Size distributions, sources, and dry deposition of atmospheric particles in southern Lake Michigan

Size segregated aerosol concentrations of more than 40 elements were measured in the southern Lake Michigan basin to quantify urban/industrial contributions to dry deposition fluxes to the Lake. Twenty-five MOI and 38 PM{dollar}sb{lcub}2.5{rcub}{dollar} samples collected over a two week period in July, 1994 at an urban, offshore, and rural downwind site were analyzed by X-ray fluorescence and instrumental neutron activation analysis (INAA). The highly resolved elemental size spectra constructed from the impactor data revealed that concentrations of the anthropogenic elements As, Sb, V, Se, and S were concentrated in fine particles with modal diameters ranging from 0.2 to 0.74 {dollar}mu{dollar}m, that Fe and Mn were concentrated on coarse particles with modal diameters of 6.9 {dollar}mu{dollar}m, and that Zn was concentrated on intermediate sized particles of 1 {dollar}mu{dollar}m. Concentrations of fine particle anthropogenic elements were greater at the offshore site than at the urban site under prevailing south and south-westerly winds, while those at the rural downwind site were comparable to the offshore concentrations. Under westerly winds, coarse particle concentrations were 2-10 times greater at the urban site.; Size-specific dry deposition velocities (v{dollar}sb{lcub}rm d{rcub}{dollar}) determined by fitting the elemental size distribution data to surrogate surface dry deposition flux measurements made offshore showed that under stable conditions and a mean wind speed of 4.3 m s{dollar}sp{lcub}-1{rcub},{dollar} minimum deposition velocities for particles with physical diameters between 0.09 and 0.53 {dollar}mu{dollar}m averaged 0.007 {dollar}pm{dollar} 0.006 cm s{dollar}sp{lcub}-1{rcub}.{dollar}; Chemical mass balance calculations were used to apportion the size-segregated aerosol to 11 source categories. In addition, dry deposition fluxes to Lake Michigan were calculated by particle size and by source category using an expanding stirred box gradient deposition model. Urban dust was predicted to contribute {dollar}>{dollar}70% of the Mn and {dollar}>{dollar}60% of the Cr dry deposition flux, while steel emissions contributed {dollar}>{dollar}70% of the Fe, 50% of the V, 15% of the As, and {dollar}>{dollar}10% of the Se flux. Thirty to fifty percent of the arsenic flux was apportioned to rural soil. A new fine particle (mmad 0.2-0.3 {dollar}mu{dollar}m) As and Sb source was identified, contributing 5 and 20% to the As and Sb deposition fluxes, respectively, while incinerator emissions contributed {dollar}>{dollar}80% of the Zn flux. Fine particle V peaks attributed to oil combustion were measured in 1/2 of the MOI samples, but contributed negligibly to the fluxes.