AEROSOL ABUNDANCES AND OPTICAL CHARACTERISTICS IN THE PACIFIC BASIN FREE TROPOSPHERE

During NASA's Global Backscatter Experiment (GLOBE) mission flights in November 1989 and May 1990, a DC-8 research aircraft probed the Pacif ic Basin free troposphere for about 90 flight hours in each month betw een +72 and -62 degrees latitude, +130 and -120 degrees longitude, and up to 39,000 feet pressure altitudes. Aerosols were sampled continuou sly in situ by optical particle counters to measure concentration and particle size, and during 48 10-min intervals during each mission by w ire impactors for concentration, size, composition, phase and shape an alyses. The optical particle counters cover a particle diameter range between 0.3 and 20 mum; wire impactors extend the range down to 0.03 m um. Results of particle number, size, shape, together with the assumpt ion of a refractive index corresponding to (NH4)2SO4 to account for th e prevalence of aerosol sulfur, were utilized in a Mie algorithm to ca lculate aerosol extinction and backscatter for a range of wavelengths (0.385 < lambda < 10.64 mum). Computations for 22 randomly selected si ze distributions yield coefficients of extinction E0.525 = (2.03 +/- 1 .20) x 10(-4) km-1 and backscatter beta0.525 = (6.45 +/- 3.49) x 10(-6 ) km-1 sr-1 in the visible, and E10.64 = (8.13 +/- 6.47) x 10(-6) km-1 and beta10.64 = (9.98 +/- 10.69) x 10(-8) km-1 sr-1 in the infra-red, respectively. Large particles (D > 0.3 mum) contribute two-thirds to the total extinction in the visible (lambda = 0.525 mum), and almost 1 00% in the infra-red (lambda = 10.64 mum). These results have been use d to define an IR optical aerosol climatology of the Pacific Basin fre e troposphere, from which it follows that the infra-red backscatter co efficient at lambda = 9.25 mum wavelength fluctuates between 5.0 x 10( -10) and 2.0 x 10(-7) km-1 sr-1 with a modal value 2.0 x 10(-8) km-1 s r-1.