In spite of nearly three decades of research, significant gaps in our understanding of processes in Polar Stratospheric Clouds (PSC) still exist. A limiting factor in advancing our understanding of PSCs has been the relative sparseness of observations, primarily long-term, but spatially limited data from ground-based stations and solar occultation satellites, interspersed with data from occasional intensive, but still localized field campaigns. However, the observational database has expanded greatly over the past decade with the advent of three satellite missions.
What we know is that PSCs play key roles in the depletion of stratospheric ozone. Heterogeneous reactions on PSCs convert the benign chlorine reservoirs HCl and ClONO2 to chlorine radical species that are involved in catalytic ozone destruction. Rates of these heterogeneous reactions depend on particle surface area and composition, which includes liquid binary H2SO4/H2O droplets (background stratospheric aerosol); liquid ternary HNO3/H2SO4/H2O droplets; solid nitric acid trihydrate (NAT) particles; and H2O ice particles. PSCs also affect ozone chemistry through the removal of HNO3 from the polar stratosphere (denitrification) via the formation and sedimentation of large NAT PSC particles. Denitrification enhances ozone depletion by delaying the reformation of the benign chlorine reservoirs.
What we do not know is how NAT particles form and evolve and lead to denitrification. Furthermore, it is uncertain to what degree chlorine activation occurs on cold background aerosol prior to the formation of PSCs. These uncertainties limit our ability to accurately represent PSC processes in global models and call into question our prognostic capabilities concerning future ozone loss in a changing climate. This is of particular concern in the Arctic, where winter temperatures hover near the PSC threshold and, hence, future stratospheric cooling could lead to enhanced cloud formation and substantially greater ozone losses.
The recent satellite missions that shed new light on PSC processes are: the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat launched in 2002, the Microwave Limb Sounder (MLS) on Aura launched in 2004, and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on CALIPSO launched in 2006. These datasets, which provide measurements of PSC occurrence and composition and relevant gas species on unprecedented vortex-wide scales, have ushered in a new era in PSC research. Numerous new research endeavors have started utilizing these satellite datasets, challenging our present knowledge of PSC processes and modeling capabilities.
This activity will bring together key scientists from the different satellite missions and in situ measurement experts, as well as theoreticians and modelers to address current key questions related to PSCs. The main objectives are to: assess recent research developments, compare remote and in situ datasets to identify their strengths and limitations, identify the key PSC characteristics required by global models and available from measurements, synthesize the new datasets into a state of the art PSC climatology, and identify remaining open science questions. This activity will ultimately lead to improved representation of PSC processes in global climate models.
The end products will include several papers describing our current understanding of PSCs as well as and a review paper on the overall state of PSC science.