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Richard J. Vong and David S. Covert

Abstract

Simultaneous field measurements of aerosol and cloud droplet concentrations and droplet diameter were performed at a maritime site on the coast of Washington State. The aerosol and droplet spectra were compared for estimating cloud condensation nucleus concentration (N ccn) as the number of particles with diameters greater than 80 nm, that is, N ccnN(D p > 80 nm). Several analytical approaches were developed and applied to the data, including a stratification of the observations into periods of high and low liquid water content (LWC) based on a threshold value of 0.25 g m−3. The aerosol data were corrected for inertial losses of cloud droplets at the inlet using wind speed and droplet size; this correction improved the measured relationships between N ccn and droplet number concentration (N d). These measurements, when coupled with the range of possible aerosol chemical compositions, imply a cloud supersaturation of 0.24%–0.31% at the Cheeka Peak sampling site during periods of high LWC.

The observations of droplet and aerosol spectra supported Twomey’s cloud brightening hypothesis in that N ccn was highly correlated (r 2 = 0.8) with N d in apparent 1:1 proportions. For the investigated range (50 cm−3 < N d < 600 cm−3) droplet effective diameter (D eff) was very sensitive to variation in N ccn for 50 cm−3 < N ccn < 200 cm−3, somewhat sensitive for 200 cm−3 < N ccn < 400 cm−3, but not very sensitive to variation in aerosol number for N ccn > 400 cm−3. A model was applied to the aerosol and droplet data to predict droplet size, as D eff, from N−0.33ccn and LWC. Predicted values for D eff agreed (r 2 = 0.8) with D eff determined directly from the cloud droplet spectra, suggesting that this approach should be useful in climate modeling for predicting cloud droplet size from knowledge of N ccn and LWC.

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Kathleen K. Crahan, Dean A. Hegg, David S. Covert, Haflidi Jonsson, Jeffrey S. Reid, Djamal Khelif, and Barbara J. Brooks

Abstract

Although the importance of the aerosol contribution to the global radiative budget has been recognized, the forcings of aerosols in general, and specifically the role of the organic component in these forcings, still contain large uncertainties. In an attempt to better understand the relationship between the background forcings of aerosols and their chemical speciation, marine air samples were collected off the windward coast of Oahu, Hawaii, during the Rough Evaporation Duct project (RED) using filters mounted on both the Twin Otter aircraft and the Floating Instrument Platform (FLIP) research platform. Laboratory analysis revealed a total of 17 species, including 4 carboxylic acids and 2 carbohydrates that accounted for 74% ± 20% of the mass gain observed on the shipboard filters, suggesting a possible significant unresolved organic component. The results were correlated with in situ measurements of particle light scattering (σ sp) at 550 nm and with aerosol hygroscopicities. Principal component analysis revealed a small but ubiquitous pollution component affecting the σ sp and aerosol hygroscopicity of the remote marine air. The Princeton Organic-Electrolyte Model (POEM) was used to predict the growth factor of the aerosols based upon the chemical composition. This output, coupled with measured aerosol size distributions, was used to attempt to reproduce the observed σ sp. It was found that while the POEM model was able to reproduce the expected trends when the organic component of the aerosol was varied, due to large uncertainties especially in the aerosol sizing measurements, the σ sp predicted by the POEM model was consistently higher than observed.

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Sarah J. Masonis, Theodore L. Anderson, David S. Covert, Vladimir Kapustin, Antony D. Clarke, Steven Howell, and Kenneth Moore

Abstract

Ground-based aerosol optical measurements made at near-ambient relative humidity (RH) under clean marine sampling conditions are presented and compared to 1) almost identical optical measurements made at a polluted continental site and 2) optical properties calculated from measured size distributions and Mie theory. The use of Mie theory (which assumes homogeneous spheres) is justified based on the fact that the sea-salt aerosol was measured in a hydrated state. This study focuses on the extinction-to-backscatter ratio S, an optical property required to interpret remote measurements by elastic backscatter lidar. For clean marine conditions, S is found to be 25.4 ± 3.5 sr at 532 nm (central value ± 95% confidence uncertainty). Other optical properties reported include single-scattering albedo, wavelength dependence of scattering, fraction of scattering due to submicrometer particles, and hemispheric-backscatter fraction, as well as the extensive properties (e.g., scattering coefficient) upon which these intensive properties are based. In addition, correlation scale lengths are examined via the autocorrelation function. Except during deliberate drying experiments that lowered the measurement RH below 43%, S exhibited little variation with RH. A subtle but clearly detectable change in optical properties was observed at the onset of volcanically influenced sampling conditions.

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