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 NO_x and NO concentrations at these traffic sites showed significant decreasing trends during the period 2003-2014, especially at Barcelona (BAR_TR) and Madrid (MAD_TR) traffic stations. The NO₂ concentrations showed statistically significant downward trends at BAR_TR and MAD_TR and remained unchanged at Granada traffic station (GRA_TR) during the study period. Despite the significant decrease in NO₂ concentrations in BCN_TR and MAD_TR 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 NO_x emissions will help to comply with the NO₂ European air quality standards. Reduction of 78%, 56% and 16% on NO_x emissions in Barcelona, Madrid and Granada were estimated to be necessary to comply with the NO₂ annual limit of 40 μg m^-3.