This paper is a very useful addition to the science of detecting total column NO₂ in the atmosphere using widely available Brewer Mark IV spectrophotometers with a newly developed algorithm. The paper is mostly well written with minor language corrections needed (see attached document with corrections marked in green). The description of the equations used in the new algorithm is deficient. This needs to be fixed before publication. The derivation of the algorithm can be in the main paper, appendix, or in supplemental material. As it is now, it is not possible to follow the derivation of equations 1 to 3 and more detail for the remaining equations 4 - 7. The results are important enough so that the included equations must be understandable for use by others.  The comparison with Pandora 117 is quite good, However, even more interesting is a comparison of Brewer 067 with the daily files from Pandora 117. I have included in my review graphs for such a comparison for 8 days in 2017. The new algorithm does an excellent job.  I will be happy to share the originals of these figures if you wish (see the end of the attached document).  My recommendation is that this paper needs an improved/extended description of the equations before being published. Other than that, this paper is an important addition to atmospheric experimental science.

Jay Herman   drjrherman@gmail.com

This is the review of the paper “Advanced NO2 retrieval technique for the Brewer spectrophotometer applied to the 20-year record in Rome, Italy” submitted by H. Diemoz and coauthors to the ESSD.

The paper provides a very detailed review of the algorithm update to derive the NO2 column from the Brewer MK IV instrument in Italy. The authors have designed and performed the error propagation analyses that provide information on data uncertainty and stability of the 20-year record.

Validation of the new algorithm retrievals is provided through 1) comparison of the station Brewer observations done during an intensive campaign against another Brewer (#066), which was calibrated independently, and 2) through comparisons against a co-located Pandora spectrometer (#117) in Rome, over a 1-year long period (2016-2017). The second comparison is of the first kind where NO2 products from Brewer MK IV and Pandora were compared over a year-long time period.

The paper is well constructed, results are summarised in Figures and Tables. The logic of the presentation is easy to follow. The reference to the previous algorithm is provided and the upgrades are well described. I agree with the first reviewer – the equations need to be written out further to explain intermediate steps. If needed, this can be addressed in the Appendix.

The new NO2 record is of great value (from the accuracy and temporal length points of view) to assess the air quality changes in Italy over a 20-year time period. This record provides information to the policymakers to address the efforts to improve air quality in the region. The record is also of value for satellite verification efforts, including OMI and TROPOMI observations. Another important value of this work is that the method applies to other Brewer MK IV observations. The new method allows historical data reprocessing that minimized instrumental interferences, such as wavelength misalignments and changes in the internal temperature of the instrument. I would be looking forward to seeing it implemented in other long-term records.