Polar sunrise experiment 1995: Hydrocarbon measurements and tropospheric Cl and Br-atoms chemistry

As part of the Polar Sunrise Experiments (PSE) 1995, we report our results on measurement of non-methane hydrocarbons (NMHC) collected at Alert, North west Territories, Canada (82.5 degrees N, 62.3 degrees W) from Julian days 57 to 113, 1995 along with our data on continuous methane and ozone measure ments during the same period. The concentration of NMHCs such as alkanes co rrelated well with that of methane during the dark period before polar sunr ise. However, no correlation was observed after the sunrise. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations rel ative to their background levels were observed. In all ozone depletion peri ods, concentration changes of alkanes and toluene were consistent with the occurrence of Cl-atom reactions. The characteristics of alkane isomer conce ntration changes suggest the dominance of HO chemistry prior to, and a swit ch to CI chemistry after, the polar sunrise (during the ozone depletion eve nts). The changes in ethyne concentration from their background level were in excess of those expected from Cl-atom kinetics alone and are attributed to additional Br-atom reactions. The time integral for Br mixing ratios is a few orders of magnitudes higher than that for Cl-atoms, suggesting much h igher Br-atom concentrations compared to Cl-atom concentrations, if they ar e present simultaneously. (C) 1999 Elsevier Science Ltd. All rights reserve d.