PINATUBO AND PRE-PINATUBO OPTICAL-DEPTH SPECTRA - MAUNA-LOA MEASUREMENTS, COMPARISONS, INFERRED PARTICLE-SIZE DISTRIBUTIONS, RADIATIVE EFFECTS, AND RELATIONSHIP TO LIDAR DATA

The Ames airborne tracking sunphotometer was operated at the National Oceanic and Atmospheric Administration (NOAA) Mauna Loa Observatory (M LO) in 1991 and 1992 along with the NOAA Climate Monitoring and Diagno stics Laboratory (CMDL) automated tracking sunphotometer and lidar. Ju ne 1991 measurements provided calibrations, optical-depth spectra, and intercomparisons under relatively clean conditions; later measurement s provided spectra and comparisons for the Pinatubo cloud plus calibra tion checks. June 1991 results are similar to previous MLO springtime measurements, with midvisible particle optical depth tau(p)(lambda = 0 .526 mum) at the near-background level of 0.012 +/- 0.006 and no signi ficant wavelength dependence in the measured range (lambda = 0.38 to 1 .06 mum). The arrival of the Pinatubo cloud in July 1991 increased mid visible particle optical depth by more than an order of magnitude and changed the spectral shape of tau(p)(lambda) to an approximate power l aw with an exponent of about -1.4. By early September 1991, the spectr um was broadly peaked near 0.5 mum, and by July 1992, it was peaked ne ar 0.8 mum. Our optical-depth spectra include corrections for diffuse light which increase postvolcanic midvisible tau(p) values by 1 to 3% (i.e., 0.0015 to 0.0023). NOAA- and Ames Research Center (ARC)-measure d spectra are in good agreement. Columnar size distributions inverted from the spectra show that the initial (July 1991) post-Pinatubo cloud was relatively rich in small particles (r<0.25 mum), which were progr essively depleted in the August-September 1991 and July 1992 periods. Conversely, both of the later periods had more of the optically effici ent medium-sized particles (0.25<r<1 mum) than did the fresh July 1991 cloud. These changes are consistent with particle growth by condensat ion and coagulation. The effective, or area-weighted, radius increased from 0.22 +/- 0.06 mum in July 1991 to 0.56 +/- 0.12 mum in August-Se ptember 1991 and to 0.86 +/- 0.29 mum in July 1992. Corresponding colu mn mass values were 4.8 +/- 0.7, 9.1 +/- 2.7, and 5.5 +/- 2.0 mug/cm2, and corresponding column surface areas were 4.4 +/- 0.5, 2.9 +/- 0.2, and 1.1 +/- 0.1 mum2/cm2. Photometer-inferred column backscatter valu es agree with those measured by the CMDL lidar on nearby nights. Combi ning lidar-measured backscatter profiles with photometer-derived backs catter-to-area ratios gives peak particle areas that could cause rapid heterogeneous loss of ozone, given sufficiently low particle acidity and suitable solar zenith angles (achieved at mid- to high latitudes). Top-of-troposphere radiative forcings for the September 1991 and July 1992 optical depths and size distributions over MLO are about -5 and -3 W m-2, respectively (hence comparable in magnitude but opposite in sign to the radiative forcing caused by the increase in manmade greenh ouse gases since the industrial revolution). Heating rates in the Pina tubo layer over MLO are 0.55 +/- 0.13 and 0.41 +/- 0.14 K d-1 for Sept ember 1991 and July 1992, respectively.