Measurements of Total Aerosol Concentration in the Stratosphere: A New Balloon-Borne Instrument and a Report on the Existing Measurement Record

The Stratospheric Total Aerosol Counter (STAC) is a lightweight balloon-borne instrument that utilizes condensational growth techniques to measure the total aerosol concentration. STAC is a miniaturized version of the legacy Wyoming condensation particle counter that operated from 1974 through 2020 in the middle latitudes and polar regions, with a few measurements in the tropics. Here we provide a description of the STAC instrument and the total aerosol measurement record, demonstrating that typical total aerosol profiles exhibit a peak in number mixing ratio, with values between 800 and 2,000 particles per mg of air (mg-1), just below the lapse rate tropopause (LRT). In the tropics and middle latitudes, mixing ratios decrease above the LRT likely due to coagulation and scavenging that results in a transfer of mass to the fewer but larger aerosol particles of the Junge layer. Exceptions to this occur in the spring time in the middle latitudes where a new particle layer between 20 and 25 km is frequently observed. In the poles, total aerosol profiles exhibit two distinct features: new particle formation in austral spring, and an increasing mixing ratio above 17 km likely due to the presence of meteoric smoke that has been concentrated within the polar vortex. High observed stratospheric particle mixing ratios, in excess of 2,000 mg-1, are observed in the polar new particle layer and at the top of polar profiles. Particles in the stratosphere range in size from a few nanometers to many micrometers in diameter. These particles are important to the radiation balance of Earth as well as chemical processes that occur in the atmosphere. Particles with diameters greater than about 50 nm are detectable by a wide variety of scientific instruments using various techniques to measure light scattered by the particles. Particles smaller than this need to be grown to larger diameters before their concentration can be measured, and therefore these measurements must be done in situ. This paper describes the design and capabilities of an instrument, the Stratospheric Total Aerosol Counter, that makes in situ measurements from high-altitude balloons of the total aerosol concentration. Versions of this instrument have been flown around the globe since 1974. The second portion of this paper provides an overview of the total aerosol concentration record since 1989, comprising more than 250 profiles. The key findings are that in tropical regions there is a significant source of particles from the total aerosol concentration maximum at the top of the troposphere. In polar regions, a significant source of particles is from meteorites burning up when they strike the Earth's atmosphere. The hardware and operation of a balloon-borne Stratospheric Total Aerosol Counter (STAC) is detailed for measurements in the lower and middle stratosphere Some of the highest total aerosol mixing ratios are found in the polar mid-stratosphere suggesting an under appreciation of this region's importance Volcanic injections of sulfur have minimal impact on the total aerosol concentration, but significantly increase the mass of larger aerosol