Stratospheric Aerosol Measurements over Mauna Loa Observatory, Hawaii

The stratospheric aerosol layer has been monitored above Mauna Loa Observatory (MLO) by lidar since late 1974. The original lidar used a ruby laser (694 nm) and since 1994 a Nd:YAG laser (532 nm) has been in operation. MLO is one of four baseline stations operated by the NOAA/Climate Monitoring and Diagnostics Laboratory. At 19.6 degrees north, MLO is generally on the edge of the tropical atmosphere but experiences mid-latitude air masses as well. The observatory lay in the path of the two largest eruptions (in terms of stratospheric aerosol) in the past 35 years, El Chichon in 1982 and Mount Pinatubo in 1991. Several other smaller eruptions have also been observed.

The exponential decay rates of stratospheric aerosol from the El Chichon and Mount Pinatubo eruptions are both about one year when calculated over an interval of several years. On smaller time scales there are other distinct decay rates attributable to the phase of the quasibiennial oscillation (QBO) in the tropical stratospheric winds. The aerosol is sequestered during the easterly phase of the QBO and transport is restricted. The aerosol layer is also lofted during this phase and the top of the layer can reach 35 km. During the transition to the westerly phase, distinct decreases in the aerosol loading are observed and the top of the layer decreases to 27 km at times. For the past four years the stratospheric aerosol above MLO has remained at a uniform background level with distinct annual variations. There is a clear seasonal cycle in the background integrated (15.8-33 km) backscatter level, which peaks in the winter at 1.7E-4 per sr and has a minimum of 0.6E-4 per sr in the summer.

There is a variation in backscatter levels of about +/- 25% on a week to week time scale, which is a result of varying transport. Lidar observations of aerosol backscatter at northern mid-latitude sites currently are on average about 10 to 30% lower in magnitude than the average backscatter at MLO, which suggests that MLO is influenced to a greater extent by a tropical background stratospheric aerosol source. Another indication of differing aerosol characteristics in different latitudes is in particle size. Since October 1996, measurements of backscatter at 1064 nm have been possible and can be used with the 532 nm measurements to provide particle size information. Air parcel trajectory calculations indicate that at times when MLO is under the influence of mid-latitude air, the average particle size is larger than in tropical air. This difference is quite constant throughout the stratospheric aerosol layer (16 to 30 km) and would be consistent with a younger aerosol in the stratospheric tropical reservoir than present in aged mid-latitude air.