Dehydration in the Arctic stratosphere during the SOLVE/THESEO‐2000 campaigns

Abstract

show clear evidence for dehydration inside the polar vortex. At 30-50 hPa, total hydrogen is reduced by approximately 0.5 ppmv. This phenomenon is apparent in all five in situ balloon observations of this period; therefore it is probable that dehydration occurred over extended regions and a long period of this winter which was characterized by a wellconfined vortex and low stratospheric temperatures. At altitudes below 50 hPa, where dehydration was strongest in previous Arctic observations and in the austral spring, total hydrogen values (2ÁCH 4 + H 2 O + H 2 ) were similar to those found in Arctic profiles from other years where there was no dehydration and to those found at midlatitudes. In some of the dehydrated air masses, small solid particles were found whose crystallization might be connected to the earlier formation of ice particles. Back trajectory calculations for the January observations indicate that the probed air masses had experienced temperatures below the ice frost point in a synoptic-scale cold region several days before the observations. Most likely, the air was dehydrated there. In addition, temperatures in these air masses dropped below ice saturation several hours prior to the observations in the lee of the Scandinavian mountain ridge. For the March measurements, no ice saturation was apparent in the recent history of the air masses, again indicating that dehydration in the Arctic winter 1999/2000 was not a local phenomenon.

On the accuracy of analysed low temperatures in the stratosphere

“…Below T NAT the 68% fractiles become larger and most of the outliers occur here. T NAT is calculated by assuming a LIMS January 76 • N profile of nitric acid and water vapour is assumed to be 4.5 ppmv below 450 K, 6.5 ppmv above 650 K, and to vary linearly in between (Buss, 2003;Schiller et al, 2002;and references therein). The temperature thresholds T NAT and T ICE as well as the concurrent HNO 3 and H 2 O mixing ratios are shown for the mid points of the four layers used in this study in Table 1.…”

Severe ozone depletion in the cold Arctic winter 2004–05

“…Although the H 2 O measurements themselves do not show any obvious dehydration (Figure 3e), the quantity H 2 O + 2 CH 4 commonly used as a measure of dehydration [Schiller et al, 2002] is reduced by up to 0.5 ppm between 440 and 470 K and for some individual data points below (Figure 3e), suggesting permanent removal of H 2 O by sedimenting ice particles in the Arctic vortex 2004/5, confirming an earlier study by Kivi et al [2005].…”

Nitric Acid Trihydrate (NAT) formation at low NAT supersaturation in Polar Stratospheric Clouds (PSCs)

“…Water vapour is measured with the fast in situ stratospheric hygrometer FISH (Schiller et al, 2002) using the Lyman-α photofragment fluorescence technique. The instrument is calibrated before and after the flight.…”