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Atmos. Chem. Phys., 19, 10675–10696, 2019
https://doi.org/10.5194/acp-19-10675-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
Size-resolved composition and morphology of particulate matter
during the southwest monsoon in Metro Manila, Philippines
Melliza Templonuevo Cruz1,2, Paola Angela Bañaga1,3, Grace Betito1,3 , Rachel A. Braun4, Connor Stahl4,
Mojtaba Azadi Aghdam4, Maria Obiminda Cambaliza1,3, Hossein Dadashazar4, Miguel Ricardo Hilario1,3 ,
Genevieve Rose Lorenzo1, Lin Ma4, Alexander B. MacDonald4, Preciosa Corazon Pabroa5, John Robin Yee5,
James Bernard Simpas1,3, and Armin Sorooshian4,6
1Manila Observatory, Quezon City 1108, Philippines
2Institute of Environmental Science and Meteorology, University of the Philippines,
Diliman, Quezon City 1101, Philippines
3Department of Physics, School of Science and Engineering, Ateneo de Manila University,
Quezon City 1108, Philippines
4Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
5Philippine Nuclear Research Institute, Commonwealth Avenue, Diliman, Quezon City 1101, Philippines
6Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
Correspondence: Melliza Templonuevo Cruz (liz@observatory.ph)
Received: 20 March 2019 – Discussion started: 12 April 2019
Revised: 17 July 2019 – Accepted: 18 July 2019 – Published: 23 August 2019
Abstract. This paper presents novel results from size-
resolved particulate matter (PM) mass, composition, and
morphology measurements conducted during the 2018
southwest monsoon (SWM) season in Metro Manila, Philip-
pines. Micro-orifice uniform deposit impactors (MOUDIs)
were used to collect PM sample sets composed of size-
resolved measurements at the following aerodynamic cut-
point diameters (Dp): 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32,
0.18, 0.10, and 0.056 µm. Each sample set was analyzed for
composition of the water-soluble fraction. Analysis for mass
was carried out on two sample sets, whereas black carbon
(BC) and morphology analysis were analyzed on a single
sample set. The bulk of the PM mass was between 0.18 and
1.0 µm with a dominant mode between 0.32 and 0.56 µm.
Similarly, most of the black carbon (BC) mass was found be-
tween 0.10 and 1.0 µm, peaking between 0.18 and 0.32 µm.
These peaks are located in the Greenfield gap, or the size
range between 0.10 and 1.0 µm, where wet scavenging by
rain is relatively inefficient. In the range between 0.10 and
0.18 µm, BC constituted 78.1% of the measured mass. Com-
parable contributions of BC (26.9 %) and the water-soluble
fraction (33.4 %) to total PM were observed and most of the
unresolved mass, which amounted to 39.6 % in total, was
for diameters exceeding 0.32 µm. The water-soluble ions and
elements exhibited an average combined concentration of
8.53 µg m−3, with SO2−
4, NH+
4, NO−
3, Na+, and Cl−as the
major contributors. Positive matrix factorization (PMF) was
applied to identify the possible aerosol sources and estimate
their contribution to the water-soluble fraction of collected
PM. The factor with the highest contribution was attributed
to “aged aerosol” (48.0 %), while “sea salt” (22.5 %) and
“combustion” emissions (18.7 %) had comparable contribu-
tions. “Vehicular/resuspended dust” (5.6 %) and “waste pro-
cessing” emissions (5.1 %) were also identified. Microscopy
analysis highlighted the ubiquity of nonspherical particles re-
gardless of size, which is significant when considering cal-
culations of parameters such as single scattering albedo, the
asymmetry parameter, and the extinction efficiency.
The significant influence from aged aerosol to Metro
Manila during the SWM season indicates that local sources
in this megacity do not fully govern this coastal area’s aerosol
properties. The fact that the majority of the regional aerosol
mass burden is accounted for by BC and other insoluble com-
ponents has important downstream effects on the aerosol hy-
groscopic properties, which depend on composition. The re-
sults are relevant for understanding the impacts of monsoonal
Published by Copernicus Publications on behalf of the European Geosciences Union.
10676 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
features on size-resolved aerosol properties, notably aque-
ous processing and wet scavenging. Finally, the results of
this work provide contextual data for future sampling cam-
paigns in Southeast Asia such as the airborne component of
the Cloud, Aerosol, and Monsoon Processes Philippines Ex-
periment (CAMP2Ex) planned for the SWM season in 2019.
1 Introduction
Ambient atmospheric aerosol particles impact human health,
visibility, climate, and the hydrological cycle. Major factors
governing these behaviors, such as the deposition fraction in
the respiratory system and activation into cloud condensa-
tion nuclei (CCN), include size and chemical composition.
Therefore, size-resolved measurements of ambient aerosol
particles can lend additional insights to the behavior and im-
plications of particulate matter (PM) in the atmosphere. One
region of interest for the characterization of PM is Southeast
Asia due to increasing urbanization and the exposure of the
population to a variety of aerosol sources, both natural and
anthropogenic (Hopke et al., 2008). However, use of space-
borne remote-sensing instrumentation presents a challenge
for characterization of aerosol in this region, due to issues
such as varying terrain and cloud cover (Reid et al., 2013).
The Philippines represents a country in Southeast Asia
with a developing economy, rapid urbanization, old vehicu-
lar technology, and less stringent air quality regulations (e.g.,
Alas et al., 2017). It is also highly sensitive to the effects
of climate change, including prolonged dry periods and re-
ductions in southwest monsoon (SWM) rainfall in recent
decades (e.g., Cruz et al., 2013). Metro Manila is the coun-
try’s capital and the center of political and economic activi-
ties. Also referred to as the National Capital Region, Metro
Manila is composed of 16 cities and a municipality that col-
lectively occupy a land area of ∼619 km2. As of 2015, Metro
Manila had a population of approximately 12.88 million
(Philippine Statistics Authority, 2015). Of the cities compris-
ing the Metro Manila area, the one that is the focus of this
study, Quezon City, is the most populated (2.94 million peo-
ple) with a population density of ∼17 000 km−2as of 2015
(Philippine Statistics Authority, 2015).
The rainfall pattern in Southeast Asia is governed by topo-
graphic effects and the prevailing surface winds brought by
the monsoons. Mountain ranges in the Philippines are gener-
ally oriented north to south on the eastern and western coasts.
As such, northeasterly winds during the East Asian winter
monsoon that starts in November cause wetness (dryness)
on the eastern (western) coasts of the country. In contrast,
the rainy season starts in May when the western North Pa-
cific subtropical high moves northeast and the Asian sum-
mer monsoon enables the propagation of southwesterly wind
through the Philippines (Villafuerte et al., 2014). Therefore,
Metro Manila, located on the western side of the Philippines,
experiences wet (May–October) and dry (November–April)
seasons. The large seasonal shift in the prevailing wind di-
rections can cause changes in the source locations of aerosol
transported to the Philippines and the subsequent direction in
which emissions from the Philippines are transported, such
as to the northwest (e.g., Chuang et al., 2013) or southwest
(e.g., Farren et al., 2019). However, one interesting feature of
Metro Manila is the consistency of PM2.5/PM10 mass con-
centrations during both the dry (44/54 µg m−3) and wet sea-
sons (43/55 µg m−3) (Kim Oanh et al., 2006), which stands
in contrast to typical assumptions that increased wet scav-
enging during rainy seasons would lead to decreases in mea-
sured PM (e.g., Liao et al., 2006). While similar results are
observed in Chennai, India, this behavior is different from
other cities in Asia, including Bandung (Indonesia), Bangkok
(Thailand), Beijing (China), and Hanoi (Vietnam), which ex-
hibit reduced PM2.5levels during the wet season compared
with the dry season (Kim Oanh et al., 2006). While the total
PM levels may stay constant across the wet and dry seasons,
seasonally resolved analyses will provide additional insights
into how the composition, morphology, and sources (trans-
ported vs. local emissions) change on a seasonal basis.
Metro Manila has been drawing growing interest with re-
spect to PM research owing to the significant levels of black
carbon (BC). A large fraction of PM in Metro Manila can
be attributed to BC (e.g., ∼50 % of PM2.5; Kim Oanh et
al., 2006), with previously measured average values of BC
at the Manila Observatory (MO) reaching ∼10 µg m−3for
PM2.5(Simpas et al., 2014). The impacts of these high levels
of BC present on human health have also received attention
(Kecorius et al., 2019). Identified major sources of BC in-
clude vehicular, industrial, and cooking emissions (Bautista
et al., 2014; Kecorius et al., 2017). Vehicular emissions are
especially prevalent along roadways where personal cars and
motorcycles, commercial trucks, and motorized public trans-
portation, including powered tricycles and “jeepneys”, are
plentiful. For instance, measurements of PM2.5at the Na-
tional Printing Office (NPO) located alongside the major
thoroughfare Epifanio de los Santos Avenue (EDSA) were
72 µg m−3on average; this value is twice the average con-
centration at MO, an urban mixed site located approximately
5 km from NPO (Simpas et al., 2014). In addition to local
emissions, long-range transport of pollution, such as biomass
burning, can also impact the study region (e.g., Xian et al.,
2013; Reid et al., 2016a, b). However, most of the past work
referenced above has focused on either total PM2.5or PM10
composition; therefore, detailed size-resolved composition
information has been lacking in this region. Like other mon-
soonal regions (Crosbie et al., 2015; Qu et al., 2015), it is
of interest, for instance, to know if products of aqueous pro-
cessing (e.g., sulfate, organic acids) during the monsoonal
period, promoted by the high humidity, become more promi-
nent in certain size ranges to ultimately enhance hygroscop-
icity, which is otherwise suppressed with higher BC influ-
ence.
Atmos. Chem. Phys., 19, 10675–10696, 2019 www.atmos-chem-phys.net/19/10675/2019/
M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10677
A year-long sampling campaign, the Cloud, Aerosol, and
Monsoon Processes Philippines Experiment (CAMP2Ex)
weatHEr and CompoSition Monitoring (CHECSM) study,
was established in July 2018 to collect size-resolved aerosol
measurements in Metro Manila. The aim of this study was to
report size-resolved PM measurements taken over the course
of the SWM (July–October) in 2018 in Quezon City, Metro
Manila, Philippines as part of CHECSM. The results of this
study are important for the following reasons: (i) they pro-
vide size-resolved analysis of BC in an area previously char-
acterized as having one of the highest BC mass percentages
in the world; (ii) they provide a basis for better understanding
the unusual phenomenon of similar observed PM levels dur-
ing a wet and dry season; (iii) they provide contextual data
that can be contrasted with both other coastal megacities and
other monsoonal regions; and (iv) they can lend insights into
the characteristics of aerosol transported both into and out of
Metro Manila and how important local sources are in Metro
Manila relative to transported pollution.
Outcomes of this study include (i) the first size-resolved
characterization of both aerosol composition and morphol-
ogy in Metro Manila for the SWM, with implications in
terms of PM effects on climate, visibility, the hydrological
cycle, and public health owing to the dependence of these
impacts on particle size; (ii) archival data that contribute to
the time line of aerosol research in Metro Manila, and more
broadly Southeast Asia, where there is considerable concern
regarding air pollution; and (iii) baseline data for aerosol
composition to be used to inform and assist research to be
conducted during future field campaigns in Southeast Asia
including the same seasonal period (i.e., SWM) in 2019 as
part of CAMP2Ex, which will involve both surface and air-
borne measurements.
2 Experimental methods
2.1 Sample site
Sampling was performed at MO in Quezon City, Philippines
(14.64◦N, 121.08◦E). Two MOUDIs were placed inside an
unoccupied room on the third floor of the MO administra-
tion building (∼87 m a.s.l.). The inlet, located just outside
the window, consisted of a 2 m long stainless steel tube and
a reducer that was connected directly to the MOUDI inlet.
Figure 1 shows the sampling location and potential surround-
ing aerosol sources. Past work focused on PM2.5suggested
that the study location is impacted locally mostly by traffic,
various forms of industrial activity, meat cooking from local
eateries, and, based on the season, biomass burning (Cohen
et al., 2009). This is consistent with another source appor-
tionment study which reported that potential sources at six
sites across Metro Manila included traffic, secondary parti-
cles, and biomass burning (Kim Oanh et al., 2013).
Meteorological data were collected using a Davis Van-
tage Pro2 Plus weather station located on the roof (∼90 m
above sea level, ∼15m above ground level) above where
the MOUDIs were located. Except for precipitation, which
is reported here as accumulated rainfall, reported values for
each meteorological parameter represent averages for the
sampling duration of each aerosol measurement. The mean
temperature during the periods of MOUDI sample collection
ranged from 24.9 to 28.1 ◦C, with accumulated rainfall rang-
ing widely from no rain to up to 78.4 mm. To identify sources
impacting PM via long-range transport to the Metro Manila
region, Figure 1a summarizes the 5 d back-trajectories for
air masses arriving at MO on the days when samples were
being collected, calculated using the NOAA Hybrid Single-
Particle Lagrangian Integrated Trajectory (HYSPLIT) model
(Stein et al., 2015; Rolph, 2016). Trajectory calculations
were started at 00:00, 06:00, 12:00, and 18:00 h at MO using
a model run height of 12 m above ground level and meteoro-
logical files from the NCEP/NCAR reanalysis dataset. Tra-
jectory cluster analysis was conducted using TrajStat (Wang
et al., 2009). The back-trajectories in Figure 1a show that
66 % of the wind indeed came from the southwest during the
sampling periods.
2.2 MOUDI sample sets
PM was collected on Teflon substrates (PTFE membrane,
2 µm pore, 46.2 mm, Whatman) in micro-orifice uniform de-
posit impactors (MOUDI, MSP Corporation, Marple et al.,
2014). Size-resolved measurements were taken at the fol-
lowing aerodynamic cut-point diameters (Dp): 18, 10, 5.6,
3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.10, and 0.056 µm. A total
of 14 sample sets were collected during the SWM season
(July–October 2018), with details regarding the operational
and meteorological conditions during each sample set shown
in Table 1. To determine the optimum sampling time required
to collect enough sample for subsequent analyses, the col-
lection times for the first four samples ranged from 24 to
119 h. Subsequent sample collection was then fixed at 48 h,
with one sample set collected every week. The sample col-
lection was designed to include samples from each day of the
week; therefore, the collection cycled between Monday and
Wednesday, Tuesday and Thursday, Wednesday and Friday,
and Saturday and Monday, starting at 14:00LT (local time)
for the weekday samples and 05:00 LT for the weekend sam-
ples. The Teflon substrates were pretreated by washing with
deionized water and air drying in a covered box. Substrates
were placed in and retrieved from the cascade impactor in-
side the laboratory in an adjacent building and transported to
and from the sampling site using an impactor holder (Csavina
et al., 2011). Samples were immediately placed in the freezer
upon retrieval.
On two occasions, two pairs of MOUDI sets
(sets MO3/MO4 and MO13/MO14) were collected si-
multaneously so that one set in each pair could undergo
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10678 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
Figure 1. (a) Location of Metro Manila, Philippines relative to Southeast Asia. Also shown are 5d backward trajectory frequencies during the
sampling duration based on HYSPLIT cluster analysis; note that 15 % correspond to trajectories within the black square. (b) Close-up view
of Metro Manila showing the location of the Manila Observatory sampling site (black rectangle). The base map shows roads, commercial
centers, and major transit lines in the city. (c) Land use classification in the vicinity of the sampling site. (Sources: GADM, Snazzy Maps,
©OpenStreetMap contributors 2019. Distributed under a Creative Commons BY-SA License, NOAA HYSPLIT, & TrajSat).
Table 1. Summary of average operating parameters, meteorological conditions, and total resolved water-soluble mass concentration for each
MOUDI sample set collected at Manila Observatory (MO) during the 2018 southwest monsoon period. On two occasions, simultaneous
MOUDI sets were collected so that one set could undergo gravimetric analysis (MO3 and MO13) and be compared with the mass resolved
from chemical speciation of the water-soluble fraction (MO4 and MO14). One additional MOUDI set devoted to microscopy analysis was
collected using aluminum substrates for 1 h on 1 August at 30 LPM (flow rate).
Sample Dates Duration Flow Wind Wind TRain Water-soluble
set name (h) rate speed direction (◦C) (mm) mass
(LPM) (m s−1) (◦) (µg m−3)
MO1 19–20 Jul 24 30 3.3 90.1 24.9 47 4.6
MO2 23–25 Jul 54 30 1.3 95.8 26.7 7.8 6.5
MO3/4 25–30 Jul 119 29/30 1.2 111.8 26.7 49.6 5.2
MO5 30 Jul–1 Aug 42 29 2.6 98.1 27.5 52.8 9.2
MO6 6–8 Aug 48 27 0.9 127.5 26.1 30.4 5.1
MO7 14–16 Aug 48 28 3.0 107.8 27.8 2.8 13.7
MO8 22–24 Aug 48 29 3.5 108.7 28.1 1 12.8
MO9 1–3 Sep 48 27 0.7 98.6 26.6 51.6 6.2
MO10 10–12 Sep 48 29 1.0 94.7 26.2 78.4 6.4
MO11 18–20 Sep 48 27 0.5 290.2 27.8 0 2.7
MO12 26–28 Sep 48 27 1.2 96.3 27.8 6.8 13.5
MO13/14 6–8 Oct 48 30/26 0.6 108.2 27.8 0.8 16.6
Atmos. Chem. Phys., 19, 10675–10696, 2019 www.atmos-chem-phys.net/19/10675/2019/
M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10679
different types of analyses. Sets 3 and 13 underwent gravi-
metric analysis using a Sartorius ME5-F microbalance.
Substrates were conditioned for at least 24 h at a mean
temperature of 20–23 ◦C and a mean relative humidity
of 30 %–40 % before pre- and post-weighing (US Envi-
ronmental Protection Agency, 2016). MOUDI set 13 was
additionally examined with a multi-wavelength absorption
black carbon instrument (MABI; Australian Nuclear Science
and Technology Organisation). This optically based instru-
ment quantifies absorption and mass concentrations at seven
wavelengths between 405 and 1050 nm; however, results are
only reported for 870 nm to be consistent with other studies,
as BC is the predominant absorber at that wavelength (e.g.,
Ramachandran and Rajesh, 2007; Ran et al., 2016). One
additional sample set for microscopy analysis was collected
for 1 h on 1 August using aluminum substrates.
2.3 Chemical composition analysis
A total of 12 sample sets, composed of 11 samples each,
were analyzed for water-soluble ions and elements (Table 2).
In order to preserve samples for additional analysis, each
Teflon substrate was cut in half. A half of each substrate was
extracted in 8 mL of Milli-Q water (18.2 Mcm) via son-
ication for 30 min in a sealed polypropylene vial. A blank
substrate was processed using the same method to serve as
a background control sample. Subsequent chemical analy-
ses of the water-soluble components in the aqueous extracts
were performed using ion chromatography (IC; Thermo Sci-
entific Dionex ICS-2100 system) for the following species:
cations – including Na+, NH+
4, Mg2+, Ca2+, dimethylamine
(DMA), trimethylamine (TMA), and diethylamine (DEA);
and anions – including methanesulfonate (MSA), pyruvate,
adipate, succinate, maleate, oxalate, phthalate, Cl−, NO−
3,
and SO2−
4. Owing to co-elution of TMA and DEA in the IC
system, a cumulative sum of the two is reported here, which
represents an underestimate of their total mass concentration
owing to overlap in parts of their peaks. Limits of detection
(LODs) were calculated for each species based on their re-
spective calibration curve (Table S1), with the LOD being 3
times the standard deviation of the residuals (predicted signal
minus measured signal) divided by the slope of the calibra-
tion curve (Miller and Miller, 2018).
The aqueous extracts were simultaneously characterized
for elemental composition using triple quadrupole induc-
tively coupled plasma mass spectrometry (ICP-QQQ; Agi-
lent 8800 Series) for the following species: K, Al, Fe, Mn,
Ti, Ba, Zn, Cu, V, Ni, P, Cr, Co, As, Se, Rb, Sr, Y, Zr, Nb,
Mo, Ag, Cd, Sn, Cs, Hf, Tl, and Pb. Limits of detection of
the examined elements were calculated automatically by the
ICP-QQQ instrument and were in the parts per trillion (ppt)
range (Table S1 in the Supplement). The sample concentra-
tions represent an average of three separate measurements
with a standard deviation of 3 % or less.
Note that some species were detected by both IC and ICP-
QQQ (i.e., Na+, K+, Mg2+, and Ca2+), and that the IC
concentrations are used here for all repeated species with
the exception of K+owing to better data quality from ICP-
QQQ. All IC and ICP-QQQ species concentrations for sam-
ples have been corrected by subtracting concentrations from
background control samples. For more examples of the ap-
plication of these methods used for substrate collection and
IC/ICP analysis, the reader is referred to other recent work
(Braun et al., 2017; Ma et al., 2019; Schlosser et al., 2017).
2.4 Microscopy analysis
As already noted, one MOUDI set on 1 August was de-
voted to microscopy analysis. Morphology and additional
elemental composition analysis was carried out on this set
of aluminum substrates using scanning electron microscopy
equipped with energy dispersive X-ray spectroscopy (SEM-
EDX) in the Kuiper Imaging cores at the University of Ari-
zona. Secondary electron (SE) imaging and EDX elemen-
tal analysis were performed using a Hitachi S-4800 high-
resolution SEM coupled to a NORAN System Six X-ray mi-
croanalysis system (Thermo Fisher Scientific). EDX anal-
ysis on individual particles was performed with 30 kV ac-
celerating voltage to obtain weight percentages of individ-
ual elements. SEM-EDX results showed that the background
control aluminum substrate was dominated by Al (88.27 %),
with minor contributions from Ag (5.34 %), C (4.87 %), O
(0.79 %), Fe (0.67 %), and Co (0.05 %). Such contributions
were manually subtracted from spectra of individual particles
on sample substrates, with the remaining elements scaled up
to 100 %. Image processing was conducted with ImageJ soft-
ware to measure particle dimensions and adjust the contrast
and brightness of images to provide better visualization.
2.5 Computational analysis
This study reports basic descriptive statistics for chemical
concentrations and correlations between different variables.
Hereafter, statistical significance corresponds to 95 % signif-
icance based on a two-tailed Student’s ttest. To complement
correlative analysis for identifying sources of species, pos-
itive matrix factorization (PMF) modeling was carried out
using the United States Environmental Protection Agency’s
(US EPA) PMF version 5. A total of 132 samples from the
12 sets analyzed for water-soluble ions and elements were
used in the PMF analysis. Species concentrations were ex-
amined before being inputted into the PMF analysis. Species
considered as “strong” based on high signal-to-noise ratios
(S/N > 1) and those with at least 50 % of the concentrations
above the LOD were used in the PMF modeling (Norris et
al., 2014). This resulted in a 132 (samples) ×30 (species)
data matrix that was inputted into the PMF analysis. Data
points with concentrations exceeding the LOD had uncer-
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10680 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
Table 2. Charge balance slopes (cations on the yaxis; anions on the xaxis) for the MOUDI sets shown, including the averages of all sets
(All) for three size ranges: sub-micrometer stages spanning 0.056–1.0 µm, super-micrometer stages (> 1.0 µm), and all stages (> 0.056 µm).
The species used in the charge balance analysis include those speciated with the IC (listed in Sect. 2.3) plus K from the ICP-QQQ analysis.
Sample set 0.056–1.0 µm > 1 µm > 0.056 µm
MO1 0.87 1.37 0.89
MO2 1.46 1.26 1.41
MO4 1.25 1.17 1.21
MO5 1.35 1.43 1.41
MO6 1.29 1.45 1.31
MO7 1.40 1.23 1.36
MO8 1.35 1.33 1.36
MO9 1.28 1.55 1.26
MO10 1.37 1.36 1.35
MO11 0.97 1.60 1.27
MO12 1.37 1.19 1.33
MO14 1.31 1.28 1.29
All 1.35 1.24 1.33
tainty quantified as
σij =0.05 ×Xij +LODij ,(1)
where σij ,Xij, and LODij are the respective uncertainty,
concentration, and LOD of the jth species in the ith sample
(Reff et al., 2007). When concentration data were not avail-
able for a particular stage of a MOUDI set for a species, the
geometric mean of the concentrations for that MOUDI stage
and species was applied with uncertainty counted as 4 times
the geometric mean value (Polissar et al., 1998; Huang et
al., 1999). A 25 % extra modeling uncertainty was applied
to account for other sources of error, such as changes in the
source profiles and chemical transformations (Dumanoglu et
al., 2014; Norris et al., 2014). The model was run 20 times
with a randomly chosen starting point for each run.
3 Results
3.1 Total mass concentrations and charge balance
The average total mass concentration (±standard devia-
tion) of water-soluble species across all MOUDI stages (Ta-
ble 1) during the study period was 8.53 ±4.48 µg m−3(with
a range of 2.7–16.6 µg m−3). The species contributing the
most to the total water-soluble mass concentration during
the SWM included SO2−
4(44 % ±6 %), NH+
4(18 % ±5 %),
NO−
3(10 % ±3 %), Na+(8 % ±3 %), and Cl−(6 % ±3 %).
The meteorological parameters from Table 1 best correlated
with total water-soluble mass concentrations were temper-
ature (r=0.64) and rainfall (r= −0.49). The highest to-
tal mass concentration (set MO13/14 with 16.6 µg m−3) oc-
curred during the period with one of the highest average tem-
peratures (27.8 ◦C) and second lowest total rainfall (0.8 mm).
Other sampling periods with high mass concentrations (sets
MO7, MO8, and MO12) coincided with the highest tempera-
ture and lowest rainfall observations. High temperatures, and
thus more incident solar radiation, presumably enhanced pro-
duction of secondary aerosol species via photochemical reac-
tions as has also been observed in other regions during their
respective monsoon season (Youn et al., 2013).
Low rainfall is thought to have been coincident with re-
duced wet scavenging of aerosol at the study site, as has been
demonstrated for other regions such as North America (Tai
et al., 2010) and megacities such as Tehran (Crosbie et al.,
2014). However, set MO11 exhibited a very low concentra-
tion even with high temperature and lack of rainfall, which
may be due to changes in the source and transport of aerosol
as this sample set coincided with a significant change in the
average wind direction (290.2◦for MO11 vs. 90.1–127.5◦for
all other MOUDI sets). While the reported rainfall measure-
ments were taken at MO, inhomogeneous rainfall patterns in
the regions surrounding the Philippines could also contribute
to the wet scavenging of PM, thereby lowering the quantity
of transported particles reaching the sample site. Future work
will address the influence of spatiotemporal patterns of pre-
cipitation on PM loadings in the Philippines as a point mea-
surement at an aerosol observing site may be misleading.
On two occasions, two simultaneous MOUDI sets (sets
MO3/MO4 and MO13/MO14) were collected in order to
compare different properties that require separate substrates.
The total mass concentrations based on the gravimetric anal-
ysis of sets MO3 and MO13 were 18.3 and 49.6 µg m−3, re-
spectively (Fig. 2). Both sets exhibited a dominant concen-
tration mode between 0.32 and 0.56 µm and the MO3 set was
different in that it exhibited bimodal behavior with a second
peak between 1.8 and 3.2 µm. The sum of speciated water-
soluble species accounted for only 28.3 % and 33.4 % of the
total gravimetric mass of sets MO3 and MO13, respectively,
indicative of significant amounts of water-insoluble species
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M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10681
Figure 2. Mass size distributions of total PM (blue dots) and re-
solved chemical species (colored bars) for MOUDI sets (a) MO3/4
and (b) MO13/14. Note that set MO13 was the only MOUDI set
where BC was quantified. ICP-QQQ represents the sum of water-
soluble elements except K; amines represent the sum of DMA,
TMA, and DEA; organic acids represent the sum of oxalate, suc-
cinate, adipate, pyruvate, phthalate, and maleate.
undetected by IC and ICP-QQQ. When adding the total mass
of BC (13.4 µg m−3) to the other resolved species from set
MO13 (the only time that BC was measured), there was still
19.7 µg m−3of unresolved mass (39.6 % of total PM). Most
of the unaccounted mass was for Dp> 0.32 µm.
The observation that BC accounts for 26.9 % of total PM
(13.4 µg m−3) is consistent with past work highlighting the
significant fraction of BC in the ambient aerosol in Manila
(Kim Oanh et al., 2006; Bautista et al., 2014; Simpas et al.,
2014; Kecorius et al., 2017). However, this fraction of BC
is very high compared with measurements during the mon-
soon season in other parts of the world. The mass fraction
of BC in total suspended PM (TSPM) was 1.6 %/2.2 % for
the monsoon season in 2013/2014 in Kadapa in southern In-
dia, even though the TSPM measured was comparable to
that in Manila (64.9 and 49.9 µg m−3, for 2013 and 2014 in
Kadapa, respectively) (Begam et al., 2017). Multiple studies
during the monsoon season in a coastal region in southwest
India showed BC mass contributions of 1.9 %–5 % (Aswini
et al., 2019 and references therein). Airborne measurements
around North America and in Asian outflow revealed that
BC accounted for only ∼1 %–2 % of PM1.0(Shingler et al.,
2016) and ∼5 %–15 % of accumulation-mode aerosol mass
(Clarke et al., 2004), respectively.
To investigate the missing species further, a charge bal-
ance was carried out for all MOUDI sets (Table 2) to com-
pare the sum of charges for cations vs. anions based on IC
analysis including Kfrom ICP-QQQ analysis (species listed
in Sect. 2.3). The slope of the charge balances (cations on the
Figure 3. (a) Mass size distribution of BC retrieved from the MABI
optical measurement at 870 nm for set MO13. Missing values were
below detection limits. (b) Photographs of each stage of set MO13
with numbers below each image representing the aerodynamic di-
ameter ranges in units of micrometers (µm).
yaxis) for the cumulative dataset was 1.33 and ranged from
0.89 to 1.41 for the 12 individual MOUDI sets that had IC
and ICP-QQQ analysis conducted on them. A total of 11 of
the 12 sets exhibited slopes above unity, indicating that there
was a deficit in the amount of anions detected, which presum-
ably included species such as carbonate and various organics.
To further determine if there were especially large anion or
cation deficits in specific size ranges, slopes are also reported
for 0.056–1 and > 1 µm. There were no obvious differences
other than the fact that two MOUDI sets exhibited slopes be-
low 1.0 for the smaller diameter range (0.056–1 µm), while
all slopes exceeded unity for > 1 µm.
3.2 Mass size distributions and morphology
3.2.1 Black carbon
The size-resolved nature of BC has not been characterized in
Manila, and MOUDI set MO13 offered a view into its mass
size distribution (Fig. 3a). There was a pronounced peak be-
tween 0.18 and 0.32 µm (4.7 µg m−3), which is visually ev-
ident in the substrate’s color when compared with all other
stages of that MOUDI set (Fig. 3b). This observed peak in
the mass size distribution of BC is similar to previous studies
of the outflow of East Asian countries (Shiraiwa et al., 2008),
biomass burning and urban emissions in Texas (Schwarz et
al., 2008), measurements in the Finnish Arctic (Raatikainen
et al., 2015), and airborne measurements over Europe (Red-
dington et al., 2013). In contrast, measurements in Uji, Japan,
showed a bimodal size distribution for the mass concentra-
tion of BC in the sub-micrometer range (Hitzenberger and
Tohno, 2001).
In the present study, there were significant amounts of
BC extending to as low as the 0.056–0.1 µm MOUDI stage
(0.26 µg m−3) and extending up in the super-micrometer
range, with up to 0.23 µg m−3measured between 1.8
and 3.2 µm. Remarkably, BC accounted for approximately
78.1 % (51.8 %) by mass of the total PM in the range from
0.10 to 0.18 µm (0.18–0.32 µm). For comparison, the mass
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10682 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
percent contribution of BC measured in the megacity of Nan-
jing, China, was 3.3 % (1.6 %) at 0.12 (0.08) µm (Ma et al.,
2017). Based on visual inspection of color on all MOUDI
sets, MO13 appears to be representative of the other sets
based on the relative intensity of the color black on sub-
strates with different cut-point diameters (Fig. 3b); the 0.18–
0.32 µm substrate was always the blackest, with varying de-
grees of blackness extending consistently into the super-
micrometer stages.
Microscopy analysis revealed evidence of nonspherical
particles in each MOUDI stage below 1 µm (Fig. 4), which
is significant as the common theoretical assumption is that
sub-micrometer particles are typically spherical (e.g., Mielo-
nen et al., 2011). Errors in this assumption impact numeri-
cal modeling results and the interpretation of remote sensing
data for aerosol particles (e.g., Kahnert et al., 2005), owing
to incorrect calculations of parameters such as single scatter-
ing albedo, the asymmetry parameter, and the extinction ef-
ficiency (e.g., Mishra et al., 2015). Some studies have noted
that sub-micrometer particles could be composed of an ag-
glomeration of small spherical particles originally formed
through gas-to-particle conversion processes (Almeida et al.,
2019), which could potentially explain the appearance for
some of the observed particles in Fig. 4. As only single par-
ticles were examined that may not be fully representative of
all particles on a particular MOUDI substrate, it is notewor-
thy that all five particles shown between 0.056 and 1µm were
irregularly shaped with signs of both multi-layering and con-
stituents adhered to one another.
The images show that a potentially important source of BC
in the area could be soot aggregates, which are formed by a
vaporization–condensation process during combustion often
associated with vehicular exhaust (e.g., Chen et al., 2006;
Chithra and Nagendra, 2013; Wu et al., 2017). Kecorius et
al. (2017) projected that 94 % of total roadside refractory
PM with number concentration modes at 20 and 80 nm was
linked to jeepneys, the most popular and inexpensive mode of
public transport in Metro Manila. They associated the larger
mode with soot agglomerates, which is consistent with the
smallest MOUDI size range examined here (0.056–0.1 µm;
Fig. 4b) exhibiting signs of agglomeration.
The total BC mass concentration integrated across all
stages of MOUDI set MO13 (13.4 µg m−3) was remark-
ably high in contrast to BC levels measured via either
filters, Aethalometers, or single particle soot photome-
ters in most other urban regions of the world (Metcalf
et al., 2012 and references therein): Los Angeles Basin
(airborne: 0.002–0.53 µg m−3); Atlanta, Georgia (ground:
0.5–3.0 µg m−3); Mexico City (airborne: 0.276–1.1 µg m−3);
Sapporo, Japan (ground: 2.3–8.0 µg m−3); Beijing, China
(ground: 6.3–11.1 µg m−3); Bangalor, India (ground: 0.4–
10.2 µg m−3); Paris, France (ground: 7.9 µgm−3); Dushanbe,
Russia (ground: 4–20 µg m−3); Po Valley, Italy (ground:
0.5–1.5 µg m−3); and Thessaloniki, Greece (ground: 3.3–
8.9 µg m−3). This is intriguing in light of extensive precipita-
tion, and, thus, wet scavenging of PM, during the study pe-
riod, which is offset by enormous anthropogenic emissions
in the region, such as those from powered vehicles like the
jeepneys that are notorious for BC exhaust (Kecorius et al.,
2017).
A possible explanation for the large contribution of BC to
PM, and the persistence of PM after rain events (Kim Oanh
et al., 2006), is that the BC is not efficiently scavenged by
precipitating rain drops. Small particles enter rain drops via
diffusion whereas large particles enter via impaction. How-
ever, particles with a diameter in the range from 0.1 to 1 µm
(known as the Greenfield gap) are too large to diffuse effi-
ciently and too small to impact; therefore, they are not ef-
ficiently scavenged (Seinfeld and Pandis, 2016). Absorption
spectroscopy of set MO13 (Fig. 2b) reveals that 95 % of the
BC mass is concentrated in the Greenfield gap; thus, the re-
moval of BC due to precipitation is inefficient. The Green-
field gap contains 66 ±11 % of the total mass (calculated for
MO3/MO13) and 65 ±10 % of the water-soluble mass (cal-
culated for the other 12 MO sets). As noted earlier, BC obser-
vations discussed in this paper were based only on a single
MOUDI set and the effect of inefficient scavenging in the
Greenfield gap could just be one of the many potential pro-
cesses affecting the BC mass size distribution. Subsequent
work that will include BC measurements in the dry season
will further investigate this hypothesis.
3.2.2 Water-soluble ions
There were two characteristic mass size distribution pro-
files for the water-soluble ions speciated by IC, depend-
ing on whether the species were secondarily produced via
gas-to-particle conversion or associated with primarily emit-
ted super-micrometer particles. The average IC species mass
concentration profile across all MOUDI sets is shown in
Fig. 5. Secondarily produced species exhibited a mass con-
centration mode between 0.32 and 0.56 µm, including com-
mon inorganic species (SO2−
4, NH+
4), MSA, amines (DMA,
TMA+DEA), and a suite of organic acids, such as ox-
alate, phthalate, succinate, and adipate, produced via precur-
sor volatile organic compounds (VOCs). Two organic acids
with peaks in other size ranges included maleate (0.56–1 µm)
and pyruvate (0.1–0.18 µm). Sources of the inorganics are
well-documented: SO2−
4and NH+
4are produced by precur-
sor vapors SO2and NH3, respectively, with ocean-emitted
dimethylsulfide (DMS) as an additional precursor to SO2−
4
and the primary precursor to MSA.
Precursors leading to secondarily produced alkyl amines
such as DMA, TMA, and DEA likely originated from a
combination of industrial activity, marine emissions, biomass
burning, vehicular activity, sewage treatment, waste incin-
eration, and the food industry (e.g., Facchini et al., 2008;
Sorooshian et al., 2009; Ge et al., 2011; VandenBoer et al.,
2011); another key source of these species – animal hus-
bandry (Mosier et al., 1973; Schade and Crutzen, 1995;
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M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10683
Figure 4. SEM image of a (a) blank filter and (b–f) individual particles in different sub-micrometer aerodynamic diameter ranges sampled
by the MOUDI: (b) 0.056–0.1, (c) 0.1–0.18, (d) 0.18–0.32, (e) 0.32–0.56, and (f) 0.56–1.0 µm.
Figure 5. Average mass size distribution of water-soluble ions speciated via IC in addition to potassium from ICP-QQQ analysis.
Sorooshian et al., 2008) – was ruled out owing to a scarcity
of such activity in the study region. Secondarily produced
amine salts were likely formed with SO2−
4as the chief anion
owing to its much higher concentrations relative to NO−
3or
organic acids.
Dimethylamine was the most abundant amine, similar to
other marine (Müller et al., 2009) and urban regions (Youn
et al., 2015); the average concentration of DMA integrated
over all MOUDI stages for all sample sets was 62.2 ng m−3
in contrast to 29.8 ng m−3for TMA+DEA. For reference,
the other key cation (NH+
4) participating in salt formation
with acids such as H2SO4and HNO3was expectedly much
more abundant (1.64 µg m−3). With regard to the competi-
tive uptake of DMA vs. NH3in particles, the molar ratio
of DMA:NH+
4exhibited a unimodal profile between 0.1 and
1.8 µm with a peak of 0.022 between 0.32 and 0.56 µm and
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10684 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
the lowest values at the tails (0.004 between 0.1–0.18 and
1–1.8 µm); DMA was not above detection limits for either
Dp< 0.1 µm or Dp> 1.8 µm. The molar ratios observed were
consistent with values measured in the urban air of Tucson,
Arizona, and coastal air in Marina, California (0–0.04; Youn
et al., 2015) and near the lower end of the range measured in
rural and urban air masses sampled near Toronto (0.005–0.2:
VandenBoer et al., 2011).
The most abundant organic acid was oxalate (195±
144 ng m−3), followed by succinate (21 ±41 ng m−3), ph-
thalate (19 ±25 ng m−3), maleate (17 ±15 ng m−3), and adi-
pate (5 ±8 ng m−3). The observation of mass concentrations
increasing with decreasing carbon number for dicarboxylic
acids (i.e., oxalate > succinate > adipate) is consistent with
many past studies for other regions as larger chain acids
undergo oxidative decay to eventually form oxalate (e.g.,
Kawamura and Ikushima, 1993; Kawamura and Sakaguchi,
1999; Sorooshian et al., 2007). Maleate is an unsaturated di-
carboxylic acid emitted from gas and diesel engines (Rogge
et al., 1993) and a product of the photooxidation of benzene
(Kawamura and Ikushima, 1993). The aromatic dicarboxylic
acid phthalate is a known photooxidation product of naph-
thalene and stems largely from plastic processing and fuel
combustion (Fraser et al., 2003; Kautzman et al., 2010; Fu
et al., 2012; Kleindienst et al., 2012). The oxidation product
(MSA) of ocean-derived DMS exhibited an overall average
concentration of 11 ±7 ng m−3, which is near the lower end
of the range of levels reported in other coastal and marine
environments (from undetected up to ∼200 ng m−3) (e.g.,
Saltzman et al., 1983, 1986; Berresheim 1987; Watts et al.,
1987; Burgermeister and Georgii, 1991; Sorooshian et al.,
2015; Xu and Gao, 2015).
Water-soluble species exhibiting a peak in the super-
micrometer range, usually between 1.8 and 5.6 µm, include
those with known affiliations with sea salt (Na+, Cl−, K+,
Mg2+) and crustal materials such as dust (Ca2+). Nitrate
peaked between 1.8 and 3.2 µm, and was best correlated
with Na+and Mg2+, suggestive of HNO3partitioning to
sea salt as has been observed in other coastal regions (e.g.,
Prabhakar et al., 2014a). There was very little NO−
3in
the sub-micrometer range (0.05 ±0.04 µg m−3) in contrast
to super-micrometer sizes (0.78 ±0.47 µg m−3). More sub-
micrometer NO−
3in the form of NH4NO3would be expected
if there was an excess of NH3after neutralizing SO2−
4. The
mean ammonium-to-sulfate molar ratio for sub-micrometer
sizes was 2.32 ±0.52 (range of 1.11–2.78), with full neutral-
ization of SO2−
4in 10 of 12 MOUDI sets. Thus, there was
a non-negligible excess in NH3that presumably participated
in salt formation with HNO3and organic species. The sig-
nificant levels of NO−
3in the same mode as Na+and Cl−
contributed to the significant Cl−depletion observed, as the
mean Cl−:Na+mass ratio between 1 and 10 µm (i.e., range
of peak sea salt influence) was 0.81 ±0.28, which is much
lower than the ratio for pure sea salt (1.81) (Martens et al.,
1973). The subject of Cl−depletion in this region will be
investigated more thoroughly in subsequent work.
Figure 6 shows SEM images of representative single par-
ticles in each super-micrometer stage. As would be expected
for sea salt and crustal material, most of the particles shown
are not spherical. Interestingly, only the particle shown be-
tween 1 and 1.8 µm was close to being spherical. Its composi-
tion based on EDX analysis was accounted for mostly by car-
bon (93.7 %) with lower amounts of oxygen (5.8 %) and Fe
(0.5 %). Sea salt particles were found in the next two stages
owing to the highest combined weight percentages of Na+
and Cl−based on EDX analysis: 1.8–3.2 µm which equates
to 36.9 %; and 3.2–5.6 µm which equates to 46.9 %. The salt
particles are not necessarily cubical but more rounded with
signs of agglomeration. These two particles were the only
ones among the 11 MOUDI stages exhibiting an EDX sig-
nal for S, with contributions amounting to ∼2 % in each
particle. This may be linked to natural SO2−
4existing in sea
salt particles. Also, the particle between 3.2 and 5.6 µm con-
tained a trace amount of Sc (1 %). The largest three particles
(≥5.6 µm) were irregularly shaped, as expected, with both
sharp and rounded edges, comprised mostly of oxygen, Al,
Fe, and Ca based on EDX analysis.
3.2.3 Water-soluble elements
Averaged data across all MOUDI sets reveal that ICP-QQQ
elements exhibited a variety of mass concentration profiles
ranging from a distinct mode in either the sub- or super-
micrometer range to having multiple modes below and above
1 µm (averages across all MOUDI sets shown in Fig. 7).
There were several elements with only one distinct peak,
which was observed in one of the two stages between 0.18
and 1.0 µm, including As, Cd, Co, Cr, Cs, Cu, Hf, Mn, Mo,
Ni, Rb, Se, Sn, Tl, V, Pb, and Zn. In contrast, the follow-
ing elements exhibited only one distinct peak in the super-
micrometer range: Al, Ba, P, Sr, Ti, Y, and Zr. The rest of the
elements exhibited more complex behavior with two distinct
peaks in the sub- and super-micrometer range (Ag, Fe, and
Nb). The following section discusses relationships between
all of the ions and elements with a view towards identifying
characteristic sources.
3.3 Characteristic sources and species relationships
A combination of PMF and correlation analysis helped iden-
tify clusters of closely related species stemming from distinct
sources. The PMF solution with five factors (Fig. 8) was cho-
sen because it passed the criteria of physical meaningfulness
and it had a calculated ratio of Qtrue :Qexpected (1.2) that
was very close to the theoretical value of 1.0. There was a
high coefficient of determination between measured and pre-
dicted mass concentration when summing up all species for
each MOUDI stage (r2=0.79; sample size, n=132), which
added confidence in relying on the PMF model for source ap-
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M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10685
Figure 6. Same as Fig. 4, but for different super-micrometer aerodynamic diameter ranges sampled by the MOUDI: (a) 1.0–1.8, (b) 1.8–3.2,
(c) 3.2–5.6, (d) 5.6–10, (e) 10-18, and (f) > 18 µm.
portionment of PM. The five distinct clusters were named for
their most plausible sources based on the species included
in the groupings, with their overall contributions to the total
mass based on PMF analysis shown in parenthesis (Table 3):
aged (48.0 %), sea salt (22.5 %), combustion (18.7 %), vehic-
ular/resuspended dust (5.6 %), and waste processing (5.1 %).
For reference, a previous study near the northwestern edge
of the Philippines identified six source factors for PM2.5that
are fairly similar to those here (Bagtasa et al., 2018): sea salt,
resuspended fine dust, local solid waste burning, and long-
range transport of (i) industrial emissions, (ii) solid waste
burning, and (iii) secondary sulfate. Each of our five group-
ings will be discussed in detail below in decreasing order of
contribution to total measured mass concentrations.
3.3.1 Aged aerosol
Although not due to one individual source, there was a dis-
tinct PMF factor that included species commonly produced
via gas-to-particle conversion processes (NH+
4, SO2−
4, MSA,
and oxalate). Correlation analysis (Table 4) also pointed to a
large cluster of species significantly related to each other, in-
cluding the aforementioned ions and a suite of other organic
acids (phthalate, succinate, and adipate), MSA, and DMA.
The latter three inorganic and organic acid ions exhibited sig-
nificant correlations with each other (r≥0.68), but also with
several elements (r≥0.36: K, V, Rb, Cs, Sn), which were
likely co-emitted with the precursor vapors of the secondar-
ily produced ions. Although BC concentrations were quan-
tified from set MO13 only, the results showed that BC was
significantly correlated (r: 0.61–0.92) with 15 species, in-
cluding those mentioned above (owing to co-emission) and
also a few elements that were found via PMF to be stronger
contributors to the combustion source discussed in Sect. 3.3.3
(Ni, Cu, As, Se, Cd, Tl, and Pb).
This PMF source factor is referred to as aged aerosol ow-
ing to its characteristic species being linked to secondary
particle formation from emissions of regional and distant
sources. The presence of NH+
4and SO2−
4could be attributed
to precursors from various local and regional combustion
sources, while MSA and DMA are secondarily produced
from ocean-derived gaseous emissions (e.g., Sorooshian et
al., 2009). Biomass burning emissions from distant upwind
regions such as Sumatra and Borneo (Xian et al., 2013) are
likely sources of K. Previous studies (Reid et al., 2012; Wang
et al., 2013) have shown that phenomena such as SWM and
El Niño events not only influence biomass burning activi-
ties on the Malay Peninsula but also impact the transport and
distribution of emissions in the study region. For instance,
Reid et al. (2016b) showed that enhancement in monsoonal
flow facilitates the advection of biomass burning and anthro-
pogenic emissions to the Philippines from Sumatra and Bor-
neo. Subsequent work will more deeply investigate the im-
pact of biomass burning from those upwind regions on the
sample site during the SWM.
While NH+
4and SO2−
4require time for production ow-
ing to being secondarily produced from precursor vapors
(i.e., SO2, NH3), oxalate is the smallest dicarboxylic acid
and requires lengthier chemistry pathways for its produc-
tion; thus, it is more likely produced in instances of aerosol
transport and aging (e.g., Wonaschuetz et al., 2012; Ervens
et al., 2018). The various elements associated with this clus-
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10686 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
Figure 7. Average mass size distribution of water-soluble elements speciated via ICP-QQQ.
Table 3. Contributions (in weight percentage) of each PMF source factor to the total mass in different diameter ranges.
Diameter Aged Sea Salt Combustion Vehicular/ Waste
range resuspended processing
(µm) aerosol dust
> 0.056 48.0 % 22.5 % 18.7 % 5.6 % 5.1 %
0.056–1.0 68.9 % 0.6 % 23.9 % 1.5 % 5.1 %
> 1.0 18.6 % 53.5 % 11.3 % 11.3 % 5.3 %
ter are co-emitted with the precursors to the aforementioned
ions and are linked to a variety of sources: metallurgical pro-
cesses (Anderson et al., 1988; Csavina et al., 2011; Youn
et al., 2016), fuel combustion (Nriagu, 1989; Allen et al.,
2001; Shafer et al., 2012; Rocha and Correa, 2018), resid-
ual oil combustion (Watson et al., 2004), biomass burn-
ing (Maudlin et al., 2015), marine and terrestrial biogenic
emissions (Sorooshian et al., 2015), and plastics processing
(Fraser et al., 2003). In addition, there is extensive ship traf-
fic in the general study region, which is a major source of
emissions in this cluster of species, particularly V and SO2−
4
(e.g., Murphy et al., 2009; Coggon et al., 2012).
PMF analysis suggested that the aged aerosol factor con-
tributed 48.0 % to the total water-soluble mass budget dur-
ing the study period. Most of the contribution resided in the
sub-micrometer range (68.9 %) unlike the super-micrometer
range (18.6 %), which is consistent with the overall mass
size distribution of total PM peaking in the sub-micrometer
range (Fig. 2). The reconstructed mass size distribution for
this PMF source factor shows the dominance of the mass
in the sub-micrometer range with a peak between 0.32 and
0.56 µm (Fig. 9). The correlation matrices for the sub- and
super-micrometer size ranges also show that the correlations
between the species most prominent in the aged aerosol cat-
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M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10687
Table 4. Correlation matrix (rvalues) between water-soluble species based on total MOUDI-integrated mass concentrations (>0.056 µm).
Blank cells represent statistically insignificant values. Results for the sub- and super-micrometer ranges are shown in Tables S2–S3. Panels
(a)–(e) represent important species from each of the source profiles identified in Sect. 3.3: (a) represents aged aerosol, (b) represents sea salt,
(c) represents combustion, (d) represents vehicular/resuspended dust, and (e) represents waste processing. DMA represents dimethylamine,
MSA represents methanesulfonate, PH represents phthalate, OX represents oxalate, MA represents maleate, SU represents succinate, and
AD represents adipate.
egory are stronger for the former size range (Tables S2–S3).
The contribution of this PMF factor to the super-micrometer
range is likely associated with species secondarily produced
on coarse aerosol such as dust and sea salt. This is evident
in the individual species mass size distributions where there
is a dominant sub-micrometer mode but also non-negligible
mass above 1 µm.
Even though the PM in a heavily populated urban region,
such as Metro Manila, is typically thought to be dominated
by local sources of aerosol, the current PMF results show that
contribution from long-range transport is still discernible.
This finding is contrary to the expectation that the signal of
transported aerosol would be lost in the noise of locally pro-
duced aerosol.
3.3.2 Sea salt
As the MO sampling site is approximately 13 km from the
nearest shoreline (Fig. 1a) and downwind of Manila Bay
in the SWM season, there was a great potential for marine
emissions to impact the samples. There were several species
with similar mass size distributions (mode: 1.8–5.6 µm) and
highly correlated total mass concentrations (r≥0.51) that
are linked to sea salt: Cl−, Na+, Ca2+, Mg2+, Ba, and Sr.
The correlations between these species were stronger when
examining just the super-micrometer range compared with
the sub-micrometer range (Tables S2–S3). The majority of
these species were used in PMF analysis and formed a dis-
tinct cluster amounting to 22.0 % of the total study period’s
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10688 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
Figure 8. Overview of the PMF five-factor solution with blue bars
representing mass concentrations and red squares signifying the
percentage of mass concentration contributed to constituents by
each source factor.
Figure 9. Reconstructed mass size distributions using PMF for the
five major source profiles.
mass budget. This source contributed only 0.6 % to the sub-
micrometer mass concentration but 53.5 % for the super-
micrometer size range. The reconstructed mass size distri-
bution for this source factor is shifted farthest to the larger
diameters compared with the other four sources with a peak
between 1.8 and 3.2 µm (Fig. 9).
It is noteworthy that this factor has the highest share of
NO−
3among all identified sources. This result is consistent
with mass size distributions shown in Figure 5 in which NO−
3
peaks in the super-micrometer range similar to sea salt con-
stituents (e.g., Na+and Cl−). Although sea salt particles nat-
urally contain NO−
3(Seinfeld and Pandis, 2016) (a mass ratio
of NO−
3:Na+of 9.8×10−8–6.5×10−5), the extremely high
ratio of NO−
3:Na+(mass ratio ∼1.8) suggests that only a
negligible portion of NO−
3in this factor originated from pri-
mary sea salt particles. Thus, the majority of NO−
3is most
likely due to HNO3partitioning to existing sea salt particles
(e.g., Fitzgerald, 1991; Allen et al., 1996; Dasgupta et al.,
2007; Maudlin et al., 2015). In addition, the Cl−:Na+mass
ratio in this profile (0.65) is smaller than that in sea salt par-
ticles (1.81), indicating high Cl−depletion mainly due to re-
actions of HNO3with NaCl (Ro et al., 2001; Yao et al., 2003;
Braun et al., 2017). Moreover, elevated loadings of trace ele-
ments (e.g., Ba, Cu, Zn, and Co) could be linked to mixing of
marine emissions with urban sources (e.g., vehicle and indus-
trial emissions) during their transport inland to the sampling
site (Roth and Okada, 1998). This process of aging is con-
sistent with the observed morphology of the sea salt particles
in this study, revealing non-cubical shapes that are rounded
owing to the likely addition of acidic species such as HNO3
(Fig. 6).
3.3.3 Combustion
There are numerous sources of combustion in the study re-
gion, including a variety of mobile sources (e.g., cars, util-
ity vehicles, trucks, buses, and motorcycles) and stationary
sources (e.g., power stations, cement works, oil refineries,
boiler stations, and utility boilers). Consequently, the next
highest contributor to total mass during the study period ac-
cording to PMF (18.7 %) was the cluster of species including
Ni, As, Co, P, Mo, and Cr, which is defined as the combus-
tion factor. These species have been reported to be rich in
particles emitted from combustion of fossil fuel and resid-
ual oil (Linak and Miller, 2000; Allen et al., 2001; Wasson
et al., 2005; Mahowald et al., 2008; Mooibroek et al., 2011;
Prabhakar et al., 2014b). Although not included in PMF anal-
ysis, other species significantly correlated with the above-
mentioned species include maleate and Ag, which also stem
from fuel combustion (Kawamura and Kaplan, 1987; Lin et
al., 2005; Sorooshian et al., 2007). Ag in particular is an ele-
ment in waste incinerator fly ash (Buchholz and Landsberger,
1993; Tsakalou et al., 2018), and its strong correlation with
Co (r=0.85) and Mo (r=0.64) provides support for this
source factor being linked to combustion processes. Maleate
is commonly found in engine exhaust (Kawamura and Ka-
plan, 1987), whereas Cr is a tracer for power plant emissions
(Singh et al., 2002; Behera et al., 2015). Of all species exam-
ined in this study, BC was best correlated with As (r=0.92),
while its correlation with Ni (r=0.85) was among the high-
est.
Atmos. Chem. Phys., 19, 10675–10696, 2019 www.atmos-chem-phys.net/19/10675/2019/
M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10689
As the elements in this cluster peaked in concentration in
the sub-micrometer mode, the weight percentage of this fac-
tor is more than double below 1 µm (23.9 %) compared with
above 1 µm (11.3 %). The reconstructed mass size distribu-
tion for this source factor peaks between 0.18 and 0.32 µm,
which is smaller than the modal diameter range for the aged
source factor (0.32–0.56 µm) likely owing to closer sources
and, thus, less time for growth to occur via condensation and
coagulation.
3.3.4 Vehicular/resuspended dust
The next PMF source factor contains chemical signatures of
dust due to high contributions to Al, Ti, Ca, and Fe. These
crustal elements are strongly related to the resuspension of
dust by traffic and construction activities (Singh et al., 2002;
Harrison et al., 2011). Other elements that were prominent in
this factor included Zr, Y, Mn, Cr, and Ba, which are asso-
ciated with tire and brake wear (Adachi and Tainosho, 2004;
Gietl et al., 2010; Song and Gao, 2011; Harrison et al., 2012;
Vossler et al., 2016), although some of them can be linked to
exhaust as well (e.g., Lin et al., 2005; Song and Gao, 2011).
This source is named vehicular/resuspended dust and con-
tributed 5.6 % to the total study period’s mass concentrations.
The weight percentage contribution of this factor was
much higher for the super-micrometer range (11.3 %) com-
pared with the sub-micrometer range (1.5 %), which is con-
sistent with the sea salt source factor owing to similar mass
size distributions of the individual species associated with
the two source categories (Figs. 5 and 7). Additional species
correlated significantly with the crustal species included Hf
and Nb, which also exhibited mass peaks between 1.8 and
3.2 µm. The reconstructed mass size distribution for this
source factor is similar to that of sea salt in that there is a peak
between 1.8 and 3.2 µm, but there is less of a unimodal pro-
file owing to what appears to be a secondary mode between
0.56 and 1.0 µm (Fig. 9), which could be linked to some of
the non-dust components of vehicular emissions.
3.3.5 Waste processing
The final PMF source factor, contributing the least over-
all to total mass (5.1 %), featured Zn, Cd, Pb, Mn, and Cu
as its main components. These species are linked to waste
processing, including electronic waste (e-waste) and bat-
tery burning and recycling in particular (Gullett et al., 2007;
Iijima et al., 2007), which has previously been reported for
Manila (Pabroa et al., 2011). The latter study reported that
although there are a few licensed operations for battery recy-
cling, there are numerous unregulated cottage melters across
Manila that regularly melt metal from batteries and discard
the waste freely. Fujimori et al. (2012) additionally showed
that e-waste recycling led to emissions of the following el-
ements (in agreement with this PMF cluster) around Metro
Manila: Ni, Cu, Pb, Zn, Cd, Ag, in, As, Co, Fe, and Mn.
This was the only PMF factor exhibiting comparable
weight percentages both below (5.1 %) and above 1 µm
(5.3 %). This is reflected in the mass size distributions of the
species included in this cluster being fairly uniformly dis-
tributed below and above 1µm. This is also demonstrated in
the reconstructed mass size distribution of this source factor
as it clearly exhibits a mode between the other four sources
(0.56–1.0 µm) and is the broadest mode (Fig. 9). The expla-
nation for this is likely rooted in the diversity of sources
contained within this source profile that lead to different
sizes of particles. Examples of such sources include process-
ing of different types of waste at varying temperatures and
through various processes (e.g., burning, melting, and grind-
ing) (Keshtkar and Ashbaugh, 2007),
4 Conclusions
This study used various analytical techniques (gravimetry,
ion chromatography, triple quadrupole inductively coupled
plasma mass spectrometry, black carbon spectroscopy, and
microscopy), meteorological data, and a source apportion-
ment model (positive matrix factorization) to characterize
the sources, chemical composition, and morphology of size-
resolved ambient particulate matter (PM) collected using
micro-orifice uniform deposit impactors (MOUDIs) in Metro
Manila, Philippines, during the southwest monsoon season
(SWM) season of 2018. The main results of this study in-
clude the following:
–The total mass concentrations were measured on two
occasions and were 18.3 and 49.6 µg m−3. Water-
soluble mass concentrations were measured on 12 occa-
sions and were 8.53 ±4.48 µg m−3on average (a range
from 2.7 to 16.6 µg m−3). Simultaneous measurements
of total, water-soluble, and black carbon (BC) mass
revealed a composition of 26.9 % BC, 33.4 % water-
soluble components, and 39.6 % unaccounted mass.
–Size-resolved BC mass concentration was measured on
one occasion, with the mass sum of all MOUDI stages
reaching 13.4 µg m−3Most of the BC mass (95 %) was
contained in the 0.1–1 µm range (i.e., the Greenfield
gap) where wet scavenging by rain is relatively ineffi-
cient. The measured BC peaked in the size range be-
tween 0.18 and 0.32 µm and accounted for 51.8 % of the
measured PM for that stage. In the range between 0.10
and 0.18 µm, the mass percent contribution of BC to the
measured PM was 78.1 %.
–Most of the total mass resided in the sub-micrometer
mode (0.32–0.56 µm); however, one MOUDI set re-
vealed an additional super-micrometer mode (1.8–
3.2 µm). Water-soluble species that peaked in the sub-
micrometer mode were associated with secondarily
produced species, including inorganic acids, amines,
www.atmos-chem-phys.net/19/10675/2019/ Atmos. Chem. Phys., 19, 10675–10696, 2019
10690 M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter
methanesulfonate (MSA), and organic acids. Water-
soluble species that peaked in the super-micrometer
mode were associated with sea salt and crustal material.
Most of the unaccounted mass was for Dp> 0.32 µm.
–The most abundant water-soluble species was SO2−
4
(44 % ±6 %), followed by NH+
4(18 % ±5 %), NO−
3
(10 %±3 %), Na+(8 % ±3 %), and Cl−(6 %±3 %). Cor-
relation analysis revealed that total water-soluble mass
was most correlated with temperature (r=0.64) and
rainfall accumulation (r= −0.49) among the meteoro-
logical factors considered, although other factors were
likely influential such as wind direction and speed.
–Regardless of particle size, the majority of the single
particles examined with energy dispersive X-ray spec-
troscopy (SEM-EDX) were nonspherical with evidence
of agglomeration.
–PMF analysis suggested that there were five factors in-
fluencing the water-soluble fraction of PM collected at
the sampling site. These factors, their contribution to to-
tal water-soluble mass, and the main species that permit
them to be linked to a physical source are as follows:
aged aerosol (48.0 %; NH+
4, SO2−
4, MSA, and oxalate),
sea salt (22.5 %; Cl−, NO−
3, Ca2+, Na+, Mg2+, Ba, and
Sr), combustion (18.7 %; Ni, As, Co, P, Mo, and Cr),
vehicular/resuspended dust (5.6 %; Al, Ti, and Fe), and
waste processing (5.1 %; Zn, Cd, Pb, Mn, and Cu). The
dominant contribution of aged aerosol to water-soluble
mass contradicts the expectation that locally produced
sources in polluted cities should drown out the signal of
transported aerosol from distant upwind areas.
Although the current study focuses exclusively on the
SWM season in Metro Manila, results of this study are ap-
plicable to the study of aerosol impacts on Southeast Asia
and other regions. First, the detection of aged aerosol not
only from regional but also distant sources confirms previ-
ous studies that PM in the region has the ability to travel
long distances during the SWM season. Therefore, charac-
terization of aerosol in Metro Manila is important for better
understanding the impacts that local emissions will have on
locations downwind of Metro Manila, including other popu-
lated cities in Southeast and East Asia. Transport of pollution
and decreased wet scavenging during the SWM season may
become increasingly important as studies have shown a de-
crease in SWM rainfall and an increase in the number of no-
rain days during the SWM season in the western Philippines
in recent decades (e.g., Cruz et al., 2013).
Second, Southeast Asia has been named “one of the most
hostile environments on the planet for aerosol remote sens-
ing” (Reid et al., 2013) because of high cloud occurrence.
Therefore, space-based remote sensing of aerosol character-
istics, such as retrievals of the aerosol optical depth (AOD),
in this region are difficult. In situ measurements are critical
for the characterization of PM in this region, especially dur-
ing seasons such as the SWM when clouds are especially
prevalent and remote sensing retrievals dependent on clear-
sky conditions are lacking.
Third, this study provides a valuable dataset to compare
to other regions impacted by monsoons where the impacts
of enhanced moisture and rainfall on size-resolved composi-
tion are not well understood. As aqueous processing results
in enhanced production of water-soluble species (e.g., sulfate
and organic acids), it is noteworthy for this monsoonal region
that the water-soluble fraction remains low relative to BC and
other insoluble components. This has major implications for
the hygroscopicity of the regional PM.
Finally, the results of this study will be used to inform fu-
ture sampling campaigns in this region, including CAMP2Ex
planned for the SWM season of 2019 based in the Philip-
pines. As the current MOUDI sampling campaign at the
Manila Observatory is expected to extend for a full year, fu-
ture work will focus on changes in aerosol characteristics and
sources on a seasonal basis as well as scavenging processes
upwind of the measurement site.
Data availability. All data used in this work are available upon re-
quest.
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/acp-19-10675-2019-supplement.
Author contributions. MTC, MOC, JBS, ABM, CS, and AS de-
signed the experiments and all co-authors carried out some aspect
of the data collection. MTC, RAB, CS, LM, HD, and AS conducted
data analysis and interpretation. MTC and AS prepared the paper
with contributions from all co-authors.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. Melliza Templonuevo Cruz acknowledges
support from the Philippine Department of Science and Technol-
ogy’s ASTHRD program. Rachel A. Braun acknowledges support
from the ARCS Foundation. Alexander B. MacDonald acknowl-
edges support from the Mexican National Council for Science and
Technology (CONACYT). We acknowledge Agilent Technologies
for their support, and Shane Snyder’s laboratories for the ICP-QQQ
data.
Financial support. This research has been supported by
the National Aeronautics and Space Administration (grant
no. 80NSSC18K0148).
Atmos. Chem. Phys., 19, 10675–10696, 2019 www.atmos-chem-phys.net/19/10675/2019/
M. T. Cruz et al.: Size-resolved composition and morphology of particulate matter 10691
Review statement. This paper was edited by Patrick Chuang and
reviewed by two anonymous referees.
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