Analysis of SOLVE Observations of PSCs and Implications for the Evolution of the Arctic Vortex

 

Research Staff: Katja Drdla

In-situ measurements of polar stratospheric clouds (PSCs) made from the ER-2 aircraft during the SAGE III Ozone Loss and Validation Experiment (SOLVE) have revealed new information about the composition and properties of these clouds. Model results have been compared with the ER-2 measurements in order to refine our understanding of the cloud microphysics.

Measurements made by the Multiangle Aerosol Spectrometer Probe (MASP) reveal that the majority of particles generally appear to be liquid-phase. Growth of these particles was observed at temperatures near 192 K, consistent with the predicted behaviour of ternary HNO3/H2SO4/H2O solutions. Even at warmer temperatures, starting as air cools below 196 K, some swelling of the particles is apparent. The correlation of this growth with total available HNO3 indicates that HNO3 condensation is responsible. The observations have been compared with predictions of several ternary solution models, with generally good agreement. The presence of liquid-phase particles has implications for the long-term evolution of the Arctic winter. The widespread persistence of liquid particles constrains our understanding of freezing processes, especially in air parcels which have experienced extensive denitrification. No significant differences in particle size distributions exist between highly denitrified and less denitrified air parcels, suggesting that very few particles are removed by the processes causing denitrification.

However, a very small fraction (<0.1 %) of the particles did freeze during the winter, forming solid-phase HNO3-containing particles which were observed both by MASP and by the NOy instrument. The presence of these frozen particles cannot be explained by current laboratory data on aerosol freezing, in which freezing only occurs at temperatures several degrees below the ice frost point. Alternative formation mechanisms, in particular homogeneous freezing above the ice frost point and heterogeneous freezing, have been explored using the microphysical model. Homogeneous freezing at a rate of 104 cm-3s-1 produces particles comparable to observations. However, the PSCs form too frequently (observations often show a lack of solid-phase PSCs well below the NAT (nitric acid trihydrate) condensation point), and the altitude variation is not well captured. For homogeneous freezing to explain the observations, the freezing rate must have a complex dependence upon the local conditions.

On the other hand, PSCs that form by heterogeneous freezing are strongly affected by the winter-long PSC processing; that is denitrification removes most of the nuclei. The resulting model correlation between denitrification and particle concentration is comparable to the ER-2 observations. In addition to providing an explanation for the occasional absence of solid-phase PSCs, this process also explains why denitrification did not exceed 80%.

To understand the winter-long implications of these findings, the model has been run from November to mid-April, using a large set of trajectories which provided representative coverage of the entire Arctic vortex through the period of PSC formation and ozone depletion. The various possible freezing processes have been shown to have different characteristics in terms of the overall extent of frozen particles, the evolution of the PSCs, denitrification, and dehydration. Scenarios with freezing above the ice frost point lead to widespread denitrification. This denitrification enhances the ozone loss at the end of the winter by up to 30%, as long as the vortex remains stable until late March.

Point of Contact: Katja Drdla, 650/604-5663, katja@aerosol.arc.nasa.gov