Lidar observations of the decline of the El Chichon stratospheric load

The eruption of the Mexican volcano El Chichon in Apr. 1982 enhanced the stratospheric aerosol load to a level which so far has not been observed by remote sensing techniques. The ruby lidar system at Garmisch-Partenkirchen recorded an increase of about two orders of magnitude above the background level of the years 1977/78. At northern midlatitudes backscattering was peaking in Jan./Feb. 1983, as illustrated. The decline of the El Chichon stratospheric perturbation is discussed. The decay of the El Chichon stratospheric cloud is characterized by an overall 1/e lifetime of about 12 months. The El Chichon decay is further characterized by seasonal variations of the integral backscattering coefficient, which exhibit a winter maximum and a summer minimum. These variations are inversely correlated with the seasonal mean movement of the tropopause height indicating transport processes

Transport studies with the IFU three-wavelength aerosol lidar during the VOTALP Mesolcina experiment

The IFU three-wavelength lidar was operated during part 1 of the VOTALP valley campaign in the Swiss Mesolcina valley near Grono. The measurements yielded information on both the three-dimensional distribution of the aerosol and the pollution transport in and above the valley. In the afternoon, a pronounced asymmetry of the transverse aerosol distribution was found which may be due to a difference in heating by the sun or to the binding of the valley near Grono. With very short delay with respect to the arrival time of the air pollution at the bottom of the valley a second step with enhanced aerosol formed every day which is ascribed to the build-up of a reverse (anti-valley) wind. During the second half of the investigations this reverse flow is expected to have been influenced by the almost parallel synoptic wind. For the final day of the lidar measurements wind data are available obtained during aircraft flights. This allows an estimate of the upward transport efficiency to be made. The flux ratio of the flow above and in the valley in the afternoon of that day was found to be roughly 90 %. This high efficiency demonstrates the importance of the vertical exchange processes in the Alps for the pollution export out of the boundary layer over Central Europe

The decay of the El Chichon stratospheric perturbation, observed by lidar at northern midlatitudes

The stratospheric perturbation caused by the April 1982 eruption of the Mexican volcano El Chichon (17.3 degrees N) passed through a maximum at northern midlatitudes ten months after the event. The following decay of the aerosol layer could be investigated until the end of 1985 when this period was terminated by renewed volcanism. Lidar backscatter measurements exhibit seasonal decay variations which can primarily be attributed to stratospheric column height variations. In addition, advection of particle loaded air masses contribute to this effect. With respect to the 1977/79 background situation the stratospheric aerosol layer decayed with an e-folding lifetime of about 1 year, when only summer observations were considered. (IFU