Marine low clouds play an important role in the global climate system (Wood, 2012), and their properties and radiative effects are very sensitive to the concentration of cloud condensation nuclei (CCN) (Carslaw et al., 2013; Rosenfeld et al., 2019). Condensational growth of pre-CCN particles (i.e., particles that are too small to form cloud droplets) (Hoppel et al., 1990; Pierce and Adams, 2006) is one major source of CCN in the remote marine boundary layer (MBL) (Pierce and Adams, 2006; Yu and Luo, 2009; Sanchez et al., 2018) and is likely the dominant one in late spring to fall (Zheng et al., 2018). Over open ocean, dimethyl sulfide (DMS) is the dominant biogenic volatile organic compound (VOC). The major oxidation products of DMS are sulfur dioxide (SO2) and methanesulfonic acid (MSA) (Andreae et al., 1985). Further oxidation of SO2 produces sulfuric acid (H2SO4), which readily condenses onto existing particles and participates in the formation of new particles (Kulmala et al., 2000). It has long been recognized that sulfate produced from DMS oxidation is a major species for particle condensational growth in the remote marine environment (Charlson et al., 1987; Raes and Van Dingenen, 1992; Pandis et al., 1994; O'Dowd et al., 1999; Sanchez et al., 2018). Earlier studies (Willis et al., 2016; Kerminen and Wexler, 1997; Karl et al., 2011) suggest that MSA may also contribute to the growth of pre-CCN particles and thus the formation of CCN. However, the effect of MSA condensation on marine CCN concentration remains unclear. Model-simulated effects range from negligible (e.g., a few percent) to significant (∼20 %), depending on the assumption of MSA volatility and the geographic location (Hodshire et al., 2019).

It has been suggested that in the remote MBL, secondary organics produced from two types of non-DMS VOCs can contribute substantially to particle condensational growth. The first type of VOCs, including isoprene, monoterpenes, and aliphatic amines (Facchini et al., 2008; Dall'Osto et al., 2012; Willis et al., 2017), is related to ocean biological activities, and secondary organic aerosol (SOA) produced from these VOCs are positively correlated with MSA (Dall'Osto et al., 2012; Willis et al., 2016, 2017; Kim et al., 2017). While the mixing ratios of isoprene and monoterpenes are typically quite low over open ocean (Hu et al., 2013) due to their weak emissions, on rare occasions, elevated monoterpene mixing ratios of up to ∼100 ppt were observed (Kim et al., 2017), possibly due to enhanced microorganism growth as a result of nutrient replenishment (Kim et al., 2017). The second type of VOCs is produced by the oxidation reactions at the air–sea interface, especially when the sea surface microlayer is enriched in organic surfactants (Mungall et al., 2017; Brüggemann et al., 2018). These water-soluble organics can come from phytoplankton but can also be from other sources, including other autotrophs and atmospheric depositions (Wurl et al., 2011). Therefore, this type of oceanic VOCs and thus SOA formed may not correlate with MSA (Wurl et al., 2011; Mungall et al., 2017; Brüggemann et al., 2018).

At present, the contribution of secondary organics to the growth of pre-CCN particles in the MBL and the seasonal variation of this contribution remain unclear, largely due to the scarcity of pre-CCN particle composition measurements. Previous studies of pre-CCN growth in the MBL were typically within relatively short time periods (i.e., about 1 month) (Dall'Osto et al., 2012; Willis et al., 2016, 2017; Kim et al., 2017; Mungall et al., 2017; Vaattovaara et al., 2006; Modini et al., 2009; Bzdek et al., 2014; Lawler et al., 2014; Swan et al., 2016) and were often conducted in coastal regions (Vaattovaara et al., 2006; Modini et al., 2009; Dall'Osto et al., 2012; Bzdek et al., 2014; Lawler et al., 2014; Swan et al., 2016) with substantial influences from continental emissions. Here we present the first long-term observational constraint on the importance of secondary organics to the growth of pre-CCN particles in the remote MBL. Hygroscopicity of size-classified particles was characterized over a period of 14 months in the Eastern North Atlantic. By taking advantage of the contrasting hygroscopicity values of sulfate, MSA, and other secondary organic species, we constrain and identify the major species that are responsible for the growth of pre-CCN particles. Our results show that the organics represent an important or even dominant condensing species during ∼80 % of growth events.

The Eastern North Atlantic (ENA) atmospheric observatory was established by the Atmospheric Radiation Measurement (ARM) Climate Research Facility (https://www.arm.gov/capabilities/observatories/ena, last access: 15 October 2020) in October 2013. This remote oceanic site, located on Graciosa Island, the Azores, Portugal (39∘5′30′′ N, 28∘1′32′′ W; 30.48 m above mean sea level) (Mather and Voyles, 2013), straddles the boundary between the subtropics and midlatitudes in the Eastern North Atlantic. The ENA is a region of persistent but diverse marine low clouds, the albedo and precipitation of which are highly susceptible to perturbations of aerosol properties (Wood, 2012; Carslaw et al., 2013). Air masses arriving at this site can originate from North America, northern Europe, the Arctic, and the Atlantic (Wood et al., 2015; Wang et al., 2016; Zheng et al., 2018). Routine measurements at the ENA site include meteorological parameters, trace gas mixing ratios, and aerosol and cloud properties (Zheng et al., 2018). The relevant routine measurements used in this study are summarized in Sect. 2.3.

From June 2017 to August 2018, the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign (Wang et al., 2016) was conducted in the Azores to investigate the aerosol–cloud interactions in the remote marine boundary layer (MBL). As a key part of this campaign, additional aerosol measurements were carried out at the ENA site, including aerosol size distribution and size-resolved CCN activated fractions (Mei et al., 2013b; Thalman et al., 2017). The instruments and calibration procedures are detailed elsewhere (Zheng et al., 2020) and are briefly described below. The data from these measurements are available at https://www.arm.gov/research/campaigns/aaf2017ace-ena (last access: 15 October 2020).

Continuous growth of Aitken-mode particles is identified from the aerosol size distribution time series. As a result of the condensational growth, aerosol chemical compositions and thus the hygroscopicity of Aitken-mode particles are expected to evolve with time during the growth events. As potential condensing species (Table 1) have contrasting hygroscopicity parameters, the variation of hygroscopicity parameter during the growth events can therefore be used to infer the major condensing species.

Figure 3 shows two examples of the identified growth events, with the dominant condensing species being sulfate and organics, respectively. While the measured κCCN values of the Aitken-mode particles (i.e., pre-CCN particles that are below ∼80 nm) are similar (∼0.45) at the start of both events, the variations of κCCN with growing particle size show opposite trends. For the July case (Fig. 3a, b), κCCN increased with the volume fraction of condensed species (fV,cond, Fig. 3b), indicating that the hygroscopicity of the condensed species, κc,CCN, exceeds that of the original particles. The derived κc,CCN value is 0.7, which is typical of sulfates (Table 1). In contrast, during the September growth event (Fig. 3c, d), κCCN decreased as the particles grew. The derived κc,CCN value is ∼0.3, indicating organics as the dominant condensing species. We note that κc,CCN is derived from the volume-weighted mixing law (Petters and Kreidenweis, 2007) (i.e., ideal Zdanovskii–Stokes–Robinson (ZSR) mixing). Organic surfactants may facilitate CCN activation by lowering surface tension of growing droplets (Ovadnevaite et al., 2017). In scenarios when particles contain organic surfactants, particle hygroscopicity κCCN may be greater than the simple volume average of participating species. As a result, the derived κc,CCN value based on the volume-weighted mixing law may be overestimated, therefore leading to an underestimation of the contribution of organics to the particle condensational growth.

A total of 62 growth events are identified during the 14-month campaign (Fig. 4). These events are classified into three categories according to the derived κc,CCN value (Table 1): (1) low hygroscopicity (i.e., κc,CCN<0.45), indicating organics as being the dominant condensing species, (2) high hygroscopicity (i.e., κc,CCN>0.65), with acidic sulfate (i.e., H2SO4 or NH4HSO4) dominating the particle condensational growth, and (3) intermediate hygroscopicity (i.e., 0.45<κc,CCN<0.65), when (NH4)2SO4 and/or mixtures of organics and acidic sulfate contribute to the particle growth.

The monthly distribution of the identified growth events and the dominant condensing species are shown in Fig. 4. Relatively more events were observed during the summer seasons due to favorable synoptic conditions. In summer, there is a stronger influence by the Azores High, while the influence from midlatitude cyclones and the corresponding wet scavenging are much weaker (Zheng et al., 2018). The distribution of the event categories shows that NH4HSO4/H2SO4 dominated the condensational growth during only 18 % of the growth events. This is less than the events dominated by organics at 24 %. The majority (58 %) of the growth events exhibit intermediate κc,CCN values, suggesting that (NH4)2SO4 or a mixture of organics and sulfate is responsible for the particle condensational growth.

To further constrain the condensing species for the intermediate κc,CCN category, we compare the κc,CCN value with the hygroscopicity under subsaturated conditions (κc,GF), which is derived from measured particle hygroscopic growth (Sect. 3). For (NH4)2SO4, the difference between κc,CCN and κc,GF is relatively small (within 20 %) (Petters and Kreidenweis, 2007), while the difference is usually substantially larger (Wex et al., 2009; Rastak et al., 2017; Petters et al., 2009; Pajunoja et al., 2015; Ovadnevaite et al., 2011; Massoli et al., 2010) for organic species. The large difference has been attributed to the solution non-ideality (Petters et al., 2009), the formation of hydrogels (Ovadnevaite et al., 2011), and the solubility and phase states (Pajunoja et al., 2015; Rastak et al., 2017). One example of the intermediate κc,CCN category is shown in Fig. S1 in the Supplement. For this case, the derived κc,CCN and κc,GF values are 0.59 and 0.45, respectively (Fig. S1). The difference is close to the measurement uncertainty (i.e., 20 %), and therefore the major condensing species for this example is classified as (NH4)2SO4.

Figure 5 compares the values of κc,CCN and κc,GF for all available events in the intermediate κc,CCN category. For most of these events, κc,GF is at least 20 % lower than κc,CCN, indicating organics likely played an important role in particle condensational growth. In addition, the chemical composition of submicron non-refractory aerosol (NR-PM1; aerodynamic diameters below 1 µm) indicates an ammonium-poor condition over the ENA (color bar in Fig. 5), typical of the remote marine environment (Adams et al., 1999). Therefore, sulfate is not fully neutralized as (NH4)2SO4. Note that the bulk NH4+/SO42- molar ratio shown in Fig. 5 is dominated by that of accumulation-mode particles, whereas the κc,CCN and κc,GF values are derived from growing Aitken-mode particles. Whereas the degrees of neutralization for accumulation and Aitken modes are expected to correlate with each other (e.g., lower neutralization degrees for both accumulation and Aitken modes under more ammonium-poor conditions), it is possible that the neutralization degree may exhibit size dependence under some circumstances. For example, in-cloud formation of SO42- influences the neutralization degree of accumulation-mode particles only (Seinfeld and Pandis, 2016), possibly leading to a lower degree of neutralization for the accumulation mode. This could explain a few data points that exhibit a lower degree of neutralization but similar κc,CCN and κc,GF values. This evidence suggests that the condensed species are a mixture of sulfates and organics during most of the intermediate κc,CCN events. These events are not dominated by (NH4)2SO4. Based on a κCCN value of 0.9 for acidic sulfates (H2SO4 and/or NH4HSO4, Table 1), the average contribution of organics during the intermediate κc,CCN events ranges from 42 % to 63 %, depending on the κCCN values of organics assumed (0.1–0.36; Table 1). Therefore, organics played an important role during the intermediate κc,CCN events and dominated the particle condensational growth for the low κc,CCN category. Together, these two categories represent a total of ∼80 % of the growth events that occurred throughout the year.

Given the importance of secondary organics to particle condensational growth, the potential sources of the condensing organics are investigated by examining the air mass origins (Sect. S1 in the Supplement). Here we classify the origin of air mass during the growth events into four types: (1) continental air masses from North America or Europe, (2) the Arctic, (3) the subtropics, and (4) the midlatitude Atlantic. Note that an air mass is denoted as continental if it passed over North America or Europe, so the non-continental types represent the air masses that had stayed over oceans or clean continental areas (i.e., Arctic region) for at least 10 d (Sect. S1). The mixing ratios of CO and O3 measured at the ENA site exhibit the highest and lower values for the continental and midlatitude ocean air masses, respectively (Fig. S3), supporting the effectiveness of the classifications.

Growth events of midlatitude Atlantic or Arctic type were observed exclusively from May to September, a period that coincides with the phytoplankton blooms in the midlatitude Atlantic or Arctic but not the subtropics (Sapiano et al., 2012). For these events, κc,CCN is anti-correlated with the MSA/SO2 ratio (Fig. 6a), which is from MERRA-2 reanalysis data (Sect. 2.3). As fixed yields of SO2 and MSA from DMS oxidation are assumed in MERRA-2 data (Chin et al., 2000; Randles et al., 2017), a lower MSA/SO2 ratio suggests that other SO2 sources in addition to DMS oxidation contribute to these events. These other sources could include volcanic emissions and combustion products from international shipping (Randles et al., 2017). As MSA is a tracer of biogenically derived SOA in marine environment (Seinfeld and Pandis, 2016), the anti-correlation also indicates that the condensed organics are likely SOA produced from VOCs emitted from ocean biological activities (e.g., phytoplankton blooms). The value of κc,CCN is not correlated with the NR-PM1 organic / sulfate ratio (Fig. 6b), suggesting different sources of the condensed species in pre-CCN and the accumulation-mode particle composition.

Among the remaining growth events, only four of them are subtropical cases, which occurred outside the bloom periods. During the other events, air masses were potentially influenced by continental emissions (Fig. S2). For these events, κc,CCN is instead positively correlated with the MSA/SO2 ratio (Fig. 6c), indicating that secondary organics formed from phytoplankton-emitted VOCs likely played a minor role in the observed particle condensational growth. The κc,CCN value generally decreases with increasing NR-PM1 organic / sulfate ratios (Fig. 6d), suggesting that the formation of SOA led to an increased organic fraction for both pre-CCN and accumulation-mode particles. Possible sources of the condensed organics include SOA generated from long-range-transported continental VOCs and VOCs released by the sea-surface microlayer oxidation that are not directly related to phytoplankton emissions.

As continentally emitted VOCs are removed by oxidation during long-range transport, it is expected that in situ SOA production from these VOCs is low and plays a minor role in particle condensational growth over the remote oceans (Kelly et al., 2019; D'Andrea et al., 2013). On the other hand, aromatic compounds were detected in pre-CCN particles in clean air masses at a coastal site (Lawler et al., 2014), indicating the potential contribution of SOA from anthropogenic VOCs with a long lifetime. Oxidation reactions at the air–sea interface can produce VOCs, which lead to subsequent SOA formation (Mungall et al., 2017; Brüggemann et al., 2018). This VOC source is present all-year round, even during winter when there is little biological activity in the ocean (Brüggemann et al., 2018). Therefore, the secondary organics produced via this pathway can contribute to the growth of pre-CCN particles during the seasons when there is little biological activity.

In summary, we show that during all seasons, secondary organics play an important role in the condensational growth of pre-CCN particles and, by extension, the formation of CCN in the remote marine boundary layer. The secondary organic species likely derive from a variety of precursors, including VOCs produced from marine biogenic activity, continentally emitted VOCs with a long lifetime that survive the long-range transport, and VOCs formed by oxidation at the air–sea interface. Current global models typically assume that sulfates dominate the particle growth over remote oceans and therefore may substantially underestimate the formation of CCN by condensational growth in the remote marine boundary layer.