The nucleation rate constants and freezing mechanism of nitric acid trihydrate aerosol under stratospheric conditions

Abstract

Abstract. The nucleation rates for submicron-diameter nitric acid trihydrate (NAT) aerosol particles were measured under stratospheric conditions using a temperatureprogramed flow tube and Fourier transform infrared extinction spectroscopy to detect the phase change from liquid to solid. The temperature range from 175 to 155 K was 3 1 examined. The measured nucleation rate constants (in units of cm-s-) are between 3.8 +_ 1.8 x 10 •ø and 9.7 +_ 6.3 x 10 TM for the temperature range between 167.2 and 163.5 K. The experiments show directly that critical nuclei form rapidly in NAT droplets at these temperatures, but subsequent crystallization is very slow, so the nucleated droplets can persist as liquids for long times unless their temperature is raised to increase the crystal growth rate. In these experiments, complete crystallization of the nucleated droplets was achieved in the observation time (about 10 s) by raising their temperature to near 180 K. Although demonstrable in the laboratory, the freezing of NAT by homogeneous nucleation at low temperature and subsequent warming are unlikely to be important for type 1 polar stratospheric cloud formation because the temperatures required for nucleation are lower than those commonly found in the lower stratosphere. IntroductionSince the suggestion that nitric acid plays a role in the formation of polar stratospheric clouds ( , 1998]. With these measurements we hope to provide a systematic and selfconsistent data set for use in the parameterization of models and in the interpretation of PSC field measurements. ExperimentThe cryogenic flow tube and associated apparatus used for

Homogeneous nucleation rates of nitric acid dihydrate (NAD) at simulated stratospheric conditions – Part II: Modelling

“…Salcedo et al (2001) noted that the linear relationship should not be extrapolated and applied to S NAD beyond the range of their experimental data. In the same work Salcedo et al (2001) suggest another linear fit for the activation energy of NAT nucleation as function of the NAT saturation ratio in the range 50<S NAT <110 which relies on their own data set and measurements of NAT nucleation in a flow tube cooled to temperatures below 167.2 K (Bertram and Sloan, 1998b). The earlier settling chamber measurements (Disselkamp et al, 1996;Prenni et al, 1998) have been disregarded by Salcedo et al (2001).…”

“…The direct crystallisation to NAT was in some studies only observed at temperatures below 180 K and saturation ratios of gas-phase nitric acid with respect to NAT larger than 45 (Bertram and Sloan, 1998b;Salcedo et al, 2001). Such low temperatures and high saturation ratios are not reached in synoptic-scale PSCs.…”

Homogeneous nucleation of NAD and NAT in liquid stratospheric aerosols: insufficient to explain denitrification

“…(b) : 54 wt% HNO 3 (Salcedo et al, 2001); : 54 wt% HNO 3 (Bertram and Sloan, 1998b). The color coding indicates at which temperature the data were obtained.…”

“…(a) : 57 wt%, 60 wt%, and 64 wt% HNO 3 (Salcedo et al, 2001); : 64 wt% HNO 3 (Bertram and Sloan, 1998a). (b) : 54 wt% HNO 3 (Salcedo et al, 2001); : 54 wt% HNO 3 (Bertram and Sloan, 1998b). The color coding indicates at which temperature the data were obtained.…”

Modeling of flow dynamics in laminar aerosol flow tubes

“…There are several basic AFT designs (Bertram & Sloan, 1998;Chelf & Martin, 2001;Cziczo & Abbatt, 1999;Hung, Malinowski, & Martin, 2002;Norman, Miller, & Worsnop, 2002;Prenni, Wise, Brooks, & Tolbert, 2001;Wise, Garland, & Tolbert, 2004), but they differ in only four main respects: tube diameter and length, which determine the flow velocities employed; optical design (transverse or axial infrared beam); the presence or absence of right-angle bends; and orientation with respect to the gravity vector. A characteristic feature of these systems, which differentiates them from other laminar flow tubes-for example, those used to study homogeneous vapour nucleation-is the existence of large temperature steps, which, for the cases reported here, can introduce temperature differences as high as 100 K between the gas/aerosol flow and the tube wall.…”