Abstract

Abstract. The extent to which water uptake influences the light scattering ability of marine sea spray aerosol (SSA) particles depends critically on SSA chemical composition. The organic fraction of SSA can increase during phytoplankton blooms, decreasing the salt content and therefore the hygroscopicity of the particles. In this study, subsaturated hygroscopic growth factors at 85 % relative humidity (GF(85 %)) of predominately submicron SSA particles were quantified during two induced phytoplankton blooms in marine aerosol reference tanks (MARTs). One MART was illuminated with fluorescent lights and the other was illuminated with sunlight, referred to as the "indoor" and "outdoor" MARTs, respectively. Optically weighted GF(85 %) values for SSA particles were derived from measurements of light scattering and particle size distributions. The mean optically weighted SSA diameters were 530 and 570 nm for the indoor and outdoor MARTs, respectively. The GF(85 %) measurements were made concurrently with online particle composition measurements, including bulk composition (using an Aerodyne high-resolution aerosol mass spectrometer) and single particle (using an aerosol time-of-flight mass spectrometer) measurement, and a variety of water-composition measurements. During both microcosm experiments, the observed optically weighted GF(85 %) values were depressed substantially relative to pure inorganic sea salt by 5 to 15 %. There was also a time lag between GF(85 %) depression and the peak chlorophyll a (Chl a) concentrations by either 1 (indoor MART) or 3-to-6 (outdoor MART) days. The fraction of organic matter in the SSA particles generally increased after the Chl a peaked, also with a time lag, and ranged from about 0.25 to 0.5 by volume. The observed depression in the GF(85 %) values (relative to pure sea salt) is consistent with the large observed volume fractions of non-refractory organic matter (NR-OM) comprising the SSA. The GF(85 %) values exhibited a reasonable negative correlation with the SSA NR-OM volume fractions after the peak of the blooms (i.e., Chl a maxima); i.e., the GF(85 %) values generally decreased when the NR-OM volume fractions increased. The GF(85 %) vs. NR-OM volume fraction relationship was interpreted using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule and used to estimate the GF(85 %) of the organic matter in the nascent SSA. The estimated pure NR-OM GF(85 %) values were 1.16 ± 0.09 and 1.23 ± 0.10 for the indoor and outdoor MARTS, respectively. These measurements demonstrate a clear relationship between SSA particle composition and the sensitivity of light scattering to variations in relative humidity. The implications of these observations to the direct climate effects of SSA particles are discussed.

Highlights

  • Aerosols impact climate directly by scattering and absorbing solar radiation and indirectly by modifying cloud properties (IPCC, 2013)

  • The observed concentration of supermicron particles was somewhat lower than that previously reported from a marine aerosol reference tanks (MARTs) (Stokes et al, 2013) and likely reflects greater gravitational losses of supermicron particles in the long sampling line used here (Fig. S4)

  • Since the hygroscopicity measurements discussed in this study are based on measurements made using polydisperse distributions, it is useful to determine the effective scattering-weighted particle diameters that characterize the MART size distributions

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Summary

Introduction

Aerosols impact climate directly by scattering and absorbing solar radiation and indirectly by modifying cloud properties (IPCC, 2013). Breaking waves in the ocean entrain air into seawater, leading to the formation of bubbles, which burst at the ocean’s surface and produce SSA particles (Lewis and Schwartz, 2004). The climate impacts of SSA particles depend critically on their composition, shape, and size (Carslaw et al, 2013; Pilinis et al, 1995). The size of SSA particles is modified through water uptake and loss, which are strong functions of chemical composition (Saxena et al, 1995). Since organic compounds are universally less hygroscopic than inorganic sea salt (Petters and Kreidenweis, 2007), their transfer to SSA will lead to less water uptake and, less scattering than in the case of pure inorganic sea salt particles of the same size

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