Effect of Sodium Sulfate, Ammonium Chloride, Ammonium Nitrate, and Salt Mixtures on Aqueous Phase Partitioning of Organic Compounds

Dissolved inorganic salts influence the partitioning of organic compounds into the aqueous phase. This influence is especially significant in atmospheric aerosol, which usually contains large amounts of ions, including sodium, ammonium, chloride, sulfate, and nitrate. However, empirical data on this salt effect are very sparse. Here, the partitioning of numerous organic compounds into solutions of Na₂SO₄, NH₄Cl, and NH₄NO₃ was measured and compared with existing data for NaCl and (NH₄)₂SO₄. Salt mixtures were also tested to establish whether the salt effect is additive. In general, the salt effect showed a decreasing trend of Na₂SO₄ > (NH)₂SO₄ > NaCl > NH₄Cl > NH₄NO₃ for the studied organic compounds, implying the following relative strength of the salt effect of individual anions: SO₄^2– > Cl^– > NO₃^– and of cations: Na⁺ > NH₄⁺. The salt effect of different salts is moderately correlated. Predictive models for the salt effect were developed based on the experimental data. The experimental data indicate that the salt effect of mixtures may not be entirely additive. However, the deviation from additivity, if it exists, is small. Data of very high quality are required to establish whether the effect of constituent ions or salts is additive or not

Measuring and Modeling the Salting-out Effect in Ammonium Sulfate Solutions

The presence of inorganic salts significantly influences the partitioning behavior of organic compounds between environmentally relevant aqueous phases, such as seawater or aqueous aerosol, and other, nonaqueous phases (gas phase, organic phase, etc.). In this study, salting-out coefficients (or Setschenow constants) (<i>K</i>_<i>S</i> [M^–1]) for 38 diverse neutral compounds in ammonium sulfate ((NH₄)₂SO₄) solutions were measured using a shared headspace passive dosing method and a negligible depletion solid phase microextraction technique. The measured <i>K</i>_<i>S</i> were all positive, varied from 0.216 to 0.729, and had standard errors in the range of 0.006–0.060. Compared to <i>K</i>_<i>S</i> for sodium chloride (NaCl) in the literature, <i>K</i>_<i>S</i> values for (NH₄)₂SO₄ are always higher for the same compound, suggesting a higher salting-out effect of (NH₄)₂SO₄. A polyparameter linear free energy relationship (pp-LFER) for predicting <i>K</i>_<i>S</i> in (NH₄)₂SO₄ solutions was generated using the experimental data for calibration. pp-LFER predicted <i>K</i>_<i>S</i> agreed well with measured <i>K</i>_<i>S</i> reported in the literature. <i>K</i>_<i>S</i> for (NH₄)₂SO₄ was also predicted using the quantum-chemical COSMO<i>therm</i> software and the thermodynamic model AIOMFAC. While COSMO<i>therm</i> generally overpredicted the experimental <i>K</i>_<i>S</i>, predicted and experimental values were correlated. Therefore, a fitting factor needs to be applied when using the current version of COSMO<i>therm</i> to predict <i>K</i>_<i>S</i>. AIOMFAC tends to underpredict the measured <i>K</i>_S((NH₄)₂SO₄) but always overpredicts <i>K</i>_S(NaCl). The prediction error is generally larger for <i>K</i>_S(NaCl) than for <i>K</i>_S((NH₄)₂SO₄). AIOMFAC also predicted a dependence of <i>K</i>_<i>S</i> on the salt concentrations, which is not observed in the experimental data. In order to demonstrate that the models developed and calibrated in this study can be applied to estimate Setschenow coefficients for atmospherically relevant compounds involved in secondary organic aerosol formation based on chemical structure alone, we predicted and compared <i>K</i>_S for selected α-pinene oxidation products

Large Bubbles Reduce the Surface Sorption Artifact of the Inert Gas Stripping Method

Accurate Henry’s law constants between air and water (<i>H</i>) are crucial for understanding a chemical’s environmental behavior. During inert gas stripping (IGS) <i>H</i> is derived from the rate of a chemical’s disappearance from aqueous solution as a result of air bubbling through a water-filled column. While <i>H</i> of many semivolatile organic compounds has been measured by IGS, inconsistent results between different studies have been attributed to chemical adsorption to the bubble surface. This surface adsorption artifact is expected to increase with a chemical’s interface–air partition coefficient (<i>K</i>_IA) and decreasing bubble size. Previous work with normal alkanols of variable chain length identified a <i>K</i>_IA threshold of approximately 0.001 m, above which IGS is compromised by the surface sorption artifact. In this study, we repeated IGS measurements of <i>H</i> of normal alkanols at different temperatures of 298.15 K, 305.65 K, 323.15 K, and 343.15 K using a modified gas inlet mechanisms that results in the formation of large bubbles (diameter approximately 5.5 mm). The new <i>H</i> values agreed very well with those measured with a head space technique that is much less susceptible to surface adsorption. The method is judged suitable for measuring <i>H</i> of surface active chemicals with <i>K</i>_IA values below 0.02 <i>m</i>

Calculating Equilibrium Phase Distribution during the Formation of Secondary Organic Aerosol Using COSMO<i>therm</i>

Challenges in the parametrization of compound distribution between the gas and particle phase contribute significantly to the uncertainty in the prediction of secondary organic aerosol (SOA) formation and are rooted in the complexity and variability of atmospheric condensed matter, which includes water, salts, and a multitude of organic oxidation products, often in two separated phases. Here, we explore the use of the commercial quantum-chemistry-based software COSMO<i>therm</i> to predict equilibrium partitioning and Setchenow coefficients of a suite of oxidation products of α-pinene ozonolysis in an aerosol that is assumed to separate into an organic-enriched phase and an electrolyte-enriched aqueous phase. The predicted coefficients are used to estimate the phase distribution of the organic compounds, water and ammonium sulfate, the resulting phase composition, and the SOA yield. Four scenarios that differ in terms of organic loading, liquid water content, and chemical aging are compared. The organic compounds partition preferentially to the organic phase rather than the aqueous phase for the studied aerosol scenarios, partially due to the salting-out effect. Extremely low volatile organic compounds are predicted to be the dominant species in the organic aerosols at low loadings and an important component at higher loadings. The highest concentration of oxidation products in the condensed phase is predicted for a scenario assuming the presence of non-phase-separated cloud droplets. Partitioning into an organic aerosol phase composed of the oxidation products is predicted to be similar to partitioning into a phase composed of a single organic surrogate molecule, suggesting that the calculation procedure can be simplified without major loss of accuracy. COSMO<i>therm</i> is shown to produce results that are comparable to those obtained using group contribution methods. COSMO<i>therm</i> is likely to have a much larger application domain than those group contribution methods because it is based on fundamental principles with little calibration

A High-Precision Passive Air Sampler for Gaseous Mercury

Passive air samplers (PASs) provide an opportunity to improve the spatial range and resolution of gaseous mercury (Hg) measurements. Here, we propose a sampler design that combines a sulfur-impregnated activated carbon sorbent, a Radiello diffusive barrier, and a protective shield for outdoor deployments. The amount of gaseous Hg taken up by the sampler increased linearly with time for both an 11-week indoor (<i>r</i>² = 0.990) and 12-month outdoor (<i>r</i>² = 0.996) deployment, yielding sampling rates of 0.158 ± 0.008 m³ day^–1 indoors and 0.121 ± 0.005 m³ day^–1 outdoors. These sampling rates are close to modeled estimates of 0.166 m³ day^–1 indoors and 0.129 m³ day^–1 outdoors. Replicate precision is better than for all previous PASs for gaseous Hg, especially during outdoor deployments (2 ± 1.3%). Such precision is essential for discriminating the relatively small concentration variations occurring at background sites. Deployment times for obtaining reliable time-averaged atmospheric gaseous Hg concentrations range from a week to at least one year

Field Evaluation of a Flow-Through Sampler for Measuring Pesticides and Brominated Flame Retardants in the Arctic Atmosphere

A flow-through sampler (FTS) was codeployed with a super high volume active sampler (SHV) between October 2007 and November 2008 to evaluate its ability to determine the ambient concentrations of pesticides and brominated flame retardants in the Canadian High Arctic atmosphere. Nine pesticides and eight flame retardants, including three polybrominated diphenyl ether (PBDE) replacement chemicals, were frequently detected. Atmospheric concentrations determined by the two systems showed good agreement when compared on monthly and annually integrated time scales. Pesticide concentrations were normally within a factor of 3 of each other. The FTS tended to generate higher PBDE concentrations than the SHV presumably because of the entrainment of blowing snow/ice crystals or large particles. Taking into account uncertainties in analytical bias, sample volume, and breakthrough estimations, the FTS is shown to be a reliable and cost-effective method, which derives seasonally variable concentrations of semivolatile organic trace compounds at extremely remote locations that are comparable to those obtained by conventional high volume air sampling. Moreover, the large sampling volumes captured by the FTS make it suitable for the screening of new and emerging chemicals in the remote atmosphere where concentrations are usually low

Mountain Cold-Trapping Increases Transfer of Persistent Organic Pollutants from Atmosphere to Cows’ Milk

Concentrations of long-lived organic contaminants in snow, soil, lake water, and vegetation have been observed to increase with altitude along mountain slopes. Such enrichment, called “mountain cold-trapping”, is attributed to a transition from the atmospheric gas phase to particles, rain droplets, snowflakes, and Earth’s surface at the lower temperatures prevailing at higher elevations. Milk sampled repeatedly from cows that had grazed at three different altitudes in Switzerland during one summer was analyzed for a range of persistent organic pollutants. Mountain cold-trapping significantly increased air-to-milk transfer factors of most analytes. As a result, the milk of cows grazing at higher altitudes was more contaminated with substances that have regionally uniform air concentrations (hexachlorobenzene, α-hexachlorocyclohexane, endosulfan sulfate). For substances that have sources, and therefore higher air concentrations, at lower altitudes (polychlorinated biphenyls, γ-hexachlorocyclohexane), alpine milk has lower concentrations, but not as low as would be expected without mountain cold-trapping. Differences in the elevational gradients in soil concentrations and air-to-milk transfer factors highlight that cold-trapping of POPs in pastures is mostly due to increased gas-phase deposition as a result of lower temperatures causing higher uptake capacity of plant foliage, whereas cold-trapping in soils more strongly depends on wet and dry particle deposition. Climatic influences on air-to-milk transfer of POPs needs to be accounted for when using contamination of milk lipids to infer contamination of the atmosphere