• Fifth largest city in The Netherlands (89 km2) (Westerink et al., 2017)
• Population density of (2639) xxxx hab/km2 (CBS, 2019)
• City centre is comprised of residential and commercial buildings, mainly low rise and few high rise (Blocken et al., 2016)
• Climate is temperate oceanic climate influenced by the north sea and atlantic ocean, cool summers, moderate winters, typically high humidity.
• Annual avg temp 9.4 C
• annual avg rainfall 776 mm (KNMI, 2019).

The ozone target value for the protection of human health (120 μg/m3; maximum daily eight-hour mean), established by the Air Quality Framework Directive, is exceeded 10 to 15 times every year, making O3 one of the most problematic pollutants in the city. As NO2 is one of the precursors to O3, the pair was assessed (the hourly limit-value for NO2 is 200 μg/m3).

NO2 daily patterns exhibit a clear urban influence linked to the traffic diurnal profile, with peaks in the morning and late afternoon and a decrease of concentrations during the weekend. The increase in concentrations during the winter could be explained by atmospheric stability and an increase in emissions linked to heating processes. This pattern is more notorious in the background station (blue line), which indicates that this increase is not due to more traffic emissions. O3 patterns have a negative correlation with NO2, explained by the photochemical reactions involving these compounds. Throughout the day, the increase in solar radiation and the presence of precursors (volatile organic compounds (VOC) and NO2) promote ozone formation and concentrations increase. During the night, the accumulation of NO2 leads to O3 consumption. A similar pattern is observed during the year with hotter months having higher O3 concentrations, with longer and higher intensity solar radiation. The average O3 concentrations during weekends are higher than during weekdays. This phenomenon is called the “weekend effect” (Pudasainee et al., 2006; Roberts-Semple et al., 2012). The negative correlation with NO2 suggests that VOC contributed to elevated O3 concentrations. Lower nitric oxide (NO) levels and VOC emissions dur- ing weekend mornings consume less O3, which accumulates later by photochemical reactions. Weekday/weekend differences in O3 are intricately related to interactions with its chemical precursors: nitrogen oxides (NOx) and VOC, respectively. Similar conclusions can be derived from the trend analysis. Fig. 3 displays long-term temporal trends for NO2 and O3 concentrations estimated for mean monthly values based on data from the urban traffic monitoring station (NL00237). NO2 shows a clear tendency for reduction, which could be the cause for the increasing O3 trend. The reduction in NOx could promote higher O3 concentrations during the night, as less O3 is consumed. However further research is needed to fully explain this behaviour in Eindhoven. In sum, Eindhoven’s air quality is heavily influenced by traffic emissions and probably by heating processes emissions during winter time, and although NO2 concentrations have been decreasing, this could be leading to an increase in ozone concentrations that are already high in the city