Grab samples for nonmethane hydrocarbons (NMHCs) were collected from three sites: daily at Alert, Northwest territories (82.5¿ N, 62.3¿ W) from January 21 to April 19, daily at an ice flow 150 km north of Alert from April 9 to 25, and on an aerial survey conducted in April over the Arctic archipelago. In situ measurements of n-butane and i-pentane were also made hourly at Alert from April 2 to April 15. During the dark period (January to March), C2-C6 hydrocarbon concentrations correlated with those of methane. Concentrations declined gradually from January to April, consistent with removal by HO radicals. On the other hand, during low-ozone periods in April and additional decrease in NMHC concentrations and change in distribution were observed. Concentration changes of alkanes were correlated to Cl atom reaction rate constants. Acetylene displayed a greater change in concentration than predicted from chlorine kinetics, possibly indicating additional removal by Br atoms. The Br atom concentration derived from the depletion of acetylene can account for the low-ozone concentrations periodically observed at Alert. The estimated Cl atom concentration is too small to be a significant loss mechanism for ozone. Thus the data from Alert and the ice floe site provide evidence for Cl and Br atom chemistry during the ozone depletion episodes observed at polar sunrise. ¿ American Geophysical Union 1994