Condensation nuclei measurements in the midlatitude (1982–2012) and Antarctic (1986–2010) stratosphere between 20 and 35 km

Balloon‐borne stratospheric condensation nuclei (CN) measurements have been made from McMurdo Station, Antarctica (78°S), 1986–2010, and from Laramie, Wyoming (41°N), 1982 to the present. In the Antarctic region, the measurements show the formation of a layer of enhanced concentrations of stratospheric CN, between 21 and 27 km, around mid August, reaching its maximum extent between September and early October. CN concentrations increase from backgrounds of 10–20 cm −3 to over 100 cm −3 in the layer. In the northern midlatitudes, the measurements show a quasi‐annual and smaller layer of enhanced CN concentrations between 25 and 31 km in late winter and early spring. In the quasi‐annual layers, CN concentrations increase from backgrounds of 1–10 cm −3 to over 20 cm −3 . Volcanic eruptions appear to enhance the CN layers observed over Laramie and McMurdo. The Arctic Oscillation 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 likely CN formation mechanism in both locations. Bimonthly measurements above Laramie support coagulation as the main reason for the dissipation of the CN layer. Air parcel trajectory modeling confirms that the CN layer forms locally to McMurdo and that it is related to solar exposure, while above Laramie trajectory analysis indicates that Arctic conditions and ambient temperature changes during northerly transport impact the magnitude of the CN layer above Laramie.