Airborne Sun photometer measurements of aerosol optical depth and columnar water vapor during the Puerto Rico Dust Experiment and comparison with land, aircraft, and satellite measurements

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 Ångström 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.