Multiangle light-scattering measurements of refractive index of submicron atmospheric particles

Multiangle light scattering (MLS) measurements of monodisperse atmospheric particles ranging in diameter from 0.2 to 0.8 mu m were made with the DAWN-A optical detector during the Southeastern Aerosol and Visibility Study (SEAVS). The study was conducted on the southwestern edge of the Great Smoky Mountains National Park from July 15 to August 25, 1995. Individual particles were separated into spherical and nonspherical classes on the basis of the variability in elastic light scattering measured at eight azimuthal angles. Values of the real part of the refractive index, n, were then inferred for spherical particles by comparing Lorenz-Mie theory calculations to calibrated light scattering responses from narrow-aperture detectors positioned at seven polar angles ranging from 40-140 degrees. The instrument was calibrated with submicron laboratory particles with n ranging from 1.38 to 1.61 for an illumination wavelength of 488 nm. The overall uncertainty in measured n was estimated to be +/- 0.02. For hygroscopic particles, refractive index was found to decrease as relative humidities increased due to the addition of water; the study-average value at low humidities (dry particles) was 1.49, while the average value at high humidities (wet particles) was 1.42. The lowest and highest values measured during the study were 1.34 and 1.54. Daily size- and RH-dependent measurements are compared to indices modeled for mixtures of ammoniated sulfate, organic carbon (OC), elemental carbon (EC), and water. Size-dependent mass concentrations of these species, with the exception of water, were measured with MOUDI cascade impactors.