the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Novel use of a Bentham UV Double Monochromator for measurements of global and direct irradiance, ozone and aerosol
Ben Liley
Richard McKenzie
Michael Kotkamp
Richard Querel
Abstract. A novel ultraviolet spectrometer has been developed and tested over 10 years at Lauder, New Zealand. The system, UV2, makes alternating measurements of the global and direct UV irradiance and can therefore be used to measure ozone and aerosol optical depth. After an analysis of the stability of UV2, these measurements, along with UV irradiance are compared to relevant observations made by an additional UV spectrometer (UV4), a Dobson spectrophotometer (#72) and two radiometers measuring aerosol optical depth, a Prede skyradiometer and a Middleton Solar radiometer (SP02). UV2 irradiance is shown to be lower than UV4 by between 2.5–3.5 %, with a standard deviation of a similar magnitude. Total column ozone values are shown to agree with Dobson values with a mean bias of 2.57 Dobson units (DU) and standard deviation of 1.15 DU when using the direct sun measurements. Aerosol optical depth at 400–412 nm and 500 nm agrees to within 0.015 and is comparable to the difference between the reference radiometers. Further work is needed, particularly in the radiometric calibration at longer wavelengths, in order to determine if this instrument can supersede or enhance measurements made by the Dobson or the aerosol radiometers.
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Alexander Geddes et al.
Status: open (until 11 Oct 2023)
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RC1: 'Comment on amt-2023-107', Anonymous Referee #1, 07 Jul 2023
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The paper discusses the performance of a spectroradiometer that has been in operation for about 10 years in Lauder, New Zealand, and presents as additional products total ozone and aerosol optical depth retrievals and comparison to other collocated standard instruments. The quality of the presentation is satisfactory, although it could have been further improved with more careful and detailed discussion of the different topics. In general the methodologies are mainly outlined instead of being presented in detail. Nevertheless, the paper is a useful documentation of the system and its performance and for this reason it can be accepted for publication in AMT as long as the specific comments below are addressed.
The introduction is severely biased to local studies neglecting the significant relevant work on spectral UV monitoring done in several other locations worldwide.
In all figures showing differences, it would help to draw a line at 0 difference to guide the eye of the reader.
65: how you cope with the time difference between global and direct spectra (and consequent change in solar zenith angle) to calculate the diffuse irradiance spectra?
69: how long it takes to acquire one full spectrum?
70: 0.65 nm is probably the “average” resolution (according to Figure 2)
87: To which instrument refers the “accuracy of the spectral calibration”? Unless it is meant “Langley calibration”.
101-103: Is there any reference to provide more details on this method? How the 45W lamps are used/mounted when measuring the direct irradiance port?
114: As you say in line 158, there is no diffuser for the direct port. Therefore the “global and direct diffuser” should probably be replaced by “global and direct ports”.
114: Figure 1 refers to the constant intensity mode which is based on measurements with a UVA photodiode. It would be interesting to discuss the stability when the constant current mode is used.
128-129: How it is ensured that the change in 2017 (and other shorter-term changes) are not caused by the 45 W lamps? How the stability of these lamps with time is ensured?
129-130: Please include some statistics for the two cases (fixed and varying sensitivity), e.g. mean and standard deviation of the differences.
161: Please elaborate on why you think that the wavelength drift indicates a linearity problem. And if such problem exists, how this could affect the actual spectra that have a large dynamic range?
165: How the performance of this instrument compares to the performance of the UV4 system? Are the observed changes in sensitivity and wavelength stability within the norms for an NDACC affiliated instrument?
177: What are the diffuser diode and the mean diode?
215: Have the ratios of the extraterrestrial intensities been calculated by Langley plots? And if yes, how often these have been done?
223: If I have understood correctly, UV2 scans in steps of 0.2 nm. Then how the wavelength with decimal 5 or 9 in table 1 are achieved?
263: I disagree with this statement. The replacement of a total ozone instrument at a particular station requires a though intercomparison and assessment a any differences and limitations. If can certainly no be based on a simple plot of differences. See for example, Groebner et al., 2021 10.5194/amt-14-3319-2021.
268: Add “of direct irradiance” after “reference spectrum”. Has this reference spectrum calculated for the specific solar zenith angle of each individual spectral measurement (400, 412 and 500 nm)?
270: Why you have to convert the direct irradiance to a horizontal surface? In fact to deduce the aerosol extinction you need the actual path of radiation. In addition, please elaborate a bit more on the methodology, so that inexperience readers can understand what you are doing.
275: 1-minute data cannot be interpolated to the times of observations by UV2 which have much coarser resolution.
309: Please add after “proven stable”, “to within XX% for global and YY % for direct irradiance”.
Technical:
64: add “radiation” after “receives”
69: replace full stop with comma at the end of the line
262: delete “the” after “its”
276: replace “+=” with “±”
Citation: https://doi.org/10.5194/amt-2023-107-RC1 -
RC2: 'RC on amt-2023-107', Anonymous Referee #2, 08 Sep 2023
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The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-107/amt-2023-107-RC2-supplement.pdf
Alexander Geddes et al.
Alexander Geddes et al.
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