Abstract
The impact of sea spray aerosols (SSAs) on Earth’s climate remains uncertain in part due to size-dependent particle-to-particle variability in SSA physicochemical properties such as morphology, composition, phase state, and water uptake that can be further modulated by the environment relative humidity (RH). The current study investigates these properties as a function of particle size and RH, while focusing on submicrometer nascent SSA (0.1–0.6 μm) collected throughout a phytoplankton bloom. Filter-based thermal optical analysis, atomic force microscopy (AFM), and AFM photothermal infrared spectroscopy (AFM–PTIR) were utilized in this regard. AFM imaging at 20% RH identified five main SSA morphologies: prism-like, core–shell, rounded, rod, and aggregate. The majority of smaller SSAs throughout a bloom were rounded, while larger SSAs were core–shell. Filter-based measurements revealed an increasing organic mass fraction with decreasing SSA size. The organic matter is shown to primarily reside in a rounded and core–shell SSA, while the prism-like and rod SSA are predominantly inorganic salts (i.e., sodium chloride, nitrates, and sulfates) with relatively low organic content, as determined by AFM–PTIR spectroscopy. AFM phase state measurements at 20% RH revealed an increasing abundance of core–shell SSA with semisolid shells and rounded SSA with a solid phase state, as the particle size decreases. At 60% RH, shells of core–shell and rounded SSA uptake water, become less viscous, and their phase states change into either semisolid or liquid. Collectively, findings reveal the dynamic and size-dependent nature of SSA’s morphology, composition, phase states, and water uptake, which should be considered to accurately predict their climate-related effects.
Highlights
IntroductionSea spray aerosols (SSAs) are generated upon the bursting of air bubbles entrained from breaking waves in the ocean and constitute a significant fraction of natural aerosol mass concentration in atmosphere.[1−5] SSA climate-relevant effects include their ability to influence Earth’s radiative budget directly, by scattering and absorbing incoming solar radiation, and indirectly, by acting as cloud condensation nuclei (CCN) or ice nuclei (IN).[1,2,6−12] Due to their abundance, SSAs provide a significant source of suspended interfaces that can facilitate heterogeneous chemical reactions with atmospheric gases.[1,13−15] During air bubble bursting at the ocean−air interface, the organic, inorganic, and biological species that are either dissolved in bulk seawater or enriched at the sea surface microlayer (SML, the uppermost organic layer with submicrometer thickness) can be transferred into the SSA.[16−22] the chemical complexity of SSA can vary significantly depending on composition and biological activity in the seawater and SML, as well as SSA generation mechanisms via bubbles bursting.[3,16,19,20,23−27] For example, submicrometer SSAs, produced via bubble-cap bursting (i.e., film drops, typical dry SSA diameter ≤ 0.25 μm) are significantly enriched with organic matter compared to SSAs produced via bubble-cavity collapse (i.e., jet drops, typical dry SSA diameter ≥ 0.25 μm), which are predominantly inorganic salts.[3,16−19,21,24,28−30] the type and concentration of organic molecules in SSA can vary with phytoplankton and heterotrophic bacteria (HB) activities in the seawater.[3,21,30,31]Measurements of chemical speciation in SSAs generated under controlled laboratory and mesocosm experimentsReceived: September 2, 2021 Revised: October 21, 2021 Accepted: November 16, 2021 Published: December 9, 2021ACS Earth and Space Chemistry http://pubs.acs.org/journal/aesccqArticle revealed the presence of organic compounds, such as alkanes, fatty acids, saccharides, proteins, and inorganic compounds containing sodium, calcium, and chloride.[1,3,16,17,20,25,30−32] This chemical complexity in SSA can govern their direct and indirect aerosol effects in atmosphere.[2,8,10,26,33−42] For example, the organic and inorganic content in SSA controls their phase state and water uptake, which alters SSA’s atmospheric aging, scattering of solar radiation, CCN and IN abilities.[2−4,6,9,10,15,26,34−40,43−49] Prior studies performed on model and nascent SSA revealed the existence of different morphologies or physicochemical mixing states (core−shell, prism-like, rod, and rounded)[2,8−10,26,34,41,42,50] at varying phase states from solid to semisolid to liquid,[2,8,38,41,42] and differing extent of water uptake.[9,35,36,40] In the context of phase state, the characteristic mass-transport time for nonvolatile organic species in a semisolid particle (viscosity range of ∼102 to 1012 Pa s, particle diameter of 100 nm) can vary from seconds to years; while for a liquid particle, it only takes microseconds to milliseconds.[15,51,52] the rate of atmospheric chemical aging of semisolid SSAs is relatively lower compared to a liquid SSA.[15,52,53] solid SSAs are more likely to be activated as IN, while semisolid and liquid SSA are more likely to be activated as CCN.[15,44,45,54] The comprehensive information about SSA phase state and water uptake is important to accurately predict their climatic relevance
Article revealed the presence of organic compounds, such as alkanes, fatty acids, saccharides, proteins, and inorganic compounds containing sodium, calcium, and chloride.[1,3,16,17,20,25,30−32] This chemical complexity in SSA can govern their direct and indirect aerosol effects in atmosphere.[2,8,10,26,33−42] For example, the organic and inorganic content in SSA controls their phase state and water uptake, which alters SSA’s atmospheric aging, scattering of solar radiation, cloud condensation nuclei (CCN) and ice nuclei (IN) abilities.[2−4,6,9,10,15,26,34−40,43−49] Prior studies performed on model and nascent SSA revealed the existence of different morphologies or physicochemical mixing states[2,8−10,26,34,41,42,50] at varying phase states from solid to semisolid to liquid,[2,8,38,41,42] and differing extent of water uptake.[9,35,36,40]
The SSA samples collected on July 26th, Aug 2nd, and Aug 6th were selected to investigate the size-dependent relative distribution of bulk ensemble-averaged and single particle organic enrichment, single particle morphologies, compositions, phase states, and water uptake properties of SSA
Summary
Sea spray aerosols (SSAs) are generated upon the bursting of air bubbles entrained from breaking waves in the ocean and constitute a significant fraction of natural aerosol mass concentration in atmosphere.[1−5] SSA climate-relevant effects include their ability to influence Earth’s radiative budget directly, by scattering and absorbing incoming solar radiation, and indirectly, by acting as cloud condensation nuclei (CCN) or ice nuclei (IN).[1,2,6−12] Due to their abundance, SSAs provide a significant source of suspended interfaces that can facilitate heterogeneous chemical reactions with atmospheric gases.[1,13−15] During air bubble bursting at the ocean−air interface, the organic, inorganic, and biological species that are either dissolved in bulk seawater or enriched at the sea surface microlayer (SML, the uppermost organic layer with submicrometer thickness) can be transferred into the SSA.[16−22] the chemical complexity of SSA can vary significantly depending on composition and biological activity in the seawater and SML, as well as SSA generation mechanisms via bubbles bursting.[3,16,19,20,23−27] For example, submicrometer SSAs, produced via bubble-cap bursting (i.e., film drops, typical dry SSA diameter ≤ 0.25 μm) are significantly enriched with organic matter compared to SSAs produced via bubble-cavity collapse (i.e., jet drops, typical dry SSA diameter ≥ 0.25 μm), which are predominantly inorganic salts.[3,16−19,21,24,28−30] the type and concentration of organic molecules in SSA can vary with phytoplankton and heterotrophic bacteria (HB) activities in the seawater.[3,21,30,31]Measurements of chemical speciation in SSAs generated under controlled laboratory and mesocosm experimentsReceived: September 2, 2021 Revised: October 21, 2021 Accepted: November 16, 2021 Published: December 9, 2021ACS Earth and Space Chemistry http://pubs.acs.org/journal/aesccqArticle revealed the presence of organic compounds, such as alkanes, fatty acids, saccharides, proteins, and inorganic compounds containing sodium, calcium, and chloride.[1,3,16,17,20,25,30−32] This chemical complexity in SSA can govern their direct and indirect aerosol effects in atmosphere.[2,8,10,26,33−42] For example, the organic and inorganic content in SSA controls their phase state and water uptake, which alters SSA’s atmospheric aging, scattering of solar radiation, CCN and IN abilities.[2−4,6,9,10,15,26,34−40,43−49] Prior studies performed on model and nascent SSA revealed the existence of different morphologies or physicochemical mixing states (core−shell, prism-like, rod, and rounded)[2,8−10,26,34,41,42,50] at varying phase states from solid to semisolid to liquid,[2,8,38,41,42] and differing extent of water uptake.[9,35,36,40] In the context of phase state, the characteristic mass-transport time for nonvolatile organic species in a semisolid particle (viscosity range of ∼102 to 1012 Pa s, particle diameter of 100 nm) can vary from seconds to years; while for a liquid particle, it only takes microseconds to milliseconds.[15,51,52] the rate of atmospheric chemical aging of semisolid SSAs is relatively lower compared to a liquid SSA.[15,52,53] solid SSAs are more likely to be activated as IN, while semisolid and liquid SSA are more likely to be activated as CCN.[15,44,45,54] The comprehensive information about SSA phase state and water uptake is important to accurately predict their climatic relevance. No existing studies have directly probed and correlated the size, morphology, and seawater biological activity-dependent phase state and water uptake of submicrometer nascent SSA under subsaturated RH conditions
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