Trace elements in atmospheric wet precipitation in Detroit metropolitan area: Levels and possible sources

Rain and snow samples were collected in the Detroit metropolitan area and analyzed by inductively coupled plasma-mass spectrometry (ICP-MS). Twenty-two elements were detected in a concentration range from ∼0.03 to ∼1.8 × 103 μg/kg. An enrichment factor (EF), defined as, EF=(X/Al)sample(X/Al)crust , was estimated for each element (X) detected, and used to determine possible origins of the element. Based on the hypothesis that crustal material is the only source of aluminum (Al) found in the atmospheric environment, an EF value near unity for an element suggests that crustal material is a major source of this element. If EF>10, an element is enriched in the atmosphere relative to its concentration in the earth's crust, implying a source other than the crust. Alkali, alkaline earth and lanthanide metals detected in the samples exhibit low EF values, indicating mainly a crustal source. Six elements (As, Cd, Cu, P, Pb and Zn) were significantly enriched in the samples as their EF values were greater than 10, thus originated likely from anthropogenic emissions. The relative order of the significantly enriched elements is estimated as follows: Cd > Pb > Zn > As > Cu > P. A high enrichment (EF∼100) for phosphorus was observed and caused plausibly by extensive usage of phosphorus-containing fertilizers and pesticides.