the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Mar 2025
Ångström exponent impact on the aerosol optical properties obtained from vibrational-rotational Raman lidar observations
Abstract. Vertical profiles of aerosol properties are essential for assessing the impact of aerosols on cloud formation and the Earth's radiation budget. Lidars can provide profiles of the particle backscatter and extinction coefficients and the extinction-to-backscatter ratio (lidar ratio). An Ångström exponent has to be assumed when computing these profiles from nitrogen vibrational-rotational Raman signals. This assumption introduces uncertainties. An alternative approach is the rotational Raman lidar method, which does not need an Ångström exponent as input. This study presents a quantitative comparison between the pure rotational and vibrational-rotational Raman lidar approaches to assess the impact of the Ångström exponent assumption on the vibrational-rotational Raman lidar solutions. In this short article, we present four contrasting case studies based on observations of wildfire smoke, Saharan dust, residential wood combustion smoke, and a cirrus layer. The optical properties are derived at a wavelength of 532 nm, with the rotational Raman signals measured at 530 nm and the vibrational-rotational Raman signals measured at 607 nm. It was found that the use of an Ångström exponent, deviating by 1 from the true value, introduces relative uncertainties of 5 % and less (backscatter coefficient), 5–10 % (extinction coefficient), and around 10 % (lidar ratio) in the vibrational-rotational Raman lidar solutions.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1386', Anonymous Referee #1, 15 Apr 2025
Manuscript considers the influence of the Angstrom exponent on calculation of the backscattering and extinction coefficients from measurements of Mie-Raman lidar. Several measurement cases related to different types of aerosol are considered.
This subject is not new, and is well familiar to every researcher working with Mie-Raman lidar. On another hand, it is useful to summarize it and to provide the expected errors for different aerosol types. Thus, from my point of view, the manuscript is suitable for AMT.
Authors work in this field for long time and are good experts, so I have not much to add. Just several technical comments.
p.5 ln.117 “In general, as the Ångström exponent A increases, the extinction coefficient…”
Should be reformulated.
Fig.3. Extinction at 3 km height is low and drop of the lidar ratio at 3 km probably has no physical meaning, So, by my opinion, no reason to show it. Within aerosol layer at 4 km, backscattering and extinction coefficients have very different profiles. And looks like extinction is strongly smoothed. Thus, profile of lidar ratio, probably, makes sense only in the center of the layer. I would show only averaged value (near the center of the layer) of the lidar ratio.
Citation: https://doi.org/10.5194/egusphere-2025-1386-RC1 -
AC2: 'Reply on RC1', Gladiola Malollari, 24 May 2025
We thank the reviewer for the positive feedback and are pleased that the manuscript is considered suitable for AMT. We have carefully addressed the technical comments in the revised version.
Reviewer: p.5 ln.117 “In general, as the Ångström exponent A increases, the extinction coefficient…” Should be reformulated.
Authors: This sentence is reformulated: Generally, an increase in the Ångström exponent value shifts the extinction coefficient and the lidar ratio to slightly larger values and the backscatter coefficient to lower values.
Reviewer: Fig.3. Extinction at 3 km height is low and drop of the lidar ratio at 3 km probably has no physical meaning, So, by my opinion, no reason to show it. Within aerosol layer at 4 km, backscattering and extinction coefficients have very different profiles. And looks like extinction is strongly smoothed. Thus, profile of lidar ratio, probably, makes sense only in the center of the layer. I would show only averaged value (near the center of the layer) of the lidar ratio.
Authors: We thank the reviewer for the recommendation. Since we aim to highlight the profile variations and deviations among different Ångström Exponent values, we believe it is more informative to present the profiles rather than only mean values. However, we agree that the drop in the LR at 3 km has no physical meaning. We decided to cut the LR profiles from 2.4 km to 3.8 km and exclude them from the plot. Figure 3 has been revised accordingly.
Citation: https://doi.org/10.5194/egusphere-2025-1386-AC2
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AC2: 'Reply on RC1', Gladiola Malollari, 24 May 2025
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RC2: 'Comment on egusphere-2025-1386', Anonymous Referee #2, 29 Apr 2025
The authors perfrom a sensitivity study concerning the impact of the assumed Angstrom Exponent, usually chosen as 1, on the Raman lidar invresions. Their lidar system, combining vibrational and rotation Raman channels allows to use as a reference retrieval the one from the rotational channel, which makes feasible such a sensitivity study. The approach is simple and clear, well presented and documented. The results are useful for the lidar community especially for the estimation of the uncertainties in the retrievals from the vabrational Raman channels. The only comment I have is, that I would expect in the conclusions, apart from the quantification of the uncertainties, a recommendation for the choice of the Angstrom exponent in the operatinal processing of lidar measurements.
Citation: https://doi.org/10.5194/egusphere-2025-1386-RC2 -
AC1: 'Reply on RC2', Gladiola Malollari, 24 May 2025
We thank the reviewer for the supportive evaluation and appreciate the positive comments on the clarity, usefulness, and relevance of our work.
We added the following paragraph: An Ångström Exponent (AE) of 0–0.5 is recommended for pure or polluted dust. AE values of 1.0 and 1.5 are more appropriate for non-dust aerosols, such as anthropogenic pollution. In the case of wildfire smoke, it is more complex. For very aged wildfire smoke, the extinction-related AE approaches 0.0, while the backscatter-related AE is 1.5, resulting in higher lidar ratios at 532 nm than at 355 nm, as observed for Canadian (Haarig et al., ACP, 2018), Australian (Ohneiser et al., ACP, 2020), and Siberian (Ohneiser et al., ACP, 2021) smoke. For black carbon-rich aerosols, such as those from residential wood combustion, the AE value can be set between 1.5 and 2.0.
Citation: https://doi.org/10.5194/egusphere-2025-1386-AC1
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AC1: 'Reply on RC2', Gladiola Malollari, 24 May 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1386', Anonymous Referee #1, 15 Apr 2025
Manuscript considers the influence of the Angstrom exponent on calculation of the backscattering and extinction coefficients from measurements of Mie-Raman lidar. Several measurement cases related to different types of aerosol are considered.
This subject is not new, and is well familiar to every researcher working with Mie-Raman lidar. On another hand, it is useful to summarize it and to provide the expected errors for different aerosol types. Thus, from my point of view, the manuscript is suitable for AMT.
Authors work in this field for long time and are good experts, so I have not much to add. Just several technical comments.
p.5 ln.117 “In general, as the Ångström exponent A increases, the extinction coefficient…”
Should be reformulated.
Fig.3. Extinction at 3 km height is low and drop of the lidar ratio at 3 km probably has no physical meaning, So, by my opinion, no reason to show it. Within aerosol layer at 4 km, backscattering and extinction coefficients have very different profiles. And looks like extinction is strongly smoothed. Thus, profile of lidar ratio, probably, makes sense only in the center of the layer. I would show only averaged value (near the center of the layer) of the lidar ratio.
Citation: https://doi.org/10.5194/egusphere-2025-1386-RC1 -
AC2: 'Reply on RC1', Gladiola Malollari, 24 May 2025
We thank the reviewer for the positive feedback and are pleased that the manuscript is considered suitable for AMT. We have carefully addressed the technical comments in the revised version.
Reviewer: p.5 ln.117 “In general, as the Ångström exponent A increases, the extinction coefficient…” Should be reformulated.
Authors: This sentence is reformulated: Generally, an increase in the Ångström exponent value shifts the extinction coefficient and the lidar ratio to slightly larger values and the backscatter coefficient to lower values.
Reviewer: Fig.3. Extinction at 3 km height is low and drop of the lidar ratio at 3 km probably has no physical meaning, So, by my opinion, no reason to show it. Within aerosol layer at 4 km, backscattering and extinction coefficients have very different profiles. And looks like extinction is strongly smoothed. Thus, profile of lidar ratio, probably, makes sense only in the center of the layer. I would show only averaged value (near the center of the layer) of the lidar ratio.
Authors: We thank the reviewer for the recommendation. Since we aim to highlight the profile variations and deviations among different Ångström Exponent values, we believe it is more informative to present the profiles rather than only mean values. However, we agree that the drop in the LR at 3 km has no physical meaning. We decided to cut the LR profiles from 2.4 km to 3.8 km and exclude them from the plot. Figure 3 has been revised accordingly.
Citation: https://doi.org/10.5194/egusphere-2025-1386-AC2
-
AC2: 'Reply on RC1', Gladiola Malollari, 24 May 2025
-
RC2: 'Comment on egusphere-2025-1386', Anonymous Referee #2, 29 Apr 2025
The authors perfrom a sensitivity study concerning the impact of the assumed Angstrom Exponent, usually chosen as 1, on the Raman lidar invresions. Their lidar system, combining vibrational and rotation Raman channels allows to use as a reference retrieval the one from the rotational channel, which makes feasible such a sensitivity study. The approach is simple and clear, well presented and documented. The results are useful for the lidar community especially for the estimation of the uncertainties in the retrievals from the vabrational Raman channels. The only comment I have is, that I would expect in the conclusions, apart from the quantification of the uncertainties, a recommendation for the choice of the Angstrom exponent in the operatinal processing of lidar measurements.
Citation: https://doi.org/10.5194/egusphere-2025-1386-RC2 -
AC1: 'Reply on RC2', Gladiola Malollari, 24 May 2025
We thank the reviewer for the supportive evaluation and appreciate the positive comments on the clarity, usefulness, and relevance of our work.
We added the following paragraph: An Ångström Exponent (AE) of 0–0.5 is recommended for pure or polluted dust. AE values of 1.0 and 1.5 are more appropriate for non-dust aerosols, such as anthropogenic pollution. In the case of wildfire smoke, it is more complex. For very aged wildfire smoke, the extinction-related AE approaches 0.0, while the backscatter-related AE is 1.5, resulting in higher lidar ratios at 532 nm than at 355 nm, as observed for Canadian (Haarig et al., ACP, 2018), Australian (Ohneiser et al., ACP, 2020), and Siberian (Ohneiser et al., ACP, 2021) smoke. For black carbon-rich aerosols, such as those from residential wood combustion, the AE value can be set between 1.5 and 2.0.
Citation: https://doi.org/10.5194/egusphere-2025-1386-AC1
-
AC1: 'Reply on RC2', Gladiola Malollari, 24 May 2025
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Gladiola Malollari
Albert Ansmann
Holger Baars
Cristofer Jimenez
Julian Hofer
Ronny Engelmann
Nathan Skupin
Seit Shallari
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2176 KB) - Metadata XML