Intake Fraction for Benzene Traffic Emissions in Helsinki - Highway and Urban Environment - page 68

Fig. 1 Spatial distribution of benzene emissions from mobile sources in 2000
benzene concentrations were not substantially high, varying from 0.2 to 0.5 mg/m 3 .
These concentrations are considerably lower than the results for the HMA which is
mainly attributable to the low height/width (0.66) ratio of the studied street canyon.
The annual average benzene intake results for the working-age population within
HMA during 2000 and 2005 were calculated using the EXPAND model. Only results
for 2005 are shown in Fig. 2 , as the results for both years were almost identical.
Higher values of total intake fraction, ranging between 10 −10 and 10 −7 , are concen-
trated in residential and commercial areas, where people spend most of their time.
Therefore, there is an increase of receptors but also sources (traffic amount increases)
in these specific grid cells. The average intake fractions due to exposure in home,
work, traffic, and other microenvironments for the working-age population within
the HMA are 2.5 × 10 −6 , 1.4 × 10 6 , 5.9 × 10 −6 and 0.2 × 10 −6 , respectively. The total
intake fraction that corresponds to the exposure of the working-age population
summed across these different microenvironments in the HMA is 1.0 × 10 −5 .
Population exposure was higher during commuting, because of the relatively higher
concentrations in the vicinity of the main roads and streets, and subsequently higher
concentrations of benzene inside vehicles. People are also more exposed to benzene
traffic related emissions in the home and work environments, since these account for
around 90% of people's time. About 20% of the subjects have an intake fraction of
zero from combined exposure in traffic and outdoor microenvironments. These null
values can be attributed to either no reported time spent in these microenvironments
or nonexistent traffic related benzene concentrations, according to the dispersion
computations. Additionally, a total intake fraction of 3.7 × 10 −5 was calculated for
the street canyon. Using measured data from EXPOLIS as the average, individual
intake fraction of benzene from traffic sources over a 48-h period was 8.0 × 10 −11 ,