Abstract
Abstract. Satellite-based observations during the Arctic winter of 2009/2010 provide firm evidence that, in contrast to the current understanding, the nucleation of nitric acid trihydrate (NAT) in the polar stratosphere does not only occur on preexisting ice particles. In order to explain the NAT clouds observed over the Arctic in mid-December 2009, a heterogeneous nucleation mechanism is required, occurring via immersion freezing on the surface of solid particles, likely of meteoritic origin. For the first time, a detailed microphysical modelling of this NAT formation pathway has been carried out. Heterogeneous NAT formation was calculated along more than sixty thousand trajectories, ending at Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP) observation points. Comparing the optical properties of the modelled NAT with these observations enabled a thorough validation of a newly developed NAT nucleation parameterisation, which has been built into the Zurich Optical and Microphysical box Model (ZOMM). The parameterisation is based on active site theory, is simple to implement in models and provides substantial advantages over previous approaches which involved a constant rate of NAT nucleation in a given volume of air. It is shown that the new method is capable of reproducing observed polar stratospheric clouds (PSCs) very well, despite the varied conditions experienced by air parcels travelling along the different trajectories. In a companion paper, ZOMM is applied to a later period of the winter, when ice PSCs are also present, and it is shown that the observed PSCs are also represented extremely well under these conditions.
Highlights
The discovery of the Antarctic ozone hole (Farman et al, 1985) was rapidly followed by the recognition that polar stratospheric clouds (PSCs), by providing the necessary surface for heterogeneous halogen activation reactions, played a central role in the observed ozone depletion (e.g. Solomon et al, 1986)
Solid PSC particles were detected at temperatures well above the frost point, and it was proposed that these particles contained HNO3 (Toon et al, 1986; Crutzen and Arnold, 1986)
Experimental work suggested that the composition could be that of nitric acid trihydrate (NAT) (Hanson and Mauersberger, 1988), and, more recently, observational work, such as the balloon-borne mass spectrometry measurements reported on by Voigt et al (2000), have shown that solid PSC particles observed above the frost point do have H2O and HNO3 stoichiometries consistent with a NAT composition
Summary
The discovery of the Antarctic ozone hole (Farman et al, 1985) was rapidly followed by the recognition that polar stratospheric clouds (PSCs), by providing the necessary surface for heterogeneous halogen activation reactions, played a central role in the observed ozone depletion (e.g. Solomon et al, 1986). Biermann et al (1996) showed, based on bulk samples of ternary solution containing solid meteoritic nuclei, that these nuclei could at best have 1/e freezing times of around 20 months While these authors aimed at excluding meteoritic material as responsible for dense (n > 0.1 cm−3) NAT clouds, their derived upper bound corresponds to an airvolume-based rate of JNheAtT,vol,upper = 7 × 10−4 cm−3 air h−1 (assuming a stratospheric smoke surface area density of 5 μm cm−3), which is certainly high enough to produce observable NAT clouds. Using balloon-based measurements carried out in the Arctic in December 2002, Larsen et al (2004) found that observed PSC properties can be reproduced by applying the surface nucleation rate of Tabazadeh et al (2002), reduced by a factor of 10–20, to calculate NAD (nitric acid dihydrate) formation from liquid aerosol, followed by instantaneous transformation of NAD to NAT The rate they determined corresponds to an air-volume-based rate of JNvoAlT = 2.5 × 10−5 cm−3 air h−1. In a companion paper (Part 2; Engel et al, 2013) the role of heterogeneous nucleation in ice formation is discussed
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