Extensive measurements of cloud condensation nuclei (CCN)
and condensation nuclei (CN) concentrations in the South Asian outflow to
the northern Indian Ocean were carried out on board an instrumented research
vessel, as part of the Integrated Campaign for Aerosols, gases and Radiation
Budget (ICARB) during the winter season (January–February 2018).
Measurements include a north–south transect across the South Asian plume
over the northern Indian Ocean and an east–west transect over the equatorial
Indian Ocean (∼2∘ S), which is far away from the
continental sources. South Asian outflow over the northern Indian Ocean is
characterized by the high values of CCN number concentration
(∼5000 cm-3), low CCN activation efficiency
(∼25 %) and a steep increase in CCN concentration with the
increase in supersaturation. In contrast, low CCN concentration
(∼1000 cm-3) with flat supersaturation spectra was found
over the equatorial Indian Ocean. The CCN properties exhibited significant
dependence on the geometric mean diameter (GMD) of the aerosol number size
distribution, and CCN activation efficiency decreased to low values
(<20 %) at the time of new-particle formation events over
near-coastal and remote oceanic regions. The analysis of the activation
efficiencies for the “similar” aerosol size distributions over the northern
Indian Ocean indicated the primary role of aerosol number size distribution
on CCN activation efficiency. The dependence of CCN properties and
activation efficiency on size-segregated aerosol number concentration,
especially during the ultrafine (<100 nm) particle events, is
investigated in detail for the first time over the region.
Introduction
Aerosol–climate interaction is one of the major uncertain components of the
Earth–atmosphere system, which includes several pathways like
aerosol–radiation (scattering and absorption of solar radiation),
aerosol–cloud (modification of cloud properties due to aerosols),
aerosol–cryosphere (snow albedo reduction due to aerosol deposition) and
aerosol-biosphere interactions having significant radiative forcing (IPCC,
2013; Rosenfeld et al., 2014; Li et al., 2016). IPCC (2013) has estimated
the global mean radiative forcing due to aerosol–radiation and aerosol–cloud
interaction to be -0.9 (-1.9 to -0.1) W m-2, which compensates for nearly
30 % of the warming due to well-mixed greenhouse gases. The fundamental
parameter relevant for understanding the aerosol–cloud interaction is the
cloud condensation nuclei (CCN), which are those aerosols that become activated at supersaturations pertinent to atmospheric conditions (Rosenfeld et al.,
2014). Hence, the large uncertainty in the estimates of aerosol–cloud
interaction points to the necessity of dedicated field campaigns and
modelling efforts to improve the level of scientific understanding on CCN
activation and to accurately quantify the change in microphysical properties
of clouds due to anthropogenic aerosols (Rosenfeld et al., 2014). Regional
meteorology also plays a major role in the aerosol–cloud interaction and the
effect of aerosols on clouds varies with meteorological regimes (Reutter et
al., 2009; Kerminen et al., 2012; Schmale et al., 2018). Since the oceans
cover about 70 % of the Earth surface and the low-level marine clouds
(stratus and stratocumulus) are highly sensitive to aerosol perturbations
(Rosenfeld et al., 2014), understanding the CCN properties and their
dependence on the physicochemical properties of aerosols is crucial to understanding the aerosol–cloud interactions over the oceanic regions lying
downwind of continental outflow (Furutani et al., 2008; Kim et al.,
2014; Snider and Brenguier, 2000).
The south Asian region, especially northern India, experiences high aerosol
loading during the winter season (Nair et al., 2007; Bharali et al., 2019).
These aerosols are mostly confined to within the atmospheric boundary layer and
have significant implications for air quality, visibility, human health and
the radiation budget (Lelieveld et al., 2001; Bharali et al., 2019). Due to the
favourable prevailing wind system, these continental aerosols are being
transported over the northern Indian Ocean (Arabian Sea and Bay of Bengal),
and the effects of these anthropogenic aerosols on regional climate have been
the major scientific theme for the several field campaigns and modelling
studies carried out during the last 2 decades (Moorthy et al., 2009;
Ramanathan et al., 2001; Lelieveld et al., 2001; Ackerman et al., 2000; Nair
et al., 2013). South Asian outflow characterized during the Indian Ocean
Experiment (INDOEX) revealed that high concentrations of black carbon (BC) and
organic carbon (OC) aerosols are emitted from biomass and fossil fuel burning
over South Asia and get transported to the Indian Ocean during winter
(Mayol-Bracero et al., 2002). Several studies have shown that the transport
of anthropogenic aerosols, especially carbonaceous aerosols, to the marine
atmosphere perturbs the regional radiation balance through aerosol–radiation
(Ramanathan et al., 2001; Moorthy et al., 2009) and aerosol–cloud
interactions (Ackerman et al., 2000; Chylek et al., 2006; Hudson and Yum,
2002). The INDOEX observations indicated that CCN characteristics of the
northern Indian Ocean are impacted by the outflow of South Asian aerosols as
seen from the widespread nature of the high CCN number concentrations over
the region (Cantrell et al., 2000, 2001). Following INDOEX, even though
there were several field experiments like ICARB-2006, ARMEX (Arabian Sea
Monsoon Experiment) and ICARB-2009 to characterize the aerosol properties,
simultaneous measurements of the CCN and aerosol properties have not been attempted in the South Asian outflow region for the last 2 decades.
The CCN properties exhibit considerable spatial and temporal heterogeneities
similar to the extrinsic properties of aerosols (Andreae, 2009;
Jefferson, 2010). Hence, aerosol-CCN studies for distinct aerosol types (like
polluted, marine, biogenic and dust) assume importance (Schmale et al.,
2018). Even though several studies have addressed the activation properties
of aerosols using extensive measurements over distinct aerosol environments
(Furutani et al., 2008; Rose et al., 2011; Kerminen et al., 2012;
Pöhlker et al., 2016; Schmale et al., 2018), studies are limited over
South Asia and its outflow regions. Size-segregated measurements of CCN
concentration along with physical and chemical properties of aerosols are
essential to unravel the complex dependence of size and chemistry of
aerosols on CCN activation (Dusek et al., 2006a; Pöhlker et al., 2016;
Rose et al., 2011; Furutani et al., 2008; Hudson, 2007), which is essential
for the better prediction of CCN concentration, and thus to reduce climate
forcing uncertainty due to aerosol–cloud interactions. Studies on the relative
importance of aerosol size distribution and chemical composition (mixing
state) on the CCN properties are rather limited over the Indian
subcontinent, except for a few studies (Jayachandran et al., 2018, references
are therein). At a microscopic level, a fraction of the total aerosols become activated as CCN, which depends primarily on the size, followed by chemical
composition and mixing state of the aerosol system (Jimenez et al., 2009;
Dusek et al., 2006a; Kerminen et al., 2012; Rose et al., 2011). There exists
a substantial uncertainty in the activation properties of particles,
especially the contribution of ultrafine particles (size below 100 nm) and
new-particle formation events to the global CCN concentration (Pierce and
Adams, 2007; Merikanto et al., 2009). Further, the role of higher levels of
ultrafine aerosols in CCN number concentration and activation efficiency is
not investigated over the northern Indian Ocean, especially when the
continental outflow dominates the marine aerosol system.
In this study, we present the results from the dedicated shipborne
measurements onboard the oceanographic research vessel (ORV) Sagar Kanya (SK) during winter 2018 (hereafter ICARB-2018) carried out as part of
Integrated Campaign for Aerosols gases and Radiation Budget (ICARB)
experiment with a broad objective to characterize the South Asian outflow.
The measurements were carried out within the continental outflow and remote
oceanic regions far away from South Asia. The present study focusses on
wintertime measurements of the CCN concentrations at different
supersaturations, along with simultaneous measurements of the aerosol
properties when the entire northern Indian Ocean was under the influence of
continental outflow from South Asia. The northern Indian Ocean is an ideal
and unique region to study the role of anthropogenic aerosols in CCN
activation, where aged continental plume having a high concentration of
carbonaceous aerosols and volatile vapours mix with marine aerosols during
the northern hemispheric winter (Mayol-Bracero et al., 2002; Nair et al.,
2007, 2013). This paper discusses the general characteristics of the CCN
concentrations, activation properties and their association with aerosol
number size distribution at different regions of the South Asian outflow.
The relative role of aerosol number size distribution on the variability of
CCN activation and the contribution of ultrafine particles to the CCN
concentration will be discussed in detail.
Campaign, instruments and general meteorology
During the winter months, the prevailing large-scale circulation over South
Asia is favourable for the transport of anthropogenic aerosols to the
Arabian Sea and Bay of Bengal (Ramanathan et al., 2001; Nair et al., 2007;
Moorthy et al., 2009). The spatial extent of the aerosol transport to the
Indian Ocean is qualitatively depicted by the climatological (2002–2017)
mean aerosol optical depth (AOD) derived from MODIS (Moderate Resolution
Imaging Spectroradiometer) observations over the northern Indian Ocean
(contours in Fig. 1). High aerosol loading (AOD >0.3) is
observed over the northern Bay of Bengal and the southeastern Arabian Sea
during winter. The ICARB-2018 measurements were planned to be carried out within
this plume area over the southeastern Arabian Sea and the regions far away
from the continental outflow. The ship cruise on board ORV Sagar Kanya
started from Goa (15∘ N, 73.8∘ E) on 16 January 2018 and ended at Tuticorin (8.8∘ N, 78.2∘ E) on 13 February 2018. The cruise track is shown in Fig. 1. The ICARB-2018 cruise
experiment mainly has three phases: (i) latitudinal (16–22 January 2018),
(ii) longitudinal (23–31 January 2019) and (iii) return (1–13 February 2019)
phases. Aerosol properties reported during each of the measurement phases
have a strong association with large-scale meteorology, as reported in the
earlier field experiments (Lelieveld et al., 2001; Ramanathan et al., 2001;
Nair et al., 2013). Depending on the origin of air masses, measurements made
during ICARB-2018 are classified into three different groups (Fig. 1). The first
phase of the cruise over the southeastern Arabian Sea (SEAS) is divided into
“SEAS1” and “SEAS2” regions where the former is influenced by the air masses
from peninsular India and the latter by air masses from the Bay of Bengal. The SEAS1
region has proximity to coastal India, while SEAS2 is relatively far away from
the mainland. The second phase of the cruise over the remote equatorial
Indian Ocean (EIO), where the continental influence is rather less compared
to SEAS1 and SEAS2, is regarded as the third group. Mostly calm and clear-sky conditions prevailed during the campaign except for a few rainfall spells
on 4, 6 and 7 February 2018 when the ship was sailing over the
southeastern Arabian Sea (4∘ N, 67.2∘ E). These
widespread rainfall events associated with the western disturbances are also
observed over the peninsular and western part of the Indian subcontinent
during this period. Due to these weather events, we have not considered the
data collected during the return phase (phase 3) in this study. The mean
values of air temperature and relative humidity during ICARB-2018 from the
automatic weather station observations were 28.0±0.8∘C
and 74±5 % respectively.
The cruise track of ICARB 2018 over the northern Indian Ocean. The
colour of the cruise track indicates the CCN number concentration
(cm-3) at 0.2 % supersaturation (SS). CCN characteristics at three
distinct regions marked as SEAS1, SEAS2 and EIO are discussed in this study.
Noontime ship location is marked by dates suffixed with J and F, where “J”
for January and “F” for February. Grey coloured contours indicate the
climatological mean values of aerosol optical depth at 550 nm derived from
the MODIS satellite for the winter season. Typical air mass back trajectories
arriving at the ship location on 18, 22 and 30 January 2018 representing
the SEAS1 (red colour), SEAS2 (green colour) and EIO (blue colour) region
respectively, estimated using HYSPLIT model are shown as solid lines.
Aerosol instruments were installed in a customized laboratory on the top
deck (∼15 m above sea level) of the ship. The instruments
aspirated ambient air from a manifold sampling inlet having a 10 µm
size cut-off at a flow rate of 16.6 L min-1. The flow rate was
frequently monitored and maintained using a flow controller and an external
pump. To avoid the contamination of the ship's exhaust with the aerosol
measurements, the bow of the ship was mostly aligned with the upwind
direction, and those measurements when the wind was blowing from the rear side
of the ship were excluded. All the measurements were averaged hourly for
uniformity, after applying the necessary quality checks and instrument-specific
data correction procedures, and then geolocated using the time-stamped
position information available from the GPS receiver installed onboard. The
simultaneous measurements of CCN number concentration using a CCN counter and
aerosol number size distribution from a scanning mobility particle sizer
(SMPS) form the primary dataset for this study.
CCN number concentration measurements at different supersaturations ranging
from 0.2 % to 1.0 % were carried out using a single-column continuous-flow
CCN counter (model: CCN-100; make: DMT) at a time resolution of 1 Hz. The
difference in the radial diffusion rate of heat and water vapour is used
to develop specific supersaturations along the centre line of the instrument
column, depending on the sheath-to-sample flow rate and the temperature
gradient along the column (Roberts and Nenes, 2005). The aerosols are
introduced into the centre line of the filtered, humidified sheath flow. The
aerosols of a size greater than the critical diameter at the set
supersaturation will grow as CCN, which are counted further by an optical
particle counter with a diode laser source at 660 nm wavelength. In the
present study, a constant flow rate of 0.5 L min-1 and a steady sheath to sample the flow ratio of 10:1 was maintained throughout the campaign. The duration of each
cycle of CCN measurements (0.2 % to 1.0 % supersaturation) spanned 30 min (Jayachandran et al., 2017, 2018). Considering the instability
inside the column during the supersaturation changes, more time is allocated
to the lowest supersaturation (0.2 %) and the first 2 min of data of each
supersaturation are excluded from the analysis.
Aerosol number size distribution measurements were carried out using a
scanning mobility particle sizer (make: TSI), which measured the size-segregated number concentration of particles from ∼9 to
420 nm. The SMPS consists of a differential mobility analyser (DMA, model: TSI 3081) and a water-based condensation particle counter (CPC, model: TSI 3786). The
particles segregated according to their electrical mobility by the DMA are
allowed to grow in the condensation chamber of the CPC to the optically
detectable size range. These particles are counted by using an optical
particle counter (Wang and Flagan, 1990). All the instruments were
calibrated prior to the campaign following the standard protocols.
Results and discussionCCN number concentrations
The number concentration of CCN at 0.2 % supersaturation, which represents
the concentration of aerosols that are hygroscopic and sufficiently large
(size >100 nm) to become activated at low (0.2 %)
supersaturations, is shown along the cruise track in Fig. 1. High
concentrations of CCN (∼2000 cm-3) are observed in the
plume area (SEAS1), and lower CCN concentrations (<500 cm-3)
are seen over the southwest part of the cruise track (∼2∘ S, 65 to 72∘ E). The low values observed during the
return phase of the cruise are attributed to the effective wet scavenging of
aerosols due to thunderstorms experienced on 4, 6 and 7 February 2018. The
SEAS2 region, which is mostly affected by the advection from the Bay of
Bengal region (lying downwind of the Indo-Gangetic Plain; Fig. 1), also
showed high CCN concentrations of up to 2500 cm-3 indicating the
widespread influence of South Asian outflow. The latitudinal gradient of CCN
at different supersaturations from 15∘ N to the Equator, between
the longitudes 74 to 75∘ E, shows distinctly
different patterns for the CCN concentrations at low (0.2 %) and high
(1.0 %) supersaturations as shown in Fig. 2. At 0.2 %
supersaturation, CCN values observed over SEAS1 (1683±435 cm-3)
are lower or comparable to that over SEAS2 (1868±276 cm-3).
This is contrary to the CCN values at 1.0 % supersaturation, where CCN
values are 2-fold higher over SEAS1 (5954±1493 cm-3) compared
to SEAS2 (2513±668 cm-3). The systematic decrease in the CCN
number concentration with latitude is clearly seen below the southern tip
(<8∘ N) of peninsular India, whereas measurements above
8∘ N are modulated by the heterogeneous sources located along the
west coast of India. The CCN concentration at 1.0 % supersaturation was 5-
to 7-fold higher than that at 0.2 % supersaturation over SEAS1, whereas
the rate of change in CCN concentration with supersaturation is
insignificant over SEAS2.
Latitudinal variation in the CCN number concentration at
0.2 %, 0.4 % and 1.0 % supersaturations (SS) over the southeastern Arabian Sea
(SEAS). The vertical bars on each data point indicate the standard deviation
of the measurements. The SEAS region is further divided into SEAS1 and SEAS2
based on the origin of air mass back trajectories and proximity to the
mainland.
Mean CCN number concentration with the standard deviation
at different supersaturations over the northern Indian Ocean (SEAS1, SEAS2
and EIO) during ICARB-2018. The CCN spectra measured at a coastal station,
Thumba, are also shown in the figure for comparison. The graph is shown on a
log–log scale.
The regional variation in the mean CCN concentration with respect to
supersaturation (commonly called CCN spectra) over SEAS1, SEAS2 and EIO is
shown in Fig. 3. The mean CCN spectra observed during December 2017 at
Thumba (8.5∘ N, 77∘ E; Jayachandran et al., 2018), which
is located on the west coast of peninsular India (close to SEAS1) and
experienced the same synoptic conditions as that during the ICARB-2018
campaign, are also shown in the figure for comparison. In general, the CCN
concentration will be higher or remain constant as supersaturation
increases. As the set supersaturation inside the instrument increases, more
and more particles will become activated since the supersaturation inside the
instrument column is higher than the minimum supersaturation (called
critical supersaturation) required for the particles to become activated. Hence
CCN at higher supersaturation are cumulative, and all particles with
critical supersaturation lower than the set supersaturation are activated as
CCN. This variation in CCN with supersaturation is parameterized using the
Twomey empirical relationship, NCCN=CSSk, where SS is
supersaturation and k is the Twomey exponent, which is also mentioned in the
figure. Twomey's exponent (k) indicates the qualitative information on the
CCN active aerosol size distribution, with high k values implying the
dominance of ultrafine-mode aerosols and low k values indicating the fine-mode (>100 nm) aerosol dominance in the measured number size
distribution (Fang et al., 2016; Jayachandran et al., 2017; Gunthe et al.,
2009). In this study, k values are estimated in the supersaturation range
from 0.2 % to 1.0 %. As shown in Fig. 2, the CCN concentration
measured in the South Asian outflow is very sensitive to the supersaturation
over SEAS1 (coastal regions adjoining India), whereas CCN concentration is
less dependent on supersaturation over the regions far away from the
continental sources (SEAS2 and EIO). The higher (0.83±0.22) value of
Twomey's exponent (k) over SEAS1 (steep CCN spectra) is attributed to the
dominance of ultrafine-mode aerosols (or somewhat hydrophobic aerosols) in
the continental outflow. The lower k values of 0.21±0.19 and 0.13±0.11 estimated for SEAS2 and EIO respectively suggested larger
particle dominance due to the extended transit of aerosols over the oceanic
regions leading to enhanced sizes.
It would be interesting to compare the present values of CCN with the
previous observations, which are rather scarcely reported over the oceanic
regions surrounding South Asia. Based on the shipborne measurements during
INDOEX, Cantrell et al. (2000) reported the CCN concentration of 1000 to
2500 cm-3 (at 0.5 % supersaturation) over 10 to 15∘ N. Similarly, airborne measurements on board a research flight
(NCAR C-130) during INDOEX reported the values of CCN at 1.0 %
supersaturation as ∼1190 cm-3 for polluted air masses and
less than 176 cm-3 for clean marine conditions over the southern Indian
Ocean (Hudson and Yum, 2002). The CCN values over the SEAS1 region during
ICARB-2018 are (i) higher than the CCN values reported during INDOEX, (ii) comparable to the values at the coastal site (Thumba) in southern peninsular
India and (iii) less than the values reported from polluted continental
locations in the Indo-Gangetic Plain (Jayachandran et al., 2020). The CCN
values observed over SEAS are mostly lower or comparable to the values over
polluted continental sites (2900±2800 cm-3 at 0.4 %
supersaturation) as reported by Andreae (2009) based on the compilation of
CCN observations carried out worldwide. Interestingly, the CCN values over
EIO (which is the relatively remote oceanic region) during ICARB-2018 are
almost 7-fold higher than the clean marine CCN values (107±56 cm-3) and lower than the polluted marine conditions (1060±400 cm-3) reported by Andreae (2009). Cantrell et al. (2001) also reported
high CCN concentrations (in the range of 300 to 1000 cm-3) at 0.3 %
supersaturation over Kaashidhoo climate observatory over the equatorial
Indian Ocean during February–March 1999. The higher concentration of CCN at
the Equator and further south of it clearly indicated the widespread
influence of continental outflow over this region (Ramanathan et al., 2001).
In general, as we move away from the continental sources, aerosol abundance
(AOD, black carbon and total mass concentration) decrease toward the open ocean
as reported by several studies over the northern Indian Ocean (Ramanathan et
al., 2001; Moorthy et al., 2009; Hudson and Yum, 2002). Similarly, Chylek et
al. (2006) have reported a latitudinal decrease in the CCN concentration at
1 % supersaturation from 1850 cm-3 at 4∘ N to 700 cm-3 at 0∘ N over the Indian Ocean during the INDOEX
campaign. Hudson and Yum (2002) reported that the influence of pollution
outflow from South Asia to the Indian Ocean ceased at 5∘ S due to
the Intertropical Convergence Zone. Over the same latitudinal sector, the
rate of decrease in CCN concentration with latitude was lower during ICARB
experiment (1.4 times) compared to the INDOEX values, which further
highlighted the persistent and widespread impact of continental outflow over
the northern Indian Ocean during ICARB-2018. Besides, CCN variation with
supersaturation also depicted a latitudinal gradient with high k values over
SEAS1 and low values over EIO. It is interesting to note that, irrespective
of the large decrease in aerosol loading and change in aerosol microphysical
properties over the region, the CCN concentrations at 0.2 % are comparable
at all locations (SEAS1, SEAS2 and Thumba) except over EIO (Fig. 3). The
high k values are observed at the coastal site, Thumba (Jayachandran et al.,
2018) and measurements from Kaashidhoo Climate Observatory (4.97∘ N, 73.5∘ E) during INDOEX (Cantrell et al., 2001). Similar to
SEAS1, an average k value of 0.8 was reported over the northeast Atlantic by
Snider and Brenguier (2000) during the ACE2 campaign. Schmale et al. (2018) also reported a similar finding based on the data collected from
several distinct locations in Europe. The low k values observed over the
SEAS2 and EIO also could be due to the cloud processing of maritime clouds
(Noble and Hudson, 2019; Hudson et al., 2015). Since the k value of the CCN
spectrum depends highly on the dominance of the ultrafine particles in the
aerosol size distribution and hygroscopicity of the aerosol system, the k
values estimated from the CCN measurements for a short range of
supersaturations should be interpreted carefully. It should be noted that
carbonaceous particles and newly formed ultrafine particles have a low
contribution to the CCN concentration and require extremely high
supersaturation conditions for the CCN activation (Dusek et al., 2006b;
Pierce and Adam, 2007).
CCN activation efficiency and geometric mean diameter
The fraction of total aerosol concentration (CN) that can act as CCN at a
specific supersaturation (CCN(SS)/CN) is termed CCN activation fraction or
activation efficiency, which is governed mainly by the number size
distribution and composition of the aerosol system (Schmale et al., 2018;
Dusek et al., 2006a). The scatter plot between the CN and CCN over SEAS1,
SEAS2 and EIO is shown in Fig. 4a, and the colour scale
indicates a geometric mean diameter (GMD) corresponding to the composite
aerosol number size distribution. CCN being a specific subset of CN, the CCN
concentrations increased with increasing total aerosol concentrations during
most of the observations. Regression analysis of CN and CCN at 0.4 %
(slope ∼0.2 and R2∼0.45) and 1.0 %
(slope ∼0.29 and R2∼0.44)
supersaturations has a poor association during the entire campaign period.
This is in contrast to the earlier observations (Gunthe et al., 2009;
Jayachandran et al., 2018), where the association between CN and CCN
increased with supersaturation. When the CN concentration was very high and
ultrafine-mode aerosols contributed significantly to the CN concentrations,
as evident from the lower values of the GMD of the aerosol size distribution
(Fig. 4a, colour scale), a weak association between CCN and CN was
observed. For GMD values greater than 100 nm, CCN and CN followed an
excellent relationship with R2∼0.99 and mean activation
efficiency of 69±10 % over all the regions (SEAS1, SEAS2 and EIO).
This highlighted that most of the particles in this size range become activated as CCN irrespective of the regional heterogeneities in the aerosol
composition. As aerosol system having a GMD of less than 60 nm (when ultrafine
particles dominate the aerosol size distribution) has deviated significantly
from the regression line. This implies that the abundance of ultrafine
particles has a direct impact on activation efficiency, since most of these
particles may not become activated at 0.4 % and 1.0 % supersaturation
levels (Pierce and Adam, 2007).
(a) Scatter plot between total aerosol (CN) and CCN at
0.4 % supersaturation over SEAS1, SEAS2 and EIO. The colour shows the
geometric mean diameter (GMD) of the aerosol size distribution. Regression
fit for a GMD greater than 100 nm is shown as a black dotted line. (b) Temporal variation in CCN concentration at 0.4 % supersaturation (SS) measured
using a CCN counter and estimated using the empirical relationship between
total aerosol number concentration and geometric mean diameter.
The “p” and “n” are empirical constants estimated using the regression
technique. The scatter plot of measured and estimated CCN concentration is
shown inset.
The influence of the GMD on the CN–CCN association is further investigated using
the regression analysis between the observed CCN at 0.4 % supersaturation
with the product of CN and GMD (CN ⋅ GMD). The coefficient of determination
(R2) had improved from 0.44 to 0.84 when CN is multiplied with the GMD. The
CCN concentration is estimated from the empirical relationship between the CN
and GMD (CCNest=n⋅CN⋅GMD, where n is a constant), which showed an excellent association with the measured CCN concentration (at 0.4 %
supersaturation). The regression coefficient between measured and estimated
CCN further increased to 0.94 for a power law, CCNest=n⋅CN⋅GMDp, where p (1.5) and n (2000) are constants estimated
iteratively for the highest value of R2. The temporal variation and
scatter plot of measured and estimated CCN concentrations at 0.4 %
supersaturation are shown in Fig. 4b. By accounting for the effect of the GMD
on CCN concentration, this analysis demonstrated the primary role of aerosol
number size distribution on CCN activation. Since the ultrafine-mode
aerosols have lesser hygroscopicity compared to the fine-mode (>1µm) aerosols (Pöhlker et al., 2016; Gunthe et al., 2009; Pierce and
Adams, 2007; Rose et al., 2011), the empirically estimated CCN number
concentration is overestimated during the periods of ultrafine-particle
dominance in the size distribution. The low activation efficiencies during
the new-particle formation events and for the higher abundance of ultrafine-mode aerosols have been reported in the literature (Pöhlker et al., 2016).
The present analysis of estimating the CCN concentration from the product of
CN and GMD for widely varying aerosol size distributions is analogous to the
better association between the product of the aerosol scattering coefficient and
its spectral dependence (Ångström exponent) with CCN concentration rather
than the regression between scattering coefficient and CCN concentration
(Jayachandran et al., 2018).
In general, aerosol size distribution plays a major role in CCN activation.
The ultrafine particles significantly decrease the activation efficiency
compared to the fine-mode aerosols. Since the CN number concentration
decreased from the southeastern Arabian Sea to the equatorial Indian Ocean,
delineating the periods of ultrafine-particle dominance from a threshold CN
value is difficult (see Fig. 4a). The CN values over the equatorial Indian
Ocean (>5000 cm-3) during ultrafine-particle events are
lower than the CN values without ultrafine-particle events over the
southeastern Arabian Sea region. The association of CCN activation
efficiency with the CN number concentration over different regions is shown
in Fig. 5. For a constant CN concentration (5000 cm-3), the CCN
activation efficiency at EIO (∼15 %) is much lower than
that of SEAS1 (∼65 %) due to the presence of more ultrafine
particles in EIO. Figure 5 highlights that the activation efficiency
decreases with CN concentration following power-law dependence (AE =a⋅CNb, where a and b are constants) with similar coefficients (b=-0.84, R2=0.85) over both SEAS1 and EIO regions. Yum et al. (2007)
also showed that CCN efficiency decreases with the increase in ultrafine-particle concentrations irrespective of air mass type, which implies the
lower hygroscopicity of these particles (Pierce and Adams, 2007).
Association of activation efficiency at 0.4 %
supersaturation (SS) with CN number concentration during SEAS1, SEAS2 and EIO of
ICARB-2018. Dotted lines indicate regression fit to the data.
The scatter plot between the CCN activation efficiency at 0.4 %
supersaturation and the GMD of aerosol size distribution segregated for
different marine regions is shown in Fig. 6a. It is clear that the higher CCN
activation efficiency is observed for particle size distributions dominated
by larger particles (GMD >130 nm) irrespective of the regions
and air mass patterns. Wherever the contribution of ultrafine particles to
the total aerosol concentration is significant, the activation efficiency
decreased. In general, low (high) GMD values resulted in low (high) CCN
activation efficiency, irrespective of aerosol composition, in the south
Asian outflow. For a given GMD of 90 nm, the activation efficiency varied
over a wide range from 20 % to 60 %, which is more notable over the EIO
region (stars), where the ultrafine particles contribute 20 % to 50 % to
the total number concentrations. Earlier, Kim et al. (2014) also
observed a significant spread in the association between CCN at 0.6 %
supersaturation and a GMD in the range of 40–70 nm, which is attributed to the
heterogeneities in aerosol chemical composition. The association between the GMD
and activation efficiency weakened during the new-particle formation events
and aerosol size distributions having multiple modes, especially in the
coarse- and ultrafine-particle regime. Supersaturation spectra of CCN
activation efficiency for low (25–50 nm) and high (125–150 nm) values of the GMD
are shown in Fig. 6b. In contrast to SEAS1 and EIO, low-GMD cases were not
observed over SEAS2. For low-GMD cases, activation efficiency at 1.0 %
supersaturation is mostly less than 30 % over SEAS1 and EIO, which implies
the low hygroscopicity of the ultrafine particles. In contrast, higher
activation efficiencies (>60 %) are observed for high-GMD
cases over all the regions irrespective of the supersaturation conditions.
The very low activation efficiency at 0.2 % supersaturation
(∼30 %) observed over the SEAS1 region increased drastically to
∼100 % at 1.0 % supersaturation for high-GMD conditions.
Regionally, SEAS1 aerosols are more CCN active, especially at higher
supersaturations (1.0 %) compared to the EIO aerosols (Fig. 6b).
(a) Association of geometric mean diameter (GMD) with CCN
activation efficiency at 0.4 % supersaturation (SS) over SEAS1, SEAS2 and EIO
during ICARB-2018. (b) Supersaturation spectra of activation efficiency for
low (25–50 nm) and high (125–150 nm) GMD values during SEAS1 and EIO. All
the size distribution measurements during SEAS2 have a GMD above 50 nm.
Vertical bars on the symbol indicate the standard deviation of the
measurements.
The above discussions highlighted the significant role of the GMD and, in turn, the
abundance of ultrafine aerosols in the activation efficiency of the aerosol
system. This analysis broadly confirms the primary role of aerosol size
distribution in deciding the CCN concentration and activation efficiency
over the region. During ICARB-2018, GMD values varied from 25 to 160 nm, and
nearly 40 % of the measurements have a GMD of less than 100 nm. The high
activation values are observed whenever the contribution of ultrafine-mode
aerosols to the CN is negligible. Also, higher CN values (>104 cm-3) with low GMD values (<60 nm) are seen over both
the SEAS1 and EIO regions suggesting new-particle formation events. A low CCN
efficiency at 0.2 % supersaturation at SEAS1 reinstates the dominance of
ultrafine particles or the hydrophobic nature of the particles resulting
from the continental outflow. It is well known that bimodal distribution is
a characteristic of the aerosol system over the marine environment (one mode
emanating from possible nucleation events and the other one in the lower
size range of the fine mode), whereas aged continental outflows over the
oceans show a unimodal distribution with a mode around 80 to 200 nm (Yum et al.,
2007). The measurements of aerosol particle size distributions carried out
over the Bay of Bengal during winter 2009 (ICARB-2009) revealed a unimodal
size distribution with a mode diameter close to 100 nm. This further supports
the lack of ultrafine particles over SEAS2, where air masses mostly
originate from the Bay of Bengal. At low supersaturations, marine aerosols
have a higher CCN activation efficiency compared to the continentally influenced
aerosol system, which flip-flops at higher supersaturations. This aspect is
further investigated using the aerosol size distribution and critical
diameter in the following section. The present study is also in line with
the earlier observations reported over various oceanic regions, which
depicted a higher activation efficiency for continental aerosols than clean
marine aerosols (Yum et al., 2007). A strong association is observed between
the GMD and a hygroscopicity parameter for different mixing state conditions
(external, internal and internal with non-growth) over the Korean peninsula (Kim
et al., 2018). The present study highlight that the higher activation
efficiencies are observed for the continental or marine aerosols mostly in
the absence of ultrafine-particle burst events.
Aerosol size distribution and CCN activation
To understand the effect of ultrafine particles on CCN activation, two
typical cases have been considered over SEAS1 and EIO, where aerosol size
distribution varied drastically within a few hours due to ultrafine-particle
events which are characterized by low GMD values and a high number
concentration of ultrafine particles (diameters <100 nm). Figure 7
shows the CCN spectra and activation efficiency along with the corresponding
aerosol number size distribution over SEAS1 and EIO for high and low
concentrations of ultrafine particles (high UFP and low UFP). Corresponding k
values (Twomey's empirical slope) are also mentioned in the figure. Over
SEAS1, the variation in CCN with supersaturation for high- and low-UFP cases
showed an almost similar pattern with a relatively steeper increase in CCN
with supersaturation for high UFP compared to low UFP. The CCN activation
efficiency for low-UFP case increased from ∼33 % at 0.2 %
supersaturation to almost 100 % at 1.0 % supersaturation, whereas for
high UFP the activation efficiency at the highest supersaturation (1.0 %) is
only ∼45 % over SEAS1. This significant difference in the
activation efficiency for the two cases can mostly be attributed to the
higher concentrations of ultrafine particles, as already mentioned in the
earlier section and Fig. 7b. The particles below 100 nm contributed almost
37 % to the total number concentration for the low-UFP case, and all
these particles are activated at high supersaturation in contrast to the
high-UFP case. The large amounts of ultrafine-particle concentrations (with
a GMD of ∼50 nm in Fig. 7b) seen in the high-UFP case did not
contribute significantly to the CCN concentrations even at 1.0 %
supersaturation.
(a, c) Typical cases of supersaturation spectra of
CCN number concentration and activation efficiency (AE) for high and low
concentration of ultrafine particles (UFP) over SEAS1 and EIO. (b, d) The aerosol number size distributions corresponding to (a) and (c).
Over EIO, though the CCN values at lower supersaturations are comparable for
both the cases, a 100 % increase is observed in CCN concentration at
1.0 % supersaturation for high UFP compared to low UFP (Fig. 7c). The
activation efficiency depicted an entirely distinct pattern with low values
(20 %) at all supersaturations for high UFP compared to the activation
efficiency of >75 % for the low-UFP case. These typical events
over SEAS1 and EIO clearly demonstrate the lower activation efficiency of
ultrafine aerosols. However, there exist strong differences in the
activation properties of CCN over SEAS1 and EIO. For both high- and low-UFP
cases, SEAS1 aerosols are more CCN active at higher supersaturations than
EIO, which is also clearly reflected in the regional mean values. The
absolute magnitude of CCN concentration is relatively less over EIO with no
significant increase in CCN concentration with increasing supersaturation
(especially for low UFP, k∼0.05). These flat CCN spectra
(lower k values <0.5 in both the cases) observed over EIO represent
maritime aerosol. The high k values (>0.8) over SEAS1 represent
polluted marine conditions (Jayachandran et al., 2017). The aerosol system
can have a high and low activation efficiency or k depending on the
contribution of ultrafine-mode aerosols to the total number concentration.
This is contrary to the general classification of the aerosol system based
on k (Jayachandran et al., 2017). The inverse relationship between the Twomey
exponent (k) and activation efficiency reported by Jayachandran et al. (2017) over coastal-location Thumba, which is geographically closer to SEAS1, does not hold during ICARB-2018 because of the new-particle formation events
and the abundance of ultrafine particles. This is similar to high-UFP cases, as
shown in Fig. 7.
To evaluate the relative importance of aerosol number size distribution in
CCN activation, similar aerosol size distributions are grouped by regressing
each individual size distribution with all the other size distributions. The
size distribution having the maximum number of occurrences of the
coefficient of determination (R2) greater than 0.9 is identified as
the first group. Then, the first group of distributions are removed, and
regression analysis is repeated to estimate the next major prominent size
distribution (Fig. 8a). The mean number size distributions and
corresponding mean activation efficiency at 0.4 % supersaturation for each
of the groups are shown in Fig. 8b. This analysis highlighted that the majority
of the hourly mean size distributions (number N∼119) depicted
a broad peak with the mode around 150–300 nm (Type 1) with the highest
activation ratio of 76 %. Whenever the mode of the size distribution falls
below 100 nm, i.e., the mode is in the ultrafine-particle regime (Type 2, Type 5
and Type 6), a lower activation efficiency of value below 20 % was observed.
The Type 3 and Type 4 size distributions, which are similar to Type 1 but
with a small difference in the particle concentration above 100 nm, also
showed a higher activation efficiency (>70 %). The spread seen
in activation efficiency (standard deviation) for a “fixed” aerosol size
distribution could be due to the difference in aerosol chemical composition.
It should be kept in mind that, even though we grouped the similar
distributions (R2>0.9), the small variations observed
between the member distributions (standard deviation of size distributions
shown in Fig. 8a) can also contribute to the standard deviation of the
activation efficiency. The difference between the activation efficiencies
for two distinct size distributions is more substantial than their standard
deviations, which initially implies the primary role of number size
distribution on the CCN activation.
(a) Most frequently occurring aerosol number size
distributions (normalized) during ICARB-2018. Vertical bars represent the
standard deviation of the data, and the number of hourly observations
averaged for each distribution is given in (b). (b) Activation
efficiency at 0.4 % supersaturation of the normalized distributions shown
in panel (a).
We have further examined the association of CCN concentration with aerosol
number concentration (dN) at different size ranges. Figure 9a shows the
scatter plot of CCN concentration with the cumulative aerosol number
concentration above the size range of 50 nm (N50), 100 nm (N100)
and 150 nm (N150). The slope of the regression fit and coefficients of
determination (R2) estimated for the CCN and aerosol concentrations
above different size ranges (up to N250) are shown in Fig. 9b. The
slope of the regression analysis increased as the lower size cut of the aerosol
number concentration increased from 50 to 250 nm. The coefficient of
determination (R2) was low for N50 and increased to 0.98 for
N100. The CCN varied with CN concentration where the slope of the
variation strongly depended on the size distribution and is stronger for
larger aerosols (GMD values >100 nm as shown in Fig. 4) and
weaker for ultrafine-mode aerosols. The scatter between CCN at 0.4 %
supersaturation and CN100 shows better association (slope
∼0.95 and R2∼0.92). This implies that
most of the particles above 100 nm are CCN at 0.4 % supersaturation and
N100 is a good proxy for CCN concentration at 0.4 % supersaturation.
The contribution of particles to the CCN concentration at 0.4 %
supersaturation is lower above 100 nm. For the particles above 200 nm, CCN
concentration is nearly 2.5 times higher than the N200.
(a) Scatter plot between CCN number concentrations at
0.4 % supersaturation (SS) with aerosol number concentration (CN) for particles
size larger than 50, 100 and 150 nm. (b) Variation in regression
coefficients (slope and R2) estimated for CCN at 0.4 %
supersaturation with aerosol number concentration at different size ranges.
The N50 indicate aerosol number concentration above 50 nm.
The above analysis indicates that the critical activation diameter, which is
the size above which all the aerosols become activated as CCN (at 0.4 %
supersaturation) is close to 100 nm for the continental outflow of aerosols
to the northern Indian Ocean during wintertime. The critical activation
diameter of the aerosol system can be experimentally estimated from size-segregated CCN measurements (Rose et al., 2010) or from the simultaneous
observations of CCN and aerosol number size distributions (Furutani et al.,
2008). Assuming an internally mixed aerosol system, we have estimated the
“apparent critical diameter” for a particular supersaturation by integrating
the aerosol number concentration (dN) from a higher to a lower size range in
such way that estimated aerosol number concentration equals CCN
concentration (Burkart et al., 2011) and the diameter corresponding to that
is called the critical diameter. The cumulative number concentration (summing
from higher size to lower) and critical diameter for different
supersaturations over SEAS1, SEAS2 and EIO are shown in Fig. 10a and b. As
seen in Fig. 10a, the contribution of ultrafine particles to the total
concentration is relatively high over SEAS1 compared to SEAS2 and EIO due to
the continental proximity. As expected, the critical diameter was lower for
higher supersaturations for all the regions. This is consistent with the
Kohler theory. A relatively large decrease in critical diameter with
supersaturation was observed over SEAS1 compared to SEAS2 and EIO. As shown
in Fig. 2, though the aerosol loading is high over SEAS1, CCN
concentration at 0.2 % supersaturation is lower over SEAS1 compared to
that of EIO. The high value of the critical diameter (∼185 nm)
over SEAS1 further confirms that only a small portion of the aerosol
population is activated as CCN at 0.2 % supersaturation. This is mostly
attributed to the presence of less hygroscopic aerosols (possibly
carbonaceous) above 100 nm size range, which requires higher supersaturation
to become activated. In contrast, the lower critical diameter at 1.0 %
supersaturation over SEAS1 could be attributed to the water-soluble aerosol
in the ultrafine-particle mode. It is interesting to note that the
critical diameter at 1.0 % supersaturation is higher for EIO than SEAS1 in
contrast with 0.2 % and 0.4 % supersaturations. This observation is in
line with the high activation efficiency at 1.0 % supersaturation over
SEAS1 compared to EIO (Fig. 6).
(a) Cumulative aerosol number concentration from higher
size range to lower over SEAS1, SEAS2 and EIO. The value at 9 nm indicates
the total number concentration. The apparent critical activation diameter is
shown as dashed vertical lines. (b) Variation in critical diameter for
0.2 %, 0.4 % and 1.0 % supersaturations (SS) over SEAS1, SEAS2 and EIO.
Simultaneous measurements of aerosol number size distribution and CCN at
different supersaturation are useful in understanding CCN characteristics.
The size distribution over SEAS1 has a mode close to 100 nm, which
represents the influence of polluted continental air mass. Ueda et al. (2016) reported bimodal size distributions as a typical maritime aerosol
system based on the extensive measurements of aerosol number size
distributions over the Pacific Ocean covering 40∘ N to
40∘ S. However, most of the measurements during ICARB-2018
depicted broad mono-modal distribution indicating the aged, continental
outflow aerosols. The bimodal distributions are less frequent and mostly
associated with ultrafine-particle bursts (Fig. 8). A fine mode has been
consistently present in the aerosol number size distribution over SEAS,
which is contributing to the high CCN activation. The regional difference in
the aerosol number size distribution and its role in determining the
activation efficiency of CCN are vivid such that the dominance of
fine or ultrafine particles decreases the CCN efficiency from the expected
values. Kalivitis et al. (2015) investigated the association of CCN at
0.2 % supersaturation with aerosol concentrations larger than the threshold
diameter (from 80 to 130 nm) over the eastern Mediterranean marine atmospheric
boundary layer. These authors reported particle concentrations above 100 nm
as the best indicator for CCN at 0.2 % supersaturation over the region,
and the present study supports this finding (Fig. 9). Burkart et al. (2011) attributed the high critical diameter of aerosols at 0.5 %
supersaturation over Vienna to the presence of insoluble aerosols. Rose et
al. (2010) have reported a dry activation diameter of 200 to 30 nm for a wide
range of supersaturations of 0.068 % to 1.27 %.
The presence of significant amounts of ultrafine particles observed over the
northern Indian Ocean could be attributed to the in situ new-particle
formation events and/or transport from the free troposphere. The ICARB-2018
measurements indicate that these ultrafine-particle events are mostly
observed during the early morning and evening hours. A nearly 20 % to 40 %
increase in the mass concentration of organic carbon (OC) was observed
during such conditions, implying the formation of secondary organic
aerosols. A large-scale anticyclonic system prevailed during the wintertime
over the region and also supported the amalgamation of distinct air masses and
intrusion of ultrafine particles formed in the free troposphere into the
lower atmosphere. The absence of a very high concentration of nucleation
aerosols (<10 nm) further confirms the possibility of the transport
of free-tropospheric aerosols. Nair et al. (2013) have reported ultrafine-particle dominance over the northern Arabian Sea during spring 2006, which
was strongly associated with the variation in chlorophyll concentrations
implying a role of ocean biogeochemistry. However, the role of ocean
biogeochemistry and that of the semi-volatile organic vapours in the
formation of ultrafine particles over the northern Indian Ocean needs to be
investigated in detail.
Since the CCN activity depends mainly on the aerosol size and composition
(Dusek et al., 2006a), the critical diameter can be regarded as a proxy for the
variations in the chemical composition of the aerosol system. It should be
emphasized that the aerosol size distribution and chemical composition are
intrinsically coupled with each other, and any change in the aerosol size
distribution may have also been due to aerosol composition differences.
Quinn et al. (2008) have correlated the critical diameter with the
hydrogenated organics aerosol (HOA) mass concentration and found that HOA
can explain about 40 % of the variance in the critical diameter. For
anthropogenic and marine environments, Furutani et al. (2008) have reported
a critical diameter of 70–110 and 50–60 nm respectively at
0.6 % supersaturation. The critical diameter estimated for the South Asian
outflow is comparable to the values of 70–90 nm (at 0.44 %
supersaturation) reported by Quinn et al. (2008) for marine regions. It
should be noted that the freshly emitted carbonaceous combustion particles
have a high critical diameter (∼350 nm) even at a high
supersaturation (0.7 %) as reported by Dusek et al. (2006b). The presence
of soluble aerosols largely enhances the CCN activity of insoluble particles
such as BC and dust (Dusek et al., 2006b) due to effective mixing. Thus,
these inferences underline the need for observations of
mixing state that are more realistic and size-segregated aerosol composition measurements in regions
like SEAS, where primary as well as secondary particles are present from
both natural and anthropogenic sources.
This study highlighted the high concentration of CCN over the South Asian
outflow regions. The relative importance of aerosol size distribution and
chemical composition in CCN activation has been the topic of investigation
for several experiments (Dusek et al., 2006a; Kerminan et al., 2012; Hudson,
2007). A significant understanding of the critical diameter (Kerminan et al.,
2012) and size-segregated hygroscopicity parameter (κ) (Petters and
Kreidenweis, 2007) enabled us to predict the CCN concentration based on the
measurement of physical and chemical properties of aerosols carried out at
distinct environments (Schmale et al., 2018). Paramonov et al. (2015) have
shown that the hygroscopicity parameter decreases with particle size, with the
ultrafine- and fine-mode aerosols having a statistically significant
difference in hygroscopicity values. Our observations with the lower activation
efficiency observed for distributions with ultrafine-particle dominance agree with Paramonov et al. (2015). Several studies have
considered an average size distribution of aerosols with a varying
hygroscopicity parameter to predict CCN concentration (Rose et al., 2010;
Gunthe et al., 2009; Jurányi et al., 2010). These studies also
emphasized that aerosol size distribution has a major role in deciding the
number concentration of CCN. Meng et al. (2014) reported that
hygroscopicity is more critical at low supersaturations and not at high
supersaturations. While the present study described the CCN characteristics
over the different parts of the northern Indian Ocean and its association
with particle number size distributions, further studies are required to
examine the climate implications of these observations.
Conclusions
Extensive measurements of aerosol and CCN properties in South Asian outflow
to the northern Indian Ocean were carried out as a part of the ICARB-2018
experiment during January–February 2018. The influence of continental
outflow on the CCN characteristics over the marine atmospheric boundary
layer extending from 15∘ N to 2∘ S (southeast Arabian
Sea and the equatorial Indian Ocean) close to the Indian subcontinent were
investigated. The major highlights of this study are the following.
High CCN concentrations are seen over the southeastern Arabian Sea, with steep
(k=0.83) CCN spectra, while low k values are observed at the equatorial
Indian Ocean;
Most of the aerosols (>75 %) in the South Asian outflow (over
SEAS) are CCN at high (1.0 %) supersaturations, whereas the aerosol system
over the equatorial Indian Ocean is less CCN efficient (53±28 %) at
higher supersaturations.
The CCN efficiency depicts a strong association with the geometrical mean
diameter of the aerosol number size distribution. The activation efficiency
decreases with greater dominance of ultrafine particles.
Fine-mode particles (>100 nm) contribute to the high activation
efficiency (69 %) over the northern Indian Ocean. The number concentration
of particles above 100 nm is a good proxy for predicting CCN number
concentration at 0.4 % supersaturation.
The formation and transport pathways of ultrafine particles over the region
during the winter season remain an open question at present, and more
dedicated field experiments and detailed investigations are required to
address this in detail. Though the variations in the GMD of the aerosol number
size distributions account for the variability in CCN activation efficiency
over the southeastern Arabian Sea and the equatorial Indian Ocean, the
change in aerosol chemistry associated with the ultrafine-particle burst
events during the early morning and evening hours needs further
investigation using the size-segregated CCN measurements and online
measurements of aerosol chemistry.
Data availability
ICARB-2018 data are available upon request from the contact author,
Vijayakumar S. Nair (vijayakumar_s@vssc.gov.in).
Author contributions
SSB and VSN designed the experiment. VSN, JV, SKK, MMG and SSB involved in
the data collection on-board ship. VSN did the scientific analysis of the
data and drafted the paper. SSB edited the paper.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Interactions between aerosols and the South West Asian monsoon”. It is not associated with a conference.
Acknowledgements
The ICARB-2018 experiment was carried out under the ISRO Geosphere-Biosphere
Programme. Authors acknowledge the National Centre for Polar and Ocean
Research (NCPOR) of the Ministry of Earth Sciences of the government of India for providing the shipboard
facilities onboard ORV Sagar Kanya. We acknowledge NOAA ARL for providing
the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT)
transport and dispersion model used in this study.
Review statement
This paper was edited by B. V. Krishna Murthy and reviewed by James Hudson and one anonymous referee.
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