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).
Highlights
Few trace gases are as ubiquitous in the atmospheric chemistry as nitrogen oxides
The first method by Herman et al (2009), called “minimum-amount Langley extrapolation” (MLE), assumes that the background NO2 vertical column density (VCD) (XNb O2 ) measurable along the atmospheric column is constant over a considered portion of the data set and that this minimum amount does not depend on the considered air mass
To obtain the final series of extraterrestrial calibration (ETC), we take the results of this last method further interpolated with a locally estimated scatterplot smoothing (LOESS) curve to only keep the slowly varying component of the calibration data set and not to introduce the short-term statistical noise
Summary
Few trace gases are as ubiquitous in the atmospheric chemistry as nitrogen oxides. These are released by both natural sources, such as microbiological activity, biomass burning, lightning, and volcanoes (Bates and Hays, 1967; Lee et al, 1997; Schumann and Huntrieser, 2007), and anthropogenic sources, like high-temperature combustion in vehicle engines, power plants, industry, and use of fertilisers. We estimate that more than 50 MkIV Brewer spectrophotometers from 22 countries have been collecting raw (unprocessed) data in the visible range and sending them to the World Ozone and Ultraviolet Radiation Data Center (WOUDC) since the establishment of the Brewer network in the 1990s These data sets can be potentially used to extract information on past NO2 total columns. The instrument has been providing UV global irradiance (Campanelli et al, 2019; Vitt et al, 2020) and ozone retrievals (Siani et al, 2018) since 1992 and has been measuring direct solar radiation in the visible range for NO2 estimates since 1996 (Francesconi et al, 2004) This series is of particular interest owing to the characteristics of the station, located in the Italian capital city, a populated and trafficked metropolis, and to the continuity and length of the data set. The final data set is provided in moles per square metre (mol m−2)
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