Recent increase in NO2 levels in the southeast of the Iberian Peninsula

We report the evolution of tropospheric NO2 over the south-east of the Iberian Peninsula from 2005 to 2017. We have used hourly NO2 levels measured at air-quality stations in urban and suburban environments. Annual averages ranged between 14 and 45 μg m−3, with peaks above 200 μg m−3. A monthly variation was observed, with higher concentrations in cold months (40–60 μg m−3) and lower levels in the warm season (13–17 μg m−3). A diurnal pattern was found in urban and suburban areas. The upward trend in NO2 observed during the whole period contrasts with the upward trend reported in 2013–2017. The NO2 tropospheric column levels measured by the Ozone Monitoring Instrument over the Iberian Peninsula indicated a similar behaviour; nevertheless, the largest Spanish metropolitan areas did not show this increase. The mean sea level pressure and wind field data of ERA5 (European Centre for Medium-Range Weather Forecasts) were used to investigate the weather conditions, the NO2 outputs of the Copernicus Monitoring Services being used for the assessment of the NO2 spatial distribution. NO2 regional events, with concentrations in the range 140–150 μg m−3, and which occurred both in the winter and summer season under anticyclonic conditions, are also described. A local origin is identified in winter, whereas in summer, they are associated with a high-pressure system that blocks Mediterranean outflows towards the Atlantic Ocean. The high NO2 levels are attributed mainly to two factors: i) local emissions, rather than contributions from the western Mediterranean (or even North Africa), and ii) an increase in the pressure gradient between the Atlantic and the Mediterranean pressure systems, associated with a decrease in wind speed, was found during the last five years compared with the previous eight. Meteorological and chemical changes in mid-latitudes associated with global warming should also be investigated in the future. [Display omitted] •NO2 measurements from 2005 to 2017 were investigated in the SE of the Iberian Peninsula.•Negative NO2 trends were found in 2005–2017; however, an increase was measured from 2013 onwards.•Trend of NO2 on the surface showed good agreement with OMI NO2 tropospheric column trends.•NO2 events occurred in both winter and summer under a weak pressure gradient.•The change in NO2 trends could be associated with changes in weather patterns.