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

The nucleation of NAD and NAT from HNO₃/H₂O and HNO₃/H₂SO₄/H₂O solution droplets is investigated both theoretically and experimentally with respect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nucleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles recently observed in the Arctic stratosphere. This conclusion is based on new droplet freezing experiments employing optical microscopy combined with Raman spectroscopy. The homogeneous nucleation rate coefficients of NAD and NAT in liquid aerosols under polar stratospheric conditions derived from the experiments are < 2 × 10^-5 cm^-3 s^-1 and < 8 × 10 ^-2 cm^-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 linear extrapolation of laboratory nucleation data to stratospheric conditions (Tabazadeh et al, Science, 291, 2591-2594, 2001). We show that this linear extrapolation is in disagreement with thermodynamics and with experimental data and, therefore, must not be used in microphysical models of PSCs. Our analysis of the experimental data yields maximum hourly production rates of nitric acid hydrate particles per cm³ of air of about 3 × 10^-10 cm^-3 (air) h^-1 under polar stratospheric conditions. Assuming PSC particle production to proceed at this rate for two months we arrive at particle number densities of < 5 × 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 rate inferred from our data is much smaller than that required to reproduce the observed denitrification in the modelling study mentioned above. This clearly shows that homogeneous nucleation of NAD and NAT from liquid supercooled ternary solution aerosols cannot explain the observed polar denitrification.