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, Northwest Territories, Canada (82.5° N, 62.3° W) from Julian days 57 to 113, 1995 along with our data on continuous methane and ozone measurements during the same period. The concentration of NMHCs such as alkanes correlated well with that of methane during the dark period before polar sunrise. However, no correlation was observed after the sunrise. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with the occurrence of Cl-atom reactions. The characteristics of alkane isomer concentration changes suggest the dominance of HO chemistry prior to, and a switch to Cl chemistry after, the polar sunrise (during the ozone depletion events). 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 higher Br-atom concentrations compared to Cl-atom concentrations, if they are present simultaneously.