Abstract
The atmospheric extinction of San Pedro Mártir (SPM) is analyzed using 13-color determinations from 294 nights of observations over the years 1973-1983, plus the extinction measures from 272 nights of uvby observations over the years 1984-1999. The general behavior of the normal extinction at SPM is given and analyzed as a function of wavelength and as a function of time. The average atmospheric extinction at SPM, excluding volcanic outbursts, has not changed significantly over the period 1973-1999. A simple 3-component model for the extinction above SPM is derived and presented; the normal, average, non-volcanic aerosols above SPM are well fit by kp(λ) = 0.0254λ-0.87. The extinction determinations for the periods following the volcanoes El Chichón and Pinatubo are given: 13C data show the effects of El Chichón and uvby for Pinatubo. The extinction curves and their variations are analyzed to study the volcanic aerosols and their evolution with time. The Apr/May'98 observing run, when large non-volcanic extinction variations occurred, is also studied and deductions drawn concerning these unusual aerosols.
