Supercooling of type 1 polar stratospheric clouds: The freezing of submicron nitric acid aerosols having HNO3 mol fractions less than 0.5

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Abstract
We report the freezing temperatures for nitric acid/water aerosol particles having radii about 0.2 μm and concentrations in the range 0.24 < xHNO3 < 0.46 (xHNO3 = nitric acid mol fraction). The aerosols supercool by up to 85 K. The droplets having xHNO3 = 0.33 have the highest freezing point in this concentration range; they supercool by 60 K and produce nitric acid dihydrate (NAD) upon freezing. We describe a simplified principal components analysis that improves the detection of the freezing point and allows the identification of the solid that precipitates upon freezing. This procedure shows that for the temperature range and experimental conditions explored, only NAD precipitates in the concentration range 0.33 < xHNO3 < 0.42 and both NAD and nitric acid trihydrate (NAT) precipitate for the range 0.24 < xHNO3 < 0.33. The temperature dependences of our measured freezing points correlate strongly with NAD saturation ratios, but have no relationship with NAT supersaturation at any concentration. Thus NAD nucleates preferentially from supercooled aerosols in this composition range, probably because of kinetic constraints on the nucleation of NAT. We conclude that NAD nucleation is possible during rapid cooling events in the polar stratosphere, if temperatures lower than 185 K are reached. During the freezing of stratospheric aerosol droplets in this concentration range, it seems likely that NAD nucleates first, thereby providing a surface on which NAT may crystallize.

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