Many soil‐derived particles dominated by insoluble material, including Saharan dusts, are known to act as ice nuclei. If, however, dust particles can compete with other atmospheric particle types to form liquid cloud droplets, they have a greater potential to change climate through indirect effects on cloud radiative properties and to affect the hydrological cycle through precipitation changes. By directly collecting and analyzing the residual nuclei of small cloud droplets, we demonstrate that Saharan dust particles do commonly act as cloud condensation nuclei (CCN) in the eastern North Atlantic. Droplet activation calculations support the measurements by showing that due to its slightly hygroscopic nature, even submicron dust can be important as CCN. Given the dual nature of Saharan dust particles as CCN and ice nuclei, this infusion of dust is expected to impact not only droplet size and albedo in small clouds, but ice formation in deep convective clouds.

Polar stratospheric cloud (PSC) distributions in the wintertime Arctic stratosphere and their optical characteristics were measured with a multi‐wavelength airborne lidar system as part of the 1989 Airborne Arctic Stratospheric Expedition. PSCs were observed on 10 flights between January 6 and February 2, 1989, into the polar vortex. The PSCs were found in the 14–27 km altitude range in regions where the temperatures were ≤195 K. Two types of aerosols with different optical characteristics (Types Ia and Ib) were observed in PSCs thought to be composed of nitric acid trihydrate. Type Ia PSCs typically exhibited low scattering ratios (1.2–1.5) and high aerosol depolarizations (30–50%) at 603 run, while Type Ib PSCs had higher scattering ratios (3–8) and lower aerosol depolarizations (0.5–2.5%). Water ice PSCs (Type 2) were observed to have high scattering ratios (>10) and high aerosol depolarizations (>10%) at temperatures ≤190 K.

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A differential absorption lidar has been built to measure CO2 concentration in the atmosphere. The transmitter is a pulsed single-frequency Ho:Tm:YLF laser at a 2.05-microm wavelength. A coherent heterodyne receiver was used to achieve sensitive detection, with the additional capability for wind profiling by a Doppler technique. Signal processing includes an algorithm for power measurement of a heterodyne signal. Results show a precision of the CO2 concentration measurement of 1%-2% 1sigma standard deviation over column lengths ranging from 1.2 to 2.8 km by an average of 1000 pulse pairs. A preliminary assessment of instrument sensitivity was made with an 8-h-long measurement set, along with correlative measurements with an in situ sensor, to determine that a CO2 trend could be detected.