Using New Optical Scattering Measurements to Identify Atmospheric Aerosols, Dusts, and Ice Crystals
Abstract
While the availability of recent satellites such as Moderate-resolution Imaging Spectroradiometer (MODIS) and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) offer improved accuracy and global coverage of nonspherical aerosol and cloud particles, such measurements still rely on gross assumptions in determination of particle type. In particular, the size and composition-dependent scattering properties of nonspherical dust and ice crystals are needed to determine the individual contributions of dust and ice to the scattering of sunlight and the earth's radiative budget. An added challenge is that the presence of dust and ice often coincide in the atmosphere because dust is an effective ice nucleus. A new in-situ instrument, the Cloud and Aerosol Spectrometer with Polarization (CASPOL) from Droplet Measurement Technologies measures light scattered by aerosols in the forward and backward directions, with an additional polarized detector in the backward direction. Scattering by a single particle can be measured by all three detectors for aerosols in a broad range of sizes, 0.6 micrometers < diameter < 50 micrometers. The CASPOL is a unique measurement tool, since unlike most in-situ probes, it measures these optical properties on a particle-by-particle basis. In this laboratory study, single particle CASPOL measurements for thirteen atmospherically relevant dusts were obtained and their optical scattering signatures were evaluated. In addition, a Continuous Flow Diffusion Chamber (CFDC) was used as an ice crystal generator to produce ice crystals via both homogenous and heterogeneous nucleation mechanisms under well-controlled laboratory conditions. Optical scattering properties of the nucleated ice crystals were then measured by the CASPOL. The total and polarized backscatter intensities were found to vary with particle size for all dust types. Using a new optical signature technique all but one dust type could be categorized into one of three optical scattering groups. Significant differences between the optical properties of single dust and ice particles of the same size were observed. Differences between the optical signatures of homogeneously and heterogeneously nucleated ice crystals were not statistically significant. In addition, assuming size distributions representative of dust and cirrus ice clouds in the atmosphere, we used the CASPOL single particle data to estimate the additive composite backscatter intensity and depolarization ratio for these populations of non spherical particles in the atmosphere, and hence their contributions to the Earth's radiative budget. Our results suggest that atmospheric ice crystals can be identified and quantified independent from the dust particles on which they form based on analysis of their backscatter and depolarization signals. Information provided by the CASPOL measurements could improve interpretation of remote sensing measurements of many types of aerosol and cloud particles.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.A21F0119B
- Keywords:
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- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE Aerosols and particles;
- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE Instruments and techniques;
- 0319 ATMOSPHERIC COMPOSITION AND STRUCTURE Cloud optics;
- 0320 ATMOSPHERIC COMPOSITION AND STRUCTURE Cloud physics and chemistry