Snowpack production of formaldehyde and its effect on the Arctic troposphere

The oxidative capacity of the atmosphere determines the lifetime and ultimate fate of atmospheric trace species. It is controlled by the presence of highly reactive radicals, particularly OH* formed as a result of ozone photolysis. The dramatic depletion of ozone in Arctic surface air during polar sunrise therefore offers an opportunity to improve our understanding of the processes controlling ozone abundance and hence the oxidative capacity of the atmosphere. Ozone destruction is catalysed by bromine atoms and is terminated once bromine reacts with formaldehyde to form relatively inert hydrogen bromide, but neither the activation of bromine nor the contribution of formaldehyde are fully understood. Particularly troubling is the failure of current models to simulate the high formaldehyde concentrations in Arctic surface air. Here we report measurements in Arctic snow and near-surface air, which suggest that photochemical production at the air-snow interface accounts for the discrepancy between observed and predicted formaldehyde concentrations. The strength of this source is comparable to that of the dominant formaldehyde source in the free troposphere (the reaction between OH* and methane) and implies that formaldehyde photolysis can be a dominant source of oxidizing free radicals in the lower polar troposphere. We expect that formaldehyde will also affect photochemistry at the snow surface to facilitate the release of bromine into the lower troposphere - the initial step in Arctic tropospheric ozone depletion.