Multiwavelength fluorescence lidar observations of fresh smoke plumes
Abstract. A five-channel fluorescence lidar was developed for the study of atmospheric aerosol. The fluorescence spectrum induced by 355 nm laser emission is analyzed in five spectral intervals using interference filters. Central wavelengths and the widths of these five interference filters are respectively: 438/29, 472/32, 513/29, 560/40 and 614/54 nm. The relative calibration of these channels has been performed using a tungsten-halogen lamp with color temperature 2800K. This new lidar system was operated during Summer – Autumn 2022, when strong forest fires occurred in the Moscow region and generated a series of smoke plumes analyzed in this study. Our results demonstrate that, for urban aerosol, the maximal fluorescence backscattering is observed in 472 nm channel. For the smoke the maximum is shifted toward longer wavelengths, and the fluorescence backscattering coefficients in 472 nm, 513 nm and 560 nm channels have comparable value. Thus, from the analysis of the ratios of fluorescence backscattering in available channels, we show that it is possible to identify smoke layers. The particle classification based on single channel fluorescence capacity (ratio of the fluorescence backscattering to elastic one), has limitations at high relative humidity (RH). Fluorescence capacity is indeed decreasing when water uptake of particles enhances the elastic scattering. However, the spectral variation of fluorescence backscattering does not evidence any dependence on RH and can be therefore considered for aerosol identification.
Igor Veselovskii et al.
Status: final response (author comments only)
RC1: 'Comment on amt-2023-5', Anonymous Referee #2, 22 Feb 2023
- AC1: 'Reply on RC1', Igor Veselovskii, 19 Mar 2023
RC2: 'Comment on amt-2023-5', Anonymous Referee #1, 28 Feb 2023
- AC2: 'Reply on RC2', Igor Veselovskii, 19 Mar 2023
Igor Veselovskii et al.
Igor Veselovskii et al.
Viewed (geographical distribution)
The authors present multi-spectral fluorescence backscatter profiles measured with a lidar. The study is of high importance to the lidar community as the fluorescence topic is still not sufficiently explored with lidar systems and the aerosol intensive properties measured here will most probably be used in future aerosol classification studies. Therefore, it is important that the values and respective uncertainties are well reported.
I would recommend the publication of this paper after some revisions regarding the following points:
-- Ratios of the fluorescence backscattering are calculated over a broad spectral region (438-614nm) but the authors do not specify how they treat the atmospheric attenuation in the different wavelengths. These spectral effects are not simplified by dividing the two backscatter coefficients and are certainly not negligible. The aerosol extinction cross-section at 614nm is 0.5 times less compared to 438nm for an Angstrom exponent of 2 (typical for biomass burning). Likewise, the molecular extinction cross-section at 614nm is 0.35 times less than the 438nm one. This introduces optical-depth dependent effects that become more and more important as the beam goes deeper in the atmosphere. For instance, the backscatter ratio among 472 and 614nm will be biased by ~4% above an aerosol layer with 0.1 AOD and ~12% above an aerosol layer with 0.3 AOD without including the molecular contribution at all. The authors must include a correction for the atmospheric attenuation in their technique (if not already applied), at least for molecules and for typical Angstrom exponent values. They should also address the error introduced when aerosol with different Angstrom exponent values are present. In Fig. 11 we may probably already see such an effect in the ratios B472/B513 and B472/B560 as the backscatter (and therefore the extinction) increases with humidity.
-- Protective windows for the telescopes are usually deployed in lidar systems. Their spectral reflectance, that also dependends on the incidence angle) must be taken into account when calculating backscatter ratios over such a broad region. Are there protective windows installed in this system? Is their effect measured and subtracted, or at least included in the calibration with the lamp? The authors must address any such potential issues and if present, correct for them or include them in the error calculation.
-- Uncertainties are not sufficiently addressed in the manuscript. There are some error bars in the figures but they are only there for some of the lines. The authors should include them for all. It is also not clear whether these correspond to random noise errors from the signal analysis or from some other source uncertainty as there is no relevant discussion. The authors should comment on how they performed the error estimation.
Minor comments are also included directly inline in the manuscript. The use of English can be in general improved. I would recommend the authors to go through the document again and correct minor grammatical/phrasing issues.