Abstract
Abstract. Marine low clouds strongly influence global climate, and their radiative effects are particularly susceptible to the concentration of cloud condensation nuclei (CCN). One major source of CCN is the condensational growth of pre-CCN particles, and sulfate has long been considered the major condensing species in the remote marine boundary layer. While some studies have suggested that secondary organic species can contribute to particle growth, its importance remains unclear. Here we present the first long-term observational evidence that organics play an important role in particle growth over remote oceans. To the contrary of traditional thinking, sulfate dominated condensational growth for only a small (∼18 %) fraction of the 62 observed growth events, even fewer than the organic-dominated events (24 %). During most (58 %) growth events, the major condensing species included both organics and sulfate. Potential precursors of the secondary organics are volatile organic compounds from ocean biological activities and those produced by the air–sea interfacial oxidation. Our results indicate that the condensation of secondary organics contributes strongly to the growth of pre-CCN particles and thereby the CCN population over remote oceans.
Highlights
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)
Routine measurements at the Eastern North Atlantic (ENA) site used in this study include the non-refractory submicron aerosol (NR-PM1) composition characterized by an aerosol chemical speciation monitor (ACSM; Aerodyne Research, Inc., Billerica, MA, USA) (Watson, 2017) and particle hygroscopic growth measured by a humidified tandem differential mobility analyzer (HTDMA; Brechtel Manufacturing Inc., CA, USA) (Uin, 2016)
Κc,CCN is instead positively correlated with the methanesulfonic acid (MSA)/SO2 ratio (Fig. 6c), indicating that secondary organics formed from phytoplankton-emitted volatile organic compound (VOC) likely played a minor role in the observed particle condensational growth
Summary
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). 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 be from other sources, including other autotrophs and atmospheric depositions (Wurl et al, 2011). This type of oceanic VOCs and SOA formed may not correlate with MSA (Wurl et al, 2011; Mungall et al, 2017; Brüggemann et al, 2018). Our results show that the organics represent an important or even dominant condensing species during ∼ 80 % of growth events
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