Concentrations of NO
2, NO, and O
3 from a rooftop monitoring station in Gothenburg, Sweden (2002–2006) were analysed to characterise NO
2 pollution. [NO
2] was shown to correlate strongly and non-linearly with [NO
x ] (NO
x ?=?NO?+?NO
2), in line with observations in other cities. The [NO
2] to [NO
x ] fraction fell initially with increasing [NO
x ]. At [NO
x ] levels >200 ppb, the decline in [NO
2]/[NO
x ] with increasing [NO
x ] levelled out and [NO
2]/[NO
x ] converged towards approximately 0.15–0.16, independent of [NO
x ]. Data from a traffic route site showed the same pattern. This value of [NO
2]/[NO
x ] at high [NO
x ] can be interpreted as the NO
2 fraction of the NO
x emissions from vehicle exhaust. Situations with high NO
x pollution and minimum [NO
2]/[NO
x ] were always associated with [O
3] close to zero. Plotting [Ox] (Ox?=?NO
2?+?O
3) vs. [NO
x ] provided a strong linear correlation for situations dominated by local pollution ([NO]/[NO
2]>1). The slope of the regression, a measure of the primary NO
2 fraction in NO
x emissions, was 0.13 during the day and 0.14 during the night. With stronger winds, the rooftop monitoring station became more similar, in terms of NO
2 pollution, to a city street site and a traffic route site, although [NO
2] was almost always higher at the street/traffic route locations. The EU standard for the annual average of [NO
2] (40 μg m
?3) was exceeded, while the hourly standard (200 μg m
?3, not to be exceeded more than 18 times per year by 2010) was not exceeded at any of the sites.
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