Impact of primary NO2 emissions at different urban sites exceeding the European NO2 standard limit.

A large part of the European population is still exposed to ambient nitrogen dioxide (NO2) levels exceeding the European Union (EU) air quality standards, being a key challenge to reduce NO2 concentrations across many European urban areas, particularly close to roads. In this work, a trend analysis of pollutants involved in NO2 processes was done for the period 2003-2014 in traffic sites from three Spanish cities (Barcelona, Madrid and Granada) that still exceed the European NO2 air quality standard limits. We also estimated the contributions of primary NO2 emissions and photo-chemically formed NO2 to the observed ambient NO2 concentrations in order to explore their possible role in the observed NO2 concentration trends. The NOx and NO concentrations at these traffic sites showed significant decreasing trends during the period 2003-2014, especially at Barcelona (BARTR) and Madrid (MADTR) traffic stations. The NO2 concentrations showed statistically significant downward trends at BARTR and MADTR and remained unchanged at Granada traffic station (GRATR) during the study period. Despite the significant decrease in NO2 concentrations in BCNTR and MADTR during the analysed period, the NO2 concentrations observed over these sites still above the annual NO2 standard limit of 40 μg m-3 and, therefore, more efficient measures are still needed. Primary NO2 emissions significantly influence NO2 concentrations at the three analysed sites. However, as no drastic changes are expected in the after-exhaust treatment technology that can reduce primary NO2 emissions to zero in the near future, only a substantial reduction in NOx emissions will help to comply with the NO2 European air quality standards. Reduction of 78%, 56% and 16% on NOx emissions in Barcelona, Madrid and Granada were estimated to be necessary to comply with the NO2 annual limit of 40 μg m-3.