Arctic Reactive Bromine Events Occur in Two Distinct Sets of Environmental Conditions: A Statistical Analysis of 6 Years of Observations

Abstract

Tropospheric bromine radicals in the Arctic efficiently remove ambient ozone and oxidize gaseous elemental mercury. Ground-based bromine monoxide (BrO) observations from the Arctic Ocean and Utqiaġvik (formerly Barrow) are combined with Modern Era Retrospective Analysis for Research and Applications version 2 reanalysis meteorological fields to determine how BrO varies with environmental conditions. The mean seasonal BrO abundance varies from year to year (p < 0.001), while regional variance in mean BrO is not statistically significant (p > 0.11). Principal component analysis derived three important principal components from the environmental data set. The third principal component explains the most variance in BrO and is correlated with low ozone and cold temperatures. This principal component is consistent with high BrO during ozone depletion events at cold temperatures and can work concurrently with each of the other two principal components to generate two distinct environmental types of high BrO events. The first principal component consists of a less-stable, thick, mixed layer and low atmospheric pressure and is consistent with observations of high BrO in low-pressure systems (e.g., storms). The second principal component consists of cold and stable conditions and is consistent with high BrO under surface-based temperature inversions. Our principal component regression model predicted the both the vertical column density of BrO in the lowest 2 km of the troposphere (R = 0.45) and the vertical column density of BrO in the lowest 200 m (R = 0.54). This statistical description of two types of reactive bromine events may help to harmonize space-based and ground-based observations.