Comparison of aerosol extinction coefficients, surface area density, and volume density from SAGE II and in situ aircraft measurements

Abstract

Aerosol size distributions measured in the lower stratosphere by the University of Denver's focused cavity aerosol size spectrometer (FCAS) were used to calculate aerosol extinction coefficients, surface area densities, and volume densities for comparison with the Stratospheric Aerosol and Gas Experiment (SAGE) II version 6.20 data products. Profiles from ten high‐altitude aircraft flights, ranging approximately 8.5 to 20.5 km altitude, are compared with nearby SAGE II occultation events. Nine of the flights date from late 1995 through 1997, and one is from early 2004. Extinctions are found to agree within instrumental uncertainties except at 386 nm below 17 km, where the SAGE II values are systematically low despite their large errors. Above 12 km, where the comparisons are statistically significant, the SAGE II volume density is about 35% lower than the FCAS measurements, though just within combined uncertainties, while the SAGE II surface area density is low compared to FCAS by a factor of 1.5 to 3. The limitations of principal component analysis may account for up to two thirds of the discrepancy in surface area density.