Stubborn aerosol: Why particulate mass concentrations do not drop during the wet season in Metro Manila, Philippines

Abstract

Wet scavenging is the most important sink for particulate matter (PM) and is expected to decrease PM concentrations in the wet season. However, Metro Manila, Philippines has highly similar PM mass across seasons despite large differences in seasonal rainfall. It is important to identify factors contributing to seasonally consistent PM mass as these may be present in similar developing megacities besides Metro Manila, leading to PM accumulation and posing significant health risks. We use size-resolved aerosol composition, aerosol optical depth, and meteorological data to reveal that the seasonally consistent PM mass in Metro Manila is due to (1) opposing seasonal cycles of black carbon and water-soluble PM, (2) inefficient scavenging by short rain events (<1 h), and (3) the high frequency (50%) of these short rain events. Water-soluble PM was most sensitive to scavenging within the 0.18–1.0 μm and 1.8–5.6 μm size ranges but more clearly for rain events lasting over an hour, pointing to the importance of rain duration for efficient scavenging. We demonstrate that the presence of rain does not imply wet scavenging is taking place efficiently and rain characteristics are critical to properly estimating wet scavenging. In a changing climate, our understanding of factors such as rain duration and aerosol accumulation will become more important for guiding air quality-related policymaking and ensuring sustainable growth in developing megacities.

Full text

Stubborn aerosol: why particulate mass
concentrations do not drop during the wet season
in Metro Manila, Philippines†
Miguel Ricardo A. Hilario,
a
Paola Angela Ba˜
naga,
bc
Grace Betito,
bc
Rachel A. Braun,‡
d
Maria Obiminda Cambaliza,
bc
Melliza Templonuevo Cruz,
b
Genevieve Rose Lorenzo,
a
Alexander B. MacDonald, §
d
Preciosa Corazon Pabroa,
e
James Bernard Simpas,
bc
Connor Stahl,
d
John Robin Yee
e
and Armin Sorooshian *
ad
Wet scavenging is the most important sink for particulate matter (PM) and is expected to decrease PM
concentrations in the wet season. However, Metro Manila, Philippines has highly similar PM mass across
seasons despite large differences in seasonal rainfall. It is important to identify factors contributing to
seasonally consistent PM mass as these may be present in similar developing megacities besides Metro
Manila, leading to PM accumulation and posing significant health risks. We use size-resolved aerosol
composition, aerosol optical depth, and meteorological data to reveal that the seasonally consistent PM
mass in Metro Manila is due to (1) opposing seasonal cycles of black carbon and water-soluble PM, (2)
inefficient scavenging by short rain events (<1 h), and (3) the high frequency (50%) of these short rain events.
Water-soluble PM was most sensitive to scavenging within the 0.18–1.0 mmand1.8–5.6 mm size ranges but
more clearly for rain events lasting over an hour, pointing to the importance of rain duration for efficient
scavenging. We demonstrate that the presence of rain does not imply wet scavenging is taking place
efficiently and rain characteristics are critical to properly estimating wet scavenging. In a changing climate,
our understanding of factors such as rain duration and aerosol accumulation will become more important
for guiding air quality-related policymaking and ensuring sustainable growth in developing megacities.
Environmental signicance
Wet scavenging is the most important sink for particulate matter (PM) and is expected to decrease PM concentrations during the high rainfall season. However,
Metro Manila, Philippines has highly similar PM mass across seasons despite large differences in seasonal rainfall. It is important to identify factors
contributing to seasonally consistent PM mass as these may be present in similar developing megacities besides Metro Manila, leading PM accumulationand
posing signicant health risks. This work uses a combination of size-resolved aerosol composition, meteorology, and aerosol optical depth over Metro Manila to
demonstrate that the presence of rain does not imply wet scavenging is taking place efficiently and rain characteristics are critical to properly estimating wet
scavenging. In a changing climate, our understanding of factors contributing to wet scavenging will become more important for guiding air quality-related
policymaking and ensuring sustainable growth in developing megacities. This study motivates further research into circumstances when wet scavenging
becomes inefficient, which is especially important over megacities where pollution accumulation poses serious health risks for millions of people.
1 Introduction
Wet scavenging is the dominant particle removal mechanism in
the atmosphere
1
and is an important process for global-scale
scavenging of anthropogenic pollutants.
2
However, the effect
of precipitation on particulate matter (PM) is dependent on
several factors. Previous work associated light rain amounts
(<1 h; <0.5 mm) with elevated PM and heavy rain amounts (>10
mm) with decreased PM.
3
While some studies have shown that
aerosol scavenging is sensitive to rain intensity,
4
others argue
that rain frequency is more important than intensity in modu-
lating aerosol removal.
5–7
Such ndings demonstrate the
a
Department of Hydrology and Atmospheric Sciences, University of Arizona, Tucson, AZ
85721, USA. E-mail: armin@arizona.edu
b
Manila Observatory, Quezon City 1108, Philippines
c
Department of Physics, School of Science and Engineering, Ateneo de Manila
University, Quezon City 1108, Philippines
d
Department of Chemical and Environmental Engineering, University of Arizona,
Tucson, AZ 85721, USA
e
Philippine Nuclear Research Institute –Department of Science and Technology,
Commonwealth Avenue, Diliman, Quezon City 1101, Philippines
†Electronic supplementary information (ESI) available. See
https://doi.org/10.1039/d2ea00073c
‡Now at: Healthy Urban Environments Initiative, Global Institute of
Sustainability and Innovation, Arizona State University, Tempe, AZ, USA.
§Now at: Department of Environmental Sciences, University of California,
Riverside, CA, 92521, USA.
Cite this: Environ. Sci.: Atmos., 2022, 2,
1428
Received 21st June 2022
Accepted 30th August 2022
DOI: 10.1039/d2ea00073c
rsc.li/esatmospheres
1428 |Environ. Sci.: Atmos., 2022, 2,1428–1437 © 2022 The Author(s). Published by the Royal Society of Chemistry
Environmental Science:
Atmospheres
PAPER

complexity of wet scavenging, as characteristics of light and
heavy rain remain challenging to capture for both global
models
8
and satellite retrievals.
9
Since precipitation is expected
to become more intense and less frequent due to climate
change,
10,11
it is important to understand how these changes
may affect aerosol scavenging, particularly over highly-
populated environments wherein inefficient scavenging may
lead to the accumulation of PM and exacerbate health risks
associated with poor air quality.
Metro Manila, Philippines is a rapidly developing megacity
lacking strict air quality monitoring and regulation even though
there are many PM sources including vehicular, industrial,
rework, and cooking emissions as well as secondary forma-
tion, based on previous source apportionment work.
12–19
Recent
size-resolved measurements focusing on source apportionment
showed that most of the PM mass in Metro Manila resides
within the submicrometer range.
14
Though PM levels (i.e., mass
concentrations) are expected to be lower in the wet season
(May–October) than the dry season (November–April) due to wet
scavenging by precipitation, Kim Oanh et al.
20
revealed that PM
levels in Metro Manila are remarkably consistent between wet
(44 mgm
3
) and dry (43 mgm
3
) seasons, later corroborated by
Simpas et al.
21
Potential contributors towards the seasonally consistent PM
include inefficient scavenging, seasonal emissions, and mete-
orological inuences. Although wet scavenging is expected to be
the dominant sink for PM,
2
a large fraction of rain in Metro
Manila is stratiform
22
and light rain is dominant throughout
northern Philippines,
23
which suggests that precipitation may
not always be an efficient removal mechanism for PM. Addi-
tionally, other meteorological variables may also inuence PM
levels
24
and counter the effect of wet scavenging on PM (e.g.,
hygroscopic growth facilitated by high relative humidity (RH)),
resulting in sustained PM levels even in the presence of rain.
Black carbon (BC) particles have been shown to catalyze sulfate
formation in both laboratory
25
and in situ studies
26
by serving as
a surface for SO
2
oxidation with NO
2
and NH
3
present at
moderate RH (30–70%). Thus, BC may also play a role in
sustaining PM concentrations throughout the year due to the
predominance of both BC and sulfate in Metro Manila.
14
Though this study focuses on Metro Manila, it is important
to note that Metro Manila may not be unique in its seasonally
consistent PM and that factors promoting such a consistency
may be present in other megacities lacking adequate air quality
monitoring, contributing to PM accumulation even in the
presence of rain and posing major health risks. Deteriorating
air quality, poor infrastructure, and worsening traffic are
problems faced by megacities globally
27
such as Lagos,
28
Malaysia,
29
Jakarta,
30
Beijing,
31
and Kolkata.
32
Seasonal meteo-
rology over several Asian megacities is inuenced by the Asian
monsoon.
33
For example, Chennai experiences a two-phase
seasonal cycle of wet and dry seasons, has relatively consistent
PM levels between seasons, and shows a clear weekday PM
enhancements
34
indicative of dominant urban emissions,
similar to Metro Manila.
35
It is important to identify factors
leading to seasonally consistent PM mass as these may be
present in similar developing megacities besides Metro Manila,
promoting PM accumulation and posing signicant health
risks. The analysis will serve to guide air quality-related poli-
cymaking and the formulation of appropriate mitigation
measures towards more sustainable development in growing
megacities.
Most past in situ studies investigating the relationship
between rain and PM focused on either chemically resolved
bulk PM
36,37
or size-resolved number concentrations.
38,39
In this
study, we consider both particle composition and size using
size-resolved aerosol composition data collected in Metro
Manila, allowing for a more comprehensive analysis of PM-rain
relationships. In order to explain the occurrence of seasonally
consistent PM levels, we focus on addressing the following: (1)
what particle sizes and species contribute to the sustained PM
during the wet season despite increased rain?; and (2) What
rain characteristics may decrease scavenging efficiency during
the wet season? To answer these science questions, we propose
the following hypotheses: (1) submicrometer particles will be
inefficiently scavenged by rain, which is largely light in intensity
over Metro Manila; and (2) more conducive conditions for
secondary formation will contribute to sustained PM during the
wet season.
2 Methods
2.1. PM sampling
Size-resolved aerosol composition was sampled in Metro
Manila, Philippines in support of the Cloud, Aerosol, and
Monsoon Processes Philippines Experiment
40
(CAMP
2
Ex)
weatHEr and CompoSition Monitoring (CHECSM)
campaign.
14,41
PM was measured using Micro-Orice Uniform
Deposit Impactors (MOUDIs) stationed at the Manila Observa-
tory (MO; 14.64N, 121.08E; 87 m above sea level) between July
2018 and October 2019. The MO site is well-established in the
literature as a well-mixed urban background site for Metro
Manila emissions.
13,14,20,21
Measurements of PM at the site are
representative of Metro Manila as it is located more than 100 m
away from the nearest road, ensuring ample time for mixing to
occur prior to sampling.
Complete details of the sampling methodology and analysis
are described elsewhere
42
with a brief summary provided here.
The MOUDI sampled PM over approximately 48 hour periods at
least once every week at the following aerodynamic cutpoint
diameters: 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.10, 0.056
mm. Aqueous extracts of collected substrates using ultrapure
water were subsequently analyzed for ions by ion chromatog-
raphy (IC; Thermo Scientic Dionex ICS-2100 system) and for
elements by triple quadrupole inductively coupled plasma mass
spectrometry (ICP-QQQ; Agilent 8800 Series). MOUDI species
concentrations have relative uncertainties of <20%.
42
The water-
soluble component of PM (hereaer PM-WS) was calculated as
the total mass concentration of speciated ions (nitrate, maleate,
magnesium, phthalate, sodium, calcium, pyruvate, succinate,
adipate, oxalate, MSA, TMA/DEA, ammonium, sulfate, chloride,
and DMA) and elements (Mo, Ni, Ti, Fe, Al, Cr, Tl, Zn, V, Cs, K,
Zr, Sr, Nb, Ba, Y, Cu, As, Rb, Mn, Se, Ag, Co, Sn, Pb, Cd, and Hf).
Black carbon (BC) was analyzed with a multi-wavelength
© 2022 The Author(s). Published by the Royal Society of Chemistry Environ. Sci.: Atmos., 2022, 2,1428–1437 | 1429
Paper Environmental Science: Atmospheres

absorption black carbon instrument (MABI; Australian Nuclear
Science and Technology Organization) at a wavelength of
870 nm for consistency with previous work.
14
Note that PM-WS
does not include BC due to the less frequent measurements of
BC. Gravimetric mass was measured by a Sartorius ME5-F
microbalance (sensitivity: 1mg). Full details of the measure-
ments are provided in the data descriptor.
42
A total of 54 MOUDI sets measured ionic and elemental
concentrations while 11 sets measured gravimetric mass and
BC. In this study, we dene the wet (dry) season as May–October
(November–April), corresponding roughly to the two phases of
the Asian monsoon as established in previous studies.
23,43–47
As
a result, we have 32 (22) MOUDI sets measuring PM-WS and 8
(3) MOUDI sets measuring gravimetric mass and BC for the wet
(dry) season. Note that 23 out of 32 MOUDI sets during the wet
season were inuenced by rain.
Although 65 MOUDI sets (11 of them with gravimetric and
BC data) may be seen as a limited sample size, the MOUDI
dataset is the rst time to our knowledge that size- and
chemically-resolved measurements of PM have been collected
in Metro Manila at such a frequency (at least once and up to
three times per week) and duration (16 months with coverage of
both monsoon seasons). Thus, ndings from this dataset are
a valuable contribution to the characterization and under-
standing of mechanisms that govern a relatively understudied
but rapidly developing region of the world.
2.2. Meteorological data
Meteorological data (2010–2020) were collected at 5 min reso-
lution by a Davis Vantage Pro2™Plus automatic weather
station (AWS) on the rooop of the Manila Observatory (90 m
above sea level). The data was quality-screened for meteoro-
logical values within acceptable ranges based on previous
criteria.
22
To compare the meteorological data and MOUDI sets
more directly, meteorological data was averaged between the
start and end times of each MOUDI set except for precipitation
which was accumulated per MOUDI set. Missing values such as
those from maintenance periods were replaced with measure-
ments from two nearby AWS stations (2 km and 5 km away). A
complete description of the syncing of AWS data to the MOUDI
sets is provided elsewhere.
35
Rain events are dened as consecutive timestamps with non-
zero rain amounts. For each rain event, we calculated rain
amount (total rain amount during each rain event), rate (mean
rain rate during each rain event), and duration (number of
hours). For the MOUDI analysis, these characteristics were
either summed or averaged per MOUDI set (e.g., if three rain
events occurred during a single MOUDI set (48 hours), their
respective rain amounts and duration were added while their
intensities were averaged).
2.3. AERONET data
To complement the long-term AWS data with a proxy of aerosol
loading, we used Level 2 aerosol optical depth (AOD; 500 nm)
data collected by the NASA AErosol RObotic NETwork (AERO-
NET)
48
photometer on the rooop of the Manila Observatory.
The AOD data has a reported uncertainty of <0.02.
49,50
In order to
quantify the response of AOD to rain event characteristics, we
calculated the percent difference between AOD averaged three
hours aer (i.e.,aer the end of the rain event until three hours
later) and three hours before each rain event, termed DAOD
(unit: %). If two rain events occurred with less than one hour
between them, we considered the two as a single rain event. We
use a percent difference in AOD in order to normalize for
differing AOD values between rain events and to isolate the
effect of wet scavenging by rainfall. The use of percent differ-
ences also accounts for the natural variability in sources over
long time periods. Varying the averaging window between two
and six hours reveals that the overall trends are robust to the
width of the AOD averaging window. There was a total of 51 rain
events across wet and dry seasons between 2010–2019 with
useable AERONET data before and aer the event (29 during the
wet season between 2012–2019). The strict ltering down to 51
rain events was necessary to reduce possibility of confounding
effects (e.g., immediately consecutive rain events) in order to
capture the full impact of wet scavenging on PM. We note that
although AERONET and MOUDI data span different timescales
(10 years and 16 months, respectively), both sources of data are
robust and reveal very similar results, providing further con-
dence in our conclusions.
2.4. Curve-tting
To provide a more quantitative description of the relationships
between aerosol variables (AOD and PM concentration) and
meteorology, we performed curve-tting based on a simple
exponential decay function y¼be
ax
where xis the meteoro-
logical variable (e.g., rain duration), yis the aerosol variable
(e.g., PM concentration), and aand bdepend on aerosol char-
acteristics. This can be linearized such that aand ln(b) are
interpretable as the slope and intercept, respectively. When
tting curves involving DAOD, an offset (c), dened as the
absolute value of the minimum DAOD, was added to all DAOD
values prior to curve-tting to account for the non-negative
constraints of exponential decay functions. Aer obtaining the
curve-t parameters (a,b), the offset was subtracted to revert
DAOD values back to their original range. To quantify the
goodness-of-t of the resulting curves, we use scatter index (SI)
which is the root mean squared error (RMSE) divided by the
mean DAOD (with offset) or PM concentrations and has been
used in previous work on error quantication.
51–53
3 Results & discussion
3.1. Consistent PM across wet and dry seasons
The seasonal consistency in PM observed by Kim Oanh et al.
20
motivates the question of what factors contribute to this
feature, with possibilities including inefficient wet scavenging
and enhanced secondary production during the wet season. The
average gravimetric mass is nearly identical across seasons for
both PM
1
and PM
10
size ranges (Fig. 1), corroborating previous
work in Metro Manila.
20,21
The consistent gravimetric mass is
explained by the opposing seasonal trends in PM-WS and BC
1430 |Environ. Sci.: Atmos., 2022, 2,1428–1437 © 2022 The Author(s). Published by the Royal Society of Chemistry
Environmental Science: Atmospheres Paper

(Fig. 1): while BC was higher during the dry season, PM-WS was
higher during the wet season. Combined, these two major
components of total PM result in seasonally consistent gravi-
metric mass. We assume gravimetric mass to be mainly
composed of BC and PM-WS with the unaccounted fraction
attributable to non-BC and non-water-soluble components of
PM that were not measurable by IC or ICP-QQQ. Since BC is
relatively inert, its main sink is wet scavenging.
54
While the
increased BC in the dry season is expected due to the lower
levels of rainfall and therefore precipitation scavenging, the
increased PM-WS in the wet season is initially counter-intuitive.
Since scavenging efficiency is composition-dependent
6,55
and
PM-WS in Metro Manila is mostly composed of sulfate and
ammonium,
14
PM-WS is expected to be susceptible to wet
scavenging and should have lower levels during the wet season.
This interesting seasonal feature is the focus of the analysis in
this study as the enhancement in PM-WS is enough to offset the
reduced BC concentrations in the wet season.
We considered if seasonal differences in PM in Metro Manila
could arise from emissions that exhibit seasonal cycles.
However, seasonal differences in meteorology may be seen as
the chief driver of seasonal trends in PM due to the following
reasons: (1) sampling was conducted within Metro Manila with
the main PM sources including vehicular emissions, industry,
cooking, sea salt, and waste processing,
12–19
which are consis-
tent throughout the year given the tropical urban setting; (2)
urban emissions are expected to be consistent throughout the
year as Metro Manila has a tropical climate with relatively stable
year-round temperatures (primarily between 25–30 C),
35
thus
emissions from heating or cooling remain consistent
throughout the year; and (3) although the Philippines does
receive long-range transport from East Asia and the Maritime
Continent,
13,35,40,56–58
Metro Manila itself is largely insulated by
mountains and ocean from these sources. Previous work has
shown that the long-range transport of PM to the Philippines is
naturally transient and PM levels return to normal values aer
episodes of long-range transport.
58
Since peaks in PM related to
transport are both occasional and transient, a seasonal PM
average measured within Metro Manila will mainly reect local
emissions, which are consistent throughout the year. This is in
contrast to cities such as Delhi, India which are situated close to
biomass burning areas and experience seasonal cycles in
emissions as a result.
59
Besides wet scavenging, we do not
discount the presence of other factors that may affect seasonal
PM averages. Although the MOUDI dataset is rich in both size-
and chemically-resolved information, the determination of
causation in the observed relationships in this study is a natural
limitation, and future work is encouraged to employ high-
resolution modeling to isolate contributions of different
factors to total PM.
Seasonal differences in the size distributions of gravimetric
mass (Fig. 2a) revealed that submicrometer concentrations were
slightly higher in the dry season, especially between 0.18–0.32
mm, while supermicrometer concentrations were slightly higher
during the wet season, notably between 1.8–5.6 mm. A wet
season enhancement is observed for gravimetric mass between
0.32–1.0 mm (Fig. 2a). Aged aerosol particles from secondary
processes have been identied as the most important source of
PM in this size range;
14
thus, gravimetric mass in this size range
may be enhanced due to more conducive meteorological
conditions for secondary aerosol processing in the wet season,
which have been shown to be a factor in Metro Manila (i.e.,
higher RH, lower boundary layer height).
35
A lack of signicant wet season enhancement in typical
supermicrometer aerosol tracers (e.g., sodium, calcium)
(Fig. S1d and f†) suggests that the supermicrometer peak in
gravimetric mass during the wet season (Fig. 2a) is largely
unaccounted for by the species measured in this study. Aer
considering contributions from PM-WS and BC, an average of
27.1% of PM
10
gravimetric mass remained unaccounted for,
reecting a similar result in Cruz et al.,
14
while an average of
only 6.8% was unaccounted for in the PM
1
range. This suggests
that there exists a supermicrometer mode component of total
gravimetric mass that cannot be fully explained by the MOUDI
data (i.e., water-soluble species and BC). One possible source for
the enhanced supermicrometer mode are particles that are
relatively water-insoluble (i.e., not detected by IC/ICP-QQQ
analyses) and do not contain BC (i.e., not detected by the
MABI). The investigation of this possibility and its source is le
to future work.
BC and PM-WS (Fig. 2b and c) experienced the greatest
seasonal changes in the submicrometer range. The heightened
PM
1
-WS concentrations during the wet season are well-
explained by similar wet season enhancements of PM
1
sulfate
Fig. 1 Seasonal medians of gravimetric mass, black carbon (BC), and
total water-soluble species (PM-WS) for PM
1
and PM
10
size ranges. Wet
(dry) season is defined as May to October (November to April). PM-WS
was measured for 32 (22) sets during the wet (dry) season; gravimetric
mass and BC were measured for 8 (3) MOUDI sets during the wet (dry)
season. Lower and upper ends of error bars represent the 25th and
75th percentile concentrations, respectively.
© 2022 The Author(s). Published by the Royal Society of Chemistry Environ. Sci.: Atmos., 2022, 2,1428–1437 | 1431
Paper Environmental Science: Atmospheres

and ammonium (Fig. S1a and b†). Higher PM
1
sulfate and
ammonium concentrations may point to meteorological
conditions during the wet season that are conducive for
secondary production (e.g., higher RH, lower boundary layer
height
35
) despite increased rain. Lower boundary layer heights
over Metro Manila in the wet season
35
may also increase surface
PM concentrations, which may also contribute to sustained wet
season PM-WS; however, the lack of a similar enhancement in
BC suggests boundary layer height may not be an important
factor for explaining the seasonal consistency in gravimetric
mass. Additionally, due to BC's high contribution to total PM in
Metro Manila,
14
BC-catalyzed sulfate formation may also serve
as an important contributor to sulfate concentrations.
25,26
In summary, total PM mass was consistent across seasons
due to opposing seasonal cycles of BC and PM-WS concentra-
tions. While BC is generally higher in the dry season, PM-WS
concentrations are higher in the wet season. The largest
seasonal enhancements in BC and PM-WS occur in the sub-
micrometer range, suggestive of the contributions of secondary
formation
14
due to conducive meteorology (i.e., high RH).
3.2. Relationships between PM and rain duration
Compared to rain amount (i.e., accumulated rain during event)
and rain intensity (i.e., mean rain rate during event) (Fig. S2†),
rain duration showed the clearest relationships with AOD and
PM
1
-WS (Fig. 3a and b), suggestive of wet scavenging, and is
thus the focus of this section. Rain events with duration less
than one hour are generally associated with heightened AOD
aer the event, perhaps due to RH-related effects on AOD,
60
while rain events longer than one hour tend towards lower AOD
aerwards (Fig. 3a). Similarly, MOUDI sets with mean rain
durations over one hour have notably lower concentrations of
PM
1
-WS (Fig. 3b), sulfate (Fig. S3a†), and ammonium
(Fig. S3b†). This suggests that one hour may serve as a rough
threshold for efficient wet scavenging.
To provide long-term context for rain in the sampling area,
a multi-year characterization of rain duration during the wet
season (2010–2020) revealed that 50% of rain events last less
than one hour (Fig. 3c). Furthermore, rain has been shown to be
mostly light in intensity over Metro Manila
22
and over the island
of Luzon (northern Philippines) where Metro Manila is located
(0–2.5 mm h
1
).
23
Considering the trends in Fig. 3a and b, the
predominance of light and short rain events and their ineffi-
cient scavenging may be partly why PM-WS is higher in the wet
season than the dry season. This is supported by Sun et al.
3
who
showed that light rain (<1 h and <0.5 mm) did not scavenge PM
efficiently. In addition to inefficient wet scavenging, we also
note that elevated RH (Fig. S4†) and lower boundary layer
heights
35
during the wet season in Metro Manila may also
counteract scavenging by creating conducive conditions for
secondary production of sulfate and ammonium. Besides
sulfate and ammonium, secondary production is an important
source of several species such as organic acids;
61
however, PM
1
-
WS in Manila is composed largely of sulfate (58% of PM
1
-WS
mass, on average) and ammonium (28%), which are much more
abundant than other secondarily produced species such as
organic acids (2%). Note that we focus on PM
1
because most of
the PM mass resides within this range in Metro Manila based on
previous source apportionment work.
14
Stratifying Fig. 3a by rain rate reveals specic rain charac-
teristics under which AOD can increase or decrease (Fig. 4).
Note that in Fig. 4 we included data from the dry season to
increase the number of data points for stratication; however,
the inclusion of dry season data did not change the overall
trends. We use percent differences in AOD rather than absolute
differences in AOD to isolate the impact of each rain event on
AOD while accounting for the confounding factor of pre-rain
AOD levels which will naturally vary between rain events. Note
that the use of percent differences also accounts for possible
variability in sources over different time periods.
Fig. 2 Size-resolved differences in seasonal median concentrations (dry minus wet) of (a) gravimetric mass, (b) black carbon (BC), and (c) total
water-soluble species (PM-WS). A positive (negative) value indicates higher dry (wet) season concentrations. Lower (upper) ends of error bars
represent the differences in 25th (75th) percentile concentrations between seasons.
1432 |Environ. Sci.: Atmos., 2022, 2,1428–1437 © 2022 The Author(s). Published by the Royal Society of Chemistry
Environmental Science: Atmospheres Paper

At low rain rates (Fig. 4a), shorter rain durations were asso-
ciated with highly variable effects on AOD, with most points
falling within the range of 50% and +50%. Longer rain dura-
tions exceeding 30 minutes were associated with more consis-
tent decreases in post-rain AOD even if rain rate was low. At
moderate rain rates (Fig. 4b), AOD tends to increase slightly
post-rain, perhaps due to the enhanced moisture provided by
moderate rain leading to higher AOD.
60
When rain durations
exceed one hour, this enhancement in AOD is less pronounced.
At high rain rates (Fig. 4c), we observe the clearest dependence
of AOD on rain duration. For longer (shorter) rain events, a large
decrease (increase) in AOD is observed post-rain. We hypothe-
size that this is due to the initial supply of moisture by shorter
rain events that enhances AOD post-rain, but AOD is reduced
aer longer rain events wherein aerosol scavenging is more
efficient. The enhancement of AOD aer short rain events may
be due to a combination of the following reasons: (1) intense
rain supplies the necessary moisture for hygroscopic growth,
60
(2) intense rain causes a disturbance at the surface and
a resuspension of aerosol,
62
and (3) shorter rain events may not
scavenge aerosol particles efficiently. The overall trends in AOD
reect the ndings from the MOUDI data (Fig. 3b), which
further suggests more efficient aerosol removal occurs with
longer rain events.
As precipitation is expected to become more intense and less
frequent due to climate change,
10,11
it is possible that rain events
Fig. 3 Scatterplots of the (a) percent difference in AOD before and after rain events (DAOD; %; 2012–2019). (b) MOUDI total water-soluble
species (PM
1
-WS; mgm
3
; 2018–2019) as a function of rain duration (hours). (c) Histogram of rain duration (2010–2020). Rain duration in (b)
refers to the mean duration of all rain events during each MOUDI set with error bars along the x-axis representing one standard deviation. Note
that points in (b) without error bars consist of single rain events. The black dashed vertical lines in (a–c) mark the median rain duration based on
(c). In (c), the red lines represent the 25th–75th percentile range (interquartile range, IQR). Number of data points per panel (N) is provided. Only
data from the wet season were plotted (May–October). Curve-fitting for (a and b) is described in Section 2.4.
Fig. 4 Scatterplots of the percent difference in AOD post-rain event (DAOD; %) as a function of rain duration (hours) and colored by season. Data
are stratified by rain rate (R;mmh
1
): (a) 0 < R< 2, (b) 2 < R< 8, (c) 8 < R< 30. Wet (dry) season is defined as May to October (November to April).
Number of data points per panel (N) is provided. The black dashed vertical lines mark the median rain duration based on Fig. 3c. Curve-fitting was
performed as in Fig. 3a (see Section 2.4).
© 2022 The Author(s). Published by the Royal Society of Chemistry Environ. Sci.: Atmos., 2022, 2,1428–1437 | 1433
Paper Environmental Science: Atmospheres

will shorten in duration, which may reduce wet scavenging
efficiency and contribute to PM accumulation in other mega-
cities besides Metro Manila. We do note that the relationship
between precipitation intensity–frequency–duration is
complex,
7,63
pointing to the importance of understanding
aerosol-rain relationships in a warming world.
5
In summary, rain events longer than one hour were associ-
ated with decreases in PM
1
-WS and AOD. AOD tended to
increase (decrease) aer rain events shorter (longer) than one
hour in duration, a trend that was more apparent at higher rain
rates (>2 mm h
1
). A long-term characterization of rain duration
revealed that during the wet season 50% of rain events last less
than one hour. This nding, in addition to the insensitivity of
PM-WS to shorter rain events, implies that inefficient scav-
enging occurs approximately half the time in Metro Manila and
explains how total PM mass can remain elevated during the wet
season.
3.3. Relationships of size-resolved PM and meteorology
In order to explore how aerosol size distributions change as
a function of rain duration, we analyze size-resolved aerosol
composition grouped by rain duration. Note that adjustments to
the rain duration categories in Fig. 5 and S5†resulted in the
same general conclusions. For rain events lasting less than one
hour, PM-WS is bimodal (Fig. 5a), peaking at 0.56–1.0 mmand
1.8–3.2 mm. Rain events longer than one hourare associated with
lower PM-WS concentrations (Fig. 5a) particularly between 0.18–
1.0 mm and 1.8–5.6 mm, suggestive of more scavenging on an
absolute mass basis within these size ranges by long rain events.
The PM-WS supermicrometer mode peak experienced a relatively
large decrease (50%), likely due to the high hygroscopicity of
species that compose most of the speciated supermicrometer
mode mass (i.e., nitrate, sodium, calcium, chloride; Fig. S5†).
Based on previous source apportionment work,
14
sodium and
chloride (calcium) in Metro Manila mostly originate from sea salt
(dust) while nitrate was sourced from HNO
3
partitioning onto sea
salt particles.
64
Dust
65
and sea salt
66,67
are efficiently removed by
wet scavenging, which explains the large concentration drop in
response to higher duration precipitation.
Both sulfate (Fig. 5b) and ammonium (Fig. 5c) tend to decrease
when rain duration exceeds one hour similar to PM-WS; however,
ammonium experienced a relatively larger decrease within the
0.32–0.56 mm range wherein concentrations dropped as much as
30% (1mgm
3
) in response to longer rain durations. This is
explainable by the higher scavenging efficiency of ammonium
compared to sulfate.
68,69
We emphasize that, although most of the
PM-WS mass (Fig. 5a) is composed of hygroscopic species (i.e.,
sulfate, ammonium), PM-WS is largely unresponsive to rain
events shorter than one hour (i.e., the division of samples into
more rain duration groups (not shown) reects the patterns
shown in Fig. 5). This result supports our previous analysis
(Section 3.2) where we demonstrated that rain durations shorter
than one hour tend to be inefficient at aerosol scavenging.
4 Summary and conclusions
As wet scavenging is the main aerosol sink, its characterization
is important for modeling aerosol lifecycle and guiding air
quality-related policymaking in megacities that may be
susceptible to pollution accumulation despite the presence of
rain. It is generally expected that PM mass concentrations
during the wet season will be lower than those during the dry
season. However, previous work observed seasonally consistent
PM in Metro Manila, Philippines. Using a rich dataset of size-
resolved aerosol composition, meteorology, and AOD retrieved
by AERONET, the following major conclusions are reached:
(1) Consistent PM concentrations across wet and dry seasons
are due to opposite-phase seasonal cycles of BC and PM-WS
concentrations. While BC is generally higher in the dry
season, PM-WS concentrations are higher in the wet season.
The largest seasonal enhancements in BC and PM-WS occur in
the submicrometer range, pointing to the role of anthropogenic
emissions and secondary formation. Heightened PM-WS
concentrations during the wet season are largely due to sub-
micrometer sulfate and ammonium. Wet season enhancements
are at least partially attributable to favorable conditions (e.g.,
high RH) for secondary formation and hygroscopic growth.
(2) The presence of rain does not imply wet scavenging is
taking place efficiently. The characteristics of rain are critical to
estimating wet scavenging. Rain events longer than one hour
were associated with decreases in PM
1
-WS and AOD. AOD
Fig. 5 Size distributions of median (a) total water-soluble species (PM-
WS), (b) sulfate, and (c) ammonium grouped by mean rain duration
during each MOUDI set. Only data from the wet season were plotted
(May–Oct). Note that y-axis limits are not the same across panels. The
number of MOUDI sets per grouping is provided in parentheses in the
legend. Lower and upper ends of error bars represent the 25th and
75th percentile concentrations, respectively.
1434 |Environ. Sci.: Atmos., 2022, 2,1428–1437 © 2022 The Author(s). Published by the Royal Society of Chemistry
Environmental Science: Atmospheres Paper

tended to increase (decrease) aer rain events shorter (longer)
than one hour in duration, a trend that was more apparent at
high rain rates. A long-term characterization of rain duration
revealed that 50% of rain events during the wet season last less
than one hour. This nding, in addition to the insensitivity of
PM-WS to shorter rain events, implies that inefficient scav-
enging may occur approximately half the time in Metro Manila
and explains how gravimetric mass remains elevated during the
wet season.
(3) Size-resolved composition grouped by rain duration
revealed the sensitivity of major PM-WS components (sulfate
and ammonium) to rain events longer than one hour. On an
absolute mass basis, the size ranges 0.18–1.0 mm and 1.8–5.6
mm were the most sensitive to wet scavenging via longer rain
events. Notably, ammonium experienced a large decrease (up to
30%) in response to rain events exceeding one hour. Though
most PM-WS mass resides in the submicrometer range, the
clear decrease in PM-WS concentrations in response to longer
rain events indicates the importance of composition in
explaining wet scavenging trends.
With the expectation of more intense, less frequent precip-
itation due to climate change,
10,11
changes in rain duration with
increasing temperatures may result in inefficient wet scav-
enging over megacities aside from Metro Manila and contribute
to the accumulation of PM and degradation of air quality. Thus,
further work is needed to better understand factors leading to
inefficient aerosol scavenging as well as the potential inuence
of these factors in other parts of the world. Future work is
encouraged to: (1) examine size-resolved composition data at
higher temporal resolution to further characterize PM
responses to rain, (2) investigate the inuence of Metro Manila's
large BC contributions (26.9% of total mass
14
) on aerosol
hygroscopicity, scavenging efficiency, and sulfate formation, (3)
analyze speciated size distributions in response to different
raindrop size distributions, (4) compare aerosol & meteorolog-
ical data in other megacities to assess the prevalence of ineffi-
cient wet scavenging and to build more statistics on these
relationships, and (5) high-resolution modeling to further
explore causation behind the observed relationships between
PM and meteorology in this study.
Author contributions
Formal analysis, visualization, writing –original dra: MRAH.
Writing –review & editing: PAB, GB, RAB, MOC, MTC, GRL,
ABM, PCP, JBS, CS, JRY, AS.
Data curation: PAB, GB, RAB, MOC, MTC, GRL, ABM, PCP,
JBS, CS, JRY, AS.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by NASA grant 80NSSC18K0148 as part
of CAMP
2
Ex. A. B. MacDonald acknowledges support from the
Mexican National Council for Science and Technology (CON-
ACYT). M. T. Cruz acknowledges support from the Philippine
Department of Science and Technology's ASTHRD Program. R.
A. Braun acknowledges support from the ARCS Foundation. We
gratefully acknowledge Agilent Technologies and Shane Snyd-
er's laboratories for ICP-QQQ analysis. The MOUDI dataset is
accessible at https://doi.org/10.6084/m9.gshare.11861859.v2.
AERONET data may be accessed at: https://
aeronet.gsfc.nasa.gov/.
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