Trends of ambient concentrations and deposition fluxes of particulate trace metals in Detroit from 1982 to 1992

Abstract

Ambient air concentrations of the particulate trace metals Fe, Zn, Pb, Ni, Cr, Cd, Be and Hg have been measured since the early 1980s at seven urban sites across the metropolitan area of Detroit. Trace metals such as Fe, Zn, Ni and Cr were found to be more abundant at industrial sites, while Pb and Cd concentrations were highest at commercial sites. Be and Hg ambient concentrations did not show any large variations among land use categories on an annual average basis. The largest increases in ambient concentrations between two consecutive years were a 20 fold increase for Fe in industrial areas (Winter 1988), a 21-fold increase for Zn in a residential area (Summer 1986), a 5-fold increase for Ni in commercial areas (Winter 1992), a 10-fold increase for Cr in industrial and commercial areas (Spring 1982), a 5-fold increase for Cd in Commercial areas (Fall 1987), a 27- and 24-fold increase for Be in residential and industrial areas, respectively; and a 6-fold increase for Hg in residential areas. Pb did not show a large increase between 2 consecutive years (< 1%); however a 6-fold decrease was observed in residential areas during the summer of 1992. Selected US market parameters were used as indicators of emission source activity and their temporal trends were compared with those of the ratio between maximum ( C max) and average ( C average) ambient concentrations of trace metals observed in Detroit. Good agreement was found between the trend in the C max C average ratio and the trend in US production and consumption, import and export, and the price of metal-based products. A dynamic model was used to calculate the dry deposition flux of trace metals associated with total suspended particulate matter and its temporal variation by land-use category accounting for the variation of meteorological parameters, surface roughness, and ambient concentration. This model showed that the seasonal variability in meteorological parameters may yield a 40–50% variation in the dry deposition velocity of particulate trace metals on an annual basis. It also showed that deposition fluxes are generally higher in industrial areas than in residential and commercial areas. Finally, temporal trends of the ambient concentration and dry deposition flux of particulate trace metals are provided for the 1982–1992 period using a simple regression of seasonal data obtained at each sampling site. Ambient concentrations and dry deposition fluxes of Fe, Pb, Cr, Cd and Be are characterized by downward trends with annual decreases in the range of −1 to −11% using 1982 as the reference year. Trace metals such as Zn, Ni and Hg show an upward trend with annual increases in the range of 0.4–9%.