The evolution of volcanic material injected into the stratosphere by Mount
Pinatubo in June of 1991 [ McCormick, 1992] provided an
excellent opportunity to test models of mid-latitude chemistry. The poleward
and downward transport of aerosols observed by the orbiting Stratosphere
Aerosol and Gas Experiment II (SAGE II) instrument, as outlined by Trepte
et al. [1993] and [ McCormick and Veiga, 1992], largely
support the first-order view that ozone is
produced rapidly in the tropics and transported along quasi-horizontal surfaces
to higher latitudes where the production rate slows and where it is slowly destroyed
by catalytic reactions with O, NO
, HO
(OH and HO
), ClO
(Cl and ClO),
and BrO
(Br and BrO) radicals.
Early measurements of the Pinatubo plume by Bluth et al.
[1992], Deshler et al. [1992], Goldman et
al. [1992] and Read et al. [1993] confirmed that
it represented an enormous perturbation of SO
to the stratosphere,
increasing the aerosol mass by about a factor of 30. Column observations
[ Mankin et al., 1992 and Wallace and Livingston, 1992]
showed, however, that unlike the eruption of El Chichón, that of Pinatubo had no
noticeable effect on the column abundance of HCl. Therefore, HCl was
either scavenged efficiently in the troposphere [ Tabazadeh and
Turco, 1993], or it was a minor component of the ejecta. SO
was
subsequently oxidized to form H
SO
[ Deshler et al.,
1992], which is supersaturated under the cold conditions of the stratosphere
and readily condenses onto existing particles or forms new ones
[ Wilson et al., 1993 and Borrmann et al., 1993].
The enhanced aerosols also influenced the radiation
balance of the atmosphere, which also could have altered the photochemical
production and transport of ozone
[ Valero and Pilewskie 1992, P. Russell et al., 1993, and
Pueschel et al., 1994].