C. Kröger

In situ mountain-wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation

0.2 cm 3 , median radii of 1 to 2 mm and volumes up to 1 mm 3 cm 3 . A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain-wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric cloud (PSC), nitric acid trihydrate (NAT), ozone, gravity wave, PSC formation

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H 2 O:HNO 3 close to 3 at temperatures near and below the equilibrium temperature T NAT . The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm -3 . Below the NAT layers and above T NAT , thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Sunset observations of ClO in the Arctic Polar Vortex and implications for ozone loss

Chlorine oxide (ClO) was measured in situ in the perturbed arctic vortex from a balloon launched near Kiruna, Sweden, 67°N, at sunset on January 27, 2000. Between 20 and 23 km, the time-dependent behavior of ClO agrees with the functional form for the kinetics of ClO + ClO + M → Cl2O2 + M. The temporal change of ClO is described well by the integral of a rate expression that includes this reaction and thermal decomposition of Cl2O2. The value for the rate coefficient that best describes the observed decreases in ClO is consistent with the upper limit of the currently recommended value, and agrees well with the results of a recent laboratory study of ClO recombination at 190–195 K. These results reinforce the link between man-made chlorine compounds and ozone losses in the polar regions.