Measurement of C2-C7 hydrocarbons during the Polar Sunrise Experiment 1994: further evidence for halogen chemistry in the troposphere

Abstract

Air samples for nonmethane hydrocarbon (NMHC) analysis were collected at two ground-based sites: Alert, Northwest Territories (82.5°N, 62.3°W) and Narwhal ice camp, an ice floe 140 km northwest of Alert, from Julian days 90 to 117, 1994, and on a 2-day aerial survey conducted on Julian days 89 and 90, 1994, over the Arctic archipelago. Several ozone depletion events and concurrent decreases in hydrocarbon concentrations relative to their background levels were observed at Alert and Narwhal ice camp. At Narwhal, a long period (≥7 days) of ozone depletion was observed during which a clear decay of alkane concentration occurred. A kinetic analysis led to a calculated Cl atom concentration of 4.5 x 103 cm-3 during this period. Several low-ozone periods concurrent with NMHC concentration decreases were observed over a widespread region of the Arctic region (82°-85°N, and 51°-65°W). Hydrocarbon measurements during the aerial survey indicated that the low concentrations of these species occurred only in the boundary layer. In all ozone depletion periods, concentration changes of alkanes and toluene were consistent with Cl atom reactions. The changes in ethyene concentration from its background level were in excess of those expected from Cl atom kinetics alone and are attributed to additional Br atom reactions. A box modeling exercise suggested that the Cl and particularly Br atom concentrations required to explain the hydrocarbon behavior are also sufficient to destroy ozone.