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The nucleation of NAD and NAT fromHNO3/H2O and HNO3/H2SO4/H2O solution droplets isinvestigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysisshows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitricacid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezingexperiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NADand NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are< 2 x 10-5 cm-3 s-1 and < 8 x 10-2cm-3 s-1, respectively. These nucleation rate coefficients are smaller by orders of magnitude than the value of~103 cm-3 s-1 used in a recent denitrification modelling study that is based on a linearextrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al.,Science, 291, 2591--2594, 2001). We show that this linear extrapolation is indisagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models ofPSCs. Our analysis of the experimental data yields maximum hourly productionrates of nitric acid hydrate particles per cm3 of air of about3 x 10-10 cm-3 (air) h-1 under polar stratospheric conditions. Assuming PSC particle production toproceed at this rate for two months we arrive at particle number densities of< 5 x 10-7 cm-3, much smaller than the value of~10-4 cm-3 reported in recent field observations. In addition, the nitric acid hydrate production rateinferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling studymentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosolscannot explain the observed polar denitrification

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