Crilley 2014_Identification of the sources of primary organic aerosols a...

                                                                                    L.R. Crilley et al. / Environmental Pollution 191 (2014) 158e165                                      163


emissions may have been a contributing source to the observed                                                          Fig. 4) recorded high traffic counts. Unlike the other schools, which
levels of DHAA at the schools along with domestic wood burning                                                         were in suburban areas, S18 is in an inner-city location and this may
barbeques.                                                                                                             explain the high levels for this factor.
                                                                                                                           The first factor was assigned to biomass burning as it was
                                                                                                                       characterized by high contributions of levoglucosan, the molecular
3.4. Source apportionment using PMF
                                                                                                                       marker for biomass burning (Qadir et al., 2013; Robinson et al.,
                                                                                                                       2006a; Simoneit, 2002). The OC/EC ratio for this factor was 2.8,
    To apportion the contributions of the different sources of pri-
                                                                                                                       which is lower than the ratio of OC/EC from previous study held in
mary organic aerosols identified at each school to the OC fraction,
                                                                                                                       Australia (Keywood et al., 2011). However, this could be explained
PMF was applied to the dataset. Interpretability was the key basis
                                                                                                                       by higher contribution of EC, which is a marker for vehicle emis-
for judging the optimum solution to the PMF analysis. Interpretable
                                                                                                                       sions (Sonntag et al., 2012), to the PMF profile. The biomass burning
solutions were those that grouped source-class-specific sets of
                                                                                                                       factor was the second largest source overall during the study, ac-
markers into distinct factors. Models that grouped markers for
                                                                                                                       counting for 29  23% of the OC (Fig. 4). From Fig. 4, the highest
multiple source classes into the same factor, distributed markers for
                                                                                                                       contributions were as expected, found during the winter (e.g. S22,
one source class across multiple factors, or contained factors with
                                                                                                                       S24 and S25) at an average of 48  19%, primarily due to the
no distinctive groupings of compounds were judged less inter-
                                                                                                                       practice of prescribed burning of bushland surrounding Brisbane.
pretable and rejected. While no molecular marker pattern can be
                                                                                                                           The next source, which accounted for 16  13% of the OC was
unambiguously associated with a specific source class, this
                                                                                                                       characterized by cholesterol. As cholesterol is a known tracer for
approach provides a systematic basis for sorting through the
                                                                                                                       meat cooking (Kleeman et al., 2008), this source was assigned to
possible PMF models (Shrivastava et al., 2007). Solutions with 3e5
                                                                                                                       meat cooking emissions. Cooking emissions are increasingly being
factors were investigated and the three factor solution was rejected
                                                                                                                       identified as a significant urban source (Mohr et al., 2012; Robinson
as it could not separate out the cooking and biomass burning fac-
                                                                                                                       et al., 2006b) and the observed high contributions may be indica-
tors. The five factor solution was not used due to splitting of the
                                                                                                                       tive of localized emissions from school canteens. This is illustrated
vehicle emissions source factor into unrealistic factors. Therefore
                                                                                                                       at S24 where the high concentrations (Fig. 4) were likely due to a
the four factor solution was found to best represent the data as it
                                                                                                                       school barbeque event that occurred on one of the sampling days.
gave realistic source profiles related to distinct primary sources.
                                                                                                                       In addition, S23 had high concentration of meat cooking emissions
Source profiles are given in Fig. 3 and were assigned to biomass
                                                                                                                       because it is located near a large number of restaurants. This
burning, cooking, vehicle and plant wax emissions based upon the
                                                                                                                       probably suggests that apart from the school canteen, there was an
high concentrations of known tracer molecules for these sources.
                                                                                                                       additional source at the school.
The contributions of each identified source factors to the OC
                                                                                                                           High contributions of the alkanes C25, C27 and C29 e C31 char-
measured at each individual school are given in Fig. 4 and the
                                                                                                                       acterized the last factor, which was assigned to plant waxes. Plant
factors will be discussed in order of their contribution to the OC
                                                                                                                       waxes particles can enter the atmosphere from either leaf abrasion
rather than factor number.
                                                                                                                       (Rogge et al., 1993c) or incomplete biomass burning (Simoneit,
    The major source, vehicle emissions (See Fig. 4), was charac-
                                                                                                                       2002). Both of these source pathways were likely as the highest
terized by the C20-25 alkanes and the hopanes 29ab, 30ab, 31abS
                                                                                                                       contributions were observed at schools (S20, S21 and S22) sampled
and 31abR. C20 and C23e5 alkanes have previously been shown to be
                                                                                                                       during the driest time of the year and schools that recorded strong
associated with diesel and gasoline vehicle emissions (Rogge et al.,
                                                                                                                       biomass burning influences (Fig. 4). This factor accounted for 7  6%
1993a). This together with the fact that the hopanes determined in
                                                                                                                       of the OC and was the smallest contributing source of primary
Section 3.3.2 were deemed to be from fossil fuel combustion, points
                                                                                                                       aerosols at the schools.
to vehicle emissions as the source. The HHI for this factor was 0.85
                                                                                                                           As the schools selected for this study were not close to any other
and in the range expected for vehicle emissions, further indicating
                                                                                                                       large source of air pollution other than road traffic, the main source
vehicle emissions as a source. This factor contributed an average of
                                                                                                                       of primary organic aerosol was hypothesized to be vehicle emis-
45  30% of the OC at the schools and was therefore the main
                                                                                                                       sions. This hypothesis was confirmed based upon the PMF analysis
source in our study. The factor was only somewhat correlated with
                                                                                                                       on selected tracer organic molecules (Fig. 4). The lower contribu-
the traffic counts beside the schools (r2 of 0.3), probably due to the
                                                                                                                       tion from biomass burning source to OC mass at S15eS20 (sites
multiple variables that contribute to the observed levels of vehicle
                                                                                                                       measured in summer, Fig. 4) was due, in part, to the sampling at
emissions at the schools (Crilley, 2013). However, it was observed
                                                                                                                       these schools occurring when the contributions of biomass burning
that with the exception of S18 (Table 2), the schools with the
                                                                                                                       from prescribed burning were at a minimum. Generally, year-round
highest contributions from vehicle emissions (S15, S17, S19 and S20,
                                                                                                                       vehicle emissions were found to have a largest overall contribution
                                                                                                                       to the OC mass at the schools. Previous work on PM2.5 in urban
                             100%                                                                                      environments using an organic molecular marker approach also
                             90%                                                                                       found that vehicle emissions rather than wood combustion were
                                                                                                       Biomass
                             80%                                                                       Burning         the largest primary source of PM2.5 (Green et al., 2013; Wang et al.,
% Organic Carbon (OC) mass




                             70%
                                                                                                                       2012). Unlike in the current work where prescribed fires in sur-
                                                                                                       Cooking
                                                                                                                       rounding bushland were the source of biomass burning, the wood
                             60%
                                                                                                                       combustion was attributed primarily to residential heating in those
                             50%
                                                                                                       Vehicle         studies (Green et al., 2013; Wang et al., 2012).
                             40%                                                                       Emissions

                             30%
                                                                                                       Plant Wax       4. Conclusions
                             20%
                             10%                                                                                          Selected organic molecular markers were quantified in order to
                              0%                                                                                       apportion the contributing sources of primary organic aerosols to
                                    S15   S16   S17   S18   S19   S20   S21   S22   S23   S24   S25
                                                                                                                       children’s exposure of PM2.5 at urban schools, with vehicle emis-
Fig. 4. Percentage of contributions of the PMF source factors to organic carbon (OC)                                   sions hypothesized to be the largest contributing source. Using
mass at the 11 schools.                                                                                                PMF, four source factors were identified and assigned to vehicle