Gauging the contribution of large rivers to ice nucleating particles in the Arctic

The Arctic climate is sensitive to variations in the abundance of atmospheric ice nucleating particles (INPs). To understand these variations, INP sources have to be defined. We investigated riverine INPs active ≥ −10 °C (INP −10 ) in the Mackenzie River, the 4th largest river discharging freshwater into the Arctic Ocean, and found a high median value of 5.2 × 10 3 INP −10 mL −1 . Heat treatment indicated a relevant contribution (median 76%) of INP −10 likely originating from fungal producers. Combined, a high INP concentration and a large discharged volume suggest that large rivers with extended river plumes contribute substantially to the abundance of INPs in the Arctic Ocean and subsequently to the lower troposphere above it. This contribution is presumably largest during summer, when airborne transport of INPs from lower latitudes is impaired by the position of the polar front near 70°N.