Dioxins and furans in air and deposition: a review of levels, behaviour and processes.

This paper is a comprehensive, critical review of the levels, behaviour and processes affecting polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) in air and deposition. Aspects of sampling, analysis and quality assurance/control are discussed initially, before a review of the PCDD/F concentrations in ambient air is presented. The general trend in sigma P4-8 CDD/F (and sigma TEQ) is: remote sites < 0.5 pg/m3 (sigma TEQ < 10 fg/m3); rural sites approximately 0.5-4 pg/m3 (sigma TEQ approximately 20-50 fg/m3); and urban/industrial sites approximately 10-100 pg/m3 (sigma TEQ approximately 100-400 fg/m3). The commonly held view that a consistent mixture of PCDD/Fs in air exists is evaluated and questioned. Issues of seasonality and short-term changes in air concentrations are also critically discussed, with respect to the possibility of seasonal emission sources to air and seasonally dependent loss processes. Data on the gas-particle partitioning of PCDD/Fs in air are reviewed; the limited database to date is believed to provide evidence for an exchangeable transfer of PCDD/Fs between these two phases. The potential importance of photolytic and radical reaction degradation processes and wet/dry deposition processes in modifying the mixture of PCDD/Fs in air is discussed. Some homologue/congener specific 'weathering' of the mixture of PCDD/Fs emitted to the atmosphere clearly occurs, but in general PCDD/Fs have 'long' atmospheric residence times, rendering them subject to long-range atmospheric transport. Data are reviewed which relate the mixture of PCDD/Fs in air to that in deposition; this leads to the conclusion that different homologue groups (which are partitioned differently between the gas and particulate phase) are transferred to the earth's surface with broadly similar efficiencies.