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 τp(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 τp(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 τp 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

As determined in situ by impactor samplers flown on an ER‐2 at 16.5‐ to 20.7‐km pressure altitude and on a DC‐8 at 9.5‐ to 12.6‐km pressure altitudes, the 1991 Pinatubo volcanic eruption increased the particle surface area of stratospheric aerosols up to 50‐fold and the particle volume up to 2 orders of magnitude. Particle composition was typical of a sulfuric acid‐water mixture at ER‐2 altitudes. Ash particles coated with sulfuric acid comprised a significant fraction of aerosol at DC‐8 altitudes. Mie‐computed light extinction increased up to 20‐fold at midvisible and >100‐fold at near‐IR wavelengths. The optical thickness measured through the aerosol layer by an autotracking Sun photometer aboard a DC‐8 aircraft at 10.7‐ to 11.3‐km pressure altitudes shows a spectral shape that is similar to the Mie‐calculated spectral extinction at ER‐2 altitudes. Surface area distributions calculated by inversion of spectral optical depth measurements show characteristics that are similar to the mean surface area distribution resulting from 35 in situ measurements.

[1] Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements obtained with the six-channel NASA Ames Airborne Tracking Sunphotometer (AATS-6) mounted on a twin-engine aircraft during the summer 2000 Puerto Rico Dust Experiment are presented. In general, aerosol extinction values calculated from AATS-6 AOD measurements acquired during aircraft profiles up to 5 km above sea level (asl) reproduce the vertical structure measured by coincident aircraft in situ measurements of total aerosol number concentration. AATS-6 extinction retrievals also agree with corresponding values derived from ground-based lidar measurements for altitudes above the trade inversion. The spectral behavior of AOD within specific layers beneath the top of the aircraft profile is consistent with attenuation of incoming solar radiation by large dust particles or by dust plus sea salt, with mean Å ngström wavelength exponents of $0.20. Values of CWV calculated from profile measurements by AATS-6 at 941.9 nm and from aircraft in situ measurements agree to within $4% (0.13 g/cm 2 ). AATS-6 AOD values measured on the ground at Roosevelt Roads Naval Air Station and during low-altitude aircraft runs over the adjacent Cabras Island aerosol/radiation ground site agree to within 0.004-0.030 with coincident data obtained with an AERONET Sun/ sky radiometer located on Cabras Island. For the same observation times, AERONET retrievals of CWV exceed AATS-6 values by $21%. AATS-6 AOD values measured during low-altitude aircraft traverses over the ocean are compared with corresponding AOD values retrieved over water from upwelling radiance measurements by the Moderate-Resolution Imaging Spectroradiometer (MODIS), Total Ozone Mapping Spectrometer (TOMS), and GOES 8 Imager satellite sensors, with mixed results.

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The Ames airborne, autotracking sunphotometer has been operated aboard a Sandia Laboratories research aircraft to measure magnitudes, temporal/spatial variabilities, and wavelength dependence of optical depths in the near‐ultraviolet to near‐infrared spectrum of smoke from two forest fires and one jet fuel fire and of background air. The results were corrected for Rayleigh scattering and for estimated absorption by ozone and nitrogen dioxide. Characteristic differences in the aerosol optical depths of background atmospheres and of different types of smokes are the following: (1) the magnitude and wavelength dependence of “background” optical depths vary with the geographic location at which the measurements are performed; (2) the wavelength dependence of smoke optical depths depends on the fuels that feed the fires and on the residence time of the smoke cloud in the atmosphere. In general, the jet fuel smoke optical depths tended to be less wavelength dependent (near‐ultraviolet to near‐infrared) than background aerosol optical depths. Forest fire smoke optical depths showed a wide range of wavelength dependences, including incidents of wavelength‐independent extinction.