Infrared Optical Constants of Organic Aerosols: Organic Acids and Model Humic-Like Substances (HULIS)

<div><p>Aerosols are important atmospheric constituents as they impact the Earth's energy balance and climate. An analysis of the impact of aerosols depends on the detailed knowledge of aerosol optical properties. However, there is a lack of refractive index data for atmospherically relevant organic compounds in the infrared (IR) region which complicates the quantitative estimation of the aerosol influence on the radiative balance. In this study, we investigate the optical properties of atmospherically relevant carboxylic acids and HUmic-LIke Substances (HULIS) proxies. Aerosol size distributions are measured simultaneously with Fourier transform infrared (FTIR) extinction spectra to calculate the complex refractive index. Scanning electron microscopy (SEM) images are also collected to investigate particle shape. Analysis of SEM images shows evidence for agglomeration in some cases. The experimentally measured IR resonances do not appear to be highly sensitive to agglomeration effects. However, there is an increase in the scattering efficiency at shorter wavelengths as the result of larger overall particle size of the agglomerates. Refractive indices are retrieved from the IR extinction spectra of organic acids and HULIS proxies. Mie simulation results confirm the quality of the retrieved optical constants. Interestingly, the optical constants determined for the acids are in agreement with the published data for fire smoke plumes.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

Size Matters in the Water Uptake and Hygroscopic Growth of Atmospherically Relevant Multicomponent Aerosol Particles

Understanding the interactions of water with atmospheric aerosols is crucial for determining the size, physical state, reactivity, and climate impacts of this important component of the Earth’s atmosphere. Here we show that water uptake and hygroscopic growth of multicomponent, atmospherically relevant particles can be size dependent when comparing 100 nm versus ca. 6 μm sized particles. It was determined that particles composed of ammonium sulfate with succinic acid and of a mixture of chlorides typical of the marine environment show size-dependent hygroscopic behavior. Microscopic analysis of the distribution of components within the aerosol particles show that the size dependence is due to differences in the mixing state, that is, whether particles are homogeneously mixed or phase separated, for different sized particles. This morphology-dependent hygroscopicity has consequences for heterogeneous atmospheric chemistry as well as aerosol interactions with electromagnetic radiation and clouds

Substrate-Deposited Sea Spray Aerosol Particles: Influence of Analytical Method, Substrate, and Storage Conditions on Particle Size, Phase, and Morphology

Atmospheric aerosols are often collected on substrates and analyzed weeks or months after the initial collection. We investigated how the selection of substrate and microscopy method influence the measured size, phase, and morphology of sea spray aerosol (SSA) particles and how sample storage conditions affect individual particles using three common microscopy techniques: optical microscopy, atomic force microscopy, and scanning electron microscopy. Micro-Raman spectroscopy was used to determine changes in the water content of stored particles. The results show that microscopy techniques operating under ambient conditions provide the most relevant and robust measurement of particle size. Samples stored in a desiccator and at ambient conditions leads to similar sizes and morphologies, while storage that involves freezing and thawing leads to irreversible changes due to phase changes and water condensation. Typically, SSA particles are deposited wet and, if possible, samples used for single-particle analysis should be stored at or near conditions at which they were collected in order to avoid dehydration. However, if samples need to be dry, as is often the case, then this study found that storing SSA particles at ambient laboratory conditions (17–23% RH and 19–21 °C) was effective at preserving them and reducing changes that would alter samples and subsequent data interpretation

Analysis of Organic Anionic Surfactants in Fine and Coarse Fractions of Freshly Emitted Sea Spray Aerosol

The inclusion of organic compounds in freshly emitted sea spray aerosol (SSA) has been shown to be size-dependent, with an increasing organic fraction in smaller particles. Here we have used electrospray ionization-high resolution mass spectrometry in negative ion mode to identify organic compounds in nascent sea spray collected throughout a 25 day mesocosm experiment. Over 280 organic compounds from ten major homologous series were tentatively identified, including saturated (C₈–C₂₄) and unsaturated (C₁₂–C₂₂) fatty acids, fatty acid derivatives (including saturated oxo-fatty acids (C₅–C₁₈) and saturated hydroxy-fatty acids (C₅–C₁₈), organosulfates (C₂–C₇, C₁₂–C₁₇) and sulfonates (C₁₆–C₂₂). During the mesocosm, the distributions of molecules within some homologous series responded to variations among the levels of phytoplankton and bacteria in the seawater. The average molecular weight and carbon preference index of saturated fatty acids significantly decreased within fine SSA during the progression of the mesocosm, which was not observed in coarse SSA, sea-surface microlayer or in fresh seawater. This study helps to define the molecular composition of nascent SSA and biological processes in the ocean relate to SSA composition

Microbial Control of Sea Spray Aerosol Composition: A Tale of Two Blooms

With the oceans covering 71% of the Earth, sea spray aerosol (SSA) particles profoundly impact climate through their ability to scatter solar radiation and serve as seeds for cloud formation. The climate properties can change when sea salt particles become mixed with insoluble organic material formed in ocean regions with phytoplankton blooms. Currently, the extent to which SSA chemical composition and climate properties are altered by biological processes in the ocean is uncertain. To better understand the factors controlling SSA composition, we carried out a mesocosm study in an isolated ocean-atmosphere facility containing 3,400 gallons of natural seawater. Over the course of the study, two successive phytoplankton blooms resulted in SSA with vastly different composition and properties. During the first bloom, aliphatic-rich organics were enhanced in submicron SSA and tracked the abundance of phytoplankton as indicated by chlorophyll-a concentrations. In contrast, the second bloom showed no enhancement of organic species in submicron particles. A concurrent increase in ice nucleating SSA particles was also observed only during the first bloom. Analysis of the temporal variability in the concentration of aliphatic-rich organic species, using a kinetic model, suggests that the observed enhancement in SSA organic content is set by a delicate balance between the rate of phytoplankton primary production of labile lipids and enzymatic induced degradation. This study establishes a mechanistic framework indicating that biological processes in the ocean and SSA chemical composition are coupled not simply by ocean chlorophyll-a concentrations, but are modulated by microbial degradation processes. This work provides unique insight into the biological, chemical, and physical processes that control SSA chemical composition, that when properly accounted for may explain the observed differences in SSA composition between field studies