Particle Size Distributions within Arctic Polar Stratospheric Clouds to Complement Particle Composition and Scattering Measurements

University Of Wyoming, Laramie WY

Investigators

Abstract

Deshler OPP-0095158 A more complete quantitative understanding of polar stratospheric ozone loss requires further work in several areas. The microphysics of the nucleation, growth, composition, and sedimentation of particles in polar stratospheric clouds (PSCs) is one of these areas. PSC particles provide sites for the heterogeneous chemistry required to activate chlorine, which then destroys ozone. The pivotal role played by PSC particles is most evident in the Arctic where ozone loss is essentially modulated by temperature, a surrogate for PSCs, from year to year. The frequency, duration, and timing of PSC occurrence determines the extent of chlorine activation and denitrification in the stratospheric polar vortex, and thus the amount of ozone loss. Removing some of the present uncertainties concerning PSC formation temperatures, particle composition, and particle phase will permit more careful modeling of the amount of chlorine processed by PSCs and the extent of denitrification. The activation of chlorine and ultimately ozone loss is dependent on all of these factors. This work takes advantage of invitations from several European colleagues to collaborate on joint measurements of PSC particles during the winters of 2000/2001 and 2001/2002. The European institutions and their measurements are: Max-Planck-Institut fur Kernphysik - Heidelberg, particle composition CNRS-Laboratoire de Meteorologie Dynamique - Paris, gas phase water vapor concentration; CNR - Istituto di Fisica dell'Atmosfera - Rome, aerosol backscatter and depolarization; and Danish Meteorological Institute - Copenhagen, aerosol backscatter and microphysical/mesoscale modeling. Under previous support these four instruments, plus the particle counters used in this work, have been integrated onto one balloon gondola which will simultaneously measure, in situ, PSC particle composition, concentration, size, depolarization, backscattering, and gas phase concentrations of water. This gondola was flown for the first time in January 2000 in the Arctic. This grant will complete four more Arctic PSC measurements with this unique set of instruments. Two instruments will be used to measure the ambient concentration of condensation nuclei and of aerosol with radii >=0.15 - 10.0 mm in 12 size classes. From these measurements size distributions can be estimated. To provide estimates of the volume of the condensed nitric acid and water in the PSCs, a second aerosol counter will be included to measure the background stratospheric aerosol. This instrument will be fitted with an inlet heater, thus sampling aerosol after they have been exposed to an inlet wall temperature of 250 K for ~ 0.1 s prior to sampling. In addition to providing aerosol size distributions and estimates of condensed volume for the joint measurements, the work will continue to focus on methods to infer particle index of refraction, which is a function of composition, through comparisons of our size distribution measurements with in situ optical scattering measurements at a number of wavelengths. Estimates of particle index of refraction which result from these comparisons will be used in the interpretation of particle composition measurements. These new joint measurements will follow the first successful set of measurements by this configuration of instruments in January 2000. The January 2000 measurements constituted the first direct measurements of single particle composition within a PSC, coincident with a characterization of aerosol size, concentration, phase, and optical properties. These measurements revealed a PSC composed of layers of liquid droplets interspersed with layers of nitric acid hydrates. The agreement amongst the various instruments in capturing the fine layered structure of the cloud was impressive. Many layers of the cloud fit well within our thermodynamic understanding of PSCs while some new HNO3 rich particle types were observed.