Stratospheric condensation nuclei: A climatology in the mid-latitude and Antarctic regions

Abstract

Balloon-borne stratospheric condensation nuclei (CN) measurements are presented since 1982 over Laramie, Wyoming (41°N) and since 1986 over McMurdo Station, Antarctica (78°S). In the Antarctic region, the climatology shows the formation of a ubiquitous stratospheric CN layer between 21-27 km around mid August, reaching its maximum extent between September and early October. CN concentrations increase from 10-20 cm−3 to over 100 cm−3. In the northern mid-latitudes, the climatology shows a quasi-annual and smaller CN increase (3-10 cm−3 to over 20 cm−3), which is observed between 25-31 km in late winter and early spring. Major volcanic eruptions appear to enhance CN layers over Laramie and McMurdo. The Arctic Oscillation (AO) generally correlates with the magnitude of the Laramie CN layer, suggesting the importance of meridional transport. Volatility measurements and nucleation modeling support a sulfuric acid and water composition, and binary homogeneous nucleation as the primary CN formation mechanism in both locations. Bi-monthly CN measurements above Laramie support coagulation as the main reason for the dissipation of the CN layer, and suggest that the layer has a global extent. This topic is further investigated using the Whole Atmosphere Community Climate Model (WACCM) coupled with the Community Aerosol and Radiation Model for Atmospheres (CARMA). Air parcel trajectory analyses indicated that Arctic conditions and the associated ambient temperature changes during transport to Laramie, impacts the magnitude of the mid-latitude CN layer. The CN layer over Antarctica may be associated with the decrease in springtime ozone loss above 20 km.