Releases of DNOC
Only one company, located on the St. Clair River near Sarnia, Ontario, reported releases of DNOC to the National Pollutant Release Inventory (NPRI) (Environment Canada, 2003b). Although the facility reported only off-site transfers and no releases to water, air or soil, it was used to develop a conservative scenario to estimate the quantity of DNOC that could be released into the St. Clair River from such a site. It is assumed, in a conservative scenario, that one customer is receiving the total annual import quantity (100-1000 tonnes). The scenario assumes releases of 0.2% of the annual import quantity of DNOC into the St. Clair River, which, based on professional judgement, recognizes routine process losses and waste from equipment cleaning for a substance handled in bulk. This accounts for releases to solid waste and wastewater. Using this percentage results in an estimated annual release of 200-2000 kg. It is further assumed that DNOC is in use throughout the year and that there is continuous release (24 hours per day) over the year (350 operating days). Daily releases over a period of 350 days correspond to approximately 0.57-5.7 kg/day. Sewage treatment plant (STP) removal rates were also considered. The STP model (CEMC, 2001) estimated that 27% of DNOC would be removed and that 73% would enter the environment in the form of final wastewater effluent from an STP.
Two main sources of atmospheric nitrophenols (a category that includes DNOC) have been reported in the literature. These include secondary formation by reactions in the troposphere and emissions from automobiles. Researchers have examined the atmospheric occurrence and formation of DNOC (Nojima et al., 1976; Alber et al., 1989; Richartz et al., 1990). DNOC has been shown to form as a secondary pollutant via the reaction of toluene and 2-methylphenol with nitrogen monoxide and hydroxyl radicals. It is difficult to estimate the quantity that may result from the anthropogenic release of precursor species. Direct emission of DNOC from car exhaust is likely only of minor importance. Under experimental conditions, exhaust from an automobile motor was found to contain DNOC at <0.01 ng/m3 (Tremp et al., 1993).