Denitrification inside the stratospheric vortex in the winter of 1999–2000 by sedimentation of large nitric acid trihydrate particles

Abstract

Synoptic‐scale polar stratospheric clouds (PSCs) have been measured with an aircraft‐borne lidar south of Spitsbergen in early February 2000. The PSCs of moderately depolarizing stratified layers extended from near the tropopause (10–11 km) up to 20 km altitude. Their internal structure in backscatter ratio reflects the variability of the particles' concentration rather than manifold sizes and phases of different optical efficiency. Actually, their size distribution was nearly monodisperse. According to T‐matrix calculations, low backscatter ratios (γ < 3), color ratios of 1–1.5, and moderate depolarization ratios (δa ≈ 0.15–0.2) indicate that the observed layers were composed of aspherical nitric acid trihydrate (NAT) particles larger than 2 μm. During three turning point missions into the cold vortex core a significant settling of the particle layers was observed between aligned outgoing and incoming legs. Low stratospheric winds and quasi‐Lagrangian flight heading minimized horizontal drifts between the probed volumes. A NAT particle sedimentation velocity of 69 ± 14 m/h is derived by cross‐covariance analysis of corresponding pairs of two‐dimensional backscatter sections of the layers. This implies a particle size of rp ≈ 7–8 μm and a mixing ratio of condensed HNO3 near 2 ppbv. The downward NOy flux below 19 km altitude amounts to j ≈ 3 ppbv km/d. Based on particle back trajectory analysis, this flux, derived from the observations in different regions of the Northern European Ice Sea, has a significant denitrification potential.