Articles | Volume 15, issue 23
Atmos. Chem. Phys., 15, 13453–13473, 2015
Atmos. Chem. Phys., 15, 13453–13473, 2015

Research article 07 Dec 2015

Research article | 07 Dec 2015

Observations of the spectral dependence of linear particle depolarization ratio of aerosols using NASA Langley airborne High Spectral Resolution Lidar

S. P. Burton1, J. W. Hair1, M. Kahnert2,3, R. A. Ferrare1, C. A. Hostetler1, A. L. Cook1, D. B. Harper1, T. A. Berkoff1, S. T. Seaman1,4, J. E. Collins1,5, M. A. Fenn1,5, and R. R. Rogers1,a S. P. Burton et al.
  • 1NASA Langley Research Center, MS 475, Hampton, VA 23681, USA
  • 2Research Department, Swedish Meteorological and Hydrological Institute, Folkborgsvägen 17, 60176 Norrköping, Sweden
  • 3Department of Earth and Space Science, Chalmers University of Technology, 41296 Gothenburg, Sweden
  • 4National Institute of Aerospace, 100 Exploration Way, Hampton, VA 23666, USA
  • 5Science Systems and Applications, Inc., One Enterprise Pkwy, Hampton, VA 23666, USA
  • anow at: Lord Fairfax Community College, Middletown, VA 22645, USA

Abstract. Linear particle depolarization ratio is presented for three case studies from the NASA Langley airborne High Spectral Resolution Lidar-2 HSRL-2). Particle depolarization ratio from lidar is an indicator of non-spherical particles and is sensitive to the fraction of non-spherical particles and their size. The HSRL-2 instrument measures depolarization at three wavelengths: 355, 532, and 1064 nm. The three measurement cases presented here include two cases of dust-dominated aerosol and one case of smoke aerosol. These cases have partial analogs in earlier HSRL-1 depolarization measurements at 532 and 1064 nm and in literature, but the availability of three wavelengths gives additional insight into different scenarios for non-spherical particles in the atmosphere. A case of transported Saharan dust has a spectral dependence with a peak of 0.30 at 532 nm with smaller particle depolarization ratios of 0.27 and 0.25 at 1064 and 355 nm, respectively. A case of aerosol containing locally generated wind-blown North American dust has a maximum of 0.38 at 1064 nm, decreasing to 0.37 and 0.24 at 532 and 355 nm, respectively. The cause of the maximum at 1064 nm is inferred to be very large particles that have not settled out of the dust layer. The smoke layer has the opposite spectral dependence, with the peak of 0.24 at 355 nm, decreasing to 0.09 and 0.02 at 532 and 1064 nm, respectively. The depolarization in the smoke case may be explained by the presence of coated soot aggregates. We note that in these specific case studies, the linear particle depolarization ratio for smoke and dust-dominated aerosol are more similar at 355 nm than at 532 nm, having possible implications for using the particle depolarization ratio at a single wavelength for aerosol typing.

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Short summary
The manuscript describes measurements of particle depolarization ratio from the NASA airborne HSRL-2 at three wavelengths, for two dust cases and a smoke case. Differences in the spectral dependence of particle depolarization ratio are due to the sizes of the non-spherical particles, large for dust and small for smoke. The large depolarization at 355nm for smoke has not been previously reported and may impact aerosol typing when only a single wavelength is available.
Final-revised paper