Microphysical Modelling of Polar Stratospheric Clouds, Denitrification and Dehydration

 

Research Staff: Katja Drdla

The freezing processes that may lead to the formation of solid-phase polar stratospheric clouds (PSCs) have been examined to assess their winter-long effects, especially denitrification, in a coupled microphysical/photochemical model. Trajectory simulations using data from November 1999 to April 2000, using a large set of trajectories which provided representative coverage of the entire Arctic vortex through the period of PSC formation and ozone depletion.

A freezing process occurring at temperatures above the ice frost point is shown to be necessary to explain both the occurrence of solid-phase PSCs early in the winter and denitrification, especially without dehydration. If freezing only occurs below the ice frost point the primary contributor to denitrification is actually sedimentation of liquid-phase PSC particles. The mechanism of a second freezing process, occurring above the ice frost point, can not yet be conclusively determined. Of the cases considered, heterogeneous freezing of the aerosol to form nitric acid trihydrate (NAT) particles best reproduced solid-phase PSC formation and observations of widespread denitrification with limited dehydration. The simulations constrain the number of frozen particles to be near either 0.02% or 1% of the total aerosol number; values in between 0.02% and 1% produce more intense denitrification than observed, demonstrating that small changes in the number of frozen particles could exacerbate denitrification. However, this result was contingent upon assuming that the heterogeneous nuclei remain active, producing PSCs, throughout the winter. An idealized homogeneous freezing process was also able to produce NAT PSCs and denitrification (rates of 106-107 cm-3s-1 compared favourably with data), but differed from observations in one key aspect: denitrification was more frequently accompanied by dehydration. Nitric acid dihydrate (NAD) particles were less effective than NAT at denitrification, but heterogeneous freezing of 0.1% of the aerosol yielded results marginally consistent with measurements. An important limitation, however, of all the scenarios considered is that they produced more intense and more widespread dehydration than was observed. This suggests that model minimum temperatures (from UKMO analyses) were too cold by 1 to 3 K.

Research on this topic continues, in order to further characterize the freezing processes. Simulations are being conducted using recent laboratory measurements on the homogeneous freezing rates of liquid solutions to form NAD and NAT. These laboratory rates are also being extrapolated to understand in detail how heterogeneous freezing may occur in the stratosphere.

 

Collaborators: M. R. Schoeberl, Goddard Space Flight Center; E. V. Browell, Langley Research Center

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