Probing Individual Particles Generated at the Freshwater–Seawater Interface through Combined Raman, Photothermal Infrared, and X-ray Spectroscopic Characterization

Abstract

Sea spray aerosol (SSA) is one of the largest global sources of atmospheric aerosol, but little is known about SSA generated in coastal regions with salinity gradients near estuaries and river outflows. SSA particles are chemically complex with substantial particle-to-particle variability due to changes in water temperature, salinity, and biological activity. In previous studies, the ability to resolve the aerosol composition to the level of individual particles has proven necessary for the accurate parameterization of the direct and indirect aerosol effects; therefore, measurements of individual SSA particles are needed for the characterization of this large source of atmospheric aerosol. An integrated analytical measurement approach is required to probe the chemical composition of individual SSA particles. By combining complementary vibrational microspectroscopic (Raman and optical photothermal infrared, O-PTIR) measurements with elemental information from computer-controlled scanning electron microscopy with energy-dispersive X-ray analysis (CCSEM− EDX), we gained unique insights into the individual particle chemical composition and morphology. Herein, we analyzed particles from four experiments on laboratory-based SSA production using coastal seawater collected in January 2018 from the Gulf of Maine. Individual salt particles were enriched in organics compared to that in natural seawater, both with and without added microalgal filtrate, with greater enrichment observed for smaller particle sizes, as evidenced by higher carbon/sodium ratios. Functional group analysis was carried out using the Raman and infrared spectra collected from individual SSA particles. Additionally, the Raman spectra were compared with a library of Raman spectra consisting of marine-derived organic compounds. Saccharides, followed by fatty acids, were the dominant components of the organic coatings surrounding the salt cores of these particles. This combined Raman, infrared, and X-ray spectroscopic approach will enable further understanding of the factors determining the individual particle composition, which is important for understanding the impacts of SSA produced within estuaries and river outflows, as well as areas of snow and ice melt.

Effects of Wind Speed on Size-Dependent Morphology and Composition of Sea Spray Aerosols

“…In recent years, SEM-EDX has emerged as an important tool in offline particle analysis for characterizing the elemental composition within individual particles. , Figure A shows a representative core–shell with internal mixing containing a prism core and a shell of organic carbon where the core is characterized as sodium chloride (NaCl), with a Cl/Na ratio ranging between 0.4 and 0.8. This ratio is smaller than what is typically observed in seawater (1.2–1.8), suggesting a depletion of chloride ions (Cl – ) may have occurred during and after sampling. ,, Additionally, a slight increase in magnesium (Mg) around the core indicates a possible coating of magnesium chloride (MgCl 2 ), which is consistent with previous studies conducted on SSA core–shells. ,, Rounded particles (Figure B) exhibited a high carbon content, which is often associated with organic carbon. Previous studies have reported that some rounded particles possess a carbon coating with a sulfur-rich core, but these types of particles were not observed in the current analysis. ,, Figure C shows rod inclusion core–shells which contain a NaCl core, organic shell, and rods with elevated counts of oxygen, sulfur, calcium, and magnesium.…”

“…This ratio is smaller than what is typically observed in seawater (1.2−1.8), suggesting a depletion of chloride ions (Cl − ) may have occurred during and after sampling. 41,83,84 Additionally, a slight increase in magnesium (Mg) around the core indicates a possible coating of magnesium chloride (MgCl 2 ), which is consistent with previous studies conducted on SSA core−shells. 41,85,86 Rounded particles (Figure 6B) exhibited a high carbon content, which is often associated with organic carbon.…”

“…5,7,40,69,89 The peaks in the 1550−1750 cm and 1000 cm −1 are associated with ν(C−C) or (C−H) side group deformations. 7,69,84,90 The shell spectra of core−shells at 10 m/s showed a broad peak between 1350 and 1450 cm −1 , which corresponds to aliphatic-rich compounds (δ (CH 2 , CH 3 )), as supported by the presence of peaks in the 800−1000 cm −1 region (C−H wags). 84,91,92 A similar spectrum was also observed for core− shells at a 19 m/s wind speed.…”

“…7,69,84,90 The shell spectra of core−shells at 10 m/s showed a broad peak between 1350 and 1450 cm −1 , which corresponds to aliphatic-rich compounds (δ (CH 2 , CH 3 )), as supported by the presence of peaks in the 800−1000 cm −1 region (C−H wags). 84,91,92 A similar spectrum was also observed for core− shells at a 19 m/s wind speed. However, most of the particles for this wind speed showed the presence of oxygenated functionalized groups.…”

Vibrational spectroscopy as a probe of heterogeneities within geochemical thin films on macro, micro, and nanoscales

“…This allows for the identication of different chemical species and phases present as well as a way to distinguish some of the unique "local environments" due to hydration, complexation, different phases, and other chemical and physical heterogeneities within environmental systems. 26,27 By monitoring the peak position, shape, and intensity of vibrational bands, molecular level insights can be gained. While a great deal of the current literature has used different vibrational methods, the recent development of micro-spectroscopic probes has been found to provide information on much smaller length scales.…”