Spectral Calibration for SO2 Cameras with Light Dilution Effect Correction
Round 1
Reviewer 1 Report
A new spectral calibration method of a UV SO2 camera is proposed by incorporating the path radiance effect. The camera images are taken for two bands centered at 310 and 330 nm. The results for the Etna volcano, Italy, are compared with those with the conventional calibration based on DOAS. Although this research has the possibility of providing new insights to the volcano monitoring community, the authors should consider the following issues to improve the research quality.
1. The terminology of “light dilution effect” is not a generally recommended way of describing the path radiance that is unavoidable for the long-distance observation of a UV camera. This term has been used in the SO2 camera community, as described, e.g., by Campion et al. (2015): “light dilution, i.e., the scattering of ambient photons within the instrument's field of view (FoV) on air parcels located between the plume and the instrument.” However, the same situation always occurs for the measurements of scattered solar radiation with both ground and satellite-based sensors. This problem is not solely for the present manuscript, but in order to solve the confusion that may arise due to different wording in neighboring communities, it would be better to explain that this dilution effect is due to the radiation component traditionally called “path radiance.” See, for example,
Waquet, F., Goloub, P., Deuzé, J. L., Léon, J. F., Auriol, F., Verwaerde, C., Balois, J.-Y., and François, P. (2007). Aerosol retrieval over land using a multiband polarimeter and comparison with path radiance method. Journal of Geophysical Research: Atmospheres, 112(D11),
and references therein.
2. P3 (Sec. 2) The instrumental specifications of the UV camera should be briefly mentioned. What was the f-number of the camera lens used for the distant (10.3 km) observation? What was the time duration elapsed for taking two images at 310 and 330 nm? What was the direction in which the “dark image” (Fig. 2) was recorded? In addition, to grasp the validity of the estimation of aerosol and molecular contributions to the path radiance, the altitudes of the observation and target (volcano) sites are required, in addition to the aerosol loading information from the nearby AERONET site (Nicolosi) on the observation day (Sep. 16, 2015) [see, e.g., Sellitto, P., Salerno, G., & Briole, P. (2017). The Etna Plume Lab (EPL) research cluster: Advance the understanding of Mt. Etna plume, from source characterization to downwind impact. Annals of Geophysics.]
3. P4-P5 The description of the retrieval algorism is farfetched. Different values of tau_m (tau_m^A and tau_m^B) should be introduced in Eqs. (1) and (2), not later in Eq. (4). Using the symbol of wavelength (\lambda) in Eqs. (1) – (7) is not needed since symbols A and B refer to the center wavelengths of 310 and 330 nm, respectively. In Fig. 2, more information should be provided for the directions in which the sky background images were recorded.
4. P7 For the sake of completeness, a brief outline of the DOAS algorithm used for deriving the results in Fig. 4 should be added. What was the DOAS light path employed for the camera calibration in the present case? Was the calibration carried out only for limited pixels of the camera data? What was the method used to estimate the path radiance for the DOAS light path?, and so on. Otherwise, the reader of this paper cannot find any clues to evaluate the correctness and exact meaning of the DOAS results shown in Fig. 4.
5. P8 The algorithm and assumptions used to derive the spectral calibration results in Fig. 5 are not clear enough. What do \sigma(\lambda), S, and \tau_Mie(\lambda) stand for in Eq. (8)? Is the use of symbols consistent with that in previous equations? Why the symbol \tau_B (not \tau_Mie) is used in the caption of Fig. 5? What was the wavelength dependence of the aerosol OD used for deriving the results in Fig. 5? What was the spectral form of the path radiance assumed in the simulation? What were the accuracies of these parameters?, and so on.
6. P9 In the derivation of the parameters of the quadratic equation, what is the meaning of the variables of x and y? What were the assumptions used to derive the results shown in Fig. 6? Is there any possibility of illustrating the variation range of the resultant parameters according to the changes in aerosol type and/or changes in the path radiance (i.e., scattered solar radiation) intensity?
7. P10 The row and column ranges of the camera image should be explicitly shown in Fig. 7. Where was the point (row and column) calibrated by the DOAS method? The meaning of “areas with low particle (loading)” is unclear. Please explain the interpretation of different areas in the images in Fig. 7. Please describe the time and date information in the captions of Fig. 7 through Fig. 11.
8. P11 Please describe typical values (or range of values) for each parameter in Eq. (9). How was “the position range of the plume cross section” defined and determined? It would be better to use “C_SO2” instead of “S_SO2” to denote the gas concentration.
9. P13 Please explain the possible variation range of the slope parameter (0.87 in this case) according to the changes in observation conditions, including the solar position and aerosol type. Is it still possible to carry out the measurement even if the sky is partially cloudy? In SI, the mass unit should be “kg,” not “Kg.”
10. P14 The section of “Quantum Efficiency” (QE) will not be necessary if the difference in the QE values between the two channels (310 and 330 nm) are incorporated when taking the ratio of I_A/I_A0 and I_B/I_B0 (P4). For the calibration of vignetting (radial darkening toward the corners of a frame), laboratory testing of the camera image against a homogeneously illuminated wall will be effective. Do the authors have any information about the vignetting for the camera employed in this observation?
11. P15 The section of “Off-Axis System Sensitivity” will be meaningful only for wide FOV observations. The consideration will not be necessary if the observation distance is long enough and a narrow FOV lens is employed. So the question is the instrumental setting (especially the camera FoV) employed to obtain the original images as shown in Fig. 2, from a location 10. 3 km away from the volcanic plume source.
12. There are a number of problems in English grammar and word choices. Following is just a short list of examples. Please refine the manuscript through the native check.
L11-12 “camera” is a countable noun. “SO2 camera” and “its measurement accuracy” should be “SO2 cameras” and “their measurement accuracy.”
L17, L174, etc. Please do not start a sentence with “And.”
L25 and L27 The same word “thereby” is repeated in a paragraph.
L84-85 “This method is … several years ago” should be “This method has been … since several years ago.”
L105 “realistic radiative transfer” – For more detailed radiative transfer (RT) calculations, the authors should refer to the established code such as MODTRAN.
L114 The phrase “inside and outside the atmosphere” does not seem to make sense.
L136 and L138 No indent should appear after these equations. Periods should be used for Eqs.(2) and (3), not commas.
L171 “didn’t change” – “did not change”
L182 The acronym “4-IM” should be spelled out.
L304 A reference citation is needed before showing Eq. (9).
Please note that these are just examples, and a thorough check is mandatory throughout the manuscript.
Author Response
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Reviewer 2 Report
The manuscript is well-written and the content is technically-sound and novel. I recommend publication.
The quality of English langugae is good.
Author Response
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Reviewer 3 Report
Comments for author File: Comments.docx
At times the English could be improved, but generally it is comprehensible.
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Round 2
Reviewer 1 Report
The authors have made significant efforts to improve the presentation quality. Following is a list of remaining issues that the reviewer has found in this second version. Please consider further revising the manuscript accordingly before preparing the printing version.
L11-12 “The detection ability of SO2 cameras have been improved effectively, while the calibration is still the main factor which limits their measurement accuracy. “ – “The detection ability of SO2 cameras has been improved effectively, while the calibration is still the main factor that limits their measurement accuracy.”
L12-14 “This paper presents a nonlinear calibration theory by taking the effect of light dilution and the dependence of plume aerosol on scattering wavelength into account.” – “This paper presents a nonlinear calibration theory by considering the effect of light dilution due to the path radiance as well as the dependence of plume aerosol on scattering wavelength.”
L25 “water environment.” – “the water environment.”
L26-27 “Researches show that SO2 stimulates the respiratory system and affects the pulmonary function, thereby weakening the immune system [1-3].” – “Researchers have shown that SO2 stimulates the respiratory system and deteriorates pulmonary function, thereby weakening the immune system [1-3].”: the word “affects” has been replaced with “deteriorates” to avoid the repeated use of the same word.
L29 “ The annual volcanic eruption process injects about tens of millions of tons of SO2 into the troposphere …” – “ The annual volcanic eruption process injects tens of millions of tons of SO2 into the troposphere …”
L55-56 “and quickly measure the SO2 column density at a single point of the volcanic plume, but it cannot obtain the emission of the entire plume [15].” – “and quickly measure the SO2 column density along the laser beam path through the volcanic plume, but it cannot observe the entire volume of plume emission [15].”
L58-59 “the dispersion of sample data.” – “the availability of sample data.”
L85-86 “This method has been the most straightforward method which is commonly used since several years ago.” – “This method has been the most straightforward and commonly used since several years ago.”
L99 “under the effect of light dilution.” – “under the effect of light dilution [ref].”: Please cite a reference.
L101-102 “This dilution effect refers to the phenomenon of scattering and attenuation of light during propagation due to the presence of a radiation component called "path radiation" during long-distance observation by SO2 cameras [24].” – “This dilution effect refers to the changes in spectral features during propagation due to the presence of path radiance arising from the scattering of solar radiation [24].” : Please note that the terminology “spectral radiance” (in units of W m-2 sr-1 nm-1) is a well-established technical term. Since the component arises from scattering, we have to specify the solid angle (sr) for the detection.
L105 “a SO2 camera” – “an SO2 camera”
L117, L130, L168, L181, L438-439, L501-502, L520, etc. “at 310 nm and 330 nm” – “at 310 and 330 nm”: Please note that in English, the repeated use of the same word should be avoided, and the “nm” is no exception.
L117-118, L131-132 “the relative sun spectrum and the relative solar scattering spectrum.” – “, and the relative spectra of direct and scattered solar radiation.”: The legend in Fig. 1 should be “Direct solar spectrum” and “Scattered solar spectrum.”
L119 ”For the wavelength of less than 300 nm” – “ For wavelengths shorter than 300 nm”
L124-126 “For the signal channel, the solar-scattered light is depleted by both the scattering of the particles and the absorption of SO2. For the wavelength of 330 nm, the scattered light is only affected by the scattering of the particle.” – “For this channel, particle (aerosol) scattering and SO2 absorption reduce the intensity of scattered solar radiation. For the reference wavelength of 330 nm, on the other hand, the scattered solar intensity is affected only by the particle scattering.”: Please note that the word “deplete” is too strong to describe this situation. The word “solar-scattered light” is problematic in the sense that “surface-scattered light” indicates the light scattered by the surface. Since the “reference channel” has been defined here, “(i.e., central wavelength of 330 nm)” can be omitted at L127.
L138 “the scattered light from the sun” – “the scattered solar light”
L141 “can be obtained as follows:” – “can be obtained as”: Please note that each equation is part of a sentence. No colon (:) is necessary after “as”.
L147 “in cases that the” – “in cases where the”
L149 “in a point several kilometers away” – “at a point several kilometers away”
L154-155 “can be expressed as a numerical relation as follows [30]:” – “can be expressed as a numerical relation as”
L165 “due to the scattering of photons” – “due to the atmospheric scattering of photons”
L173 “on September 16, 2015 using” – “on September 16, 2015, using”
L174-175 “The altitude of Milo Town and crater is 803 meters and 3103 meters, respectively [32]. The main instrumental information shown in Table 1.” – “The altitude of Milo Town and the crater is 803 and 3103 m, respectively [32]. The main instrumental information is summarized in Table 1.
L180, L183 “Figure 2a and 2d represent … as shown in Figure 2b, 2e, 2c and 2f, respectively. “ – “Figures 2a and 2d represent … as shown in Figures 2b, 2e, 2c and 2f, respectively.
L186-187 “The exposure time for A and B channel plume images is 33 ms and 8 ms, respectively.” – “The exposure time for A and B channel plume images was 33 and 8 ms, respectively.
L192 “Figure 3a and 3b present the ODs of 310 nm and 330 nm channels …” – “Figures 3a and 3b present the ODs of 310 and 330 nm channels …”
L196-197 “in order to stress details of the plume, the mountain of all images have been removed.” – “ in order to stress the details of the plume, the mountain areas have been masked in all images.”
L210-211 “Aiming to improve the calibration accuracy, a new method of spectral calibration is proposed in this study.” – “Aiming to improve the calibration accuracy, we propose a new method of spectral calibration in this study.”: This suggestion is to avoid the problem of a dangling modifier.
L215-217 “Through merging a set of OD images with DOAS data on time series, the calibration curve of DOAS can be obtained by fitting the relationships of the primary function.” – “By merging …”: Here, the meaning of “the primary function” is unclear.
L221-223 “the extinction coefficients of channels A and B, which are 0.0743 km-1 and 0.0654 km-1, respectively, to correct the radiation transmission of sunlight from the plume to the cameras’ FOV.” – “the extinction coefficients of channels A and B, which are 0.0743 and 0.0654 km-1, respectively, to correct the transmission of solar radiation from the plume to the cameras’ FOV.”
L225 “the calibration curve of SO2 camera” – “the calibration curve of the SO2 camera”
L245 Just after Eq. (8), “(lambda = lambda_A, B)”
L246 “Combining Equation (7) and (8),” – “Combining Equations (7) and (8),”
L247-249 “Based on the study by Lübcke et al. and ... 1.09 was chosen [30].” – “Based on the study by Lübcke et al. [30] and ... 1.09 was chosen.”
L249 “Figure 5a and 5b show ...” – “Figures 5a and 5b show ...”, the same applies to L299, L300, ...
L251-253 “through the constructed MODTRAN radiative transfer model, which ... calibration curves [34].” – “through the constructed MODTRAN radiative transfer model [34], which ... calibration curves.”
L256-257 “While the direct linear relationship is destroyed for distant or optically thick volcanic plumes as ...” – “The direct linear relationship is destroyed for distant or optically thick volcanic plumes, as ...”
L258-259 “The larger the aerosol optical thickness is, the lower the sensitivity towards SO2 will be. What’s more, ...” – “The larger the aerosol optical thickness, the lower the sensitivity towards SO2. Moreover, ...”: Please do not use the too colloquial expression “What’s more.”
L275 ”In addition, the coefficients a, b, c of ...” – “In addition, the coefficients a, b, and c of ...”
L298 “with and without light dilution effect.” – “with and without the light dilution effect.”: Please note that “effect” is a countable noun. The same at L314.
L306 Please indicate the position of the “coordinate of (512,672)” in Fig. 7(a). (not “coordinates” at L305.)
L316 “more obvious, about 2.5 times.” – “more obvious by about 2.5 times.”
L318, L534 Please do not start a sentence with “And.” Usually, “Moreover” or “Furthermore” is preferred.
L320 “a difference less than 4%.” – “a difference of less than 4%.”
L320 “While for values between 500th-930th row” – “For values between 500-930th row”
L324 in plume.” – “in the plume.”
L334-340 “cross section” – “cross-section”. The same applies to all the similar cases.
L342-355 Please cite appropriate references explaining the optical flow method in general and the Farnebäck optical flow method in particular.
L351 “07:07 sec 18, ...” – “07:07 on September 18, 2015, ...”
L365-366 “obtained through spectral calibration method and DOAS calibration method” – “obtained through the spectral calibration method and the DOAS calibration method”
L372-373 “a linear fit of the SO2 emission rates inverse of the two calibration methods” – What is “inverse” in this context?
L383-387 “The spectral calibration method may be inferior to the DOAS calibration method in eliminating the light dilution effect, but it also takes into account the effect of light dilution, and the spectral calibration method does not have the problem of systematic errors caused by spatial matching, so the results of the inversion of the two methods have some errors.” – “The spectral calibration method may be inferior to the DOAS calibration method in eliminating the light dilution effect, though it does not have the problem of systematic errors due to spatial matching. Therefore, some errors are inevitable for the inversion results obtained from these two methods.”
L392 “the specific effects of light dilution effect and aerosols, and comparing with ...” – “the specific effects of light dilution and aerosol extinction and comparing it with ...”: Please note the duplication of “effects” and “effect.”
L403-406 “In contrast, the spectral calibration method is based on a single scattering model to correct for light dilution effect, and although the light dilution correction effect is slightly deficient, there is no difficulty in matching the FOV, so the matching accuracy is better than that of the DOAS calibration method.” – “In contrast, the spectral calibration method is based on a single scattering model to correct for the light dilution effect. Although this effect is slightly underestimated, no difficulty is found in matching the FOV. Thus, it is conceived that the matching accuracy is better than that of the DOAS calibration method.”
L410-411 “... which may be invalid when the concentration of carbon black particulate matter in the plume is particularly large.” It is necessary to cite a reference about this effect.
L412-413 “there is a large error in the SO2 emission rate results from the inversion of the two calibration methods.” – “there is a large error in the SO2 emission rate resulting from the inversion using the two calibration methods.”
L451 “causing the angle of optical incident on the filter to shift” – “causing the angle of optical incidence on the filter to shift”
L454 “Ideally, all light is incident vertically, however, in an optical imaging system, all light cannot be directed vertically.” – “Although ideally, all light is incident vertically, this condition is not always satisfied in an optical imaging system.”
L476-477 “by integrating the effective aperture radius R and all relative directions” – “by integrating Tc G over the effective aperture radius R and all relative directions”: The explanations of the integrand, Tc and G, should precede this part.
L537-538 “the new mothed is essentially free of the influence of particulate matter in plume.” – “the new method is essentially free of the influence of particulate matter in the plume.”
Please see the comments above.
Author Response
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Reviewer 3 Report
I thank the authors for comprehensively addressing my comments on the manuscript. In my opinion it is now ready for publication. Congratulations on the great work.
Author Response
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