Detailed knowledge about the urban NO2 concentration field is a key element for obtaining accurate, individual exposure estimates. These are required for improving the understanding of the impact of ambient NO2 on human health and for related air quality measures. We developed a compact and robust quantum cascade laser absorption spectrometer (QCLAS) and deployed it on a tram in the city of Zurich (Switzerland) to perform mobile real-time concentration measurements of NO2. Thorough analysis of the obtained NO2 data, for instance by comparison with data from fixed air quality monitoring (AQM) sites, revealed the instrument to be highly accurate and valuable for collection of data that can be used in statistical models for the calculation of spatio-temporally resolved NO2 concentration maps. The combination of fast mobile measurements with AQM data proved to be very suitable, but the statistical data analysis also showed that a single mobile instrument is not sufficient in the studied urban area, for mainly two reasons: (i) short residence close to sources with large short-term NO2 variations and (ii) limited representativeness of the tram tracks for the entire urban environment.
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Hundt et al. (2017) studied this question.