MULTIWAVELENGTH COMPARISON OF MODELED AND MEASURED REMOTE TROPOSPHERIC AEROSOL BACKSCATTER OVER PACIFIC-OCEAN

Aerosol concentrations and size distributions in the middle and upper troposphere over the remote Pacific Ocean were measured with a forward scattering spectrometer probe (FSSP) nn thp NASA DC-8 aircraft during NASA's Global Backscatter Experiment (GLOBE) in May-June 1990, The FS SP size channels were recalibrated based on refractive index estimates from flight-level aerosol volatility measurements with a collocated l aser optical particle counter (LOPC). The recalibrated FSSP size distr ibutions were averaged over 100-s intervals, fitted with lognormal dis tributions and used to calculate aerosol backscatter coefficients at s elected wavelengths, The FSSP-derived backscatter estimates were avera ged over 300-s intervals to reduce large random fluctuations. The smoo thed FSSP aerosol backscatter coefficients were then compared with LOP C-derived backscatter values and with backscatter measured at or near flight level from four lidar systems operating at 0.53, 1.06, 9.11, 9. 25, and 10.59 mu m. Agreement between FSSP-derived and lidar-measured backscatter was generally best at flight level in homogeneous aerosol fields and at high backscatter values. FSSP data often underestimated low backscatter values especially at the longer wavelengths due to poo r counting statistics for larger particles (> 0.8 mu m diameter) that usually dominate aerosol backscatter at these wavelengths. FSSP data a lso underestimated backscatter at shorter wavelengths when particles s maller than the FSSP lower cutoff diameter (0.35 mu m) made significan t contributions to the total backscatter.