Thirty years of in situ stratospheric aerosol size distribution measurements from Laramie, Wyoming (41°N), using balloon‐borne instruments

Abstract

Vertical profiles of size‐resolved aerosol concentrations above Laramie, Wyoming (41°N), have been measured for the past thirty years, 1971‐2001. During this period, two somewhat different optical particle counters have been used to measure particles with radii ≥0.15 μm, whereas the instrument to measure condensation nuclei (CN) has not changed significantly since the late 1970s. The two optical particle counters measure aerosols ≥0.15, 0.25 μm and aerosols ≥0.15–2.0 μm in twelve size classes. These measurements have concentration (N) uncertainties ∝ ±N−0.5, but with a minimum of ±10%. Sizing uncertainties are about ±10%. The impact of these uncertainties on size distribution fitting parameters and aerosol moments are approximately ±30% and ±40%. The long‐term record from these measurements indicates that volcanoes have controlled stratospheric aerosol abundance for 20 of the past 30 years. The present period, beginning in 1997, represents the longest volcanically quiescent period in the record. These and other measurements clearly show that stratospheric aerosol are now in a background state, a state rarely occurring in recent times, and that this background state is not significantly different than observations in 1979. Aerosol volumes and surface areas, inferred from size distributions fit to the measurements, are compared with SAGE II satellite estimates of surface area and volume. For volume the measurements are in agreement within measurement error throughout the record. For surface area there is good agreement for a volcanic aerosol laden stratosphere, but for background aerosol conditions the SAGE II estimates are about 40% less than the in situ measurements. Present aerosol surface areas are ∼1.0 (0.6) μm2 cm−3 in the 15–20 (20–25) km layer based on in situ measurements. The Laramie size distribution record is now available to the community over the internet.