SIZE-RESOLVED MEASUREMENTS OF LIGHT-SCATTERING BY AMBIENT PARTICLES IN THE SOUTHWESTERN USA

Measurements are reported from integrating nephelometers modified to s ample alternately from a fine-particle (D(aero) < 2.5 mum) cyclone and an unrestricted inlet. These nephelometers were successfully operated at two sites in the southwest U.S.A. as part of the SCENES program in the spring and summer of 1989. One of the nephelometers was collocate d with a transmissometer monitoring total extinction. Fine- and total- particle samples were collected at each site for determination of mass and light absorption. Intercomparisons of the collocated daytime scat tering, absorption, and total extinction measurements indicate that th e nephelometer reported somewhat less than half the actual scattering by coarse particles. When this under-response is corrected for, the ne phelometer and transmissometer show good agreement. The corrected data indicate that coarse particles were responsible from one-quarter to o ne-third of the total particle scattering. Predominantly coarse-partic le dusts are estimated to have contributed one-third to one-half of th e total-particle scattering.