『Abstract
　Concentrations of NO2, NO, and O3 from a rooftop monitoring station in Gothenburg, Sweden (2002-2006) were analysed to characterise NO2 pollution. [NO2] was shown to correlate strongly and non-linearly with [NOx] (NOx = NO + NO2), in line with observations in other cities. The [NO2] to [NOx] fraction fell initially with increasing [NOx]. At [NOx] levels ＞200 ppb, the decline in [NO2]/[NOx] with increasing [NOx] levelled out and [NO2]/[NOx] converged towards approximately 0.15-0.16, independent of [NOx]. Data from a traffic route site showed the same pattern. This value of [NO2]/[NOx] at high [NOx] can be interpreted as the NO2 fraction of the NOx emissions from vehicle exhaust. Situations with high NOx pollution and minimum [NO2]/[NOx] were always associated with [O3] close to zero. Plotting [Ox] (Ox = NO2 + O3) vs. [NOx] provided a strong linear correlation for situations dominated by local pollution ([NO]/[NO2]＞1). The slope of the regression, a measure of the primary NO2 fraction in NOx 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 NO2 pollution, to a city street site and a traffic route site, although [NO2] was almost always higher at the street/traffic route locations. The EU standard for the annual average of [NO2] (40 μg m^-3) was exceeded, while the hourly standard (200μg m^-3, not to be exceeded more than 18 times per year by 2010) was not exceeded at any of the sites.

Keywords: Gothenburg; Nitrogen dioxide; Nitrogen monoxide; Oxidant; Ozone; Photochemistry; Primary NO2 fraction; Urban air pollution』

1. Introduction
2. Materials and methods
3. Results
　3.1. Average [NO2] at different [NOx]
　3.2. Relationships of [NO2]/[NOx] with [NOx]
　3.3. Relationships of [NO2]/[NOx] with [O3] at different levels of global radiation
　3.4. Relationships between [Ox] and [NOx]
　3.5. Differences in [NO2] between monitoring sites
4. Discussion and conclusions
References