Observation of NOy uptake by particles in the Arctic tropopause region at low temperatures

Measurements pf gas- and condensed-phase NOy, HNO3, O-3, and particle conce ntration and size were performed on 24 January 1997 during POLSTAR on a fli ght through an ice cloud. In the ice cloud below the tropopause only 2 to 4 pptv particle NOy were detected. The observed average mixing ratio of tota l HNO3 is significantly larger than the detected particle NOy. Considering the combined error limits of the NOy and KNO3 measurements, however, it can not be ruled out that the observed uptake of NOy was limited by the availab le gas-phase HNO3. The ratio of total HNO3 to gaseous NOy measured during t he cirrus cloud penetration amounts only to 0.2 in contrast to 0.9 in the s tratospheric part of the flight.