The presence or absence of liquid-liquid phase separation (LLPS) in aerosol particles containing oxidized organic species and inorganic salts affects particle morphology and influences uptake into, diffusion through, and reactivity within those particles. We report here an accessible method, similar to ice core analyses, using solutions that are relevant for both aerosol chemical systems and aqueous two-phase extraction systems and contain ammonium sulfate and one of eight alcohols (methanol, ethanol, 1-propanol, 2-propanol, 2-butaonol, 3-methyl-2-butanol, 1,2-propanediol, or 1,3-propanediol) frozen in articulated (bendable) straws to probe LLPS. For alcohols with negative octanol-water partitioning coefficient (K OW) values and O/C ratios ≥0.5, no LLPS occurs, while for alcohols with positive K OW values and O/C ratios ≤0.33, phase separation always occurs, both findings consistent with observations using different experimental techniques. When a third species, glyoxal, is added, the glyoxal stays in the aqueous phase, regardless of whether LLPS occurs. When phase separation occurs, the glyoxal forms a strong intermolecular interaction with the sulfate ion, red-shifting the ν3(SO4 2-) peak by 15 cm-1. These results provide evidence of chemical interactions within phase-separated systems that have implications for understanding chemical reactivity within those, and related, systems.

The physical properties of aqueous secondary organic aerosol (SOA) can be influenced by the chemical composition of the solvent matrix within which SOA-forming reactions take place. The chemical structure and functionality of perturbing solutes on solution viscosity in bulk-phase SOA-modeling reaction mixtures containing glyoxal and ammonium sulfate were measured for a series of oxidized C-1-C-5 compounds. In general, as the solute concentration increased, solution viscosity increased, with the magnitude of those increases dependent upon the solute species added. The presence of glyoxal increased viscosity in about 50% of mixtures as a result of specific solute/solvent/product interactions. The oxygen/carbon (O/C) ratio was found to have a linear relationship with relative viscosity at a high solute concentration such that every -CH2CH2OH moiety addition increased solution viscosity by 25%. Solutes with identical O/C ratios but different structural characteristics (for example, acetone, 1-propanol, and 2-propanol) showed 7-8% differences in viscosity as a function of the chain length or ability to act as a hydrogen bond donor. The addition of an alcohol group increased viscosity by 10-20% over hydrogen or methyl groups in the same position. The solute structure appears to play a role in mediating intrasolvent interactions, and increases in hydrogen bonding and intersolute interactions lead to increased bulk solution viscosity, as shown in a series of experiments varying the timing and order of solute addition. These solutes are models for oxidized aerosol components, and the structure-dependent results suggest the importance of the aqueous layer functional group presence on aerosol physical properties, such as viscosity.

The chemical and physical properties of secondary organic aerosol (SOA) may be influenced by the aqueous environment within which SOA-forming reactions occur. Bulk-phase reactions between glyoxal and ammonium sulfate produce light-absorbing compounds such as imidazole-2-carboxaldehyde (IC) and biimidazole (BI) and model aerosol-phase chemistry. The effect of solvent composition on IC and BI production kinetics were examined for solutions containing a series of short-chain alcohols (methanol, ethanol, 1-propanol, 2-propanol, 2-butanol, tert-butanol, and 3-methyl-2-butanol), as functions of the species and concentration added. The addition of an alcohol likely altered the solvent by inducing the formation of micro-heterogeneities of hydrated alcohol molecules. An increased alcohol concentration in solution resulted in a general increase in the production of IC and BI, with ethanol and the secondary alcohols causing the largest changes. The rate constants for IC and BI production in solutions that do not contain alcohol are found to be (4.03 +/- 2.02) x 10(-6) s(-1) and (3.48 +/- 1.75) x 10(-6) s(-1), respectively. In solutions that do contain an alcohol, rate constant values range from 3.23 x 10(-6) s(-1) to 6.61 x 10(-6) s(-1) for IC and 0.985 x 10(-6) s(-1) to 6.87 x 10(-6) s(-1) for BI, depending on the alcohol and the concentration. While these prevailing trends of more alcohol in solution leading to amplified production of imidazoles were observed for all seven alcohols, the diversity of responses implies the mode of action depends strongly on the species of alcohol added. Properties that describe a solution in bulk, such as dielectric constant, are not found to be reliable predictors of subsequent system behavior. Solvent composition influences SOA chemistry, and interactions between relevant functional groups within solution drive this influence, which suggests the importance of understanding individual relationships between aerosol-phase chemicals and SOA properties and production.

Nitrous acid (HONO) is a photochemical source of hydroxyl radical and nitric oxide in the atmosphere that stems from abiotic and biogenic processes, including the activity of ammonia-oxidizing soil microbes. HONO fluxes were measured from agricultural and urban soil in mesocosm studies aimed at characterizing biogenic sources and linking them to indigenous microbial consortia. Fluxes of HONO from agricultural and urban soil were suppressed by addition of a nitrification inhibitor and enhanced by amendment with ammonium (NH4(+)), with peaks at 19 and 8 ng m(-2) s(-1), respectively. In addition, both agricultural and urban soils were observed to convert (15)NH4(+) to HO(15)NO. Genomic surveys of soil samples revealed that 1.5-6% of total expressed 16S rRNA sequences detected belonged to known ammonia oxidizing bacteria and archaea. Peak fluxes of HONO were directly related to the abundance of ammonia-oxidizer sequences, which in turn depended on soil pH. Peak HONO fluxes under fertilized conditions are comparable in magnitude to fluxes reported during field campaigns. The results suggest that biogenic HONO emissions will be important in soil environments that exhibit high nitrification rates (e.g., agricultural soil) although the widespread occurrence of ammonia oxidizers implies that biogenic HONO emissions are also possible in the urban and remote environment.

Nitrous acid (HONO) is an important OH radical source that is formed on both ground and aerosol surfaces in the well-mixed boundary layer. Large uncertainties remain in quantifying HONO sinks and determining the mechanism of HONO uptake onto surfaces. We report here the first laboratory determination of HONO uptake coefficients onto actual soil under atmospheric conditions using a coated-wall flow tube coupled to a highly sensitive chemical ionization mass spectrometer (CIMS). Uptake coefficients for HONO decrease with increasing RH from (2.5 ± 0.4) × 10(-4) at 0% RH to (1.1 ± 0.4) × 10(-5) at 80% RH. A kinetics model of competitive adsorption of HONO and water onto the particle surfaces fits the dependence of the HONO uptake coefficients on the initial HONO concentration and relative humidity. However, a multiphase resistor model based on the physical and chemical processes affecting HONO uptake is more flexible as it accounts for the pH dependence of HONO uptake and bulk diffusion in the soil matrix. Fourier transform infrared (FTIR) spectrometry and cavity-enhanced absorption spectroscopy (CEAS) studies indicate that NO and N2O (16% and 13% yield, respectively) rather than NO2 are the predominant gas phase products, while NO2(-) and NO3(-) were detected on the surface post-exposure. Results are compared to uptake coefficients inferred from models and field measurements, and the atmospheric implications are discussed.

Nitrate (NO3(-)) is an abundant component of aerosols, boundary layer surface films, and surface water. Photolysis of NO3(-) leads to NO2 and HONO, both of which play important roles in tropospheric ozone and OH production. Field and laboratory studies suggest that NO3¯ photochemistry is a more important source of HONO than once thought, although a mechanistic understanding of the variables controlling this process is lacking. We present results of cavity-enhanced absorption spectroscopy measurements of NO2 and HONO emitted during photodegradation of aqueous NO3(-) under acidic conditions. Nitrous acid is formed in higher quantities at pH 2-4 than expected based on consideration of primary photochemical channels alone. Both experimental and modeled results indicate that the additional HONO is not due to enhanced NO3(-) absorption cross sections or effective quantum yields, but rather to secondary reactions of NO2 in solution. We find that NO2 is more efficiently hydrolyzed in solution when it is generated in situ during NO3(-) photolysis than for the heterogeneous system where mass transfer of gaseous NO2 into bulk solution is prohibitively slow. The presence of nonchromophoric OH scavengers that are naturally present in the environment increases HONO production 4-fold, and therefore play an important role in enhancing daytime HONO formation from NO3(-) photochemistry.

The reaction of Cl atoms with CH3D proceeds either by abstraction of hydrogen to produce HCl + CH2D or by abstraction of deuterium to produce DCl + CH3. Using Cl atoms with different amounts of translational energy, produced by photolysis of Cl2 with 309, 355, or 416 nm light, reveals the influence of translational energy on the relative reaction probability for the two channels. These measurements give an estimate of the energy barrier for the reaction for comparison to theory and indicate that tunneling is the dominant reaction mechanism at low collision energies. Adding two quanta of C-H stretching vibration causes the reaction to proceed readily at all collision energies. Detecting the vibrational state of the CH2D product shows that vibrational energy initially in the surviving C-H bond appears as vibrational excitation of the product, an example of spectator behavior in the reaction. The reaction produces both stretch and stretch-bend excited products except at the lowest collision energy. A subtle variation in the reaction probability of the lowest energy rotational states with translational energy may reflect the presence of a van der Waals well in the entrance channel.

Author Institution: Chemistry Department, University of Wisconsin - Madison, Madison, Wisconsin 53706

Author Institution: Chemistry Department, University of Wisconsin - Madison, Madison, Wisconsin 53706

Author Institution: Chemistry Department, University of Wisconsin - Madison, Madison, Wisconsin 53706

Author Institution: Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706