THE ROLE OF ORGANICS IN MARINE AEROSOL CCN ACTIVATION
J. OVADNEVAITE, K. FOSSUM, D. CEBURNIS, AND C. O’ DOWD
School of Physics & Centre for Climate and Air Pollution Studies, National University of Ireland Galway,
Ireland.
Keywords: Organic Matter, CCN, Marine Aerosol.
INTRODUCTION
Marine aerosol occurring in cloud condensation nucleus (CCN) sizes suggest that it may contribute
notably to the CCN population (Meskhidze et al., 2006;Sorooshian et al., 2009), but further cloud droplet
number concentration would strongly depend on the chemical aerosol composition and ambient (cloud)
conditions, such as available water content, supersaturation and competition between the CCN of different
composition (O'Dowd et al., 1999). Since the global importance of marine aerosol particles to the cloud
formation postulated several decades ago (Charlson et al., 1987), it has progressed from the evaluation of
the nss-sulphate and sea salt effects to the acknowledgement of the significant role of organic aerosol
(O'Dowd et al., 2004). It was demonstrated that primary marine organics, despite its hydrophobic nature,
can possess the high CCN activation efficiency, resulting in the efficient cloud formation (Ovadnevaite et
al., 2011). Organic aerosol, ubiquitous in both the clean and polluted atmosphere, can be present as a pure
organic aerosol or and internally-mixed aerosol with other constituents such as sulphate and nitrate aerosol
(Kanakidou et al., 2005;Fuzzi et al., 2006). The hygroscopicity of organic aerosol in sub-saturated
humidity fields is typically less than most common salts found in the atmospheric aerosol (Liu et al.,
2010); however, the ability of organic aerosol to activate cloud droplets is predicted to be greatly
increased in supersaturated air due a lowering of the droplets surface tension, ultimately leading to more
nuclei being activated at lower supersaturations(Facchini et al., 2000). While this phenomenon has been
acknowledged for some time, it has yet to be demonstrated in the real atmosphere.
There are two major sources of marine organics –primary sea spray production and secondary new particle
formation. For the latter, organics can play a role in both formation and growth of the newly formed
particles. We have previously reported that new particle production occurs over the open North Atlantic
Ocean in polar marine air masses (Monahan et al., 2010). During these new particle production events, the
new particle mode typically injects 1,500-2,500 cm-3 new particles into the sub-100 nm size range and
over spatial scales ~1,500 km, resulting in approximate 4-fold increase in number concentration and, thus,
potentially CCN. Here we study the organic effect on primary and secondary marine aerosol activation to
CCN. Results from two intensive measurement campaigns in the Eastern North Atlantic (Mace Head) and
the Southern Ocean (PEGASO cruise) are presented here with the main focus on CCN dependence on
aerosol chemical composition and, especially, origin and sources of marine organic. We investigate the
activation of sea spray composed of the sea salt and externally mixed with nss-sulphate as well as the sea
spray highly enriched in organics, stressing the importance of the latter to the formation of the cloud
droplets. In addition, the organic effect on CCN activation of newly formed marine particles is
investigated. Moreover, the suitability of existing theories to explain the marine aerosol activation to CCN
is explored.
METHODS
CCN measurements were performed with a DMT CCN counter as well as a miniature Continuous Flow
Streamwise Thermal Gradient Chamber, which measure the fraction of aerosol that act as a CCN for a
range of supersaturations. During this study, the supersaturation spanned from 0.1% to 1 % for the former
and 0.2% to 0.82% for the latter. To perform a closure exercise, CCN concentrations were also calculated
using -Köhler theory (Petters et al., 2007) where the critical activation diameter (Dc) and  calculations
were constrained by AMS-derived chemical composition along with documented hygroscopic growthfactors for specific compounds (1.1, 1.8, 1.5, 2.4, 1.57 corresponded to OM, sulphate, nitrate, sea salt and
MSA, respectively) and associated relationships between growth-factor, Dc , , and Relative Humidity
(Petters et al., 2007). The final calculated CCN number concentration was then derived from SMPS
measurements by integrating all particles larger than Dc. The water tension of 0.072 J m-2 was used
unless otherwise specified. For the second method, critical activation diameters were derived from size
segregated CCN measurements and used similarly to the previous method - integrating all particles larger
than Dc to derive the calculated CCN.
Figure 1 shows the CCN activation dependence on aerosol chemical composition with very similar effects
in both locations, Southern Ocean as well as North East Atlantic. As expected, sea salt dominated particles
displayed the best activation or the smallest critical diameters at the same supersaturations. It was
followed by sulphate dominated particles, which activation was pretty similar to laboratory generated
ammonium sulphate particles. On average, the activation performance of marine organics was similar in
both locations, which points to a comparable biological source or, at least, the source resulting in aerosol
of the similar CCN properties. However, the CCN activation of marine organics was better if compared to
the anthropogenic organic matter activation (green line versus black in Figure 1, right side). Figure 1
contains only primary marine organics, which good activation into CCN has already been demonstrated
(Ovadnevaite et al., 2011).
Figure 1. Supersaturation and critical CCN activation diameter relationships for distinct chemical
composition particles. (Left) Southern Ocean PEGASO cruise (Right) Mace Head atmospheric research
station; Slope colours indicate the dominant aerosol compound derived from the pie charts in the upper
panel. Three clean aerosol cases for both PEGASO and Mace Head with dominant sulphate (red), sea salt
(brown) and organic matter (green) were plotted, in addition one polluted case with dominant
anthropogenic organics (black) was added to Mace Head plot. Also, laboratory activation curves for
ammonium sulphate (solid grey line) and sea salt (dashed grey line) are presented.
A potential of representing the dichotomous effects by a κ-Kohler theory was evaluated. κ-Kohler theory
is one of the most widely used CCN activation theories, which is claimed to be able to provide CCN
concentrations using either chemical aerosol composition or hygroscopisity. It usually works reasonably
well in regions dominated by anthropogenic sources yet its suitability for reproducing marine CCN has not
been properly investigated.
Similarly, a CCN activation representation by Kohler theory was investigated for organic rich newly
formed secondary particles registered at Mace Head. -Kohler equation was applied in two ways – using 
calculated from HR-ToF-AMS chemical composition and CCN activity derived from SS vs Dc equation,
obtained from the size segregated CCN measurements. Chemical composition derived  resulted in a
significant underestimation of total CCN particles if compared to the CCNC measurements during the
Open Ocean Nucleation events (O'Dowd et al., 2010). Organic matter showed a significant contribution to
both ultrafine and accumulation mode particles and its hydrophobic properties resulted in a low number of
CCN particles derived from combination of -Kohler theory and SMPS size distributions. On the other
hand, size segregated CCN indicated a different activity for ultrafine particles if compared to
accumulation mode ones. Former possessed smaller critical activation diameters than the latter at the same
supersaturations, which resulted in a higher number of CCN derived from ultrafine particles, which, in
turn, resulted in a closure between the measured and calculated CCN.
CONCLUSIONS
CCN concentrations were calculated using κ derived from aerosol hygroscopicity measurements and
compared to the ambient CCN measurements. The results show that at low super-saturation (0.3%) κKohler tends to underestimate marine CCN concentrations, pointing at primary marine organic effects.
However, calculated and measured CCN concentrations are in reasonably good agreement at high supersaturations (1%). This is due to the fact that critical particle activation diameter decreases with increasing
super-saturation and that marine organics tends to enhance the size of sea spray particles (Yoon et al.,
2007), thereby, the number of particles that haven’t been activated decreases with an increasing supersaturation. Given that models can reproduce this increase in particle size, the remaining discrepancies
between the real CCN and predicted by the κ-Kohler could be insignificant for certain super-saturations.
On the other hand, secondary marine organics exhibit similar enhancing effect on CCN activation, but
require a more sophisticated state of the art thermodynamic model to account for the liquid-liquid phase
separation (Zuend et al., 2012) and organic effect of ultrafine particles on CCN activation.
ACKNOWLEDGEMENTS
The research leading to these results has received funding from the European Union’s Seventh Framework
Programme (FP7/2007-2013) project BACCHUS under grant agreement n°603445, the Irish
Environmental Protection Agency, Spanish Ministry of Economy and Competitiveness (MINECO) as part
of the PEGASO (Ref.: CTM2012-37615) and BIO-NUC (Ref.: CGL2013-49020-R), HEA-PRTLI4.
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