Sulfate and nitrate firn concentrations on the Greenland ice sheet: 1. Large‐scale geographical deposition changes

Abstract

Eleven shallow firn cores and three deeper ice cores drilled along a meridional line over the Greenland ice sheet from 71° to 80°N have been specifically evaluated for their nitrate and sulfate concentrations. Recent average annual sulfate concentrations strongly increase from around 100 ppb in central Greenland to 175 ppb in the northeastern part of the ice sheet, while nitrate levels remain essentially constant at around 150 ppb. Recent sulfate firn concentrations exhibit a distinct seasonal cycle with maximum concentrations in spring. The seasonal amplitude is 5 times higher over the northern ice sheet compared to the central Greenland region. The corresponding nitrate cycle is only weakly defined with a tendency toward higher summer concentrations. Application of a macroscopic deposition model, developed for irreversibly deposited sulfate as well as reversibly deposited nitrate, shows that geographical variations in sulfate and nitrate firn concentrations can be explained largely by the spatial change in snow accumulation rate over the entire interior of the Greenland ice sheet both for recent and for preindustrial conditions. The recent wet sulfate deposition flux has increased by a factor of 2 since preindustrial times, while the dry deposition flux has risen by a factor of 5. In the case of nitrate, depositon fluxes have increased by a factor of 2 both for wet and for dry deposition. On the basis of the empirically determined model parameters, up to 50% of the initially deposited nitrate appears to be remobilized from the firn column.