Abstract. In situ measurements of size-dependent water uptake by atmospheric particles made with a tandem differential mobility analyzer (TDMA) and size-resolved chemical composition of aerosol samples collected with cascade impactors in the Smoky Mountains have been examined in order to ascertain the influence of organic carbon compounds on aerosol hygroscopicity. Particles were dried to -5 % relative humidity (RH) before entering the TDMA, leading us to believe that salts of ammonium and sulfate were in crystalline states for relative humidities below their expected deliquescent points. TDMA-measured water content was found to be in excess of the sulfate-associated water modeled using laboratory data for binary aqueous solutions and the method of Zdanovskii-Stokes-Robinson for multicomponent solutions over a wide range of humidities (RH=5-85%). Furthermore, excess water was observed to increase in proportion to the organic fraction of mass associated with each examined size in the range 0.05 to 0.4 gm. These data are used to obtain an empirical relationship between the amount of water associated with particulate organics and relative humidity. This analysis shows that organic-associated water content is considerably less than that of sulfate compounds, on a volume basis, for high RH, but comparable or greater for low RH. These results are consistent with laboratory data for water absorption by a range of organics vis-h-vis ammonium salts of sulfate.

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Multi-angle azimuthal measurements of light scattering by individual submicrometre particles have been made with the DAWN-A multi-angle light scattering (MALS) instrument. Variability in azimuthal scattering at a polar angle of 55 • has been used to determine spherical and nonspherical fractions of laboratory-produced and atmospheric aerosols classified at mobility diameters of 0.2-0.8 µm (size parameters of 1.3-5.1). Parameters used for shape fractionation were derived empirically by comparing scattering signatures from crystalline sodium chloride cubes and spherical dioctyl sebacate (DOS) droplets and then applied to test aerosols and atmospheric aerosols sampled at low (3-10%) and high (50-80%) values of relative humidity. DOS and NaCl contrast was great enough to permit their distinction with less than 5% uncertainty for 0.4-0.8 µm, but contrast was observed to decrease as the size parameter approached unity. Crystalline ammonium sulphate test aerosol appeared mildly aspherical, with nonspherical fraction values in the range 15-40%. Nonspherical fractions of atmospheric aerosols measured during the Southeastern Aerosol and Visibility Study (SEAVS), conducted during July and August of 1995, were generally small (<10%), providing confidence in our application of Mie theory for determination of aerosol optical properties. DAWN-A shape data have been compared with aerosol chemistry and hygroscopicity measured during SEAVS, revealing an overall strong correlation of nonspherical fractions to soil dust content and 'less hygroscopic' fractions.

Multiangle light scattering (MLS) measurements of monodisperse atmospheric particles ranging in diameter from 0.2 to 0.8 µm 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 LorenzMie 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.

The Wide-range Particle Spectrometer (WPS™) is a recently introduced commercial instrument with the unique capability to measure size distributions of aerosols from 0.01 to 10 µm in diameter. The instrument includes a Scanning Mobility Spectrometer (SMS) comprised of a Differential Mobility Analyzer (DMA) and a Condensation Particle Counter (CPC) for particle measurement from 0.01 to 0.5 µm and a Laser Particle Spectrometer (LPS) for measurement in the ~0.4 to 10 µm range. These components are small enough to fit into a small portable cabinet (~26 kg) with all accompanying control hardware and electronics. No external pumps are required and power consumption is only about 150 W.The DMA is calibrated with Standard Reference Materials (SRM) from the U.S. National Institute of Standards and Technology (NIST), including SRM 1691 and SRM 1963a. These are uniform size polystyrene latex (PSL) spheres available from NIST with mean diameters of 0.269 µm and 0.1018 µm respectively. The CPC has a dual reservoir design to prevent the working fluid from being contaminated by water due to moisture condensation in the condenser. The LPS is calibrated with four NIST-traceable PSL sphere sizes. Calibration curves are generated not only for PSL (real refractive index of 1.585), but also for discrete values real refractive index ranging from 1.30 to 1.60. This procedure allows the user to select the most appropriate curve for determination of the light-scattering-equivalent sphere size that takes into account the effect due to refractive index of real aerosols. The LPS has a wide-angle collection optics design to produce a monotonic response curve for routine measurement in the field.