Monthly balloon borne aerosol backscatter measurements at ~173¿ and wavelengths of 940 and 490 nm have been made at Lauder, New Zealand (45 ¿S), and Laramie, Wyoming (41 ¿N), since 1992. The presentation and analysis here focus on the trophosperic results which suggest that a similar quasi steady state background aerosol appears over both sites with superimposed seasonal disturbances that are probably related to arid region dust storms and biomass burning. Volcanic influences on the free troposphere are found to be minimal. In contrast to the background aerosol, the aerosol disturbances show a strong hemispheric difference with fewer perturbations over Lauder as well as significantly differing vertical profile structure, as might be expected at a remote site. Our interpretation of the higher resolution backscatter measurements substantially supports the conclusions drawn from the lower resolution stratospheric aerosol and gas experiment (SAGE) and stratospheric aerosol measurement (SAM) satellite observations.¿ 1997 American Geophysical Union