Hello everyone,
I came across a JGR paper titled "Lidar studies of paticulates in the UTLSregion at a tropical station over the Indian subcontinent" by Bijoy V Thampi et al. (2008JD010556) and found an interesting point about the altitude profiles of molecular backscatter coefficient for co-polarized and cross-polarized 532nm channel are estimated using the respective values of backscatter cross-section along with the altitude profile of molecular number density. As far as I know, the molecular (Rayleigh) particles are spherical in shape and isotropic scattering possible. If this is the case, how the cross-polarized 532nm channel molecular backscatter coefficient altitude profile can be obtained?
I'm not sure I read the sentence regarding Rayleigh scattering correctly. Did you mean that Rayleigh-induced depolarization is negligible so it is not clear how the presumably zero backscattered radiance in the perpendicular channel can be measured and interpreted?
If this is the case then you're right that the signal should be small.
But, it is not pure zero because the depolarization of molecular return includes air anisotropy, and "The depolarization of the molecular return that includes the Cabannes line and the rotational Raman lines is 1.5%" ( Young, A. T., "Rayleigh scattering," Physics Today, January 1982, 42--48, 1982.)
1.5% of the total molecular backscatter is a measurable amount given that there's no solar background and the accumulation period is long enough.
Does this make sense?
Dear Dr. Feofilov,
Thank you so much for response. From your clarification, I understand that the molecular backscatter coefficient for cross-polarized channel (532nm) cannot be completely zero. In the paper (Article Lidar studies of particulates in the UTLS region at a tropic...
), it was mentioned that the value of molecular depolarization ratio is taken as 0.028 in accordance with Bodhaine et al. (1999) - "On Rayleigh Optical Depth Calculations". By definition of molecular depolarization ratio, it is defined as the fraction of molecular backscatter coefficient in cross-polarized channel to that in copolarized channel. Will this ratio is constant vertically in the atmosphere?
At the first sight, I would say so, but one has to take into account that the atmosphere gets contaminated with fine (subvisible) aerosols and the gas mixture becomes different when we move closer to the surface, so if you need the precise numbers then I would search deeper, I can't give a reference off the top of my head.
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