Background Stratospheric Aerosol Variations Deduced from Satellite Observations

Abstract

AbstractThe Stratospheric Aerosol and Gas Experiment II (SAGE II) aerosol products from 1998 to 2004 have been analyzed for the tendency of changes in background stratospheric aerosol properties. The aerosol extinction coefficient E has apparently increased in the midlatitude lower stratosphere (LS) in both hemispheres, at an annual rate that is as great as 2%–5%. Positive changes in the aerosol surface area density S in the midlatitude LS are most distinct, with a rate of increase that is as high as 5%–6% annually. At the same time, there has been a secular decrease in aerosol effective radius R, especially in the tropical LS, at a rate of up to −2.5% yr−1. Corresponding to these trends, the aerosol number concentration is inferred to have increased by roughly 5%–10% yr−1 in the tropical LS and by 4%–8% yr−1 in the midlatitude LS. Changes in aerosol mass are also deduced, with rates of increase in the midlatitude LS that are in the range of 1%–5% yr−1. The large uncertainty in operational S product is the major factor influencing the trend in S, aerosol number concentrations, and mass. The authors’ global assessment supports the speculation of Hofmann et al. on the basis of local observations that the cause of an increase in lidar backscatter over a similar period was a consequence of aerosol particle growth due to enhanced anthropogenic sulfur dioxide emissions. Moreover, it is found that an increase in the injection rate of condensation nuclei from the troposphere to the stratosphere at tropical latitudes is required to sustain the increase in stratospheric aerosol concentrations identified in this analysis.