Because of their extensive coverage, marine low clouds greatly impact the
global climate. Presently, the response of marine low clouds to the changes
in atmospheric aerosols remains a major source of uncertainty in climate
simulations. One key contribution to this large uncertainty derives from the
poor understanding of the properties and processes of marine aerosols under
natural conditions and the perturbation by anthropogenic emissions. The
eastern North Atlantic (ENA) is a region of persistent but diverse
subtropical marine boundary layer (MBL) clouds, where cloud albedo and
precipitation are highly susceptible to perturbations in aerosol properties.
Here we examine the key processes that drive the cloud condensation nuclei
(CCN) population in the MBL using comprehensive characterizations of aerosol
and trace gas vertical profiles during the Aerosol and Cloud Experiments in
the Eastern North Atlantic (ACE-ENA) field campaign. During ACE-ENA, a total
of 39 research flights were conducted in the Azores: 20 during summer 2017
and 19 during winter 2018. During summer, long-range-transported aerosol
layers were periodically observed in the lower free troposphere (FT),
leading to elevated FT CCN concentrations (
Remote marine low cloud systems have a large spatial coverage and are particularly susceptible to aerosol perturbations because of their relatively low optical thickness and low background cloud condensation nuclei (CCN) concentrations. The response of low cloud systems to changes in aerosols is a major source of uncertainty in simulations of climate change (Bony and Dufresne, 2005; Wyant et al., 2006; Turner et al., 2007; Carslaw et al., 2013). One major contribution to this large uncertainty derives from the poor understanding of the properties and processes of marine aerosols under natural conditions and the perturbation by anthropogenic emissions. The processes that control CCN population in the marine boundary layer (MBL) have been examined by a number of studies. These processes include entrainment of free troposphere aerosols (Raes, 1995; Clarke et al., 2013), new particle formation (NPF) (Bates et al., 1998; O'Dowd et al., 2010), production of sea spray aerosols (O'Dowd et al., 2004; Russell et al., 2010; Prather et al., 2013; Quinn et al., 2017), condensational growth of Aitken-mode particles (Sanchez et al., 2018; Zheng et al., 2018, 2020a), interstitial particle scavenging by cloud droplets (Pierce et al., 2015), and the removal of CCN by coalescence scavenging (Wood et al., 2012, 2017). In addition, synoptic conditions also strongly influence entrainment and coalescence scavenging and therefore the population of MBL aerosols (Bates et al., 2000; Wood et al., 2015, 2017).
The eastern North Atlantic (ENA) is a region of persistent but diverse
subtropical MBL clouds (Wood et al., 2015; Rémillard and Tselioudis,
2015). The origins of the aerosols arriving at the ENA are diverse, varying
from clean marine air masses to those that are strongly influenced by
continental emissions (O'Dowd and Smith, 1993; Wood et al., 2015; China
et al., 2017; Zawadowicz et al., 2021). Zheng et al. (2018) examined MBL
aerosol in the ENA using 3 years of measurements (2015–2017) at the
US Department of Energy Atmospheric Radiation Measurement (ARM)
site on Graciosa Island in the Azores, Portugal. In the ENA, MBL aerosol
concentrations in different size ranges exhibit strong seasonal variations.
For example, larger accumulation-mode aerosols (
While previous studies have greatly advanced our understanding of MBL
aerosol properties and processes, they are mostly based on measurements at
ground and sea level, whereas the vertical profiles of aerosol properties are
needed to understand some of the key processes that drive CCN populations in
the MBL, including long-range transport of continental emissions,
entrainment of free tropospheric (FT) air, and the interactions between
aerosols and clouds. Airborne measurements were carried out during several
field campaigns in the 1990s, including the North Atlantic Regional
Experiment (NARE) (Parrish et al., 1998), the Atlantic Stratocumulus
Transition Experiment (ASTEX) (Albrecht et al., 1995), the Marine
Aerosol and Gas Exchange (MAGE) campaign (Huebert et al., 1996) that
organized the chemical experiment within ASTEX, and the Aerosol
Characterization Experiment (ACE-2) (Raes et al., 2000). The emphasis
of NARE was mostly on ozone chemistry, and ASTEX was focused on the
transition of marine stratocumulus clouds. During ACE-2, the variation of
aerosol size distribution and chemical composition was examined during three
Lagrangian experiments over periods of
In this study, we present comprehensive airborne measurements of aerosols and trace gases in both summer and winter seasons during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign (Wang et al., 2021). The large number of the flights provides statistically robust characterization of the vertical profiles of aerosol properties and allows for understanding of the aerosol properties under natural conditions (i.e., aerosols mostly produced by natural processes) and those strongly influenced by anthropogenic emissions. Key processes that control the population of CCN in the MBL are investigated by examining the vertical profiles of aerosol properties and their variations between the seasons. The impact of synoptic conditions on MBL structure and the vertical profiles of the aerosol populations are examined, and the implications for studying aerosol–cloud interactions using ground-based aerosol measurements are discussed.
During the ACE-ENA campaign, the ARM Aerial Facility Gulfstream-1 (G-1)
research aircraft was deployed in the Azores, Portugal, as part of two
intense operation periods (IOPs) during summer 2017 (June to July, summer
IOP) and winter 2018 (January to February, winter IOP). The G-1 aircraft was
stationed at the Lajes airport on Terceira Island, and a total of 39 flights
(20 in summer and 19 in winter) were conducted. The dates and durations of
the flights are summarized in Fig. 1. The deployments during both seasons
allow for the examination of key aerosol and cloud processes under a variety
of representative meteorological and cloud conditions. Each flight consisted
of four to six vertical profiles (excluding those leaving and arriving at
the Lajes airport), providing the aerosol and trace gas properties as a
function of altitude. The flights also included horizontal legs near the
surface of the ocean (
Date and time periods of research flights during summer
Measurements on board the G-1 included meteorological parameters, trace gas
species, aerosol, and cloud properties (Wang et al., 2021). Measurements
used in this study are summarized in Table 1 and described briefly below.
The height of the MBL is derived from the measured vertical profile of
potential temperature, from which the boundary between the MBL and FT is
often clearly defined by an abrupt increase in the potential temperature
with altitude (i.e., temperature inversion). When the inversion is not
obvious, liquid water content (LWC) and water vapor mixing ratio (
Instrumentation deployed during the ACE-ENA that are used for data analysis in this study. All measurements were made at a frequency of 1 Hz except for the HR-ToF-AMS, PILS, and thermal denuder.
BC mass concentration was characterized by a single-particle soot photometer
(SP2, DMT Inc., Longmont, CO). A high-resolution time-of-flight aerosol mass
spectrometer (HR-ToF-AMS) (Jayne et al., 2000; DeCarlo et al., 2006;
Zawadowicz et al., 2021) and a particle-into-liquid sampler (PILS) coupled
with offline ion chromatography (Orsini et al., 2003; Sullivan et al.,
2019) were deployed to characterize sub-micrometer non-refractory aerosol
composition. Both the HR-ToF-AMS and the PILS were deployed downstream of an
impactor with a cutoff size of 1
The vertical profiles of potential temperature and LWC show that the MBL
heights during the summer and winter IOPs are 1220
Cluster analysis of 10 d back trajectories arriving at 500, 1500,
and 3000 m above the ENA site during the summer
Mixing ratios of water vapor, CO, and O
Vertical profiles of
Both biomass burning and anthropogenic pollution generate the precursors of
O
The average dry aerosol size distributions within the MBL are bimodal with a
clear Hoppel minimum for both seasons (Fig. 5a). The Hoppel minimum
represents an average particle size at which particles become CCN. Although
the Hoppel minimum shows some variation from day to day, its value is
relatively constant during the same flight day. To facilitate the discussion
of aerosol processes that influence the MBL CCN population, we define
pre-CCN as particles with diameters smaller than the Hoppel minimum (i.e.,
particles that are too small to form cloud droplets under typical conditions
in the MBL). CCN are defined as the particles with diameters larger than the
Hoppel minimum. Therefore, both nucleation- and Aitken-mode particles belong
to the pre-CCN. During ACE-ENA, since the Aitken-mode particles often
dominated the pre-CCN population, the concentrations of pre-CCN
(
Aerosol size distributions measured by the FIMS on board the G-1
aircraft within
Figure 6 shows the vertical profiles of particle number concentrations
normalized to standard temperature and pressure (STP, 273.15 K and 101.325 kPa; Fig. 6a–c), mean particle diameter of Aitken and accumulation modes
(Fig. 6e–f), and the number fraction of volatile particles (Fig. 6d).
During both the summer and winter IOPs, the total aerosol number
concentration (
Vertical profiles showing the
Figure 7 shows the vertical profiles of the mass concentrations of
non-refractory species measured by HR-ToF-AMS and BC mass concentration
measured by SP2 for both seasons. Sulfate, organics, and ammonium constitute
almost 99 % of the non-refractory sub-micrometer aerosol mass, whereas
nitrate concentration is negligible. The sulfate concentration maximizes
near the ocean surface, reaching approximately 0.5
Vertical profiles of the mass concentrations of
The vertical gradients of
In contrast, during both seasons,
Once pre-CCN are entrained into the MBL, they can grow and reach CCN-active
sizes through condensation (Yoon et al., 2007; Sanchez et al., 2018;
Zheng et al., 2018, 2020a). Therefore, the entrainment of FT
Aitken-mode aerosol represents an indirect source of MBL CCN in the ENA. It
has long been recognized that sulfates produced from dimethyl sulfide (DMS)
oxidation are major species for the condensational growth of pre-CCN in
remote marine environments. Methanesulfonic acid (MSA), another product of
DMS oxidation, may also participate in particle condensational growth
(Kerminen et al., 1997; Ayers and Gillett, 2000; Karl et al., 2011;
Willis et al., 2016; Hodshire et al., 2019). Using measurements during
ACE-ENA, Zheng et al. (2020a) show that secondary organics contribute
substantially to pre-CCN condensational growth and thus the formation of CCN
in the remote marine environments, consistent with some early studies
(Meskhidze and Nenes, 2006; Facchini et al., 2008; Wurl et al., 2011;
Dall'Osto et al., 2012; Willis et al., 2017; Mungall et al., 2017;
Brüggemann et al., 2018). The higher MBL
Another mechanism for the formation of CCN within the MBL is the activation of Aitken-mode particles in a stronger than average updraft, which causes a higher peak supersaturation (Kaufman and Tanré, 1994). These Aitken-mode particles would otherwise remain in the interstitial air of clouds. Once activated, sulfate and organics can be produced through aqueous chemistry inside droplets. Unless these droplets are removed by precipitation, they become CCN upon evaporation outside the clouds and readily participate in subsequent cloud formation. The effect of this mechanism on the MBL CCN budget is difficult to evaluate with measurements only and will be a subject of future studies. The vertical profile of sulfate mass concentration indicates a surface source, consistent with the picture that over the open ocean, most submicron sulfate is derived from DMS through both gas-phase and in-cloud oxidation (Hegg and Hobbs, 1981; Gurciullo et al., 1999; Ovadnevaite et al., 2014; McCoy et al., 2015). The higher MBL sulfate mass concentration during the summer season is a result of stronger DMS emission (Zawadowicz et al., 2021) and higher oxidant (e.g., OH) concentrations. During summer, nearly half of the air masses arriving in the ENA MBL had been circulating around the Azores high over open ocean for more than 10 d, indicating that Aitken-mode aerosols have extended time to grow by condensation and in-cloud processes.
For both seasons,
Enhanced organic mass concentration was observed in the MBL during summer
(Fig. 7b). Comparison of the vertical profiles of
The spectral shape of submicron aerosol size distributions shows a strong
variability between seasons and between the MBL and FT (Fig. 5). A clear
separation between the Aitken and accumulation modes by a Hoppel minimum is
evident in the MBL. What stands out in the wintertime aerosol size
distribution is the larger proportion of particles below 20 nm, potentially
resulting from more NPF events due to low existing surface area
concentrations. A recent study showed that over the ENA, NPF takes place in
the upper part of the decoupled MBL following the passage of cold fronts
when open-cell convection and scattered cumulus clouds frequently occur
(Zheng et al., 2021). The NPF is due to the combination of low existing
aerosol surface area, cold air temperature, availability of reactive gases,
and high actinic fluxes in the clear regions between scattered cumulus
clouds. The larger fraction of particles below 20 nm in the MBL during the
winter is attributed, at least partially, to the more frequent passage of
cold fronts over the ENA and NPF in the upper MBL (Kolstad et al.,
2009). These newly formed particles can continuously grow into the Aitken
mode and contribute to the CCN in the MBL (Zheng et al., 2018, 2020a). The mean Aitken-mode particle size
(
The fitting parameters of average aerosol size distributions for the Aitken mode and accumulation mode during the summer IOP and winter IOP in the MBL and FT. The particle concentrations are normalized to standard temperature and pressure (273.15 K and 101.325 kPa; STP).
The contribution of FT entrainment to MBL particle concentrations is
estimated from the entrainment velocity and the difference in the average
particle concentrations between the lower FT and MBL. The entrainment flux
Average particle concentrations in the MBL and lower FT, as well as the change rates of MBL particle number concentrations due to the entrainment from FT. The particle concentrations and change rates are normalized to standard temperature and pressure (273.15 K and 101.325 kPa; STP).
The rate of
As shown earlier, aerosol layers with elevated
A long-range transport event observed during the research flight on
29 June 2017.
Previous ground observations at the Pico mountaintop station showed frequent
elevations of summertime CO and O
The composition of FT aerosol layers provides additional insight into the
source of the long-range-transported aerosols. There are relatively strong
correlations among the mass concentrations of BC, sulfate, and organics in
the FT aerosol layers compared to those in the MBL (Fig. S6), consistent
with previous measurements of summertime aerosols at the Pico mountaintop
station, which show that the FT aerosols are generally internal mixtures,
including soot and sulfate coated by organic matter (China et al.,
2017). During ACE-ENA, on average the sulfate mass fraction between
1600 and 2600 m (i.e., the altitude range of FT aerosol layers during the summer
IOP) outside the background conditions (
The sulfate mass fraction (
The synoptic condition strongly influences the structure of the MBL and thus
aerosol properties. The Azores consistently lie in an area of substantial
variability in synoptic configuration, thermodynamic environment, and cloud
properties. The ENA site is under a strong influence from the North Atlantic
high-pressure system (Azores high) and is periodically subject to frontal
passages (Rémillard et al., 2012). The synoptic conditions for the
39 flight days during ACE-ENA are classified as Azores high, pre-front,
front, post-front, or unclassified conditions following the method described
in Mechem et al. (2018). The classification of the synoptic conditions
is based on the reanalysis fields of geopotential height at 500 hPa pressure
levels (Gelaro et al., 2017). The 6-hourly reanalysis products are
examined to judge the category of the synoptic conditions. This
classification process is further combined with the archived surface weather
maps obtained from the National Meteorological Service of Germany (Deutscher
Wetterdienst – DWD;
Number of flight days under different synoptic conditions during the summer IOP and winter IOP. The corresponding percentages over the IOP are shown in the parentheses.
Figure 9 shows the vertical profiles of meteorological parameters, CO mixing
ratio, and aerosol properties measured on 8 July, an example of
Azores high conditions. On this day, the potential temperature and LWC
indicate a well-mixed MBL with shallow clouds below a strong temperature
inversion at around 1000 m. Inside the MBL, the bimodal aerosol size
distribution shows a clear Hoppel minimum, which is a result of cloud processing
(Hoppel et al., 1994). The aerosol size distribution was largely uniform
at different altitudes within the MBL. The aerosol in the lower FT showed a
layered structure, with properties clearly different from that in the MBL,
demonstrating the heterogeneity within the FT under a stable atmospheric
structure controlled by the Azores high. An elevated concentration of
Aitken-mode particles with mode diameter of
Vertical profiles of parameters under the synoptic condition of
Azores high on 8 July 2017.
Figure 10 shows an example of vertical profiles of meteorological
parameters, CO mixing ratio, and aerosol properties when the MBL is
decoupled. The measurements were carried out on 8 February 2018,
when the front and associated cloud band were located north of the Azores
(i.e., pre-front condition). The vertical profile of potential temperature
indicates a deep decoupled MBL that consisted of the surface mixed layer
below 500 m and the upper decoupled layer from 500 to 1900 m. A thin layer
of stratus was observed near the top of the surface mixed layer, and cumulus
clouds were observed in the upper decoupled layer (Fig. 10a). Inside the
surface mixed layer, the aerosol size distribution is bimodal and
independent of altitude. In contrast, the aerosol size distribution varied
with altitude inside the upper decoupled layer.
Vertical profiles of parameters under the synoptic condition of
pre-front on 8 February 2018.
In this study, we present aerosol properties, trace gas mixing ratios, and
meteorological parameters characterized on board the G-1 aircraft during both
the summer and winter IOPs of the ACE-ENA campaign. The key processes that
drive the CCN population in the MBL are investigated by examining the
variation of aerosol properties with altitude, season, and synoptic
condition. On average, all particle concentrations (i.e.,
The chemical composition analysis shows that sulfate, organics, and ammonium dominate the non-refractory aerosol mass concentration (around 99 % in both the summer and winter IOPs). The vertical profile of sulfate mass concentration indicates a surface source, consistent with the picture that over the open ocean sulfate in submicron aerosol is mostly derived from DMS through both gas-phase and in-cloud oxidation. Stronger DMS emissions and higher oxidant (e.g., OH) concentrations lead to a higher MBL sulfate mass concentration during summer. An enhanced organic mass concentration was also observed in the MBL during summer and is attributed to surface sources including stronger emission of primary organic marine aerosol and production of secondary organic aerosol from oceanic VOCs.
The impact of synoptic conditions on the MBL structure and aerosol
properties is examined. Under the pre-front and post-front conditions,
stronger convective activities often lead to a deeper and decoupled boundary
layer consisting of two sublayers: a surface mixed layer and an upper
decoupled layer. In comparison, a well-mixed boundary layer is more
prevalent under Azores high conditions. Aerosol in the decoupled boundary
layers exhibits strong vertical variations. Coagulation scavenging and
evaporation of drizzle below clouds lead to reduced
The data specifically related to the ACE-ENA
campaign can be found at
The supplement related to this article is available online at:
JW designed the study. YW, GZ, DAK, AL, AAM, FM, RM, AJS, JES, StS, AS, JT, RWe, RWo, MAZ, and JW collected and analyzed the aerosol and trace gas data aboard the G-1. YW, GZ, MPJ, DM, DV, RWo, and JW analyzed the cloud data and synoptic conditions. YW and JW prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
This article is part of the special issue “Marine aerosols, trace gases, and clouds over the North Atlantic (ACP/AMT inter-journal SI)”. It is not associated with a conference.
The ACE-ENA campaign was supported by the Atmospheric Radiation Measurement (ARM) Climate Research Facility and the Environmental Molecular Sciences Laboratory (EMSL); both are US Department of Energy (DOE) Office of Science User Facilities sponsored by the Office of Biological and Environmental Research. We thank Tamara Pinterich for her help in the preparation and operation of the FIMS during the IOPs. This research was supported by the Atmospheric System Research (ASR) program as part of the DOE Office of Biological and Environmental Research under award nos. DE-SC0020259, KP1701000/57131, DE-SC0013489, DE-SC0012704 (BNL), DE-SC0018948, and DE-SC0021256. The Pacific Northwest National Laboratory is operated for the DOE by the Battelle Memorial Institute (DE-AC05-76RL01830). Amy Sullivan and Rodney Weber were supported under contract DOE 333890. Daniel A. Knopf acknowledges support by the US Department of Energy, Office of Science (BER), Atmospheric System Research (DE-SC0016370). The authors thank Tamara Pinterich for her help in the preparation and deployment of the FIMS on board the G-1 aircraft and the G-1 flight and ground crews (Mike Hubbell, John Hubbe, Clayton Eveland, Mike Crocker, Pete Carroll, Matt Newburn, Mikhail Pekour, Lexie Goldberger, and Jon Ray) for their support of the ACE-ENA mission.
This research has been supported by the US Department of Energy (grant nos. DE-SC0020259, KP1701000/57131, DE-SC0013489, DE-SC0012704, DE-SC0018948, DE-AC05-76RL01830, DE-SC0016370, and DE-SC0021256).
This paper was edited by Lynn M. Russell and reviewed by two anonymous referees.