Experimental determination of the partitioning of representative organic pollutants to the air-water interface.

Using glancing-angle laser-induced fluorescence (GALIF) spectroscopy as a probe, the partitioning of naphthalene, fluoranthene, pyrene, umbelliferone, phenol red, and bisphenol A from bulk solution to the air-water interface was examined in both pure water and aqueous solutions of 6 mM octanol. Previous studies provided similar Langmuir adsorption isotherms for anthracene and imidazole 2-carboxaldehyde. The surface partitioning behaviour of each compound in both environments was well described using a Langmuir adsorption model; partitioning coefficients were derived from the fits to such isotherms. Only the PAH molecules, naphthalene, fluoranthene and pyrene, saw an enhancement in the surface partitioning in octanol solution compared to pure water. The surface partitioning to pure water surfaces could be fairly well described using a one parameter linear free energy relationship based on either solubility or KOW.

Photochemical renoxification on commercial indoor photoactive paint.

Surface chemistry plays an important role in the indoor environment owing to the large indoor surface to volume ratio. This study explores the photoreactivity of surfaces painted with a photoactive paint in the presence of NOx. Two types of experiments are performed; illumination of painted surfaces with a nitrate deposit and illumination of painted surfaces in the presence of gaseous NO. For both types of experiments, illumination with a fluorescent bulb causes the greatest change in measured gaseous NOx concentrations. Results show that relative humidity and paint composition play an important role in the photoreactivity of indoor painted surfaces. Painted surfaces could contribute to gas-phase oxidant concentrations indoors.

Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets.

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3- formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir-Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols.

Uptake of Hydrogen Peroxide from the Gas Phase to Grain Boundaries: A Source in Snow and Ice.

Hydrogen peroxide is a primary atmospheric oxidant significant in terminating gas-phase chemistry and sulfate formation in the condensed phase. Laboratory experiments have shown an unexpected oxidation acceleration by hydrogen peroxide in grain boundaries. While grain boundaries are frequent in natural snow and ice and are known to host impurities, it remains unclear how and to which extent hydrogen peroxide enters this reservoir. We present the first experimental evidence for the diffusive uptake of hydrogen peroxide into grain boundaries directly from the gas phase. We have machined a novel flow reactor system featuring a drilled ice flow tube that allows us to discern the effect of the ice grain boundary content on the uptake. Further, adsorption to the ice surface for temperatures from 235 to 258 K was quantified. Disentangling the contribution of these two uptake processes shows that the transfer of hydrogen peroxide from the atmosphere to snow at temperatures relevant to polar environments is considerably more pronounced than previously thought. Further, diffusive uptake to grain boundaries appears to be a novel mechanism for non-acidic trace gases to fill the highly reactive impurity reservoirs in snow's grain boundaries.

Ionic Composition of Particulate Matter across Canada Shows Evidence for Near-Ubiquitous Chloride Activation and Sulfate Neutralization.

Using publicly available data from the National Air Pollution Surveillance Program, water-soluble ion mole fractions in PM2.5 and PM2.5-10 were extracted for 2010, 2012, 2013, and 2015 at six locations across Canada. Fine particle ion content was dominated by ammonium sulfate, with ammonium to sulfate ratios largely approaching 2 and no seasonal or geographic trends. Sulfate and nitrate mole fractions were negatively correlated in the fine particles, consistent with their expected pathways of entry into PM2.5. Coarse particle composition varied depending on location and season. Coastal stations were dominated by sodium and chloride year-round while inland stations showed high amounts of these ions only during the colder months, where road salt is often applied. A negative correlation between nitrate and chloride mole fractions as well as a positive correlation between nitrate concentrations and "missing" chloride provides evidence for near-ubiquitous nitrate displacement of chloride. This displacement was strongly indicated in coastal sites and also evident inland, especially during cold months. Weak evidence for nitrogen dioxide as the source of nitrate was found for inland sites, but none was found for coastal sites, suggesting a nonlocal source for the particulate nitrate precursor.

Reactive Uptake of Gas-Phase NO2 by Urban Road Dust in the Dark.

Road dust constitutes a prominent source of anthropogenic particulate matter, making its heterogeneous interactions with common atmospheric gas-phase compounds important. Here, we show that three distinct samples of urban road dust-including dust samples collected from city streets in summer and winter, and an urban park in summer-react with NO2 in the dark, forming NO and surface nitrite. The loss of NO2 ranged from ∼2 to 13% of its gas-phase concentration and scaled with its concentration as well as with the mass of the road dust sample. The uptake of NO2 by the winter dust was ∼4 times greater than that seen from summer street dust, which was in turn greater than that by the park dust. The conversion ratio of NO2 → NO ranged from 0.06 to 0.8 NO produced per NO2 lost and was greatest for the summer park dust. Exposure of the summer road dust to NO2 roughly doubles the concentration of inorganic nitrite anion in the dust but does not produce nitrate. The formation of NO and photolabile nitrite products means that heterogeneous NO x reactions occurring on the surface of road dust in the dark could have wide implications for the oxidative potential of urban areas.

Experimental Confirmation of H2O2 Adsorption at the Water-Air Interface.

Recent work has reported that hydrogen peroxide is formed at the air-water interface. Given the reduced solvation environment there, this process could give rise to enhanced production of OH from H2O2 photolysis at the interface. These considerations give some importance to understanding the adsorption thermochemistry of hydrogen peroxide. Although there are two molecular dynamics studies that provide the adsorption free energy, to date there is no experimental verification that H2O2 adsorbs at the air-water interface. Here we use glancing-angle Raman spectroscopy to follow the surface adsorption behavior of this molecule. Using standard states of 1 mol L-1 for each of the bulk and surface phases yields a ΔG° of -5 kJ mol-1 at 293 K, comparable to that obtained for DMSO.

Chemical Morphology Controls Reactivity of OH Radicals at the Air-Ice Interface.

At the air-ice interface, some aromatic compounds such as benzene and anthracene are surprisingly unreactive toward OH. This may be a consequence of the poor solvation of these compounds at the interface, resulting in clustering there. We test this hypothesis by comparing the reaction of OH with pyrene, a 4-ring polyaromatic hydrocarbon (PAH), to reactions of OH with the more water-soluble compounds coumarin and 7-hydroxycoumarin (7OHC). We observe that OH reacts readily with coumarin and 7OHC at both liquid and frozen air-water interfaces. Pyrene, a much less soluble compound, reacts with OH at the liquid surface but not at the air-ice interface. We report evidence of pyrene aggregation at the ice surface based on its broadened and red-shifted emission spectrum alongside fluorescence mapping of anthracene, a closely related 3-ring PAH, which shows bunching at the ice surface. By contrast, fluorescence mapping shows that coumarin is fairly homogeneously distributed at the air-ice interface. Together, these results suggest that the limited reactivity of some compounds toward OH at the ice surface may be a consequence of their propensity to self-aggregate, demonstrating that chemical morphology can play an important role in reactions at the ice surface.

Modeling the Removal of Water-Soluble Trace Gases from Indoor Air via Air Conditioner Condensate.

Water-soluble trace gas (WSTG) loss from indoor air via air conditioning (AC) units has been observed in several studies, but these results have been difficult to generalize. In the present study, we designed a box model that can be used to investigate and estimate WSTG removal due to partitioning to AC coil condensate. We compared the model output to measurements of a suite of organic acids cycling in an indoor environment and tested the model by varying the input AC parameters. These tests showed that WSTG loss via AC cycling is influenced by Henry's law constant of the compound in question, which is controlled by air and water temperatures and the condensate pH. Air conditioning unit specifications also impact WSTG loss through variations in the sensible heat ratio, the effective recirculation rate of air through the unit, and the timing of coil and fan operation. These findings have significant implications for indoor modeling. To accurately model the fate of indoor WSTGs, researchers must either measure or otherwise account for these unique environmental and operational characteristics.

Loss of NO(g) to painted surfaces and its re-emission with indoor illumination.

Heterogeneous surface reactions play a key role in the chemistry of the indoor environment because of the large indoor surface-to-volume ratio. The presence of photocatalytic material in indoor paints may allow photochemical reactions to occur at wavelengths of light that are present indoors. One such potential reaction is the heterogeneous photooxidation of NO to HONO. NO(g) is commonly found indoors, originating from combustion sources, ventilation and infiltration of outdoor air. We studied the interaction of NO(g) with painted surfaces illuminated with indoor fluorescent and incandescent lighting. There is a loss of NO(g) to painted surfaces in the dark at both 0 and 50% RH. At 50% RH, there is a re-release of some of that NO(g) under illumination. The same behavior is observed for illumination of different colored paints. This is in contrast to what is seen with TiO2 as the substrate, where photoenhanced uptake of NO(g) and formation of NO2 (g) are observed. We hypothesize that the loss of NO(g) is due to adsorption and diffusion into the paint. The re-release of NO under illumination is thought to be due to photooxidation of NO to HONO on the painted surface at higher relative humidities and subsequent HONO photolysis.

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.

Water uptake by indoor surface films.

Indoor surfaces provide a plentiful and varied substrate on which multiphase reactions can occur which can be important to the chemical makeup of the indoor environment. Here, we attempt to characterise real indoor surface films via water uptake behaviour and ionic composition. We show that water uptake by indoor films is different than that observed outdoors, and can vary according to room use, building characteristics, and season. Similarly, preliminary investigation into the ionic composition of the films showed that they varied according to the room in which they were collected. This study highlights the importance of different types of soiling to multiphase chemistry, especially those reactions controlled by relative humidity or adsorbed water.

Seasonality of the Water-Soluble Inorganic Ion Composition and Water Uptake Behavior of Urban Grime.

Impervious surfaces, especially in urban environments, are coated with a film composed of a complex mixture of substances, referred to as urban grime. Despite its ubiquity, the factors that dictate urban grime composition are still not well understood. Here, we present the first study of the seasonal variation in composition of water-soluble inorganic ions present in urban grime, performed by analyzing samples collected in Toronto for 4-week intervals over the course of a year. A clear seasonality in the composition is evident, with NaCl dominating in the winter months and Ca2+ and NO3- dominant in the summer. We compare the grime composition to the water-soluble ion composition of PM2.5 and PM10 in order to infer chemistry occurring within the grime and find evidence that chemistry occurring within the urban grime matrix could provide a source of ClNO2 and NH3 to the urban atmosphere. The uptake of water by urban grime also shows a clear seasonality, which may be driven by the changing proportions of nitrate salts and/or oxidized organic compounds over the year.

Differences in photochemistry between seawater and freshwater for two natural organic matter samples.

We report changes in the excitation and resolved fluorescence spectra, inferred triplet formation and singlet oxygen formation abilities of two different Natural Organic Matter samples (NOM) in seawater vs. freshwater or NaCl solution. In artificial seawater solution (but not in NaCl solution), the natural water-derived NOM samples Suwannee River Natural Organic Matter (SRNOM) and Nordic Reservoir Natural Organic Matter (NRNOM) display large enhancements in fluorescence intensity. Nearly identical spectra are seen when seawater is replaced by solutions of Mg2+ at its seawater concentration, consistent with magnesium binding to ligand sites of the natural organic matter giving rise to different photophysics. Fluorescence anisotropy measurements show a decrease in anisotropy of SRNOM and NRNOM in seawater, also consistent with Mg2+ binding. Different effects of Mg2+ are seen when the different NOM samples are illuminated: NRNOM exhibits increased formation of its triplet state and also quenching of its triplet by oxygen, compared to its photochemistry in the absence of Mg2+, while SRNOM exhibits a reduction in triplet formation in the presence of Mg2+. These observations imply that the photochemistry of NOM in seawater may be very different from what is expected based on freshwater or NaCl solution measurements.

Fatty Acid Surfactant Photochemistry Results in New Particle Formation.

Organic interfaces that exist at the sea surface microlayer or as surfactant coatings on cloud droplets are highly concentrated and chemically distinct from the underlying bulk or overlying gas phase. Therefore, they may be potentially unique locations for chemical or photochemical reactions. Recently, photochemical production of volatile organic compounds (VOCs) was reported at a nonanoic acid interface however, subsequent secondary organic aerosol (SOA) particle production was incapable of being observed. We investigated SOA particle formation due to photochemical reactions occurring at an air-water interface in presence of model saturated long chain fatty acid and alcohol surfactants, nonanoic acid and nonanol, respectively. Ozonolysis of the gas phase photochemical products in the dark or under continued UV irradiation both resulted in nucleation and growth of SOA particles. Irradiation of nonanol did not yield detectable VOC or SOA production. Organic carbon functionalities of the SOA were probed using X-ray microspectroscopy and compared with other laboratory generated and field collected particles. Carbon-carbon double bonds were identified in the condensed phase which survived ozonolysis during new particle formation and growth. The implications of photochemical processes occurring at organic coated surfaces are discussed in the context of marine SOA particle atmospheric fluxes.

Water Evaporation from Acoustically Levitated Aqueous Solution Droplets.

We present a systematic study of the effect of solutes on the evaporation rate of acoustically levitated aqueous solution droplets by suspending individual droplets in a zero-relative humidity environment and measuring their size as a function of time. The ratios of the early time evaporation rates of six simple salts (NaCl, NaBr, NaNO3, KCl, MgCl2, CaCl2) and malonic acid to that of water are in excellent agreement with predictions made by modifying the Maxwell equation to include the time-dependent water activity of the evaporating aqueous salt solution droplets. However, the early time evaporation rates of three ammonium salt solutions (NH4Cl, NH4NO3, (NH4)2SO4) are not significantly different from the evaporation rate of pure water. This finding is in accord with a previous report that ammonium sulfate does not depress the evaporation rate of its solutions, despite reducing its water vapor pressure, perhaps due to specific surface effects. At longer evaporation times, as the droplets approach crystallization, all but one (MgCl2) of the solution evaporation rates are well described by the modified Maxwell equation.

Chemical Morphology of Frozen Mixed Nitrate-Salt Solutions.

Surface chemistry on ice can play an important role in atmospheric composition, but uncertainties in the chemical partitioning within ice samples has hindered the development of accurate models of these systems. Using Raman microscopy, we examine the nitrate distribution in ice and liquid samples containing environmentally relevant concentrations of sodium chloride, sodium bromide, and sea salt analogue Instant Ocean. Nitrate is enhanced at the surface compared to in the bulk in all frozen samples but to different degrees depending on the added salt. A large variation of nitrate in the horizontal dimension of frozen samples provides direct evidence that it congregates in pockets or grain boundaries within the crystal. In these pockets, nitrate experiences a solution-like environment, whereas at the surface nitrate sometimes forms solid crystals.

Nitrate Photolysis in Salty Snow.

Nitrate photolysis from snow can have a significant impact on the oxidative capacity of the local atmosphere, but the factors affecting the release of gas-phase products are not well understood. Here, we report a systematic study of the amounts of NO, NO2, and total nitrogen oxides (NOy) emitted from illuminated snow samples as a function of both nitrate and total salt (NaCl and Instant Ocean) concentration. The results provide experimental evidence that the release of nitrogen oxides to the gas phase is directly related to the expected nitrate concentration in the brine at the surface of the snow crystals. With no added salts, steady-state release of gas-phase products increases to a plateau value with increasing prefreezing nitrate concentration; with the addition of salts, the steady-state gas-phase nitrogen oxides generally decrease with increasing prefreezing NaCl or Instant Ocean concentration. In addition, for these frozen mixed nitrate (25 mM)-salt (0-500 mM) solutions, there is an increase in gas-phase NO2 seen at low added salt amounts, with NO2 production enhanced by up to 42% at low prefreezing [NaCl] (≤25 mM) and by up to 89% at prefreezing Instant Ocean concentrations lower than 200 mM [Cl-]. This enhancement may be important to the atmospheric oxidative capacity in polar regions.

Mechanistic Insights on the Photosensitized Chemistry of a Fatty Acid at the Air/Water Interface.

Interfaces are ubiquitous in the environment and many atmospheric key processes, such as gas deposition, aerosol, and cloud formation are, at one stage or another, strongly impacted by physical and chemical processes occurring at interfaces. Here, the photoinduced chemistry of an air/water interface coated with nonanoic acid-a fatty acid surfactant we use as a proxy for chemically complex natural aqueous surface microlayers-was investigated as a source of volatile and semivolatile reactive organic species. The carboxylic acid coating significantly increased the propensity of photosensitizers, chosen to mimic those observed in real environmental waters, to partition to the interface and enhance reactivity there. Photochemical formation of functionalized and unsaturated compounds was systematically observed upon irradiation of these coated surfaces. The role of a coated interface appears to be critical in providing a concentrated medium allowing radical-radical reactions to occur in parallel with molecular oxygen additions. Mechanistic insights are provided from extensive analysis of products observed in both gas and aqueous phases by online switchable reagent ion-time of flight-mass spectrometry and by off-line ultraperformance liquid chromatography coupled to a Q Exactive high resolution mass spectrometer through heated electrospray ionization, respectively.