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Atmos. Chem. Phys., 20, 2387–2405, 2020
https://doi.org/10.5194/acp-20-2387-2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
Long-range aerosol transport and impacts on size-resolved aerosol
composition in Metro Manila, Philippines
Rachel A. Braun1, Mojtaba Azadi Aghdam1, Paola Angela Bañaga2,3, Grace Betito3, Maria Obiminda Cambaliza2,3,
Melliza Templonuevo Cruz2,4, Genevieve Rose Lorenzo2, Alexander B. MacDonald1, James Bernard Simpas2,3,
Connor Stahl1, and Armin Sorooshian1,5
1Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ, USA
2Manila Observatory, Loyola Heights, Quezon City 1108, Philippines
3Department of Physics, School of Science and Engineering, Ateneo de Manila University,
Loyola Heights, Quezon City 1108, Philippines
4Institute of Environmental Science and Meteorology, University of the Philippines, Diliman, Quezon City 1101, Philippines
5Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ, USA
Correspondence: Armin Sorooshian (armin@email.arizona.edu)
Received: 7 May 2019 – Discussion started: 30 August 2019
Revised: 24 January 2020 – Accepted: 29 January 2020 – Published: 28 February 2020
Abstract. This study analyzes long-range transport of
aerosol and aerosol chemical characteristics based on in-
stances of high- and low-aerosol-loading events determined
via ground-based size-resolved aerosol measurements col-
lected at the Manila Observatory in Metro Manila, Philip-
pines, from July to October 2018. Multiple data sources, in-
cluding models, remote sensing, and in situ measurements,
are used to analyze the impacts of long-range aerosol trans-
port on Metro Manila and the conditions at the local and
synoptic scales facilitating this transport. Through the use
of case studies, evidence of long-range transport of biomass
burning aerosol and continental emissions is identified in
Metro Manila. Long-range transport of biomass burning
aerosol from the Maritime Continent, bolstered by south-
westerly flow and permitted by low rainfall, was identified
through model results and the presence of biomass burning
tracers (e.g., K, Rb) in the ground-based measurements. The
impacts of emissions transported from continental East Asia
on the aerosol characteristics in Metro Manila are also iden-
tified; for one of the events analyzed, this transport was fa-
cilitated by the nearby passage of a typhoon. Changes in the
aerosol size distributions, water-soluble chemical composi-
tion, and contributions of various organic aerosol species
to the total water-soluble organic aerosol were examined
for the different cases. The events impacted by biomass
burning transport had the overall highest concentration of
water-soluble organic acids, while the events impacted by
long-range transport from continental East Asia showed high
percent contributions from shorter-chain dicarboxylic acids
(i.e., oxalate) that are often representative of photochemical
and aqueous processing in the atmosphere. The low-aerosol-
loading event was subject to a larger precipitation accumula-
tion than the high-aerosol events, indicative of wet scaveng-
ing as an aerosol sink in the study region. This low-aerosol
event was characterized by a larger relative contribution from
supermicrometer aerosols and had a higher percent contribu-
tion from longer-chain dicarboxylic acids (i.e., maleate) to
the water-soluble organic aerosol fraction, indicating the im-
portance of both primary aerosol emissions and local emis-
sions.
1 Introduction
Better understanding of long-range transport of aerosol is
critical for determining the fate of atmospheric emissions and
improving models of atmospheric aerosol. Nutrients (e.g.,
Duce et al., 1991; Artaxo et al., 1994), bacteria (e.g., Bo-
vallius et al., 1978; Maki et al., 2019), and pollutants (e.g.,
Nordø, 1976; Lyons et al., 1978; Lindqvist et al., 1991) can
be transported through the atmosphere over large distances
across the globe. Atmospheric aerosol can undergo phys-
Published by Copernicus Publications on behalf of the European Geosciences Union.
2388 R. A. Braun et al.: Long-range aerosol transport
iochemical changes through photochemical and aqueous-
processing mechanisms such that their characteristics at the
emission source can be quite different from those farther
downwind (e.g., Yokelson et al., 2009; Akagi et al., 2012).
Large uncertainties remain in atmospheric aerosol models
due to impacts of aqueous processing and wet scavenging
on aerosol (Kristiansen et al., 2016; Xu et al., 2019).
The plethora of both natural and anthropogenic emissions
in and around Southeast (SE) Asia; the proximity of islands
and continental regions in SE and East Asia; and the large,
growing population makes SE Asia a prime candidate for the
study of long-range transport of atmospheric aerosol. More-
over, the extensive cloud coverage and precipitation during
certain times of the year in SE Asia allow for an exami-
nation of the effects of aqueous processing and wet scav-
enging. Characterizations of aerosol in mainland SE Asia
and the Maritime Continent (MC), which includes the is-
lands south of the Philippines and north of Australia (e.g.,
islands part of Malaysia and Indonesia), have found major
emission sources to be industrial activities, shipping, urban
megacities, and biomass burning (Reid et al., 2013). In addi-
tion, natural emission sources, including marine emissions,
plant life, and occasionally volcanic eruptions, intermingle
with anthropogenic emissions. Mixing of aerosol from an-
thropogenic and biogenic sources has been noted to be in-
fluential in the overall production of secondarily produced
aerosol via gas-to-particle conversion processes (Weber et
al., 2007; Goldstein et al., 2009; Brito et al., 2018). In ad-
dition, the mixing of marine and biomass burning emissions
can produce compositional changes, such as enhancements
in chloride depletion (e.g., Braun et al., 2017) and methane-
sulfonate (MSA) production (Sorooshian et al., 2015). The
mechanisms governing aerosol changes in mixed air masses
have wide-ranging and complex impacts and require further
study in regions, such as SE Asia, that are impacted by mul-
tiple aerosol emission sources.
One major contributor to atmospheric aerosol in SE Asia
and the MC that has received considerable attention is
biomass burning. Biomass burning in SE Asia appears to
be dominated by anthropogenic activities, such as peatland
burning (Graf et al., 2009; Reid et al., 2013; Latif et al.,
2018) and rice straw open-field burning (Gadde et al., 2009).
However, current satellite retrievals underestimate the true
emissions in the region (Reid et al., 2013). Identification of
biomass burning emissions in the MC using satellite-based
observations is difficult for numerous reasons, including the
characteristics of fires common to the region (e.g., low-
temperature peat burning) and abundant cloud cover (Reid
et al., 2012, 2013). However, the potential for long-range
transport of biomass burning emissions from the MC has
received considerable attention (Wang et al., 2013; Xian et
al., 2013; Reid et al., 2016a; Atwood et al., 2017; Ge et al.,
2017; Song et al., 2018). In order to better understand the
frequency, amount, and fate of biomass burning emissions in
the MC and SE Asia, both in situ measurements and model-
ing studies are needed. Insights into the fate of biomass burn-
ing emissions in the atmosphere are crucial and applicable on
a global scale, especially since studies have indicated an in-
creasing trend in biomass burning worldwide (Flannigan et
al., 2009, 2013).
As a megacity in SE Asia, Metro Manila, Philippines (pop-
ulation in 2015 ∼12.88 million; Philippine Statistics Au-
thority, 2020), is a prime location for the study of locally pro-
duced urban anthropogenic aerosol (Kim Oanh et al., 2006)
that is mixed with biogenic, natural, and anthropogenic pol-
lutants from upwind areas. Previous research conducted at
the Manila Observatory (MO) in Quezon City, Metro Manila,
characterized PM2.5(particulate matter, PM, with aerody-
namic diameter less than 2.5 µm) and sources of measured
particles, with traffic emissions being the major source at
MO (Simpas et al., 2014). Interestingly, levels of measured
PM2.5at MO showed little variance between the wet (June–
October) and dry seasons (Simpas et al., 2014). Additional
studies have further characterized vehicular emissions by fo-
cusing on black carbon (BC) particulate concentrations in
sites around the Metro Manila region, including near road-
ways (Bautista et al., 2014; Kecorius et al., 2017; Alas et al.,
2018). Due to very high population density in Metro Manila,
it is expected that many of the urban PM sampling sites are
highly affected by local anthropogenic sources as opposed to
long-range transport. However, the proximity of the Philip-
pines to other islands and continental Asia raises the question
of the relative impacts of long-range transport as opposed to
local emissions on not just Metro Manila but also downwind
regions.
Long-range transport to the Philippines varies by season
since there is a strong change in weather patterns throughout
the year (Bagtasa et al., 2018). Another study of the aerosol
over the South China Sea (SCS), which is bordered to the
east by the Philippines, found seasonal changes in aerosol
emission sources, with year-round anthropogenic pollution,
smoke from the MC between August and October, and dust
from northern continental Asia between February and April
(Lin et al., 2007). The season from approximately June to
September (Cayanan et al., 2011; Cruz et al., 2013), referred
to as the Southwest Monsoon (SWM) season, is character-
ized by increased prevalence of southwesterly winds and
precipitation. During the SWM season, biomass burning is
prevalent in the MC, while biomass burning is more common
in continental SE Asia during the winter and spring (Lin et
al., 2009; Reid et al., 2013). While variability exists in the
start dates of the different seasons, the northeast monsoon
transition generally occurs in October (Cruz et al., 2013),
and previous research has defined this season as occurring
from October to February (Bagtasa, 2011). During the north-
east monsoon, aerosol influences from northern East Asia
were measured in the northwestern edge of the Philippines
(Bagtasa et al., 2018). In addition to transport of aerosol to
the Philippines, the influence of emission outflows from the
Philippines has also been measured in the northern SCS at
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R. A. Braun et al.: Long-range aerosol transport 2389
Dongsha Island (Chuang et al., 2013) and in coastal southeast
China (Zhang et al., 2012). Long-range transported aerosol in
SE and East Asia has various sources and, therefore, different
physiochemical properties. However, the prevalence of the
signal of long-range transported aerosol in a highly polluted
megacity, such as Metro Manila, is not well characterized.
As recent studies have indicated a decline in SWM rain-
fall in the western Philippines and an increase in no-rain days
during the typical SWM season (Cruz et al., 2013), the poten-
tial for wet scavenging of aerosol during these time periods
could be decreasing. Furthermore, decreases in monsoonal
rainfall in other parts of Asia, including India (Dave et al.,
2017) and China (Liu et al., 2019), have been linked to in-
creases in aerosol, especially those of anthropogenic origin.
Reinforcing mechanisms in these interactions, such as de-
creased rainfall reducing wet scavenging, leading to higher
aerosol concentrations that in turn suppress precipitation, and
the corresponding climatic changes in monsoonal rain in the
western Philippines underscore the need to better understand
the processes governing atmospheric aerosol characteristics
and sources, especially during the monsoonal season.
The present study focuses on three high-aerosol-loading
events, contrasted with a very low aerosol event, as iden-
tified by ground-based observations collected at MO from
July to October 2018. The objectives of the study are to
(i) describe synoptic- and local-scale conditions facilitat-
ing various transport cases; (ii) characterize aerosol physic-
ochemical properties associated with long-range transport;
and (iii) identify transformational processes, especially with
regard to chemical composition, of aerosol during long-range
transport to the highly populated Metro Manila region. The
results of this work have implications for better understand-
ing (i) the fate of biomass burning emissions in a region
with prevalent wildfires that are poorly characterized by re-
mote sensing; (ii) the impact of transformational and removal
mechanisms, including aqueous processing, photochemical
reactions, and wet scavenging, on long-range transported
aerosol from multiple sources; and (iii) typical synoptic- and
local-scale behavior of aerosol in a region that is both highly
populated and gaining increasing attention due to campaigns
such as the NASA-sponsored Clouds, Aerosols, and Mon-
soon Processes Philippines Experiment (CAMP2Ex).
2 Methodology
2.1 Ground-based observations
As part of a year-long sampling campaign (CAMP2Ex
weatHEr and CompoSition Monitoring: CHECSM) at the
Manila Observatory (MO; 14.64◦N, 121.08◦E) in Quezon
City, Metro Manila, Philippines, 12 sets of size-resolved
aerosol were collected from July to October 2018 using a
micro-orifice uniform deposit impactor (MOUDI; Marple
et al., 2014). Details for the 12 size-resolved sets can be
found in Table 1. Sample Teflon substrates (PTFE mem-
brane, 2 µm pore, 46.2 mm diameter, Whatman) were cut in
half for preservation for future analysis. Half-substrates were
extracted in 8 mL of Milli-Q water (18.2 Mcm) in sealed
polypropylene vials through sonication for 30 min. Aqueous
extracts were subsequently analyzed for ions using ion chro-
matography (IC; Thermo Scientific Dionex ICS-2100 sys-
tem) and elements using triple quadrupole inductively cou-
pled plasma mass spectrometry (ICP-QQQ; Agilent 8800 Se-
ries). The list of analyzed species and limits of detection for
those species can be found in Table S1 in the Supplement,
with limits of detection in the parts per trillion (ppt) range for
ICP and the parts per billion (ppb) range for IC. Background
concentrations were also subtracted from each sample. For
each MOUDI set (naming convention: MO#), the mass con-
centration sum of the water-soluble species was calculated;
using this summation, the three high-aerosol-loading events
were identified (MO7, MO12, and MO14), as well as the
lowest aerosol event (MO11). The average ±standard de-
viation of the total water-soluble species measured for the
remaining eight sets not identified in the high or low cate-
gories is 6.99 ±2.71 µg m−3.
2.2 Remote-sensing observations
Retrievals of atmospheric profiles from the Cloud-Aerosol
Lidar with Orthogonal Polarization (CALIOP) on board the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Ob-
servation (CALIPSO) satellite were taken for select satellite
overpasses corresponding to MOUDI sample sets of inter-
est (Winker et al., 2009). Previous studies have examined the
ability of CALIOP to capture atmospheric profiles in SE Asia
and the MC, with one major challenge in this region being
the lack of cloud-free schemes (Campbell et al., 2013; Ross
et al., 2018). Overpasses corresponding to the three highest
aerosol events were analyzed, but no data were available for
the time encompassing MO11. The CALIOP Level 2 Verti-
cal Feature Mask (VFM) version 4.20 was used to distinguish
between clear air, clouds, and aerosol (Vaughan et al., 2004).
For figures of CALIOP VFM data in this study, data are plot-
ted at 30 m vertical resolution every 5km along the satellite
ground track.
2.3 Models
To describe the synoptic-scale conditions, data were used
from the Modern-Era Retrospective analysis for Research
and Applications, version 2 (MERRA-2; Gelaro et al., 2017).
Horizontal winds at 850 hPa (GMAO, 2015a) were tempo-
rally averaged over the sampling period using 3h instanta-
neous data and subsequently spatially averaged to increase
figure readability. The total cloud area fraction (GMAO,
2015b) was also temporally averaged over the sampling pe-
riod using 1 h time-averaged MERRA-2 data.
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2390 R. A. Braun et al.: Long-range aerosol transport
Table 1. Description of the MOUDI sample sets from this study. Accumulated precipitation during the sample sets was found using
PERSIANN-CCS for the area bounded by 14.6067–14.6946◦N and 121.0199–121.0968◦E.
Set name Start date (MM/DD/YY)/ End date/ Total water-soluble % of water- Precipitation
local time local time species (µg m−3) soluble mass (mm)
< 1 µm
MO1 7/19/18 7/20/18 4.61 67.3 % 27
12:40 12:43
MO2 7/23/18 7/25/18 6.52 62.1 % 14
11:29 17:10
MO4 7/25/18 7/30/18 5.17 66.4 % 35
19:16 18:12
MO5 7/30/18 8/1/18 9.17 64.8 % 11
19:17 13:19
MO6 8/6/18 8/8/18 5.11 55.8 % 50
14:33 14:38
MO7 8/14/18 8/16/18 13.70 60.3 % 3
13:59 14:04
MO8 8/22/18 8/24/18 12.73 71.6 % 10
13:46 13:53
MO9 9/1/18 9/3/18 6.23 76.7 % 64
05:00 05:05
MO10 9/10/18 9/12/18 6.36 79.5 % 20
14:42 15:02
MO11 9/18/18 9/20/18 2.70 47.3 % 26
14:12 14:24
MO12 9/26/18 9/28/18 13.49 59.9 % 1
13:53 13:53
MO14 10/6/18 10/8/18 16.55 78.4 % 0
05:00 05:05
Five-day air mass back trajectories were calculated using
the Hybrid Single Particle Lagrangian Integrated Trajectory
(HYSPLIT) model from NOAA (Stein et al., 2015) and grid-
ded meteorological data from the National Centers for En-
vironmental Prediction/National Center for Atmospheric Re-
search (NCEP/NCAR) reanalysis project. The model was run
for back trajectories terminating at the MOUDI inlet (∼85 m
above sea level) starting at the beginning of the sample set
and every 6h thereafter during each sample set, resulting
in (1 +N/6) trajectories for each set, where Nis the total
number of sampling hours. Heights above ground level for
HYSPLIT back trajectories corresponding to the three high-
aerosol-loading events (MO7, MO12, and MO14) can be
found in Fig. S1 in the Supplement. The HYSPLIT model has
been used extensively in studies focused on regions across
the globe to study aerosol transport (Stein et al., 2015).
Precipitation amounts were found using the Precipita-
tion Estimation from Remotely Sensed Information us-
ing Artificial Neural Networks–Cloud Classification System
(PERSIANN-CCS) dataset (Hong et al., 2004), which is
available from the UC Irvine Center for Hydrometeorology
and Remote Sensing (CHRS) Data Portal (Nguyen et al.,
2019). PERSIANN-CCS has previously been used to ana-
lyze precipitation events in the region of interest, as shown
by the successful characterization of rainfall during Typhoon
Haiyan over the Philippines in November 2013 (Nguyen et
al., 2014). Benefits of PERSIANN-CCS include the data
availability at 0.04◦×0.04◦spatial resolution, while uncer-
tainties in the dataset arise from sources such as a lack of bias
correction (Nguyen et al., 2014). Precipitation accumulated
during the sample sets (Table 1) was calculated to be the aver-
age found for the region surrounding MO in the box bounded
by 14.6067–14.6946◦N and 121.0199–121.0968◦E.
To further describe long-range transport activity, results
from the Navy Aerosol Analysis and Prediction System
(NAAPS) operational model are included for the selected
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R. A. Braun et al.: Long-range aerosol transport 2391
study periods (Lynch et al., 2016; https://www.nrlmry.navy.
mil/aerosol/, last access: 20 February 2019). Global meteo-
rological fields used in the NAAPS model are supplied by the
Navy Global Environmental Model (NAVGEM; Hogan et al.,
2014). The NAAPS model has previously been employed to
study aerosol in the MC (e.g., Xian et al., 2013).
3 Results
3.1 Cases of long-range aerosol transport
The following sections (3.1.1–3.1.4) describe the synoptic-
and local-scale meteorological conditions governing long-
range aerosol transport during the three highest aerosol
events (MO7, MO12, and MO14) and, for the purposes of
comparison, the lowest aerosol event (MO11). Also included
are characterizations of aerosol from remote-sensing and
model results. Results of size-resolved aerosol characteriza-
tion at MO are discussed in Sect. 3.2.
3.1.1 MO7 (14–16 August 2018): smoke transport from
the Maritime Continent
Many previous studies have focused on the prevalence of
biomass burning in the MC and the potential for transport
of smoke towards the Philippines (Wang et al., 2013; Xian
et al., 2013; Reid et al., 2016a; Atwood et al., 2017; Ge et
al., 2017; Song et al., 2018). Figure 1a shows the average
850 hPa wind vectors and cloud fraction for the MO7 sam-
pling period. The prevailing wind direction was towards the
northeast, consistent with typical SWM flow. Furthermore,
areas with lower cloud coverage were present to the south-
west of Metro Manila. The HYSPLIT back trajectory for
this sample set also shows an air mass originating around
the MC to the southwest of MO that is then transported over
the ocean towards the Philippines (Fig. 2a). As evidenced
by the name of the season (i.e., Southwest Monsoon), this
trajectory is typical for this time of the year and was the
dominating trajectory pattern for the remaining eight sample
sets not chosen for in-depth analysis (Fig. S2). Furthermore,
for MO1–MO10 (i.e., all sample sets with prevailing south-
westerly wind influence), MO7 had the lowest rain amount
for the surrounding region, followed by MO8, which had the
fourth-highest water-soluble aerosol concentration (Table 1).
This suggests that wet scavenging could have been less in-
fluential in MO7 and MO8, thereby leading to an increase
in the PM measured. Three CALIPSO overpasses near MO
occurred during the MO7 sample set and one occurred dur-
ing the nighttime after sampling ended; however, the signal
was largely attenuated in the lower 8km during the daytime
samples for the area surrounding MO (Fig. S3). In the case
of the two nighttime overpasses (Fig. 3), which sampled to
the southwest of Manila, a deep aerosol layer is observed in
the VFM extending from the surface to around 3 km (Fig. 3).
This classic case of long-range transport from the MC to the
Philippines during the SWM season is also clearly shown in
the biomass burning smoke surface concentrations from the
NAAPS model (Fig. 4a).
3.1.2 MO11 (18–20 September 2018): lowest aerosol
event
MO11 had the lowest overall water-soluble aerosol mass
concentration (2.7 µg m−3), which is over 6 times lower than
the highest aerosol MOUDI set. As evidenced by both the
850 hPa wind vectors (Fig. 1b) and the HYSPLIT back tra-
jectories (Fig. 2b) from this set, conditions are very different
from the highest three aerosol events and show transport pat-
terns with flow originating over the open ocean to the east of
the Philippines moving almost due west. The lack of anthro-
pogenic aerosol sources in the path of the back trajectories
could result in the overall low amount of aerosol observed.
This set was also characterized by high accumulated rainfall
amounts for the region in the path of the back trajectories
(Fig. 2b) and in the area surrounding MO as compared to the
highest aerosol events (Table 1), increasing the possibility
that wet scavenging effectively removed most of the trans-
ported (and, to some extent, local) aerosol. In addition, the
NAAPS model showed no smoke influence from the MC and
an isolated anthropogenic and biogenic fine aerosol plume
around Metro Manila, suggesting local sources accounted for
the majority of the measured aerosol (Fig. 4b).
3.1.3 MO12 (26–28 September 2018): impacts of
Typhoon Trami
Typhoon Trami (Category 5) passed to the northeast of the
island of Luzon in the Philippines during MO12 (Fig. 1c).
Typhoon influences on atmospheric aerosol, caused by vary-
ing factors such as wind speed and precipitation, have been
studied in China (Yan et al., 2016; Liu et al., 2018), Korea
(Kim et al., 2007), Malaysia (Juneng et al., 2011), the South
China Sea (Reid et al., 2015, 2016b), and Taiwan (Fang et al.,
2009; Chang et al., 2011; Lu et al., 2017). The influences of
typhoons on biomass burning emissions and transport in the
MC have also been examined (Reid et al., 2012; Wang et al.,
2013). In this case, the influence of this storm changed the
prevailing wind direction approaching the northern Philip-
pines, effectively pulling an air mass from the west of the is-
land and, along with it, emissions from continental East Asia
(Fig. 2c). Furthermore, the air mass passed through regions
of relatively little rainfall during transport to the Philippines
(Fig. 2c), and accumulated rainfall at MO during this sample
set was very low (Table 1). One CALIPSO overpass around
the ending time of set MO12 and one during the nighttime
after sampling ended (Fig. 3) show that, in the direction of
transport (i.e., north of the MO, from around 15–20◦N),
there is an aerosol layer extending up to around 2 km dur-
ing the day (northwest of MO) and 3 km at night (northeast
of MO). The influence of emissions from continental East
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2392 R. A. Braun et al.: Long-range aerosol transport
Figure 1. MERRA-2 data for 850 hPa wind vectors and total cloud fraction averaged over the sample set duration for (a) MO7 (14–16
August), (b) MO11 (18–20 September), (c) MO12 (26–28 September), and (d) MO14 (6–8 October). The location of the Manila Observatory
is indicated by the red circle. (Note that 850 hPa wind vectors are also averaged to increase grid spacing and improve figure readability.)
Asia is also apparent in the NAAPS model (Fig. 4c). Obser-
vations at Dongsha Island, located to the north of the Philip-
pines, have revealed influence from Gobi Desert emissions
(Wang et al., 2011) and anthropogenic sources (Atwood et
al., 2013). Farther south in the MC, aerosol measurements in
Malaysia have also indicated influence of aged, long-range
transport from sites to the north in East Asia (Farren et al.,
2019).
3.1.4 MO14 (6–8 October 2018): mixed influences
The final MOUDI set (MO14) included in this study rep-
resents a transition in meteorological regimes at the end of
the SWM season and resulted in the highest overall water-
soluble mass concentration. This event had some of the low-
est rainfall amounts in the region surrounding Metro Manila
(Fig. 2d), with zero accumulated precipitation at MO dur-
ing the sampling period (Table 1). Furthermore, low cloud
fraction was observed for regions to the northwest and east
of Metro Manila (Fig. 1d). Back trajectories from HYS-
PLIT show that the air mass appeared to be influenced by
a mix of continental sources in East Asia and local sources
(Fig. 2d). Furthermore, two CALIPSO overpasses, one dur-
ing the nighttime while sampling was occurring and the
other during the daytime after sampling ended, show a deep
aerosol layer north of MO, extending from the surface to
around 2 km on 6 October and lower on 8 October (Fig. 3).
From the NAAPS model, it appears that a mixture of MC
smoke emissions and continental East Asia emissions con-
verges around the northern Philippines (Fig. 4d).
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R. A. Braun et al.: Long-range aerosol transport 2393
Figure 2. Rainfall accumulation, extending from 5 d before the mid-
point of each sample set until the midpoint of each sample set, from
PERSIAN-CCS for (a) MO7, (b) MO11, (c) MO12, and (d) MO14.
In blue are the 5 d air mass back trajectories terminating at the
MOUDI inlet at MO (∼85 m above sea level) every 6h during
each of the sample study periods. Note that the maximum precip-
itation accumulation in the region shown during the study periods
was 955 mm; however, for figure readability, the scale was reduced
to 0–250 mm.
3.2 Ground-based aerosol chemical composition
3.2.1 Size-resolved aerosol characteristics
The water-soluble mass size distributions and the percent
contribution of each MOUDI stage to the water-soluble mass
for the four sets of interest (MO7, MO12, MO14, and MO11)
and the average (±1 standard deviation) of the remaining
sets (MO1–MO6, MO8–MO10) are shown in Fig. 5. Most
of the sets show a bimodal distribution with peaks in both
the submicrometer and supermicrometer range; one excep-
tion is the lowest aerosol event (MO11), which shows a fairly
broad size distribution. The highest aerosol event, MO14,
shows a significant peak in the submicrometer range, with a
very large drop in mass concentration in the supermicrom-
eter range. This is in stark contrast to the lowest aerosol
event (MO11), which shows that the supermicrometer range
contributes the greatest percent to the total water-soluble
mass. The second- and third-highest aerosol events, MO7
and MO12, also show significant enhancements in the su-
permicrometer range as compared to the average of the other
sets and MO14.
Figure 6 describes the major species contributing to the
water-soluble mass. MO14 had one of the highest combined
contributions of SO2−
4and NH+
4(77.2 % of water-soluble
mass), with only MO10 being slightly larger at 77.6 %. These
two species are typically associated with the submicrome-
ter range and anthropogenic origins due to their formation
through secondary processes such as gas-to-particle conver-
sion of gaseous SO2and NH3, respectively, and aqueous pro-
cessing to form SO2−
4(Ervens, 2015). In contrast, MO11 had
the lowest overall combined percent contribution of these
two species (41.4 %) to the water-soluble aerosol mass. Of
all 12 SWM MOUDI sets, MO11 had the highest percent
contributions from Ca2+(14.0 %) and Cl−(12.5 %), as well
as one of the highest contributions from Na+(10.7 %). Each
of these species is associated with primary emissions, includ-
ing dust in the case of Ca2+and sea salt for Na+and Cl−,
resulting in larger particles (i.e., > 1µm). The HYSPLIT back
trajectories for MO11 match well with the MOUDI results,
as the influence of marine aerosol (i.e., Na+, Cl−) and lack of
anthropogenic sources of SO2and NH3are apparent. Local
sources of dust most likely contribute the highest amount to
the measured Ca2+, as the back trajectories show few other
crustal sources farther upwind. Average size-resolved pro-
files for all of the species in these 12 sample sets can be found
in Cruz et al. (2019), with characteristic size distribution pro-
files agreeing with the above assessments.
3.2.2 Enhancements in tracer species
In addition to insights from the major water-soluble chem-
ical species found in aerosol, tracer aerosol species can
also be used to identify impacting emission sources (e.g.,
Fung and Wong, 1995; Allen et al., 2001; Ma et al., 2019).
For the aforementioned high-aerosol events, numerous tracer
species are elevated in some, but not all, sample sets. This
makes these species prime candidates for linking influenc-
ing sources to the measured ambient aerosol. The authors
theorize that MO8, which was the fourth-highest aerosol
event (Table 1), also was impacted by biomass burning due
to the back-trajectory analysis (Fig. S2), NAAPS model
(Fig. S4), and increases in select species described subse-
quently. Therefore, MO8 was separated from the other sam-
ple sets for the purposes of the following characterizations.
Figure 7 shows the size-resolved aerosol composition for se-
lect tracer species for the four highest aerosol events (MO7,
MO8, MO12, and MO14), the lowest aerosol event (MO11),
and the average (±standard deviation) of the remaining
seven sample sets.
Potassium is frequently used as a biomass burning tracer
(e.g., Andreae, 1983; Artaxo et al., 1994; Echalar et al.,
1995; Chow et al., 2004; Thepnuan et al., 2019). This species
shows highly elevated levels in the submicrometer range for
MO7 and MO8 (i.e., the sets influenced by biomass burn-
ing transport from the MC). Other elevated trace elements
for these two profiles include Rb, Cs, Se, and Ti (Fig. 7).
Previous studies in the western United States (Schlosser et
al., 2017; Ma et al., 2019) have also shown Rb enhance-
ments in wildfire-influenced aerosol. Rb has also been mea-
sured in flaming and smoldering biomass burning emissions
(Yamasoe et al., 2000). Enhancements in Rb and Cs in the
fine fraction of aerosol influenced by wildfire emissions have
been observed in South Africa (Maenhaut et al., 1996), with
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2394 R. A. Braun et al.: Long-range aerosol transport
Figure 3. CALIOP Vertical Feature Mask (VFM) for overpasses during or following MO7, MO12, and MO14. For the CALIPSO satel-
lite overpass tracks, the dashed lines correspond to the nighttime profiles and solid lines are for daytime. Note that nighttime overpasses
correspond to early morning times before sunrise for the listed days and daytime overpasses occurred during the early afternoon.
Figure 4. NAAPS model snapshots corresponding to conditions at the stop time of sample sets (a) MO7, (b) MO11, and (c) MO12 and
(d) 3 h after the sample stop time for MO14. The top row of figures is anthropogenic and biogenic fine aerosol (ABF) surface concentration
(µg m−3), while the bottom row is biomass burning smoke surface concentration (µgm−3).
similar results shown in this study for aerosol in the submi-
crometer size range. Se is also enhanced for these two sets in
the submicrometer range, as it is often formed through gas-
to-particle conversion processes of inorganic Se compounds
(Wen and Carignan, 2007). A wide variety of sources for
atmospheric Se exist (Mosher and Duce, 1987), including,
but not limited to, coal combustion (Thurston and Spengler,
1985; Fung and Wong, 1995; Song et al., 2001), marine emis-
sions (Arimoto et al., 1995), volcanos, and biomass burn-
ing (Mosher and Duce, 1987). In contrast to the other en-
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R. A. Braun et al.: Long-range aerosol transport 2395
Figure 5. (a) Mass size distributions for total water-soluble mass
(Cis the sum of mass concentrations for water-soluble species) and
(b) percent contribution of each size range to the total water-soluble
mass for the three MOUDI sets with the highest aerosol mass con-
centrations (MO7, MO12, and MO14), the set with the lowest con-
centration (MO11), and the average (±1 standard deviation error
bars) for the remaining eight sets.
Figure 6. Percent contribution of various species to the total water-
soluble mass concentration for each of the 12 sample sets. The sam-
ple sets with the three highest aerosol concentrations (MO7, MO12,
and MO14) and the lowest aerosol concentration (MO11) are shown
as solid bars while all other sample sets are stripes. The “organics”
category contains the sum of methanesulfonate (MSA), pyruvate,
adipate, succinate, maleate, oxalate, and phthalate.
hanced species for MO7 and MO8, the mass concentration
mode for Ti resides in the supermicrometer size range. Ti
is typically associated with crustal material that can be sus-
pended through mechanisms such as vehicle usage (Stern-
beck et al., 2002; Querol et al., 2008; Amato et al., 2009) and
lofting in wildfire plumes (Maudlin et al., 2015; Schlosser
et al., 2017). While long-range transport of biomass burning
aerosol could lead to the enhancements measured for these
Figure 7. Selected elements that showed elevated concentrations
during at least one of the highest aerosol events (MO7, MO8,
MO12, or MO14). The concentrations from the lowest aerosol event
(MO11) are also shown. The “other sets” category displays the av-
erage (±1 standard deviation) for the remaining seven sets.
biomass burning tracer species, local emission sources, such
as waste burning and wood burning for cooking, may also
play a role.
Two tracer species are included that showed enhancements
for MO12, specifically Ba in the supermicrometer range and
V in the submicrometer range (Fig. 7). One well-documented
source of aerosol Ba is nonexhaust vehicle emissions, includ-
ing brake wear (Sternbeck et al., 2002; Querol et al., 2008;
Amato et al., 2009; Jeong et al., 2019). V also has well-
characterized emission sources, most specifically fuel com-
bustion (Fung and Wong, 1995; Artaxo et al., 1999; Song
et al., 2001; Lin et al., 2005; Kim and Hopke, 2008). In
coastal environments, V is often tied to shipping emissions
(Agrawal et al., 2008; Pandolfi et al., 2011; Maudlin et al.,
2015; Mamoudou et al., 2018). As these sources are anthro-
pogenic in origin, it is difficult to determine the relative in-
fluences of long-range transport versus local emissions, espe-
cially with the proximity of the sampling site to major road-
ways and shipping in Manila Bay. However, the enhancement
in V could result from the transport of the aerosol over major
shipping lanes farther upwind.
Finally, Fig. 7 shows three selected elements that appear
enhanced in MO14, all of which are typically tied to an-
thropogenic sources. Both Pb and Sn are found mainly in
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2396 R. A. Braun et al.: Long-range aerosol transport
the submicrometer range and have been linked by previous
studies to vehicle emissions (Singh et al., 2002; Amato et
al., 2009), industrial emissions (Querol et al., 2008; Allen
et al., 2001), and waste burning (Kumar et al., 2015). Other
sources of Pb could include e-waste recycling (Fujimori et
al., 2012) and biomass burning (Maenhaut et al., 1996). The
size distribution of Mo for MO14 shows a much broader dis-
tribution, with peaks in both the sub- and supermicrometer
ranges. Sources of Mo include vehicle emissions (Pakka-
nen et al., 2003; Amato et al., 2009), combustion (Pakkanen
et al., 2001, 2003), and industrial activity, including copper
smelters (Artaxo et al., 1999). As is the case with the en-
hanced species in MO12, the anthropogenic nature of these
species makes it difficult to determine the relative contribu-
tion of long-range versus local emissions. However, as both
MO12 and MO14 show enhancements in anthropogenically
produced trace elements, the influence of long-range trans-
port from industrial and urban areas in continental East Asia
is plausible.
3.2.3 Variability of water-soluble organic species
Figure 8 shows the sum of the total measured water-soluble
organic species and the relative contributions of oxalate, suc-
cinate, adipate, maleate, pyruvate, MSA, and phthalate to
the total measured water-soluble organics for MO7, MO8,
MO11, MO12, MO14, and the average (±1 standard devia-
tion) of the remaining sets. Malonate (C3) was not character-
ized due to its low concentrations in the samples measured
and the coelution of C3 with carbonate in the IC analysis.
Glutarate (C5) was also excluded from the analysis due to
very low concentrations. For the examination of the organic
species, MO8 was again separated from the other MOUDI
sets due to it having the second-highest concentration of or-
ganic species (0.66 µg m−3) and an organic species contri-
bution profile very similar to that of MO7. The remaining
MOUDI sets included in the average category (MO1–MO6,
MO9–MO10) all have total organic species concentrations
that were lower than the four highest aerosol sets (MO7,
MO8, MO12, MO14) and greater than the lowest aerosol set
(MO11). The lowest aerosol event (MO11) has the lowest
overall concentration of organic aerosol (0.09µg m−3), while
the second-highest aerosol event (MO7) has the highest con-
centration of organic aerosol (0.70 µg m−3).
Many studies worldwide have examined the relative con-
tributions of organic species to atmospheric aerosol, with
oxalate typically having the highest contribution among
dicarboxylic acids (Kawamura and Kaplan, 1987; Kawa-
mura and Ikushima, 1993; Kawamura and Sakaguchi, 1999;
Sorooshian et al., 2007a; Hsieh et al., 2007, 2008; Aggar-
wal and Kawamura, 2008; Deshmukh et al., 2012, 2018;
Li et al., 2015; Hoque et al., 2017; Kunwar et al., 2019).
Oxalate was the dominant water-soluble organic species for
all 12 MOUDI sets, with oxalate having the highest con-
tribution to the organic aerosol in MO12 (88.7% of total
Figure 8. Pie charts showing the fraction of species contributing to
the measured water-soluble organic aerosol. Below each pie chart
title is the sum of the water-soluble organic species measured, with
the “other sets” chart showing the average ±1 standard deviation
for the remaining sets.
organic aerosol). Oxalate is often considered a byproduct
of photochemical aging of longer-chain dicarboxylic acids
(e.g., Kawamura and Ikushima, 1993; Kawamura and Sak-
aguchi, 1999), and therefore an increase in oxalate is often
considered a signature of aged aerosol in the absence of pri-
mary oxalate emissions from sources such as biomass burn-
ing. Another major pathway of oxalate formation is aqueous
processing (Crahan et al., 2004; Ervens et al., 2004, 2018;
Sorooshian et al., 2006, 2007b; Wonaschuetz et al., 2012),
which is likely prevalent during the SWM when there is fre-
quent cloud cover. Previous studies have also demonstrated
the ability for transport and photochemical aging of water-
soluble organic acids over long distances in a marine envi-
ronment (e.g., Kawamura and Sakaguchi, 1999) and the im-
portance of emissions from continental Asia in the organic
aerosol budget in the western north Pacific (Aggarwal and
Kawamura, 2008; Hoque et al., 2017). The back trajecto-
ries of the air masses terminating at MO during MO12 and
MO14 indicate origins of emissions from continental East
Asia (Fig. 2). It is plausible that the high contribution of
oxalate to the organic aerosol in MO12 and MO14 (which
had the fourth-highest percent contribution of oxalate) is
due to the degradation of both primarily emitted and secon-
darily produced longer-chain dicarboxylic acids during the
transport process through mechanisms described above, such
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R. A. Braun et al.: Long-range aerosol transport 2397
as photochemical degradation and aqueous processing, with
the former mechanism being plausible in the regions of low
cloud cover to the north and northwest of Manila (Fig. 1) and
the latter mechanism potentially being of great importance
due to the typhoon influences during transport. While the
aerosol measured in MO7 and MO8 also shows long-range
transport influences (Figs. 2a and S2), the overall signal of or-
ganic aerosol is much stronger in these two sets, such that the
absolute concentration of oxalate (MO7: 0.47 µgm−3; MO8:
0.42 µg m−3) is still greater than in MO12 (0.19 µgm−3) and
MO14 (0.37 µg m−3). However, biomass burning is a well-
documented source of both oxalate and longer-chain dicar-
boxylic acids (e.g., Falkovich et al., 2005; Nirmalkar et al.,
2015; Cheng et al., 2017; Deshmukh et al., 2018; Thepnuan
et al., 2019).
Succinate has been linked to biomass burning emissions
(Wang and Shooter, 2004; Falkovich et al., 2005; Zhao et
al., 2014; Balla et al., 2018), vehicular emissions (Kawa-
mura and Kaplan, 1987; Kawamura et al., 1996; Yao et
al., 2004), and secondary production via photochemical re-
actions of precursor organic compounds (Kawamura and
Ikushima, 1993; Kawamura et al., 1996; Kawamura and Sak-
aguchi, 1999). The two MO MOUDI sets thought to have
the most influence from biomass burning emissions (MO7
and MO8) had the highest organic aerosol mass concentra-
tions and the highest mass percent contributions of succi-
nate to the organic aerosol (MO7: 14.3 % and MO8: 17.5 %).
In contrast, the next highest contribution of succinate to the
organic aerosol was 4.2% measured in MO5. These results
agree with previous studies in Northeast China that showed
an increase in total organic aerosol mass concentration and a
strong increase (decrease) in the relative contribution of suc-
cinate (oxalate) during biomass burning periods as opposed
to non-biomass-burning periods (Cao et al., 2017). Results
from California, USA, also showed higher percent contribu-
tions of succinate to the water-soluble organic aerosol dur-
ing periods influenced by biomass burning (Maudlin et al.,
2015).
MO11 had the second-highest relative contribution of
maleate (28.5 % of water-soluble organic aerosol) out of all
12 sample sets and had a much higher percent contribution as
compared to the four highest aerosol events (<2.5 % for each
of the following: MO7, MO8, MO12, and MO14). Maleate
is linked to the oxidation of aromatic hydrocarbons, usu-
ally from anthropogenic sources such as vehicular emissions
(Kawamura and Kaplan, 1987; Kunwar et al., 2019). One ex-
planation for this result could be the higher rainfall accumu-
lation in and around the study region during MO11 as com-
pared to the three highest aerosol sets (Fig. 2). Wet scaveng-
ing could have removed aerosol from transported air masses
during their journey towards MO, thereby increasing the rel-
ative contribution of local sources to the measured aerosol
in MO11. Because of the reduced aging time associated with
emissions from local sources, the relative increase in the con-
tribution of longer-chain dicarboxylic acids and the decrease
in the relative contribution of oxalate is plausible. Hsieh et
al. (2008) showed in samples from Taiwan that the relative
contribution of oxalate to the organic acids was also higher
during periods of high aerosol loading as opposed to periods
of moderate aerosol loading when the overall PM concen-
tration was lower. MO11, which showed air mass back tra-
jectories originating to the east of the Philippines from the
open Pacific (Fig. 2b), had the lowest overall water-soluble
PM concentration, the lowest overall concentration of water-
soluble organic acids, and the second-lowest percent contri-
bution of oxalate to the organic acid mass (57.1%) of all the
sets.
Phthalate is an aromatic dicarboxylic acid often linked
to anthropogenic sources through photochemical transfor-
mation of emissions from vehicles (Kawamura and Kaplan,
1987; Kawamura and Ikushima, 1993) and waste burning
(Kumar et al., 2015), although aqueous processing has also
been proposed as a formation mechanism (Kunwar et al.,
2019). Accordingly, phthalate has been shown to have sea-
sonal and diurnal variations in concentration, with enhanced
production usually linked to times of stronger solar radiation
(i.e., summertime and daytime: Satsumabayashi et al., 1990;
Ray and McDow, 2005; Ho et al., 2006; Kunwar et al., 2019).
However, increased emissions of precursor species during
different times of the year may affect these trends (Hyder
et al., 2012). Sets MO7, MO8, MO11, and MO14 had the
highest contribution to the water-soluble organics from ph-
thalate (range: 9.5 %–10.2%). In contrast, the remaining sets
had a much lower contribution (range: 1.7%–4.9 %). How-
ever, the absolute concentration of phthalate was highest in
sets MO7, MO8, and MO14 (range: 45.3–67.0 ng m−3) and
much lower for the remaining sets (range: 2.0–8.9ng m−3).
Increased phthalate concentrations during biomass burning
episodes have been previously measured in SE Asia (Cao et
al., 2017). Furthermore, cloud coverage was fairly low dur-
ing MO14 as compared to the other sets of interest (Fig. 1),
increasing the possibility of photochemical production of ph-
thalate. For the remaining sample sets, the range of phthalate
concentrations is substantially lower and fairly consistent, in-
dicating that the measured phthalate in these samples most
likely represents the local background conditions.
While not a carboxylic acid, MSA is nonetheless an im-
portant organic aerosol species, especially in marine environ-
ments. The assumed precursor of MSA in this study is from
the oxidation of marine-emitted dimethylsulfide (DMS). In-
terestingly, all sample sets showed approximately the same
mass percent contribution of MSA to the organic aerosol,
ranging from a minimum of 3.1 % (MO6) to a maximum of
7.0 % (MO5). However, the absolute concentration of MSA
was highest in the two sets with biomass burning influence
(MO7: 23.3 and MO8: 21.4 ngm−3), with concentrations 8.4
and 7.7 times higher, respectively, than the lowest MSA con-
centration measured (MO11: 2.8 ng m−3). A previous study
showed that MSA concentrations in air masses with mixed
influence from marine and biomass burning emissions are
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2398 R. A. Braun et al.: Long-range aerosol transport
higher than the concentrations measured from either source
alone (Sorooshian et al., 2015). The results from the present
study (i.e., more MSA measured in sets with biomass burn-
ing influence) in SE Asia again highlight the complexity of
interactions between air masses with different sources and
the accompanying changes in aerosol physiochemical prop-
erties.
4 Conclusions
This study sought to characterize influences of local and
long-range transported aerosol to the Philippines during the
Southwest Monsoon (SWM) season as well as the various
synoptic- and local-scale conditions that facilitate and sup-
press long-range transport of aerosol. As a highly populated
megacity, Metro Manila is the source of a large amount
of urban, anthropogenic pollution. However, synoptic-scale
weather, including the typical SWM flow and typhoons, can
impact the transport of aerosol to and from Metro Manila.
While previous work in a rural area in the northwest edge
of the Philippines has identified seasonal aerosol transport
patterns to the Philippines using PM2.5data (Bagtasa et al.,
2018), the present study highlights case studies of in situ
size-resolved aerosol measurements from Metro Manila to
examine the potential for aerosol transport to impact this ur-
ban area as well.
For two of the sample sets with enhanced total water-
soluble aerosol mass concentration, biomass burning aerosol
transport from the Maritime Continent (MC) towards the
Philippines was identified using air mass back trajectories
and the NAAPS model. This transport followed a southwest-
erly flow pattern that is typical of this time of year (Fig. S2)
and lends its name to the SWM season. Deep aerosol lay-
ers, extending from the surface to 3km, were identified by
CALIOP to the southwest of the Philippines. The influence
on aerosol in Metro Manila was shown through enhance-
ments in biomass burning tracer species (e.g., K, Rb) and
increased concentration of organic aerosol. The challenges
in satellite-based retrievals of biomass burning in the region
(Reid et al., 2012, 2013) and the underestimation of fire activ-
ity in the region by these satellite retrievals (Reid et al., 2013)
lead to unanswered questions about the amount and fate of
biomass burning emissions in the MC and SE Asia. The abil-
ity to measure biomass burning signatures in a highly pol-
luted, urban megacity such as Metro Manila and the evidence
of long-range transport gathered through multiple methods
and data sources (i.e., in situ measurements, models, and re-
mote sensing) speak to the strong signature of biomass burn-
ing emissions in the region and the long-range transport path-
ways available for these emissions.
In contrast, transport of anthropogenic emissions from
continental East Asia was identified on two occasions with
high water-soluble aerosol mass concentrations, with one
measured instance of long-range transport having been facil-
itated by the influence of a typhoon. In these cases, it is dif-
ficult to separate urban emissions between local and distant
sources. However, the elevation of select tracer species (Ba,
V, Pb, Mo, Sn) and the water-soluble organic aerosol char-
acteristics for these two cases (i.e., high relative contribution
of oxalate to the organic aerosol) indicated that long-range
transported urban emissions could impact Metro Manila.
Finally, one low-aerosol-loading case was impacted by
air masses traveling over the open ocean to the east of the
Philippines. This case showed an enhanced fraction of super-
micrometer aerosol and a very low concentration of water-
soluble organic acids. Higher rain accumulation during this
sample set, as opposed to the sample sets with the high-
est water-soluble aerosol concentrations, could have led to
greater wet scavenging of aerosol. This case also had the
lowest overall mass concentration of water-soluble organic
species, a low percent contribution of oxalate to the water-
soluble organics, and a high percent contribution of maleate.
This result points to the relative importance of locally emit-
ted species that have not yet undergone photochemical and
aqueous processing mechanisms that lead to the degradation
of longer-chain dicarboxylic acid species into oxalate.
These results have important implications for better under-
standing the aerosol budget in and around the Philippines and
SE Asia via the identification of various tracer species (e.g.,
K and Rb for biomass burning) and the impacts of differ-
ent long-range aerosol transport pathways. In addition, the
mixing of different air mass types, resulting in changes in
aerosol characteristics (e.g., enhanced oxalate in emissions
from continental regions, enhanced MSA during periods of
biomass burning influence), is a subject that requires more
attention on a global basis. While this work has shown the
influence of mixing biomass burning emissions and urban
emissions, from both local and more distant urban centers,
additional analysis at the study site has demonstrated the in-
fluences seen from the mixing of sea salt aerosol with other
air masses (AzadiAghdam et al., 2019). As remote-sensing
measurements in this region are notoriously difficult (e.g.,
Reid et al., 2009, 2013), in situ and model results lend vi-
tal data to address the questions surrounding characteristics
of aerosol that are transported into and out of this highly
populated region. Measurements from in situ airborne cam-
paigns, such as CAMP2Ex, can further address the changes
in aerosol physicochemical characteristics that occur during
long-range transport and aging in the atmosphere in the re-
gion.
Data availability. Ground-based size-resolved aerosol
data from the Manila Observatory can be found at:
https://doi.org/10.6084/m9.figshare.11861859 (Stahl et al.,
2020).
Atmos. Chem. Phys., 20, 2387–2405, 2020 www.atmos-chem-phys.net/20/2387/2020/
R. A. Braun et al.: Long-range aerosol transport 2399
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/acp-20-2387-2020-supplement.
Author contributions. MTC, MOC, JBS, RAB, ABM, CS, and AS
designed the experiments, and all coauthors carried out some aspect
of the data collection. MTC, RAB, CS, and AS conducted the data
analysis and interpretation. RAB and AS prepared the manuscript
with contributions from all coauthors.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. Rachel A. Braun acknowledges support from
the ARCS Foundation. Melliza Templonuevo Cruz acknowledges
support from the Philippine Department of Science and Technol-
ogy’s ASTHRD Program. Alexander B. MacDonald acknowledges
support from the Mexican National Council of Science and Tech-
nology (CONACYT). We acknowledge Agilent Technologies for
their support and Shane Snyder’s laboratories for ICP-QQQ data.
Financial support. This research has been supported by the NASA
(grant no. 80NSSC18K0148).
Review statement. This paper was edited by Joshua Fu and re-
viewed by three anonymous referees.
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