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Air quality studies have indicated that particle number size distribution(NSD) is unevenly spread in urban air. To date, these studies have focussedon differences in concentration levels between sampling locations ratherthan differences in the underlying geometries of the distributions. As aresult, the existing information on the spatial variation of the NSD inurban areas remains incomplete. To investigate this variation in a largemetropolitan area in the southern hemisphere, NSD data collected at ninedifferent locations during different campaigns of varying duration werecompared using statistical methods. The spectra were analysed in terms oftheir modal structures (the graphical representation of the number sizedistribution function), cumulative distribution and number median diameter(NMD). The study found that with the exception of one site all distributionswere bimodal or suggestive of bimodality. In general, peak concentrationswere below 30 nm and NMDs below 50 nm, except at a site dominated by dieseltrucks, where it shifted to around 50 and 60 nm respectively. Ultrafineparticles (UFPs (<100 nm)) contributed to 82–90% of the particlenumber, nanoparticles (<50 nm) to around 60–70%, except at the dieseltraffic site, where their contribution dropped to 50%. Statisticalanalyses found that the modal structures heterogeneously distributedthroughout Brisbane whereas it was not always the case for the NMD. Thediscussion led to the following site classification: (1) urban sitesdominated by petrol traffic, (2) urban sites affected by the proximity tothe road and (3) an isolated site dominated by diesel traffic. Comparisonsof weekday and weekend data indicated that, the distributions were notstatistically different. The only exception occurred at one site, wherethere is a significant drop in the number of diesel buses on the weekend.The differences in sampling period between sites did not affect the results.The statistics instead suggested variations in traffic composition. However,the relative contribution of petrol vehicle emissions at each site could notbe assessed due to the limited traffic information available

Spatial variation in particle number size distributions in a large metropolitan area