AMBIENT CONCENTRATIONS, SCAVENGING RATIOS, AND SOURCE REGIONS OF ACID RELATED-COMPOUNDS AND TRACE-METALS DURING WINTER IN NORTHERN MICHIGAN

Daily measurements of the atmospheric concentrations of HNO3, NO3-, NO2, SO2, SO4(2-), NH4+, and several trace metals were made at the University of Michigan Biological Station over a 124-day period during the 1984-1985 winter. The composition of the daily precipitation was also determined. The relative contributions of scavenged NO3- and HNO3 to the precipitation was estimated by assuming that the NO3- scavenging ratio was the same as that of trace metals with a similar particle size. Similarly, the SO4(2-) and SO2 contributions were based on the scavenging ratios of NH4+ and trace metals. On this basis, it was determined that the event median NO3- and HNO3 scavenging ratios were 500 and 3500, respectively. HNO3 scavenging accounted for 83% of the total scavenged NO3-. Scavenging of SO4(2-) accounted for all the snow SO4(2-) in 67% of the events. In the remaining events, some SO2 was scavenged, with a median scavenging ratio of 219. Overall, 67% of the snowfall acidity appeared to be due to HNO3 scavenging. Backward air-mass trajectories that were calculated for each event were used to determine the general source regions of the acidic species. Snow associated with air masses from the south and west accounted for 81 and 75% of the deposited NO3- and SO4(2-), respectively.