Advanced NO<sub>2</sub> retrieval technique for the Brewer spectrophotometer applied to the 20-year record in Rome, Italy

Abstract

Abstract. A re-evaluated data set of nitrogen dioxide (NO2) column densities over Rome for the years 1996 to 2017 is here presented. This long-term record is obtained from ground-based direct sun measurements with a MkIV Brewer spectrophotometer (serial number #067) and further reprocessed using a novel algorithm. Compared to the original Brewer algorithm, the new method includes updated NO2 absorption cross sections and Rayleigh scattering coefficients, and it accounts for additional atmospheric compounds and instrumental artefacts, such as the spectral transmittance of the filters, the alignment of the wavelength scale, and internal temperature. Moreover, long-term changes in the Brewer radiometric sensitivity are tracked using statistical methods for in-field calibration. The resulting series presents only a few (about 30) periods with missing data longer than 1 week and features NO2 retrievals for more than 6100 d, covering nearly 80 % of the considered 20-year period. The high quality of the data is demonstrated by two independent comparisons. In the first intensive campaign, Brewer #067 is compared against another Brewer (#066), recently calibrated at the Izaña Atmospheric Observatory through the Langley method and there compared to reference instrumentation from the Network for the Detection of Atmospheric Composition Change (NDACC). Data from this campaign show a highly significant Pearson's correlation coefficient of 0.90 between the two series of slant column densities (SCDs), slope 0.98 and offset 0.05 DU (Dobson units; 1.3×1015 molec.cm-2). The average bias between the vertical column densities is 0.03 DU (8.1×1014 molec.cm-2), well within the combined uncertainty of both instruments. Brewer #067 is also independently compared with new-generation instrumentation, a co-located Pandora spectrometer (#117), over a 1-year-long period (2016–2017) at Sapienza University of Rome, showing linear correlation indices above 0.96 between slant column densities, slope of 0.97, and offset of 0.02 DU (5.4×1014 molec.cm-2). The average bias between vertical column densities is negligible (−0.002 DU or -5.4×1013 molec.cm-2). This, incidentally, represents the first intercomparison of NO2 retrievals between a MkIV Brewer and a Pandora instrument. Owing to its accuracy and length, the Brewer data set collected in Rome can be useful for satellite calibration/validation exercises, comparison with photochemical models, and better aerosol optical depth estimates (NO2 optical depth climatology). In addition, it can be employed to identify long-term trends in NO2 column densities in a metropolitan environment, over two decades witnessing important changes in environmental policies, emission loads and composition, and the effect of a worldwide economic recession, to offer just a few examples. The method can be replicated on the more than 80 MkIV spectrophotometers operating worldwide in the frame of the international Brewer network. The NO2 data set described in this paper can be freely accessed at https://doi.org/10.5281/zenodo.4715219 (Diémoz and Siani, 2021).

The site-specified primary calibration conditions for the Brewer spectrophotometer

“…The Brewer spectrophotometer provides data products that include column ozone (e.g., Kerr, 2002;Kerr et al, 1981), column sulphur dioxide (SO2; e.g., Fioletov et al, 1998;Zerefos et al, 2017), column nitrogen dioxide (NO2, by Mark IV only; e.g., Kerr et al, 1988;Cede et al, 2006;Diémoz et al, 2021), spectral UV radiation (e.g., Bais et al, 1996;Fioletov et al, 2002), aerosol optical depth (AOD) (e.g., Kazadzis et al, 2005;Marenco et al, 2002;Diémoz et al, 2016;López-Solano et al, 2018), and effective ozone layer temperature (Kerr, 2002). However, the main data product provided by Brewer instrument is the TCO via direct-sun (DS) observations using the method described above.…”

Characterisation and Field Test of a Simple AvaSpec Array Spectroradiometer for Solar Irradiance Measurements at an Alpine Site

“…These were required as input to the radiative transfer model. In particular, a MkIV Brewer spectrophotometer was used to retrieve ozone [56,57] and nitrogen dioxide [58,59] vertical column densities, and a sun/sky POM-02 photometer was employed to estimate the aerosol optical depth (AOD) and microphysical/optical properties [60,61]. The aerosol properties were retrieved using the inversion algorithm Skyrad pack MRI (Meteorological Research Institute) version 2 [62].…”

Aerosol optical characteristics in the urban area of Rome, Italy, and their impact on the UV index

“…The Brewer MKIV is a single monochromator spectrophotometer specifically designed to retrieve the total column ozone by measuring solar direct irradiances at selected UV wavelengths in the ozone absorption spectrum (Siani et al, 2018) and nitrogen dioxide in the visible part of the spectrum (Diémoz et al, 2021). The accuracy of the direct Sun measurements of total ozone taken with a well-maintained Brewer spectrophotometer is 1 % (Vanicek, 2006).…”