Improvements to the nitrogen dioxide observations by means of the MKIV Brewer spectrophotometer

Nitrogen dioxide (NO2) is a key component of the Earth's atmosphere, being involved in the ozone destruction and production and influencing the radiative balance of our planet. Furthermore, it impacts on human health and contributes to the tropospheric pollution. Techniques to measure atmospheric nitrogen dioxide from both the space and the ground have noticeably advanced in the last decades and provide valuable information, while also presenting some serious limitations. In 1985, an automated Brewer ozone spectrophotometer was modified to add capability to measure solar visible radiation and retrieve atmospheric nitrogen dioxide besides total ozone and sulfur dioxide. Since then, more than 60 MKIV Brewer spectrophotometers have been put in operation in a worldwide network and long-term nitrogen dioxide records have been collected at several sites. However, the original algorithm, developed in the 1980's, has never been officially updated and, although notable efforts have been done in the last years to improve the retrieval by the Brewer, high-quality estimates of nitrogen dioxide by this instrument are still difficult to acquire. This work introduces substantial innovation in measuring nitrogen dioxide with MKIV Brewers. For the first time, an instrument of this type was thoroughly characterised in its operating spectral range. A novel mathematical framework supporting observations was then developed to update the Brewer processing algorithm, to better understand its potentials and limitations and to optimise the measurement technique. The spectroscopic data sets used within the algorithm were completely updated using the most recently published absorption cross sections. Model calculations were performed with top-level radiative transfer codes to test the new algorithm, which not only allows more accurate estimates of nitrogen dioxide, but also introduces new capabilities in Brewer measurements, such as the retrieval of the oxygen dimer (O4) and the degree of the linear polarisation of skylight. To test the new method, four field campaigns were organised. The limitations in the traditional calibration techniques due to the daily evolution of nitrogen dioxide were overcome by employing an innovative method and one Brewer (#066) was successfully calibrated and compared to an instrument belonging to the Network for the Detection of Atmospheric Composition Change. The two different observation techniques in which the Brewer operates, namely by looking directly to the sun or vertically to the zenith, were proven to be equivalent within the stated uncertainty. The latter was thoroughly evaluated by means of a Monte Carlo based method. The new algorithm was effectively applied to reprocess the long-term series recorded in the European Brewer stations of Saint-Christophe and Rome (Italy), Athens (Greece), Hradec Kralove (Czech Republic). To this purpose, two empirical methods to calibrate the Brewer instruments through statistical analyses were validated. The results were compared with satellite total column data retrieved from the SCIAMACHY, GOME-2 and OMI instruments and with in-situ measurements of tropospheric concentrations to assess the impact of anthropogenic emissions.