Ice-nucleating agents in sea spray aerosol identified and quantified with a holistic multimodal freezing model.

Sea spray aerosol (SSA) is a widely recognized important source of ice-nucleating particles (INPs) in the atmosphere. However, composition-specific identification, nucleation processes, and ice nucleation rates of SSA-INPs have not been well constrained. Microspectroscopic characterization of ambient and laboratory-generated SSA confirms that water-borne exudates from planktonic microorganisms composed of a mixture of proteinaceous and polysaccharidic compounds act as ice-nucleating agents (INAs). These data and data from previously published mesocosm and wave channel studies are subsequently used to further develop the stochastic freezing model (SFM) producing ice nucleation rate coefficients for SSA-INPs. The SFM simultaneously predicts immersion freezing and deposition and homogeneous ice nucleation by SSA particles under tropospheric conditions. Predicted INP concentrations agree with ambient and laboratory measurements. In addition, this holistic freezing model is independent of the source and exact composition of the SSA particles, making it well suited for implementation in cloud and climate models.

Emerging investigator series: chemical and physical properties of organic mixtures on indoor surfaces during HOMEChem.

Organic films on indoor surfaces serve as a medium for reactions and for partitioning of semi-volatile organic compounds and thus play an important role in indoor chemistry. However, the chemical and physical properties of these films are poorly characterized. Here, we investigate the chemical composition of an organic film collected during the HOMEChem campaign, over three cumulative weeks in the kitchen, using both Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and offline Aerosol Mass Spectrometry (AMS). We also characterize the viscosity of this film using a model based on molecular formulas as well as poke-flow measurements. We find that the film contains organic material similar to cooking organic aerosol (COA) measured during the campaign using on-line AMS. However, the average molecular formula observed using FT-ICR MS is ∼C50H90O11, which is larger and more oxidized than fresh COA. Solvent extracted film material is a low viscous semisolid, with a measured viscosity <104 Pa s. This is much lower than the viscosity model predicts, which is parametrized with atmospherically relevant organic molecules, but sensitivity tests demonstrate that including unsaturation can explain the differences. The presence of unsaturation is supported by reactions of film material with ozone. In contrast to the solvent extract, manually removed material appears to be highly viscous, highlighting the need for continued work understanding both viscosity measurements as well as parameterizations for modeled viscosity of indoor organic films.

Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces.

Chemical reactions on indoor surfaces play an important role in air quality in indoor environments, where humans spend 90% of their time. We focus on the challenges of understanding the complex chemistry that takes place on indoor surfaces and identify crucial steps necessary to gain a molecular-level understanding of environmental indoor surface chemistry: (1) elucidate key surface reaction mechanisms and kinetics important to indoor air chemistry, (2) define a range of relevant and representative surfaces to probe, and (3) define the drivers of surface reactivity, particularly with respect to the surface composition, light, and temperature. Within the drivers of surface composition are the roles of adsorbed/absorbed water associated with indoor surfaces and the prevalence, inhomogeneity, and properties of secondary organic films that can impact surface reactivity. By combining laboratory studies, field measurements, and modeling we can gain insights into the molecular processes necessary to further our understanding of the indoor environment.

Photolytic Aging of Secondary Organic Aerosol: Evidence for a Substantial Photo-Recalcitrant Fraction.

Photolytic aging has been proposed as a major mass loss mechanism for atmospheric secondary organic aerosol (SOA). However, estimated mass loss rates vary by orders of magnitude, and their impacts on modeled SOA loadings and properties are highly uncertain. In this study, photolysis rates and composition changes of α-pinene SOA are analyzed in situ over multiple days in an environmental chamber. After an initial exponential decay (τ ∼ 22 h), the mass loss rate slows dramatically, with more than ∼70-90% of the SOA particulate mass undergoing an essentially negligible photolytic degradation. Scaled to ambient conditions, SOA undergoes rapid photolysis over only its first day in the atmosphere; beyond this, the remaining SOA is photo-recalcitrant, and photolysis ceases to be a major sink compared to wet deposition time scales. Thus, extrapolation of the initial photolysis loss rate to the entire aerosol mass may significantly overestimate the role of photolysis in the removal of atmospheric SOA.

Infrared Ion Spectroscopy of Environmental Organic Mixtures: Probing the Composition of α-Pinene Secondary Organic Aerosol.

Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.

Condensed-phase biogenic-anthropogenic interactions with implications for cold cloud formation.

Anthropogenic and biogenic gas emissions contribute to the formation of secondary organic aerosol (SOA). When present, soot particles from fossil fuel combustion can acquire a coating of SOA. We investigate SOA-soot biogenic-anthropogenic interactions and their impact on ice nucleation in relation to the particles' organic phase state. SOA particles were generated from the OH oxidation of naphthalene, α-pinene, longifolene, or isoprene, with or without the presence of sulfate or soot particles. Corresponding particle glass transition (Tg) and full deliquescence relative humidity (FDRH) were estimated using a numerical diffusion model. Longifolene SOA particles are solid-like and all biogenic SOA sulfate mixtures exhibit a core-shell configuration (i.e. a sulfate-rich core coated with SOA). Biogenic SOA with or without sulfate formed ice at conditions expected for homogeneous ice nucleation, in agreement with respective Tg and FDRH. α-pinene SOA coated soot particles nucleated ice above the homogeneous freezing temperature with soot acting as ice nuclei (IN). At lower temperatures the α-pinene SOA coating can be semisolid, inducing ice nucleation. Naphthalene SOA coated soot particles acted as ice nuclei above and below the homogeneous freezing limit, which can be explained by the presence of a highly viscous SOA phase. Our results suggest that biogenic SOA does not play a significant role in mixed-phase cloud formation and the presence of sulfate renders this even less likely. However, anthropogenic SOA may have an enhancing effect on cloud glaciation under mixed-phase and cirrus cloud conditions compared to biogenic SOA that dominate during pre-industrial times or in pristine areas.

Measuring Mass-Based Hygroscopicity of Atmospheric Particles through in Situ Imaging.

Quantifying how atmospheric particles interact with water vapor is critical for understanding the effects of aerosols on climate. We present a novel method to measure the mass-based hygroscopicity of particles while characterizing their elemental and carbon functional group compositions. Since mass-based hygroscopicity is insensitive to particle geometry, it is advantageous for probing the hygroscopic behavior of atmospheric particles, which can have irregular morphologies. Combining scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX), scanning transmission X-ray microscopy (STXM) analysis, and in situ STXM humidification experiments, this method was validated using laboratory-generated, atmospherically relevant particles. Then, the hygroscopicity and elemental composition of 15 complex atmospheric particles were analyzed by leveraging quantification of C, N, and O from STXM, and complementary elemental quantification from SEM/EDX. We found three types of hygroscopic responses, and correlated high hygroscopicity with Na and Cl content. The mixing state of 158 other particles from the sample broadly agreed with those of the humidified particles, indicating the potential to infer atmospheric hygroscopic behavior from a selected subset of particles. These methods offer unique quantitative capabilities to characterize and correlate the hygroscopicity and chemistry of individual submicrometer atmospheric particles.

Liquid-liquid phase separation in aerosol particles: imaging at the nanometer scale.

Atmospheric aerosols can undergo phase transitions including liquid-liquid phase separation (LLPS) while responding to changes in the ambient relative humidity (RH). Here, we report results of chemical imaging experiments using environmental scanning electron microscopy (ESEM) and scanning transmission X-ray microscopy (STXM) to investigate the LLPS of micrometer-sized particles undergoing a full hydration-dehydration cycle. Internally mixed particles composed of ammonium sulfate (AS) and either: limonene secondary organic carbon (LSOC), α, 4-dihydroxy-3-methoxybenzeneaceticacid (HMMA), or polyethylene glycol (PEG-400) were studied. Events of LLPS were observed for all samples with both techniques. Chemical imaging with STXM showed that both LSOC/AS and HMMA/AS particles were never homogeneously mixed for all measured RH's above the deliquescence point and that the majority of the organic component was located in the outer phase. The outer phase composition was estimated as 65:35 organic: inorganic in LSOC/AS and as 50:50 organic: inorganic for HMMA/AS. PEG-400/AS particles showed fully homogeneous mixtures at high RH and phase separated below 89-92% RH with an estimated 70:30% organic to inorganic mix in the outer phase. These two chemical imaging techniques are well suited for in situ analysis of the hygroscopic behavior, phase separation, and surface composition of collected ambient aerosol particles.

Reactivity of liquid and semisolid secondary organic carbon with chloride and nitrate in atmospheric aerosols.

Constituents of secondary organic carbon (SOC) in atmospheric aerosols are often mixed with inorganic components and compose a significant mass fraction of fine particulate matter in the atmosphere. Interactions between SOC and other condensed-phase species are not well understood. Here, we investigate the reactions of liquid-like and semisolid SOC from ozonolysis of limonene (LSOC) and α-pinene (PSOC) with NaCl using a set of complementary microspectroscopic analyses. These reactions result in chloride depletion in the condensed phase, release of gaseous HCl, and formation of organic salts. The reactions attributed to acid displacement by SOC acidic components are driven by the high volatility of HCl. Similar reactions can take place in SOC/NaNO3 particles. The results show that an increase in SOC mass fraction in the internally mixed SOC/NaCl particles leads to higher chloride depletion. Glass transition temperatures and viscosity of PSOC were estimated for atmospherically relevant conditions. Data show that the reaction extent depends on SOC composition, particle phase state and viscosity, mixing state, temperature, relative humidity (RH), and reaction time. LSOC shows slightly higher potential to deplete chloride than PSOC. Higher particle viscosity at low temperatures and RH can hinder these acid displacement reactions. Formation of organic salts from these overlooked reactions can alter particle physiochemical properties and may affect their reactivity and ability to act as cloud condensation and ice nuclei. The release and potential recycling of HCl and HNO3 from reacted aerosol particles may have important implications for atmospheric chemistry.