Differential rain and snow scavenging efficiency implied by ionic concentration differences in winter precipitation

Data from hourly, sequential precipitation samples taken during five winters at Brookhaven National Laboratory were analyzed to investigate differences in ionic composition between rain and snow. Concentrations of all ions measured hydrogen, sulfate, nitrate plus nitrite, ammonium, chloride and sodium, were significantly higher in melted snow than in rain. Nitrate plus nitrite averaged nearly five times and ammonium nearly three times greater in snow than in rain. Other concentrations differed by less than a factor of two. Synoptic and meteorological conditions were similar in rain and snow cases except for temperature and local wind direction. Surface temperatures during rain were extrapolated upwards assuming a moist adiabatic lapse rate to determine the probable height of the freezing level. Results suggest that all winter rain probably starts as frozen precipitation. An inverse correlation was found between concentration and precipitation fall speed as inferred from precipitation rate and type. Thus, rain-snow concentration differences appear to imply differential scavenging efficiencies or removal mechanisms in lower cloud or sub-cloud layers. Altough the chemical composition and state of the materials removed cannot be unequivocally determined from the data, particulate scavenging is strongly indicated.