MEASUREMENTS OF C-2-C-7 HYDROCARBONS DURING THE POLAR SUNRISE EXPERIMENT 1994 - FURTHER EVIDENCE FOR HALOGEN CHEMISTRY IN THE TROPOSPHERE

Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground-based sites: Alert, Northwest Territories (82.5 degrees N, 62.3 degrees W) and Narwhal ice camp, an ice flee 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2-day aerial surv ey conducted on Julian days 89 and 90, 1994 over the Arctic archipelag o. Several ozone depletion events and concurrent decreases in hydrocar bon concentrations relative to their background levels were observed a t Alert and Narwhal ice camp. At Narwhal, a long period (greater than or equal to 7 days) of ozone depletion was observed during which a cle ar decay of alkane concentration occurred. A kinetic analysis led to a calculated Cl atom concentration of 4.5 x 10(3) cm(-3) during this pe riod. Several low-ozone periods concurrent with NMHC concentration dec reases were observed over a widespread region of the Arctic region (82 degrees-85 degrees N, and 51 degrees-65 degrees W). Hydrocarbon measu rements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone de pletion periods, concentration changes of alkanes and toluene were con sistent with Cl atom reactions. The changes in ethyne concentration fr om its background level were in excess of those expected from Cf atom kinetics alone and are attributed to additional Br atom reactions. A b ox modeling exercise suggested that the Cl and particularly Br atom co ncentrations required to explain the hydrocarbon behavior are also suf ficient to destroy ozone.