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 in- vestigated both theoretically and experimentally with re- spect to the formation of polar stratospheric clouds (PSCs). Our analysis shows that homogeneous NAD and NAT nu- cleation from liquid aerosols is insufficient to explain the number densities of large nitric acid containing particles re- cently observed in the Arctic stratosphere. This conclu- sion is based on new droplet freezing experiments employ- ing 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 denitrifica- tion modelling study that is based on a linear extrapola- tion 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 thermo- dynamics and with experimental data and, therefore, must not be used in microphysical models of PSCs. Our anal- ysis of the experimental data yields maximum hourly pro- duction 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 den- sities of < 5 × 10−7 cm−3, much smaller than the value of 1 Introduction ∼10−4 cm−3 reported in recent field observations. In ad- dition, 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.