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

Abstract

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.

Potential of polarization/Raman lidar to separate fine dust, coarse dust, maritime, and anthropogenic aerosol profiles

“…The numerous field observations indicate that the total dust linear depolarization ratio is always around 0.3 at 532 nm (close to the dust sources as well as after long-range transport). In contrast, the 1064 nm dust depolarization ratio was found to vary strongly from values of 0.22 to 0.28 for lofted dust layers (and typical dust size distribution after regional as well as long-distance travel; Freudenthaler et al, 2009;Burton et al, 2015;Haarig et al, 2017a) to values around 0.4 (Burton et al, 2015) close to dust sources. Only during events close to dust emission zones when giant dust parti-3406 R.-E. Mamouri and A. Ansmann: Fine and coarse dust profiling cles and even sand particles (having diameters > 60 µm) are in the air, such very high particle linear depolarization ratios are obviously measurable at 1064 nm.…”

“…These values are mostly derived by combining the laboratory studies (of the fine dust depolarization ratios) and a variety of field observations of the total (fine + coarse) dust depolarization ratio. Burton et al (2015) reported high particle linear depolarization ratios around 0.4 for 532 and 1064 nm, as found by Sakai et al (2010), in dense dust plumes over the United States close to the ground and probably close to the dust sources with a strongly dominating coarse mode. Veselovskii et al (2016) and Hofer et al (2017) found maximum particle depolarization ratios around 0.35 at 532 nm, respectively, in dense western African and central Asian dust plumes.…”

“…The dust linear depolarization ratio at 355 nm seems to be always close to 0.25 (Groß et al, , 2015Burton et al, 2015;Haarig et al, 2017a), disregarding the distance from the dust source and strength of the outbreak and occurring particle sizes. Even during rather strong dust storms in central Asia, Hofer et al (2017) found maximum 355 nm depolarization ratios of 0.29 only.…”

Triple-wavelength depolarization-ratio profiling of Saharan dust over Barbados during SALTRACE in 2013 and 2014

“…We implemented new channels to permit simultaneous observations of dust linear depolarization ratios at 355, 532, and 1064 nm. Freudenthaler et al (2009) and Burton et al (2015) showed already that the dust linear depolarization ratio significantly changes with the transmitted laser wavelength, obviously as a result of changing contributions of fine-mode dust particles (particles with diameters < 1 µm) and coarse-mode (super-micrometer) particles to the light depolarization. Burton et al (2015) recently presented triplewavelength polarization lidar observations in an aged and fresh dust layer performed with an airborne high-spectralresolution lidar (HSRL-2).…”

“…Freudenthaler et al (2009) and Burton et al (2015) showed already that the dust linear depolarization ratio significantly changes with the transmitted laser wavelength, obviously as a result of changing contributions of fine-mode dust particles (particles with diameters < 1 µm) and coarse-mode (super-micrometer) particles to the light depolarization. Burton et al (2015) recently presented triplewavelength polarization lidar observations in an aged and fresh dust layer performed with an airborne high-spectralresolution lidar (HSRL-2). The main goal of the paper is to present for the first time ground-based triple-wavelength polarization/Raman lidar observations (case studies) of the depolarization ratio of Saharan dust after long-range transport and to provide a high-quality statistical data set of dust depolarization ratios at 355, 532, and 1064 nm.…”