A Shinyei PPD20V particle counter sensitive to particles with a
diameter larger than 1 μm was used to determine the number concentration
of PM in particles per litre. PM emissions from traffic
combustion are smaller than 0.5 μm and cannot be detected with this
device. Based on a detailed chemical characterization of particulate
matter at a nearby site (Toner et al., 2008) we expect the majority of
the particles in the N1 μm range to be sea salt and local dust from
the wind shear created by vehicular traffic and construction. Since
international health standards and PM measurements are usually
reported in μg m−3 a conversion factor was needed to allow better
intercomparison of the reported PM concentrations. Given that the
sensor used is able to identify particles between 1 μm and 10 μm and
assuming a typical PM size distribution, the average aerodynamic
diameter would be around 3 μm (Seinfeld and Pandis, 1997). Thus, for
a typical silicate particle density of 2.2 g cm−3 the conversion factor
for transforming particle counts per volume air into particle mass per
volume air would be equal to 0.028 (μg m−3 per part/l). This conversion
to mass concentration is approximate but does not affect the
analysis qualitatively.