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

                                                            L.R. Crilley et al. / Environmental Pollution 191 (2014) 158e165                                                     159


developing asthma and wheezing (Gehring et al., 2010; Ryan et al.,                             organic compounds (e.g. levoglucosan and cholesterol) occurs directly from par-
2009). Children spend a large portion of their day at school.                                  ticulate matter on the filters. Therefore, polar and non-polar compounds are
                                                                                               analyzed in one step. An automated sampling robot exchanged the complete GC
Consequently schools represent a location accounting for a signif-                             liners placed in the injector which is automatically closed and opened by a liner
icant fraction of their overall daily exposure. Schools are also                               exchanging unit (Linex, Atas GL, Netherlands). During thermal desorption (16 min,
unique urban environments that are influenced not only by local                                 300 
 C) MSTFA enriched carrier gas was passed through the liner. Desorbed and
and regional sources but also by school-related activities such as                             derivatized molecules were focused at 70 
 C on the head of a nearly non-polar
                                                                                               capillary column, BPX5, 25 m, 0.22 mm ID, 0.25 mm film (SGE, Australia) which
drop-off/pick-up traffic, which may result in increased exposure to
                                                                                               was installed in an Agilent 6890 gas chromatograph (Agilent, USA). After the
vehicle emissions. However, there is limited information on chil-                              isothermal run at 70 
 C, the temperature was increased to 130 
 C within one minute.
dren’s exposure to vehicle emissions and other sources in the                                  Then the rate was lowered to 8 
 C min1 until a temperature of 330 
 C was reached
literature (Mejía et al., 2011).                                                               followed by an isothermal time of 30 min. The detection of compounds was carried
    Considering the aforementioned gaps in knowledge, the present                              out on a Pegasus III TOFMS (LECO, USA). The data acquisition frequency was 25
                                                                                               spectra per second within a mass-to-charge range of 35e500. The detection limits
study aimed to determine the primary sources of the ambient fine                                for the targeted compounds can be found in Tables S1 and S2 (Supporting
particulate matter present at urban schools by quantifying the                                 Information).
levels of selected organic molecular markers. PMF, a receptor model                                 Details of the elemental carbon (EC) and organic carbon (OC) analysis can be
(Paatero and Tapper, 1994), was applied in order to quantitatively                             found in Crilley (2013). Briefly, PM2.5 samples were collected concurrently at the
                                                                                               same location at each school and were analyzed according to the IMPROVE method
determine the contributions of these sources to levels of ambient
                                                                                               on a thermal/optical transmittance carbon analyser (Sunset Laboratories) (Chow
particles at the schools. This enabled the driving factors to the                              et al., 2001).
levels of different primary sources to be investigated and we hy-
pothesized that vehicle emissions will be the main primary source
                                                                                               2.4. Data analysis
contributing to children’s exposure at school and this is reflected in
the schools’ selection criteria, outlined in the next section.                                     Positive Matrix Factorization (PMF) was performed in this study using the EPA
                                                                                               PMF v3.0, developed by the U.S EPA (US-EPA and Research Triangle Park N, 2008). A
                                                                                               total of 21 organic compounds were used in the initial analysis and these included n-
2. Method
                                                                                               alkanes from C20e33, four hopanes, cholesterol, dehydroabietic acid and its meth-
2.1. Sampling sites                                                                            ylester, levoglucosan, mannosan, EC and the OC fraction. Missing values in a certain
                                                                                               school site were replaced with the geometric mean at the same site and the un-
     The 11 schools selected for this study, which will be referred to as S15 to S25, are
                                                                                               certainty was set to four times the geometric mean for those values. As about 80% of
in different suburbs in the city of Brisbane, Australia. A map depicting the schools
                                                                                               n-alkanes between C18 and C21 are typically in the gas phase, therefore these
location in Brisbane is given in Figure S1, Supporting Information. This chemical
                                                                                               compounds were excluded from the PMF analysis. Species with more than 50% of
sampling was a part of a larger campaign to study the effect of ultrafine particle from
                                                                                               data below the limit of quantification (LOQ) were also excluded from the inputs.
traffic emissions on children’s health, known as UPTECH (www.ilaqh.qut.edu.au/
                                                                                               Therefore, some alkanes (C20, C21, C26, C28, C32 and C33), dehydroabietic acid and its
Misc/UPTECH%20Home.htm). In this study 25 schools participated but only at the
                                                                                               methylester, and mannosan were excluded from the PMF analyses. In other few
last 11 schools were filters collected for this analysis due to instrument availability.
                                                                                               cases the values were replaced with half of the LOQ and the uncertainties set to 5/6
The sampling campaign for the current paper was conducted from November 2011
                                                                                               of the LOQ. For the other data, the uncertainties are set according to the following
till August 2012.
                                                                                               equation:
     The schools that were chosen were not near any other large source of air
pollution, other than road traffic and were also not close to any large infrastructure             h                                            i0:5
projects. A site close to the middle of the school which gave the best overall rep-            u ¼ ðerror fraction  concentrationÞ2 þ ðLOQ Þ2                                    (1)
resentation was chosen to conduct the measurements and sampling at each school.
Inlets for the sampling were placed on the top of a trailer, which served to house all         where error fraction is the percentage of uncertainty, LOQ is the limit of quantifi-
of the instruments at the site including an automatic weather station (Monitor                 cation. Overall error fractions were estimated from sampling and analytical errors.
Sensors). Data from nearby weather stations was also obtained from the Bureau of               Species with low signal to noise ratio (S/R < 2), were assigned as weak. One hundred
Meteorology (BOM) and Queensland Department of Science, Information Technol-                   bootstrap runs were performed to assess the uncertainty of the factor loadings and
ogy, Innovation and the Arts (DSITIA) as some of the schools would have been                   profiles. Changing the Fpeak value did not improve the source profiles or Q value and
affected by local winds due to schools buildings. Sampling inlets were approxi-                so base model results (Fpeak ¼ 0) are reported. The PMF solutions are discussed in
mately 3 m off the ground. Traffic counts were taken on the busiest road next to the            Section 3.3.
school, referred to as the main road throughout. In the traffic count, vehicles that                The other data analysis technique employed was one way ANOVA used to
were classified as light were cars, motorbikes and scooters. Trucks with 2, 3 and 4             compare the concentrations between the seasons and this was performed using
axels were classified as medium vehicle and long articulated trucks classified as                SPSS v19.
heavy vehicles.


2.2. Sampling methodology                                                                      3. Results and discussion
    The particle size fraction was selected using a low-volume PM2.5 cyclone and the
required flow rate for the cyclone was maintained using critical orifices. The parti-            3.1. General school characteristics
cles were collected in polycarbonate filter holders on 47 mm, Quartz fiber filters
(Whatman) which was pre-treated at 500 
 C for 1 h prior to use. The sampling
                                                                                                   The sampling was conducted over almost a full year and covered
period was from 08:00 until 08:00 the next day, Monday to Friday for one week to
give a total of four samples at each school. This sampling regime typically gave               both winter and summer, as reflected in the meteorological con-
sampling volumes of 6 m3. After sampling each filter was placed in a Petri dish and             ditions described in Table 1. Brisbane is a subtropical city with the
covered in aluminum foil. The samples were then sealed in a ziplock bag and frozen             mean temperatures ranging from 14 to 25 
 C (Table 1). October till
until analysis. The flow rate for the cyclones was checked at the beginning of each             April is warmer with higher humidity and this period is referred to
sample and always set to within 0.1 L per minute of the desired flow rate. Field
                                                                                               as summer. May till September is cooler with typically more stable
blanks were collected at each school and all results were field-blank corrected.
                                                                                               weather and it is referred to as the winter months. A range of traffic
2.3. Chemical analysis                                                                         conditions were also observed at the schools (Table 2). Some of the
                                                                                               schools were on busy arterial roads (S19, S20) while others were in
    The organic composition of the samples was analyzed using In-situ Derivati-
zation Thermal Desorption (IDTD) followed by Gas Chromatography Time-Of-Flight                 residential areas with low traffic counts (S16, S18, S21). There were
Mass Spectrometry (GC-TOF-MS) (Orasche et al., 2011; Schnelle-Kreis et al., 2011) at           also schools alongside roads that service commercial/light indus-
the Joint Mass Spectrometry Centre, Helmholtz Zentrum München, Neuherberg,                     trial areas (S23, S25) and such schools recorded a higher proportion
Germany. In brief, filter punches were placed in an injection liner and spiked with             of heavy and medium vehicles (Table 2). Based on the organic
internal standards (isotope labeled reference compounds) and 10 ml derivatizing
agent N-Methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA). The method allows
                                                                                               compounds detected and their relative abundance, the major
derivatization using MSTFA reagent of the polar organic fraction of filter samples in-          contributing sources of primary organic aerosols were determined
situ on the filter during thermal desorption. Derivatization and desorption of polar            at the schools and discussed in the next sections.