Impact of organic coating on optical growth of ammonium sulfate particles

Phase Transitions in Organic and Organic/Inorganic Aerosol Particles

The phase state of aerosol particles can impact numerous atmospheric processes, including new particle growth, heterogeneous chemistry, cloud condensation nucleus formation, and ice nucleation. In this article, the phase transitions of inorganic, organic, and organic/inorganic aerosol particles are discussed, with particular focus on liquid-liquid phase separation (LLPS). The physical chemistry that determines whether LLPS occurs, at what relative humidity it occurs, and the resultant particle morphology is explained using both theoretical and experimental methods. The known impacts of LLPS on aerosol processes in the atmosphere are discussed. Finally, potential evidence for LLPS from field and chamber studies is presented. By understanding the physical chemistry of the phase transitions of aerosol particles, we will acquire a better understanding of aerosol processes, which in turn impact human health and climate.

Optical trapping and light scattering in atmospheric aerosol science

Aerosol particles are ubiquitous in the atmosphere, and currently contribute a large uncertainty to climate models. Part of the endeavour to reduce this uncertainty takes the form of improving our understanding of aerosol at the microphysical level, thus enabling chemical and physical processes to be more accurately represented in larger scale models. In addition to modeling efforts, there is a need to develop new instruments and methodologies to interrogate the physicochemical properties of aerosol. This perspective presents the development, theory, and application of optical trapping, a powerful tool for single particle investigations of aerosol. After providing an overview of the role of aerosol in Earth's atmosphere and the microphysics of these particles, we present a brief history of optical trapping and a more detailed look at its application to aerosol particles. We also compare optical trapping to other single particle techniques. Understanding the interaction of light with single particles is essential for interpreting experimental measurements. In the final part of this perspective, we provide the relevant formalism for understanding both elastic and inelastic light scattering for single particles. The developments discussed here go beyond Mie theory and include both how particle and beam shape affect spectra. Throughout the entirety of this work, we highlight numerous references and examples, mostly from the last decade, of the application of optical trapping to systems that are relevant to the atmospheric aerosol.

Does liquid-liquid phase separation impact ice nucleation in mixed polyethylene glycol and ammonium sulfate droplets?

Particles can undergo different phase transitions in the atmosphere including deliquescence, liquid-liquid phase separation (LLPS), melting, and freezing. In this study, phase transitions of particles/droplets containing polyethylene glycol with a molar mass of 400 g mol^-1 (PEG400) and ammonium sulfate (AS), i.e., PEG400-AS particles/droplets, were investigated at different organic-to-inorganic dry mass ratios (OIRs) under typical tropospheric temperatures and water activities (a_w). The investigated droplets (60-100 μm) with or without LLPS in the closed system froze through homogeneous ice nucleation. At temperatures lower than 200 K, multiple ice nucleation events were observed within the same individual droplets at low a_w. Droplets with and without LLPS shared similar lambda values at the same OIR according to the lambda approach indicating they form ice through the same mechanism. A parameterization of lambda values was provided which can be used to predict freezing temperature of aqueous PEG400-AS droplets. We found that adding AS reduces the temperature dependence of a_w in aqueous PEG400 droplets. Assuming incorrectly that a_w is temperature-independent for a constant droplet composition leads to a deviation between the experimental determined ice nucleation rate coefficients for droplets at OIR > 1 and the predicted values by the water-activity-based ice nucleation theory. We proposed a parameterization of temperature dependence of a_w to minimize the deviations of the measured melting temperatures and nucleation rate coefficients from the corresponding predictions for aqueous PEG400-AS system.

Coexistence of three liquid phases in individual atmospheric aerosol particles

Aerosol particles are ubiquitous in the atmosphere and play an important role in air quality and the climate system. These particles can contain mixtures of primary organic aerosol, secondary organic aerosol, and secondary inorganic aerosol. We show that such internally mixed particles can contain three liquid phases. We also demonstrate that the presence of three liquid phases impacts the time needed for the particles to reach equilibrium with the surrounding gas phase and likely impacts the ability of the particles to activate into cloud droplets. A framework is presented for predicting conditions needed for the formation of three liquid phases in the atmosphere. These results will lead to improved representations of aerosols in models for air quality and climate predictions.

Individual atmospheric particles can contain mixtures of primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA). To predict the role of such complex multicomponent particles in air quality and climate, information on the number and types of phases present in the particles is needed. However, the phase behavior of such particles has not been studied in the laboratory, and as a result, remains poorly constrained. Here, we show that POA+SOA+SIA particles can contain three distinct liquid phases: a low-polarity organic-rich phase, a higher-polarity organic-rich phase, and an aqueous inorganic-rich phase. Based on our results, when the elemental oxygen-to-carbon (O:C) ratio of the SOA is less than 0.8, three liquid phases can coexist within the same particle over a wide relative humidity range. In contrast, when the O:C ratio of the SOA is greater than 0.8, three phases will not form. We also demonstrate, using thermodynamic and kinetic modeling, that the presence of three liquid phases in such particles impacts their equilibration timescale with the surrounding gas phase. Three phases will likely also impact their ability to act as nuclei for liquid cloud droplets, the reactivity of these particles, and the mechanism of SOA formation and growth in the atmosphere. These observations provide fundamental information necessary for improved predictions of air quality and aerosol indirect effects on climate.

Exploring the formation potential and optical properties of secondary organic aerosol from the photooxidation of selected short aliphatic ethers

Secondary organic aerosol (SOA) formation potential for six kinds of short aliphatic ethers has been studied. The size distribution, mass concentration, and yield of SOA formed by ethers photooxidation were determined under different conditions. The results showed that all six ethers can generate SOA via reaction with OH radicals even under no seed and NO_x-free condition. The mass concentration for six seedless experiments was less than 10 µg/m³ and the SOA yields were all below 1%. The strong increase in the SOA formation was observed when the system contained ammonium sulfate seed particles, while SOA yield decreased under the high-NO_x condition. SOA composition was analyzed using offline methods. Infrared spectra indicated that there are complex components in the particle-phase including carbonyls acid and aldehydes species. Moreover, the aqueous filter extracts were analyzed using ultraviolet-visible spectrometer and fluorescence spectrophotometer. For the fresh methyl n-butyl ether SOA, the largest absorption peak occurs at 280 nm and there exists slightly absorption in the 300-400 nm. Excitation-emission matrices display the distinct peak at excitation/emission = 470 nm/480 nm according to the fluorescence spectrum. These findings are important considerations of formation for ether SOA that can eventually be included in atmospheric models.

Relative Humidity History Affects Hygroscopicity of Mixed Particles of Glyoxal and Reduced Nitrogenous Species

The relative humidity (RH) history that manifests the cycling of dehydration (water evaporation) and hydration (water uptake) may affect particle-phase reactions, products from which have strong influences on the physical properties and thus climatic effects of atmospheric particles. Using single-trapped particles, we show herein hygroscopic growths of mixed particles with reactive species undergoing three types of RH cycles, simulating different degrees of particle-phase reactions in the atmosphere. The reactive species are the widely known α-dicarbonyl glyoxal (GLY), and five reduced nitrogenous species, ammonium sulfate (AS), glycine (GC), l-alanine (AL), dimethylamine (DMA), and diethylamine (DEA). The results showed that the mixed particles after reactions generally had altered efflorescence relative humidity (ERH) and deliquescence relative humidity (DRH) values and reduced hygroscopic growths at moderately high RH (>80%) conditions. For example, with an additional slow drying step, the mean mass growth factors at 90% RH during dehydration dropped from 2.56 to 2.02 for GC/GLY mixed particles and from 2.45 to 1.23 for AL/GLY mixed particles. The reduced hygroscopicity with more RH cycling will thus lead to less efficient light scattering of the mixed particles, thereby resulting in less cooling and exacerbating direct heating due to light absorption by the products formed.

Phase separation in organic aerosol

Liquid–liquid phase separation is prevalent in aerosol particles composed of organic compounds and salts and may impact aerosol climate effects.

Characterization of particulate products for aging of ethylbenzene secondary organic aerosol in the presence of ammonium sulfate seed aerosol

Aging of secondary organic aerosol (SOA) particles formed from OH- initiated oxidation of ethylbenzene in the presence of high mass (100-300μg/m(3)) concentrations of (NH4)2SO4 seed aerosol was investigated in a home-made smog chamber in this study. The chemical composition of aged ethylbenzene SOA particles was measured using an aerosol laser time-of-flight mass spectrometer (ALTOFMS) coupled with a Fuzzy C-Means (FCM) clustering algorithm. Experimental results showed that nitrophenol, ethyl-nitrophenol, 2,4-dinitrophenol, methyl glyoxylic acid, 5-ethyl-6-oxo-2,4-hexadienoic acid, 2-ethyl-2,4-hexadiendioic acid, 2,3-dihydroxy-5-ethyl-6-oxo-4-hexenoic acid, 1H-imidazole, hydrated N-glyoxal substituted 1H-imidazole, hydrated glyoxal dimer substituted imidazole, 1H-imidazole-2-carbaldehyde, N-glyoxal substituted hydrated 1H-imidazole-2-carbaldehyde and high-molecular-weight (HMW) components were the predominant products in the aged particles. Compared to the previous aromatic SOA aging studies, imidazole compounds, which can absorb solar radiation effectively, were newly detected in aged ethylbenzene SOA in the presence of high concentrations of (NH4)2SO4 seed aerosol. These findings provide new information for discussing aromatic SOA aging mechanisms.