GEOPHYSICAL RF3EARCH LETI]ERS, VOL. 19, NO.2, PAGF_.S 179-182, JANUARY 24,1992 AIRBORNE OBSERVATIONS OF SO2, HC1, AND 03IN THE STRATOSPHERIC PLUME OF THE PINATUBO VOLCANO IN JULY 1991 William G.Mankin, M. T. Coffey, and Aaron Goldmanl National Center for Atmospheric Research, Boulder, Colorado Abstract. We have used a high resolution infrared spectrometer aboard the NASA Wallops Flight Facility Electra aircraft to measure the totalcolumn amount of SO2,03, and HC1 above the aircraft while flying overthe Caribbean three weeks after the June 15 eruptionof Mt. Pinatuboin the Philippines. South of 20øN latitude we observed columns of S02 ranging from 2.0 x 1016 to 3.7x 1016 molecules-cm -2. Inaddition, the column amount of HC! averaged 1.5x !015 rnolecules-cm -2 inthe region ofthe plume. This may represent a small increase in HC1 abovethe amount,estimated from our previous measurements, that wouldhavebeen present had there been•no volcanic eruption, but the increase is substantially less than thatseen following the 1982 eruptions ofE1 Chichtn[Mankin andCoffey, 1984]. Introduction Volcanoes whicherupt powerfully enough to injectlarge quantities of gases andparticles intothestratosphere can have an important impact onthe atmosphere. The radiative effects ofthe particles injected directly or, more importantly, created chemically from oxidation of SO2 can change climate globally [Hansen et al., 1992]. Small sulfuric aciddroplets cause a decrease in the tropospheric temperature by scattering incoming sunlight backto space.In addition, we have come to recognize more recently that these particles can serve as sites for heterogeneous chemistrywhich 'can deplete ozone [Hofmann and Solomon,1989, Michelangeli et al., 1989; Brasscur et al., 1991]. With increased levels of chlorine in the stratosphere, therisk of significant ozone depletion by these processes, similar to the processes that cause the Antarctic ozonehole, has increased. Therefore a major volcanic eruption into the stratosphere iscause forconcern as well as an opportunity for scientific investigation. In June 1991,the Pinatubo volcano (!5.!4øN, 120.35øE) inthe Philippines experienced a series of explosive eruptions. The eruptions were among thelargest volcanic eruptions of this century [Smithsonian Institution, 1991]. On June 15 the activity culminated in aparoxysmal eruption formore than 15 hours, which injected large amounts of gas and particles to altitudes well into the stratosphere (up to30km) [Smithsonian Institution, 1991]. The prevailing stratospheric winds transported the ash and gas in the stratosphere initially southwesterly and then westerly atrates up to75km/hr.This initial transport was somewhat different than that following the INCAR Affiliate Scientist from the University of Denver Copyright 1992 bythe American Geophysical Union. Paper number 91GL02942 0094-8534/92/91GL-02942503.00 1982 eruptions of E1 Chich6n which were morezonaland resulted in less material south of theequator. Measurements of theaerosol cloud by theAdvanced Very HighResolution Radiometer on the NOAA 11 satellite and measurements of the SO2cloud withtheTotalOzone Mapping Spectrometer on Nimbus 7 showed that by8 July1991, approximately 3 weeks after theextremely powerful mid-June eruptions, thePinatubo cloud had circled the Earth and formed a semi-continuousband fromroughly 20øS to 20øN latitudes. To observe thegaseous and particulate content and extent of the plume and its effect on the atmosphere, NASA organized an expedition aboard the NASA WallopsFlight Facility Electra aircraft. The expeditionwas based in Bridgetown, Barbados (13.25øN,59.60 ø W). The airborne instrument package comprised a 532 nm upward pointing lidar to measure aerosol distributions, from the NASA Langley ResearchCenter; a radiometerto measuredirect and diffuse solar radiation, from the NASA Ames Research Center;, a vertically looking ultraviolet correlation spectrometer (COSPEC) to measure SO2 column amounts, operated by the Atmospheric Environment Service of Canada; andaninfrared Fourier transform spectrometer to measure colurn amount of a numberof gases, providedby the National Center for Atmospheric Research. Thispaper will describe results from theNCAR Fourier transform spectrometer. Observations Six research flights wereflown between 7 July and 14 July1991. Figure ! shows the4 Electra flight paths during the times that the NCARspectrometer was employed tocollect high resolution infrared spectra of the setting sun; the direction offlight was selected tobring radiation from the sun directly in through a port side window. Although weflewat as high an altitude aswas possible withthe Electra (5.5-7.0 km), we were still in the middletroposphere and the large amount of water vapor abovethe flight altitude caused difficulty in extracting theamounts of stratospheric trace gases in spectral regions withstrong water absorption. The spectrometer viewed the sun at solar elevations between 0øand 15 øthrough aninfrared transmitting window in the side of theaircraft. The atmospheric paths contributing to an observation are all westof the aircraft flight path. For a solar elevation of 5ø an air parcel in the line of sight at 25 km altitude is approximately 180 km west of the aircraft. Typically, 250 interferograms were recorded during each flight. Each interferogram, representing 6 sec of observation, was Fourier transformed to produce an infrared spectrum with a resolution of 0.06 cm -1 (FWHM). Individual spectra were averaged in groups of 10 to produce a higher signal to noise ratioand to smooth atmospheric variability. Previous resultsfrom the NCAR FTS system include measurements of HC1and HF [Mankin andCoffey, 1983],03 179