Pinatubo and pre‐Pinatubo optical‐depth spectra: Mauna Loa measurements, comparisons, inferred particle size distributions, radiative effects, and relationship to lidar data

Abstract

The Ames airborne tracking sunphotometer was operated at the National Oceanic and Atmospheric Administration (NOAA) Mauna Loa Observatory (MLO) in 1991 and 1992 along with the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) automated tracking sunphotometer and lidar. June 1991 measurements provided calibrations, optical‐depth spectra, and intercomparisons under relatively clean conditions; later measurements provided spectra and comparisons for the Pinatubo cloud plus calibration checks. June 1991 results are similar to previous MLO springtime measurements, with midvisible particle optical depth τ p(λ = 0.526μm) at the near‐background level of 0.012 ± 0.006 and no significant wavelength dependence in the measured range (λ = 0.38 to 1.06μm). The arrival of the Pinatubo cloud in July 1991 increased midvisible particle optical depth by more than an order of magnitude and changed the spectral shape of τ p(λ) to an approximate power law with an exponent of about −1.4. By early September 1991, the spectrum was broadly peaked near 0.5 μm, and by July 1992, it was peaked near 0.8 μm. Our optical‐depth spectra include corrections for diffuse light which increase postvolcanic midvisible τ p values by 1 to 3% (i.e., 0.0015 to 0.0023). NOAA‐ and Ames Research Center (ARC)‐measured 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<0.25μm), which were progressively depleted in the August‐September 1991 and July 1992 periods. Conversely, both of the later periods had more of the optically efficient medium‐sized particles (0.25<r<1 μm) than did the fresh July 1991 cloud. These changes are consistent with particle growth by condensation and coagulation. The effective, or area‐weighted, radius increased from 0.22 ± 0.06μm in July 1991 to 0.56 ± 0.12 μm in August‐September 1991 and to 0.86 ± 0.29 μm in July 1992. Corresponding column mass values were 4.8 ± 0.7, 9.1 ± 2.7, and 5.5 ± 2.0 μg/cm 2, and corresponding column surface areas were 4.4 ± 0.5, 2.9 ± 0.2, and 1.1 ± 0.1 μm 2/cm 2. Photometer‐inferred column backscatter values agree with those measured by the CMDL lidar on nearby nights. Combining lidar‐measured backscatter profiles with photometer‐derived backscatter‐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 greenhouse gases since the industrial revolution). Heating rates in the Pinatubo layer over MLO are 0.55 ± 0.13 and 0.41 ± 0.14 K d−1 for September 1991 and July 1992, respectively.