Stratospheric Aerosol Measurements I: Time Variations at Northern Midlatitudes

The results of over 70 balloon soundings, by the University of Wyoming's Atmospheric Physics Group mostly during 1972 and 1973 from a number of stations, are being utilized in a study of the temporal and spatial distribution of the global stratospheric aerosol. This paper deals with the instrumentation, calibration, etc., and with the results of monthly soundings from the Laramie (41°N) station during the approximately two-year period of measurement. This period comprises an interval apparently free of major volcanic activity just prior to the extensive volcanic contributions to the stratospheric aerosol which occurred in late 1974. It thus may be compared to the pre-Agung era and is perhaps as close to the so-called “natural stratospheric background conditions,” if indeed such conditions ever exist, as will likely be attained in the near future. A simple seasonal variation in the total stratospheric aerosol loading below about 20 km altitude dominates the temporal variation at Laramie, resulting in a maximum in winter and a minimum in summer. A high correlation with tropopause height is observed. The seasonal variation appears to be superimposed on a long-term variation, the nature of which is unknown. Above 20 km, no seasonal variation is evident, and the natural aerosol production processes appear to be nearly in equilibrium with loss processes.