the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Feb 2025
Comparison of total ozone measurements in Melbourne, Australia, performed with a low-cost micro spectrometer and a Brewer MK-III
Abstract. A new design for a simple and very low-cost instrument for measuring total column ozone is described and its performance evaluated by comparison to a co-located MK III Brewer spectrophotometer for approximately six months. The ozone retrieval is based on the "Global Irradiance" method used to derive total column ozone from the Norwegian GUV radiometer network. While the total cost of components of the instrument was less than EUR 3000, total ozone values were found to agree with Direct Sun Brewer values with a standard deviation of 1.8 %, comparable to the agreement between DS and ZS Brewer data. The most significant limitations of the retrieval method were found to be the dependence on temperature and cloud-amount. As well as being low-cost, the instrument appears robust and easy to operate. As the instrument covers fully the UV-A/B spectrum it is fully possible to further develop the analyses software to include other data products related to UV-radiation.
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Status: open (until 25 Mar 2025)
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RC1: 'Comment on egusphere-2025-87', Anonymous Referee #1, 11 Mar 2025
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Comparison of total ozone measurements in Melbourne, Australia, performed with a low-cost micro spectrometer and a Brewer MK-III
Kåre Edvardsen, Matt Tully, Steve Rhodes
Submitted to https://doi.org/10.5194/egusphere-2025-87 Preprint. Discussion started: 17 February 2025
General comments:
This paper provides a description of the methods to derive total column ozone (TCO) from a very low-cost instrument measuring global ultra-violet irradiance. The paper is well written, well structured and the methods are clearly described. It is recommended for publication after addressing the comments below.
The paper focuses rather on the method of TCO retrial rather than a sound analysis of the performance of TCO compared to a reference instrument. Therefore, the general comments are divided into two following parts:
Method:
Commonly, TCO is derived best by direct sun measurements. The paper uses global UV measurements, which implicate parameters of uncertainty such as clouds in front of the sun, clouds in the sky, effective albedo from the surrounding terrain and cosine response of the diffusor. All these effects would not occur when using direct sun measurements. The authors argue that in many places worldwide cloud coverage hinder direct sun measurement for TCO. Are there so many places with such conditions? Maybe the performance of the low-cost instrument would be much better when using direct sun measurements. Has this ever been tested with the instrument?
The method includes many parametrizations based on empirical functions. I have lost the overview. Maybe the authors can provide a summary of all the empirical parametrizations, the number of measurements needed for the parametrization and, if possible, an estimation of the uncertainty of the parametrization and its implication of TCO.
The parametrization also arises the question if the method is also suitable for other placed world wide, once it has “calibrated” with a Brewer at a specific site. Or must the method be “calibrated” for each location worldwide separately with a Brewer?
Performance:
It is not clear if the authors have compared daily averages with quasi synchronous measurements for STS GI and Brewer DS. I suggest comparing synchronous measurements within e.g. 10 minutes intervall and also show the individual measurements to indicate the variation of TCO.
Commonly these small low cost spectrometers suffer from straylight, which biases TCO at low solar zenith angles or high ozone slant columns. In order to assess the performance of the instrument the comparison between the Brewer and STS GI depending on ozone slant column (airmass * TCO) should be provided.
Temperature dependency. Indeed, the dependency of the small array spectroradiometer is a crucial issue of these instruments. Ambient temperature causes wavelength shifts, linearity and detectability problems (signal to noise ratio etc.). I am surprised that a simple parametrization (Eq. 2) can account for that. It would be worthful to calculate the uncertainty of TCO based on the parametrization and its applicability for other instruments and other locations.
Furthermore, the model includes the ozone absorption cross section, which depends on the effective ozone temperature (basically stratospheric temperature). How sensitive is the algorithm and resulting TCO on effective ozone temperature?
Specific comments:
Page 2: line 32: How significant is the fraction of the price of the Brewer. To my estimate it is about 10% of a Brewer, which is already low cost.
Page 2, line 47: Is the MK II Brewer a double monochromator with insignificant stray light impact?
Page 2, line 60: Can the method be described as a cloud correction?
Page 3 line 89: The reference is missing
Page 4 line 95: What is the slit function and full width half maximum of the instrument, resulting from 25 micron slit?
Page 5 line 126: Do you mean ozone slant column (=airmass * TCO) - > see comment above.
Page 5: 132: and EQ 1 is this parametrization also applicable at other locations
Page 5, line 146. Again, what is the full width half maximum of the slit function?
Table 1: It would be worthful to indicate the performance in percent
Caption Figure 2: SZ -> ZS
Figure 4. The ratio between Brewer and STS would be more helpful
Page 11 line 280: Why is TCO increasing with temperature? Wavelength shift?:
Page 11 line 286: I suggest using quasi simultaneous measurements.
Page 12 line 310: At what conditions is STS comparable to the Brewer? What are the limitations?
Page 13: line 324. The conclusion should not end with a bullet list. Maybe a closing sentence would help.
Citation: https://doi.org/10.5194/egusphere-2025-87-RC1
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