the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Comparison of global UV irradiance measurements between a BTS CCD-array and a Brewer spectroradiometers
- 1Departamento de Observación de la Tierra y la Atmósfera, Instituto Nacional de Técnica Aeroespacial (INTA), El Arenosillo, Huelva, España
- 2Departamento de Física, Instituto del Agua, Cambio Climático y Sostenibilidad, Facultad de Ciencias, Universidad de Extremadura, Badajoz, España
- 3Centro de Experimentación de El Arenosillo (CEDEA), Instituto Nacional de Técnica Aeroespacial (INTA), El Arenosillo, Huelva, España
- 1Departamento de Observación de la Tierra y la Atmósfera, Instituto Nacional de Técnica Aeroespacial (INTA), El Arenosillo, Huelva, España
- 2Departamento de Física, Instituto del Agua, Cambio Climático y Sostenibilidad, Facultad de Ciencias, Universidad de Extremadura, Badajoz, España
- 3Centro de Experimentación de El Arenosillo (CEDEA), Instituto Nacional de Técnica Aeroespacial (INTA), El Arenosillo, Huelva, España
Abstract. Spectral measurements of UV irradiance are of great importance to ensure human health protection as well as to support scientific research. To perform these measurements, double monochromator scanning spectroradiometers are the preferred devices, thanks to their linearity and stray-light reduction. However, because of their high cost and demanding maintenance, CCD-array-based spectroradiometers are increasingly used for monitoring UV irradiance. Nevertheless, CCD-array spectroradiometers have specific limitations, such as a high detection threshold or stray-light contamination. To overcome these challenges, several manufacturers are striving to develop improved instrumentation. In particular, Gigahertz-Optik GmbH has developed the stray-light-reduced BTS2048-UV-S spectroradiometer series (from now on called BTS). In this study, the long-term performance of the BTS and its seasonal behavior, regarding global UV irradiance, has been assessed. To carry out the analysis, BTS’ irradiance measurements have been compared against measurements of the Brewer MK-III #150 scanning spectrophotometer during three campaigns. A total of 711 simultaneous spectra, measured under cloud-free conditions and covering a wide range of solar zenith angle (from 14° to 70°) and UV index (from 2.4 to 10.6), are used for the comparison. During the three measurement campaigns, the global UV spectral ratio BTS/Brewer was almost constant (at around 0.93) in the 300–360 nm region for solar zenith angles (SZAs) below 70°. Thus, the BTS calibration was stable during the whole period of study (∼1.5 years). Likewise, it showed no seasonal nor SZA significant dependence in this wavelength region. Regarding the UV index, a good correlation between the BTS and the Brewer #150 was found, i.e. the dynamic range of the BTS is comparable to that of the Brewer #150. These results confirm the quality of the long-term performance of the BTS array spectroradiometer to measure global UV irradiance.
Carmen González et al.
Status: open (until 25 May 2022)
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RC1: 'Comment on amt-2022-105', Anonymous Referee #2, 04 May 2022
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The paper assesses the performance of a CCD-based spectrometer (BTS) in measuring the spectral solar irradiance in the UV range using data from 3 campaigns that took place in a period of 1.5 years. The comparison was done against data of a double monochromator Brewer spectroradiometer operating regularly at the campaign site. The BTS spectrometer is a rather new instrument and such studies to assess its long term performance are useful contributions for solar UV monitoring. The paper is well structured and addresses most of the usual aspects of intercomparisons of radiation instruments. To my opinion it is in a good stage to be accepted for publication, but I believe with some extra work as suggested in my specific comments below, the results could be further improved and possibly better substantiated. The language of the paper in good, despite some small flaws; some of them are mentioned in the “technical comments” section below.
Specific comments
1, 1: In the title, I suggest adding the word “spectral” before measurements.
3, 66: You actually mean range of intensity, therefore I suggest to avoid using the term UV Index in this context.
4, 98: What is “high-end light measurements”?
4, 101: The “measurement time ranging from 0.1 to 6000 ms” applies only to the photodiode or also to the CCD?
5, 138: I suggest to draw a darker horizontal line at 1.0, to guide the eye of the reader and make the comparison amongst the three panels easier. This applies also to figures 3 and 4.
5, 143: Concerning the increasing ratio towards shorter wavelengths below 300 nm, this could be partly produced by the cosine response of the BTS diffuser, if the cosine error is larger than the Brewer’s. Please include this information in section 2.2, and if the error is larger than the Brewer’s I suggest including a brief discussion. Moreover, from figure 1, I don’t think that the 5% agreement is valid down to 300 nm. I would be more conservative to the lower limit (e.g. closer to 305 nm). This is also evident from table 1, where only the last column shows variabilities below 5%, contradicting the statement of line 148.
6, 157-159: The discussion around the noise level and its reset to 0 is not clear for inexperienced readers.
7, 173: For the plots of Figure 3, a more stringent time synchronization could be achieved for each wavelength band (of ±2.5 nm) as opposed to the general synchronization based on the time at 326.5 nm. To be clearer, I mean to compare the data based on the difference between the time the central wavelength of each band is measured and the time of the BTS spectrum. This might further improve the results, especially at larger SZAs when small time differences increase notably the irradiance level. Actually, this might explain a small part the deviations at the shorter wavelengths, in addition to stray-light and (possibly) to cosine response.
8, 183: At the caption of Figure 3 please add a note to alert the reader for the x-axis scale change in the bottom panel. The same holds for Figure 4.
8, 185: A different time synchronization could also be applied for the UV Index comparisons, instead of the time at 326.5 nm. As the erythemally weighted irradiance peaks at between 306-308 nm (depending on SZA and total ozone) the time in this wavelength range would be more appropriate for the comparison and I believe would also improve the results.
Technical Comments
2, 45: “on arrays of CCD sensors”. Do you mean “on arrays or CCD sensors”? Otherwise, just say “on CCD sensors”.
2, 54: Replace “a considerable effort” with “considerable efforts”
3, 70: Replace “calibration” by “sensitivity”
3, 84: Omit the unnecessary term (double Brewer).
4, 99: “The spectral detector is a spectrometer”. This doesn’t make sense. Maybe you can omit “a spectrometer”?
5, 149: Replace “is similar to the one other stray-light-corrected CCD-array spectroradiometers have” with “is similar to other stray-light-corrected CCD-array spectroradiometers”
10, 200: I would prefer to see Figure 5 with axes of equal length.
10, 208: I assume you mean agreement within ±5%.
11, 221: Replace “specific” with “regular”
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AC2: 'Reply on RC2', Carmen González Hernández, 20 May 2022
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The authors thank Referee #2 for the careful and constructive examination of the manuscript. The reply was uploaded in the form of a supplement.
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AC2: 'Reply on RC2', Carmen González Hernández, 20 May 2022
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RC2: 'Comment on amt-2022-105', Anonymous Referee #1, 04 May 2022
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This is a very interesting paper on the performance of the BTS CCD-array spectroradiometer , compared with a Brwer spectroradiometer.
My major comments are related:
a. With the explenation of the differences / changes of this ratio with wavelength (mainly)
b. With a conclusion of the measurement uncertainty and accuracy of the Brewer instrument and through this comparison reporting on the uncertainty of the new BTS CCD array
Speciffic commentsLine 39 maybe also include an OMI related validation publication: Arola et al., A new approach to correct for absorbing aerosols in OMI UV DOI: 10.1029/2009GL041137
Line 62 Probably a reference to the Qasume: Quality assurance of spectral ultraviolet measurements in Europe through the development of a transportable unit (QASUME) DOI: 10.1117/12.468641
Line 115 probably a table mentioning the dates, names, ozone, temperature , cloud comments of the 3 periods could be useful
Line 137 : why have you put the limits for 70 degrees and the cloudless sky ?
Line 145: Kouremeti
Table 1: Variability: is this 1 sigma ?
Section 4.1:
-Is there any idea for the low but obvious drop of the ratio going from 305 to the end of the spectrum (e.g. fig. 3) ?-Can you comment on the signal to noise ratio for low wavelengths and high solar zenith angles ?
-Is the curvature of the ratios in figure 2 due to the instrument calibration principles/sources ? or is there any other reason involved?
Figure 4c: ratios seems slightly lower than the other peiods.
Could you provide an estimation of the Brewer accuracy and uncertainty on deriving UV Index and based on this work to report also on the accuracy and uncertainty of the new instrument ?
An uncertainty estimation of the CCD-array instrument would be very useful for this work.-
AC1: 'Reply on RC1', Carmen González Hernández, 20 May 2022
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The authors thank the Referee #1 for the careful and constructive examination of the manuscript. The reply was uploaded in the form of a supplement.
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AC1: 'Reply on RC1', Carmen González Hernández, 20 May 2022
reply
Carmen González et al.
Carmen González et al.
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