Spatial variation in particle number size distributions in a large metropolitan area J. F. Mejía1, L. Morawska1, and K. Mengersen2 1International Laboratory of Air Quality and Health, Queensland University of Technology, GPO Box 2434, Brisbane, Qld 4001, Australia 2School of Mathematical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD, 4001, Australia
Abstract. Air quality studies have indicated that particle number size distribution
(NSD) is unevenly spread in urban air. To date, these studies have focussed
on differences in concentration levels between sampling locations rather
than differences in the underlying geometries of the distributions. As a
result, the existing information on the spatial variation of the NSD in
urban areas remains incomplete. To investigate this variation in a large
metropolitan area in the southern hemisphere, NSD data collected at nine
different locations during different campaigns of varying duration were
compared using statistical methods. The spectra were analysed in terms of
their modal structures (the graphical representation of the number size
distribution function), cumulative distribution and number median diameter
(NMD). The study found that with the exception of one site all distributions
were bimodal or suggestive of bimodality. In general, peak concentrations
were below 30 nm and NMDs below 50 nm, except at a site dominated by diesel
trucks, where it shifted to around 50 and 60 nm respectively. Ultrafine
particles (UFPs (<100 nm)) contributed to 82–90% of the particle
number, nanoparticles (<50 nm) to around 60–70%, except at the diesel
traffic site, where their contribution dropped to 50%. Statistical
analyses found that the modal structures heterogeneously distributed
throughout Brisbane whereas it was not always the case for the NMD. The
discussion led to the following site classification: (1) urban sites
dominated by petrol traffic, (2) urban sites affected by the proximity to
the road and (3) an isolated site dominated by diesel traffic. Comparisons
of weekday and weekend data indicated that, the distributions were not
statistically different. The only exception occurred at one site, where
there 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 not
be assessed due to the limited traffic information available.
Citation: Mejía, J. F., Morawska, L., and Mengersen, K.: Spatial variation in particle number size distributions in a large metropolitan area, Atmos. Chem. Phys., 8, 1127-1138, doi:10.5194/acp-8-1127-2008, 2008.