Stratospheric nitric acid vapour measurements in the cold Arctic vortex: implications for nitric acid condensation

Abstract

STRATOSPHERIC nitric acid condensation has recently been proposed^{1, 2} to take place in the cold winter polar vortex, priming it for chlorine-catalysed ozone destruction. This idea has been developed subsequently^3–8. Condensation, thought to proceed via a heterogeneous heteromolecular mechanism involving stratospheric water vapour and pre-existing H₂SO₄/H₂O aerosols, which serve as condensation nuclei, leads to solid nitric acid trihydrate (NAT; HNO₃.3H₂O) aerosols. It is expected^{1, 2, 8} that the condensation temperature, T_c(NAT), is significantly greater than that of water-ice, T_c(H₂O-ice), given the stratospheric abundances of gaseous nitric acid and water and the thermodynamic properties of stratospheric aerosols⁸. Knowledge of T_c(NAT) is important because it determines the spatial and temporal extent of NAT aerosols and hence possibly of polar ozone destruction. Here we report in situ measurements of the detailed height distribution of gaseous nitric acid in the cold arctic vortex, using a balloon-borne technique^9–16, which offers a much better altitude resolution (30 m) than previous satellite measurements^{17, 18}. Our data set an upper limit to T_c(NAT) of 195 K at 23 km, and are consistent with model predictions based on the present gaseous nitric acid data, typical water vapour abundances and thermodynamic data⁸ of macroscopic NAT mixtures. However, our upper limit to T_c(NAT) is markedly lower than some of the early model estimates^{1, 2}.