Atmospheric Behavior of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans and the Effect of Combustion Temperature

Abstract

Emissions from the combustion of wood chips treated with pentachlorophenol (PCP), polyvinyl chloride (PVC) plastic pipe shavings, and solid PCP were injected into 25-m 3 outdoor Teflon film chambers and aged in sunlight under typical atmospheric conditions. The behavior of particulate and gas-phase polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) and polycyclic aromatic hydrocarbons (PAHs) was monitored over time. Only the tetra-and pentachlorinated dioxins and furans were shown to clearly partition at all into the gas phase in these experiments, perhaps due to high concentrations of TSP. Little or no reactivity was observed for PCDDs and PCDFs residing on particles resulting from hightemperature combustion (760-800°C). Greater photochemical reactivity of particle-bound PCDD/Fs resulted after low-temperature combustion (350-380°C), where fairly rapid photolysis competed with a production mechanism believed to begin with PCP. Photolysis rates of PCDD/Fs appeared to increase with decreasing levels of chlorination, lending evidence to the observed enrichment of the higher chlorinated species in the natural environment. On low-temperature combustion particles, model results showed that TCDD half-lives increased from 0.4 under North Carolina summer outdoor conditions to 17 h under wintertime conditions. For hightemperature combustion particles under similar outdoor conditions, half-lives ranged from 6.8 to 62 h. For these same conditions, model OCDD half-lives increased from 5 and 38 h in low-temperature combustion experiments to 36 and 257 h in high-temperature combustion experiments. Experimental particle-phase PAH photolytic half-lives also increased from between 0.1 and 3.6 h in low-temperature combustion experiments to between 4.4 h and stability on high-temperature combustion particles. These differ-ences may be explained by differences in the particles generated from low-and high-temperature combustion.

Phototransformation rate constants of PAHs associated with soot particles

“…Numerous studies show that both direct and indirect phototransformation reactions of PAHs may proceed at different rates on particles compared to homogeneous or heterogeneous aqueous systems (Behymer and Hites, 1988; Pennise and Kamens, 1996; Miller and Olejnik, 2001; Matuzawa et al, 2001; Kahan and Donaldson, 2007; Jacobs et al, 2008). Although photodegradation on soot particles is likely a large sink for PAHs, it is difficult to measure this process experimentally because it depends on the thickness of the soot deposit and, in most cases, on transport kinetics, which need to be considered.…”

“…Although photodegradation on soot particles is likely a large sink for PAHs, it is difficult to measure this process experimentally because it depends on the thickness of the soot deposit and, in most cases, on transport kinetics, which need to be considered. Several studies have measured PAH photodegradation rates on a variety of particle types and other solid substrates (Behymer and Hites, 1985; Pennise and Kamens, 1996; Matuzawa et al, 2001; Wang et al, 2005; Niu et al, 2007). However, these past studies suffer from one or two general weaknesses: (1) PAHs were added to an existing particle, typically at high concentrations, which does not mimic ambient particles, and (2) the reactions were examined in bulk (i.e., not on suspended particles) and the data treatment did not account for light attenuation by the particles, which will reduce the apparent photodegradation rate.…”

The formation of dioxins in combustion systems

“…The interaction of PCDD/F with the environment has been summarised by Alcock and Jones [25]. The stability of PCDD/F generated by combustion and adsorbed onto airborne particles has been studied in 25 m 3 outdoor chambers [26]. The PCDD/F formed at high temperatures (760-800 • C) were more stable to atmospheric photolysis than that formed at 350-380 • C. The lower chlorinated congeners were more susceptible to decay, with TCDD having a half-life of 0.4 h under summer conditions and 17 h in winter.…”

Early history and rationale for outdoor chamber work at the University of North Carolina

“…2-nitrofluoranthene) in both the gas and particle phases by then student Zhihua Fan. [40,41] Much of this work is summarised in a 2002 text on aromatics by Cavert et al [42] Kamens and his late faculty colleague M. Judith Charles, along with their students Parag Birla, [43] Chris Lutz [44] and David Penise [45] went on to employ these techniques to study the atmospheric stability of brominated and chlorinated dioxins and furans.…”