Modeling Secondary Organic Aerosol Production from Photosensitized Humic-like Substances (HULIS)

Humic-like substances (HULIS) are ubiquitous in atmospheric aerosols. Despite experimental evidence that HULIS can catalyze secondary organic aerosol (SOA) formation through photosensitizer chemistry, the potential contribution of this pathway to ambient SOA has not been quantified. In this study, GAMMA, a photochemical box model, was used to analyze the experimental data of Monge et al. (Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 6840−6844) to quantify the kinetics of uptake of limonene by particles containing humic acid, a laboratory proxy for HULIS. The results indicate that limonene is taken up by irradiated particles containing humic acid efficiently, with a reactive uptake coefficient of 1.6 × 10^–4. Consequently, simulations of limonene–HULIS photosensitizer chemistry under ambient conditions, simultaneously with other aqueous SOA formation processes, show that this pathway could contribute up to 65% of the total aqueous SOA at pH 4. The potential importance of this pathway warrants further laboratory studies and representation of this SOA source in atmospheric models

Photochemical Aging of Light-Absorbing Secondary Organic Aerosol Material

Dark reactions of methylglyoxal with NH₄⁺ in aqueous aerosols yield light-absorbing and surface-active products that can influence the physical properties of the particles. Little is known about how the product mixture and its optical properties will change due to photolysis as well as oxidative aging by O₃ and OH in the atmosphere. Here, we report the results of kinetics and product studies of the photochemical aging of aerosols formed by atomizing aqueous solutions of methylglyoxal and ammonium sulfate. Experiments were performed using aerosol flow tube reactors coupled with an aerosol chemical ionization mass spectrometer (Aerosol–CIMS) for monitoring gas- and particle-phase compositions. Particles were also impacted onto quartz windows in order to assess changes in their UV–visible absorption upon oxidation. Photooxidation of the aerosols leads to the formation of small, volatile organic acids including formic acid, acetic acid, and glyoxylic acid. The atmospheric lifetime of these species during the daytime is predicted to be on the order of minutes, with photolysis being an important mechanism of degradation. The lifetime with respect to O₃ oxidation was observed to be on the order of hours. O₃ oxidation also leads to a net increase in light absorption by the particles due to the formation of additional carbonyl compounds. Our results are consistent with field observations of high brown carbon absorption in the early morning

Surface Disordering and Film Formation on Ice Induced by Formaldehyde and Acetaldehyde

Small aldehydes, such as formaldehyde (HCHO) and acetaldehyde (CH₃CHO), are known to contribute significantly to the OH and O₃ budgets in polar atmospheres via chemical interaction with snow and ice surfaces. However, our understanding of how these small aldehydes interact with ice surfaces is rather limited. In this work, ellipsometry was used to study the interaction of gas-phase HCHO and CH₃CHO with ice under partial pressure and temperature conditions akin to polar snowpack interstitial space. Both HCHO and CH₃CHO were found to induce surface change consistent with the formation of a disordered interfacial layer (DIL) at temperatures below which no intrinsic DIL is expected to exist (<i>T</i> < 238 K). For HCHO, induced surface disorder showed both temperature and partial pressure dependence. For CH₃CHO, temperature seemed to be the dominant factor; a DIL surface transition was observed in the presence of CH₃CHO at 223 ± 2 K, in agreement with earlier findings that partitioning behavior of CH₃CHO is different above and below this transition temperature. Exposure to HCHO or CH₃CHO was also observed to cause the formation of opaque domains on the ice surface, which may correspond to hydrate formation

Modeling Photosensitized Secondary Organic Aerosol Formation in Laboratory and Ambient Aerosols

Photosensitized reactions involving imidazole-2-carboxaldehyde (IC) have been experimentally observed to contribute to secondary organic aerosol (SOA) growth. However, the extent of photosensitized reactions in ambient aerosols remains poorly understood and unaccounted for in atmospheric models. Here we use GAMMA 4.0, a photochemical box model that couples gas-phase and aqueous-phase aerosol chemistry, along with recent laboratory measurements of the kinetics of IC photochemistry, to analyze IC-photosensitized SOA formation in laboratory and ambient settings. Analysis of the laboratory results of Aregahegn et al. (2013) suggests that photosensitized production of SOA from limonene, isoprene, α-pinene, β-pinene, and toluene by ³IC* occurs at or near the surface of the aerosol particle. Reactive uptake coefficients were derived from the experimental data using GAMMA 4.0. Simulations of aqueous aerosol SOA formation at remote ambient conditions including IC photosensitizer chemistry indicate less than 0.3% contribution to SOA growth from direct reactions of ³IC* with limonene, isoprene, α-pinene, β-pinene, and toluene, and an enhancement of less than 0.04% of SOA formation from other precursors due to the formation of radicals in the bulk aerosol aqueous phase. Other, more abundant photosensitizer species, such as humic-like substances (HULIS), may contribute more significantly to aqueous aerosol SOA production

Aerosol Brown Carbon from Dark Reactions of Syringol in Aqueous Aerosol Mimics

We performed a laboratory investigation of the chemical processing of syringol, a representative model phenolic compound emitted from wood burning, in concentrated aqueous salt solutions mimicking tropospheric aerosol particles. For solutions containing chloride salts, we observed the formation of light-absorbing organic products (“brown carbon”), accompanied by a phase separation, within 10 h under dark conditions. Products were characterized at the molecular level using ultraperformance liquid chromatography interfaced to diode array detection and high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization and matrix-assisted laser desorption/ionization interfaced to high-resolution time-of-flight mass spectrometry. The ultraviolet–visible spectra, together with high-resolution mass spectra results, suggest that syringol can be oxidized by dissolved oxygen, and the presence of Cl^– promotes this reaction. Our results provide new insights into the evolution of aerosol optical properties during aging, specifically the formation of aerosol brown carbon in biomass-burning plumes

Simulating Aqueous-Phase Isoprene-Epoxydiol (IEPOX) Secondary Organic Aerosol Production During the 2013 Southern Oxidant and Aerosol Study (SOAS)

The lack of statistically robust relationships between IEPOX (isoprene epoxydiol)-derived SOA (IEPOX SOA) and aerosol liquid water and pH observed during the 2013 Southern Oxidant and Aerosol Study (SOAS) emphasizes the importance of modeling the whole system to understand the controlling factors governing IEPOX SOA formation. We present a mechanistic modeling investigation predicting IEPOX SOA based on Community Multiscale Air Quality (CMAQ) model algorithms and a recently introduced photochemical box model, simpleGAMMA. We aim to (1) simulate IEPOX SOA tracers from the SOAS Look Rock ground site, (2) compare the two model formulations, (3) determine the limiting factors in IEPOX SOA formation, and (4) test the impact of a hypothetical sulfate reduction scenario on IEPOX SOA. The estimated IEPOX SOA mass variability is in similar agreement (<i>r</i>² ∼ 0.6) with measurements. Correlations of the estimated and measured IEPOX SOA tracers with observed aerosol surface area (<i>r</i>² ∼ 0.5–0.7), rate of particle-phase reaction (<i>r</i>² ∼ 0.4–0.7), and sulfate (<i>r</i>² ∼ 0.4–0.5) suggest an important role of sulfate in tracer formation via both physical and chemical mechanisms. A hypothetical 25% reduction of sulfate results in ∼70% reduction of IEPOX SOA formation, reaffirming the importance of aqueous phase chemistry in IEPOX SOA production

Observation of Organic Molecules at the Aerosol Surface

Organic molecules at the gas-particle interface of atmospheric aerosols influence the heterogeneous chemistry of the aerosol and impact climate properties. The ability to probe the molecules at the aerosol particle surface in situ therefore is important but has been proven challenging. We report the first successful observations of molecules at the surface of laboratory-generated aerosols suspended in air using the surface-sensitive technique second harmonic light scattering (SHS). As a demonstration, we detect trans-4-[4-(dibutylamino)­styryl]-1-methylpyridinium iodide and determine its population and adsorption free energy at the surface of submicron aerosol particles. This work illustrates a new and versatile experimental approach for studying how aerosol composition may affect the atmospheric properties

Ammonium Addition (and Aerosol pH) Has a Dramatic Impact on the Volatility and Yield of Glyoxal Secondary Organic Aerosol

Glyoxal is an important precursor to secondary organic aerosol (SOA) formed through aqueous chemistry in clouds, fogs, and wet aerosols, yet the gas-particle partitioning of the resulting mixture is not well understood. This work characterizes the volatility behavior of the glyoxal precursor/product mix formed after aqueous hydroxyl radical oxidation and droplet evaporation under cloud-relevant conditions for 10 min, thus aiding the prediction of SOA via this pathway (SOA_Cld). This work uses kinetic modeling for droplet composition, droplet evaporation experiments and temperature-programmed desorption aerosol–chemical ionization mass spectrometer analysis of gas-particle partitioning. An effective vapor pressure (<i>p</i>′_L,eff) of ∼10^–7 atm and an enthalpy of vaporization (Δ<i>H</i>_vap,eff) of ∼70 kJ/mol were estimated for this mixture. These estimates are similar to those of oxalic acid, which is a major product. Addition of ammonium until the pH reached 7 (with ammonium hydroxide) reduced the <i>p</i>′_L,eff to <10^–9 atm and increased the Δ<i>H</i>_vap,eff to >80 kJ/mol, at least in part via the formation of ammonium oxalate. pH 7 samples behaved like ammonium oxalate, which has a vapor pressure of ∼10^–11 atm. We conclude that ammonium addition has a large effect on the gas-particle partitioning of the mixture, substantially enhancing the yield of SOA_Cld from glyoxal

Aqueous-Phase Secondary Organic Aerosol and Organosulfate Formation in Atmospheric Aerosols: A Modeling Study

We have examined aqueous-phase secondary organic aerosol (SOA) and organosulfate (OS) formation in atmospheric aerosols using a photochemical box model with coupled gas-phase chemistry and detailed aqueous aerosol chemistry. SOA formation in deliquesced ammonium sulfate aerosol is highest under low-NO<i>_x</i> conditions, with acidic aerosol (pH = 1) and low ambient relative humidity (40%). Under these conditions, with an initial sulfate loading of 4.0 μg m^–3, 0.9 μg m^–3 SOA is predicted after 12 h. Low-NO<i>_x</i> aqueous-aerosol SOA (aaSOA) and OS formation is dominated by isoprene-derived epoxydiol (IEPOX) pathways; 69% or more of aaSOA is composed of IEPOX, 2-methyltetrol, and 2-methyltetrol sulfate ester. 2-Methyltetrol sulfate ester comprises >99% of OS mass (66 ng m^–3 at 40% RH and pH 1). In urban (high-NO_<i>x</i>) environments, aaSOA is primarily formed via reversible glyoxal uptake, with 0.12 μg m^–3 formed after 12 h at 80% RH, with 20 μg m^–3 initial sulfate. OS formation under all conditions studied is maximum at low pH and lower relative humidities (<60% RH), i.e., when the aerosol is more concentrated. Therefore, OS species are expected to be good tracer compounds for aqueous aerosol-phase chemistry (vs cloudwater processing)

Photoactivated Production of Secondary Organic Species from Isoprene in Aqueous Systems

Photoactivated reactions of organic species in atmospheric aerosol particles are a potentially significant source of secondary organic aerosol material (SOA). Despite recent progress, the dominant chemical mechanisms and rates of these reactions remain largely unknown. In this work, we characterize the photophysical properties and photochemical reaction mechanisms of imidazole-2-carboxaldehyde (IC) in aqueous solution, alone and in the presence of isoprene. IC has been shown previously in laboratory studies to participate in photoactivated chemistry in aerosols, and it is a known in-particle reaction product of glyoxal. Our experiments confirmed that the triplet excited state of IC is an efficient triplet photosensitizer, leading to photosensitization of isoprene in aqueous solution and promoting its photochemical processing in aqueous solution. Phosphorescence and transient absorption studies showed that the energy level of the triplet excited state of IC (³IC*) was approximately 289 kJ/mol, and the lifetime of ³IC* in water under ambient temperature is 7.9 μs, consistent with IC acting as an efficient triplet photosensitizer. Laser flash photolysis experiments displayed fast quenching of ³IC* by isoprene, with a rate constant of (2.7 ± 0.3) × 10⁹ M^–1 s^–1, which is close to the diffusion-limited rate in water. Mass spectrometry analysis showed that the products formed include IC–isoprene adducts, and chemical mechanisms are discussed. Additionally, oxygen quenches ³IC* with a rate constant of (3.1 ± 0.1) × 10⁹ M^–1 s^–1