Five polar impurity sources are proposed: anthropogenic NO-3 and SO2-4 in recent Greenland but not Antarctic snow; marine aerosols, the dominant C1- source; volcanic eruptions, which produce detectable but variable peaks in SO2-4, F-, and/or C1- concentrations; biogenic sulfurous gases from soil and coastal waters, the dominant source for SO2-4 in nonvolcanic strata; an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum. Solar activity and supernova events do not modulate NO-3 concentrations in Greenland snow.