On the difference between SO 2−4 and NO −3 in wintertime precipitation

Abstract

Winter rains have lower NO − 3 levels but higher SO 2− 4 levels than snows in the NE United States. In this study, four years of winter precipitation data from SE Michigan were examined to help understand these differences. Although NO − 3 levels were indeed higher in snow than winter rain, the higher concentrations could be attributed to the generally lower precipitation depths associated with snow events than with rain events. The NO − 3 concentrations are inversely correlated with precipitation depth. There was no evidence that snow scavenged HNO 3 in the air more efficiently than rain. Conversely, SO 2− 4 was far higher in winter rain than in snow. This could not be explained in terms of ground-level ambient S concentrations or the wind direction from which the storm originated. However, the cloud temperatures were high enough in the case of rain to suggest that the cloud hydrometeors could have been present as liquid droplets rather than ice crystals. The SO 2− 4 concentrations of the precipitation were highly correlated with the temperatures of the cloud layers. The data suggest that SO 2 is incorporated and oxidized to SO 2− 4 in clouds most efficiently when the hydrometeors are present as liquid droplets. The fact that NO − 3does not show the same relationship suggests that incorporation of N species into cloud water followed by oxidation is not as important a process for N as for S.