Atmospheric lifetimes of dibenzo-p-dioxins and dibenzofurans

The experimental and theoretical data available concerning the gas- and particle-phase reactions of polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs) are discussed. These data lead to the expectation that the dominant tropospheric loss processes of gas-phase PCDDs and PCDFs will be photolysis and reaction with the OH radical, with the OH radical reaction being the most important for the less chlorinated species. The estimated tropospheric lifetimes of gas-phase PCDFs increase significantly more rapidly with the degree of chlorination than is the case for PCDDs. For particle-associated PCDDs and PCDFs, the dominant tropospheric removal processes are expected to be photolysis and wet and dry deposition, with wet and dry deposition of the host particles being the most important. The estimated lifetimes in the lower troposphere range from less than 1 day for dibenzo-p-dioxin, the mono-, di- and trichlorodibenzo-p-dioxins, dibenzofuran and the monochlorodibenzofurans present in the gas phase, to greater than or equal to 10 days for particle-associated PCDDs and PCDFs, with a general increase in the tropospheric lifetime with the degree of chlorination. While long-range transport of PCDDs is expected to occur for those PCDDs which are totally or mainly particle associated, gas- and particle-phase PCDFs containing four or more chlorine atoms are also expected to have sufficiently long tropospheric lifetimes to undergo long-range transport.