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

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

Table 1
Average meteorological conditions of the schools studied during the sampling week. Note that temperature and relative humidity readings were taken from the school weather
station while wind speed measurements were taken from the closest BOM or DSTIA station.

                           S15          S16           S17          S18            S19            S20           S21        S22        S23         S24           S25

 Sampling date             Nov 2011     Nov 2011      Dec 2012     Mar 2012       Mar 2012       Apr 2012      Jun 2012   Jun 2012   Jul 2012    Aug 2012      Aug 2012
 Temperature (
 C)         24           25            24           22             23             20            14         14         15          15            15
 Relative humidity (%)     65           75            71           71             79             63            58         66         60          40            68
 Wind Speed (m s1)        4.1          1.0           0.91         1.0            3.3            0.7           0.8        0.6        0.7         3.0           3.5


3.2. Detected organic compounds at the schools                                             vehicle emissions than plant wax emissions. This was probably
                                                                                           because the schools sampled in summer recorded higher traffic
    A number of particulate organic compounds were detected at                             flows (e.g. S15, S19 and S20). At the schools sampled in winter there
the schools as summarized in Table 3, which demonstrates that the                          was a higher abundance of plant wax alkanes (C>27) compared to
organic species and their concentrations varied from school to                             the schools sampled in summer (Fig. 1). Plant waxes alkanes can
school. Of the analyzed species, the most abundant organic com-                            come from either abrasion of vegetation detritus and incomplete
pound was generally levoglucosan, which was detected at all of the                         biomass burning (Cass, 1998; Pietrogrande et al., 2011; Simoneit,
schools except for S16 (Table 3). Of the other anhydrous sugars                            2002). In Brisbane, most trees are evergreen and winter is the
targeted, mannosan and galactosan were detected only at schools                            driest time of year, increasing the likelihood of emissions from leaf
sampled in winter, at S21-5 and S22, S24-5, respectively (Table 3).                        abrasion. In addition, the practice of prescribed burning of bush-
The most abundant resin acids detected were dehydroabietic acid                            land, described in more detail in Section 3.3.3, would also explain
(DHAA) and dehydroabietic acid methylester (DHAA-ME), and                                  the higher abundance of alkanes from plant wax during winter.
these compounds were observed only at the schools sampled in                                   Further differentiation between vehicle emissions and biogenic
summer (Table 3). Cholesterol, a known tracer molecule for meat                            sources is possible by determining the Cmax, which refers to the
cooking emissions, was observed at the schools (Table 3) (Kleeman                          most abundant alkane at each school, from the data in Table 3.
et al., 2008), indicates that cooking was a source. The most abun-                         Diesel and gasoline vehicle emissions have a different Cmax, at C20
dant hopanes were 17a(H),21b(H)-30-norhopane (29ab) and                                    and C23e5, respectively while plant wax emissions typically have a
17a(H),21b(H)-hopane (30ab) at all of the schools. In addition,                            Cmax above C27 (Cass, 1998; Rogge et al., 1993a; Schnelle-Kreis et al.,
17a(H),21b(H)-22S-homohopane (31abS) and 17a(H),21b(H)-22R-                                2011, 2005). At the schools sampled in summer the Cmax was
homohopane (31abR) were detected at S18-25 in appreciable                                  similar to that expected for gasoline vehicles (C23e5) whereas at
quantities. The n-alkanes analyzed were octadecane (C18) to hex-                           schools sampled in the winter, the Cmax was within the range for
atriacontane (C36) and their concentrations and distributions varied                       plant waxes (C>27). The exception was S23, which had a Cmax of 24
across the schools. The average values for each alkane at the schools                      and this indicates that vehicle emissions were the most likely major
are given in Table S2 (Supporting Information), with the sum of n-                         contributing source of n-alkanes at this school. Carbon preference
alkanes summarized in Table 3. Throughout this paper, the n-al-                            index (CPI) value, which is defined as the ratio of the sum con-
kanes are referred to as Cn, where n indicates the number of carbon                        centration of odd- to even-carbon numbered n-alkanes, has been
atoms. For example, pentacosane is referred to as C25. All abbrevi-                        widely used to distinguish between sources of n-alkanes (Brown
ations are listed in Table S2. The range of n-alkanes detected at the                      et al., 2002). Vehicle emissions are expected to have a ratio
schools indicated that fossil fuel combustion was another signifi-                          around 1 whereas plant waxes are expected to have a ratio >2
cant source at the schools, which is investigated further in the next                      (Schnelle-Kreis et al., 2007). In Table 3, the schools with a CPI
section.                                                                                   around 1 were S18, S19, S20, S22 and S23, which were generally the
                                                                                           schools beside comparatively higher trafficked roads. However, the
3.3. Sources of primary PM2.5 based on the detected organic                                CPI at S15e17 would suggest an influence from plant waxes in
compounds                                                                                  addition to that from vehicle emissions. Thus, from the n-alkanes it
                                                                                           was difficult to distinguish between the contributions of sources,
3.3.1. n-alkane concentrations and distribution                                            though it is clear that the two main sources were likely to be vehicle
    The concentration of the targeted n-alkanes is presented in                            emissions and plant waxes. Further investigations of the influence
Fig. 1, averaged for schools sampled during summer and winter.                             of vehicle emissions were possible by analyzing the concentrations
Vehicles emit alkanes in the range of C19e32 (Rogge et al., 1993a).                        and distributions of the hopanes.
The majority of the detected alkanes were observed in this range,
suggesting that this was the main source of alkanes at the schools.                        3.3.2. Hopane concentrations and distribution
Another possible source which has an overlapping alkane distri-                                Generally, the same hopanes distribution was found at all of the
butions are plant waxes, which are associated with alkanes greater                         schools and the average concentration for each hopane is presented
than C27 (Cass, 1998). From Table S2 and Fig. 1 alkanes within this                        in Fig. 2. The homohopane isomerization index (HHI, defined as
range were also detected, indicating that this was another potential                       31abS/31abSþ31abR) is about 0.1e0.4 for coal burning and 0.6 for
source at all schools. From Fig. 1, the schools sampled in summer                          mineral oil emission sources, such as vehicles (Oros and Simoneit,
had a higher abundance of alkanes and a higher association with                            2000; Schnelle-Kreis et al., 2011). For the schools where these


Table 2
Mean traffic conditions from the main at the schools studied over the sampling week.

                              S15          S16         S17           S18          S19             S20            S21        S22         S23            S24           S25

 Traffic (Veh hr1)            858          46          757           35           1093            1121           151        469         657            160           332
 Light vehicles (%)            92.1        97.8         95.3         97.9           89.9            96.2          97.4       96.8        71.1           97.1          92.0
 Medium vehicles (%)            5.9         1.6          3.0          2.1            3.2             2.5           2.1        2.5        28.0            2.4           6.5
 Heavy vehicles (%)             0.9         0.4          0.6          0.0            2.0             0.8           0.4        0.6         0.6            0.5           1.2