPCB BCF in fish for selected Aroclors.

aUSEPA (2002) Dose-Response Assessment from Published Research of the Toxicity of 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Related Compounds to Aquatic Wildlife - Laboratory Studies. Cincinnati, OH: National Center for Environmental Assessment, Office of Research and Development (EPA/600/R-02/095).

bUSEPA (2003) Non-dioxin-like PCBs: Effects and consideration in ecological risk assessment. Experimental Toxicology Division National Health and Environmental Effects Research Laboratory office of Research and Development. (NCEA-C-1340. ERASC-003. June). For selected Aroclors; water solubility, vapor pressure, and Henry's law measured at 25 °C. cPCB BCF in fish for selected Aroclors.

vapor and particulate phases is related to temperature and atmospheric particle concentration. During the summer, when temperatures are higher, most of the less-chlorinated congeners tend to be in the vapor phase. In the winter, the less-chlorinated congeners tend to be split between the particulate and vapor phases. This can be important when considering human and wildlife exposure.

The most important pathway for removal of PCDDs and PCDFs from the atmosphere, and the main route through which dioxins enter aquatic and terrestrial environments, is by gravitational settling and washout in rain. Dioxins attached to particulate matter will tend to settle out under gravity, with larger, coarser particles deposited more rapidly and closer to emission sources than smaller particles. Dioxins bound to fine particulate and in gaseous form are more prone to long-range atmospheric transport. These particles will tend to be deposited by rain and snow, although they may have traveled far from the emissions source before they are eventually removed. With regard to sources of human and wildlife exposure, the inhalation pathway is generally not regarded to be a significant source of either human or ecological exposures relative to the levels likely encountered through the food chain.

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