Analysis of surface ozone and nitrogen oxides at urban, semi-rural and rural sites in Istanbul, Turkey Original Research Article

Ozone (O3) mixing ratios were measured at three different sites (urban/traffic, semi-rural and rural/island) in Istanbul from September 2007 to December 2009 in order to determine the diurnal, monthly and seasonal variations of O3 and nitrogen oxides (NOx) and to study the local and regional impacts. This is the first study that evaluates the O3 levels in semi-rural and rural sites in Istanbul in addition to the urban sites. The diurnal O3 variations are generally characterized by afternoon maxima (64 ppb at the urban, 80 ppb at the semi-rural and 100 ppb at the rural site) and the nighttime minimum being more pronounced at the polluted urban site. The monthly mean O3 mixing ratios start to increase in March, reaching their maximum values in August for the urban (~ 25 ppb) and semi-rural sites (30 ppb). However, at the rural site, the monthly mean O3 levels reach their maximum value in June (35 ppb). The O3 mixing ratios for weekends were higher than those on weekdays at each site by up to 28%, possibly due to changes in VOC sensitivity and reduction in NOx levels. In order to better understand and characterize the relationship between air masses and O3 levels, cluster analysis was applied to the back-trajectories calculated by the HYSPLIT model for the semi-rural site. The analyses clearly showed that major transport is characterized by northern and western clusters, particularly from the Eastern Europe and the Mediterranean region, as well as recirculation over Istanbul due to high pressure systems leading to accumulated levels of O3. The results clearly suggest that extended measurement networks from urban to rural sites should be considered for a more comprehensive evaluation of O3 levels.