Variability in the stratospheric background aerosol over Mauna Loa Observatory

The stratospheric aerosol layer above Mauna Loa Observatory (MLO), Hawaii, has been at low background levels for the past 5 years. This is the first t ime that an extended nonvolcanic background aerosol period has been observe d since modem measurements began in the early 1960s. Lidar backscatter at 5 32 nm shows a distinct maximum in winter and minimum in summer. The five an nual cycles have included three easterly phases and two westerly phases of the quasibiennial oscillation (QBO). Differences in aerosol backscatter ver sus altitude profiles are seen for different QBO phases. There is also a sw itching of about 25% in the magnitude of the aerosol backscatter on a weekl y time scale with varying particle size derived from multiwavelength data. Assumption of a tropical particle source at background suggests that the di ffering particle regimes are tropical and midlatitude.