Predicting the environmental fates of emerging contaminants: Synergistic effects in ozone reactions of nitrogen-containing alkenes.

While nitro and amino alkenes are common in pharmaceuticals, pesticides, and munitions, their environmental fates are not well known. Ozone is a ubiquitous atmospheric oxidant for alkenes, but the synergistic effects of nitrogen-containing groups on the reactions have not been measured. The kinetics and products of ozonolysis of a series of model compounds with different combinations of these functional groups have been measured in the condensed phase using stopped-flow and mass spectrometry methods. Rate constants span about six orders of magnitude with activation energies ranging from 4.3 to 28.2 kJ mol-1. Vinyl nitro groups substantially decrease the reactivity, while amino groups have the opposite effect. The site of the initial ozone attack is highly structure dependent, consistent with local ionization energy calculations. The reaction of the neonicotinoid pesticide nitenpyram, which forms toxic N-nitroso compounds, was consistent with model compounds, confirming the utility of model compounds for assessing environmental fates of these emerging contaminants.

Gas Phase and Gas-Solid Interface Ozonolysis of Nitrogen Containing Alkenes: Nitroalkenes, Enamines, and Nitroenamines.

Emerging contaminants are of concern due to their rapidly increasing numbers and potential ecological and human health effects. In this study, the synergistic effects of the presence of multifunctional nitro, amino and carbon-carbon double bond (C═C) groups on the gas phase ozonolysis in O2 or at the air/solid interface were investigated using five simple model compounds. The gas phase ozonolysis rate constants at 296 K were (3.5 ± 0.9) × 10-20 cm3 molecule-1 s-1 for 2-methyl-1-nitroprop-1-ene and (6.8 ± 0.8) × 10-19 cm3 molecule-1 s-1 for 4-methyl-4-nitro-1-pentene, with lifetimes of 134 and 7 days in the presence of 100 ppb ozone in the atmosphere, respectively. The rate constants for gas phase E-N,N-dimethyl-1-propenylamine and N,N-dimethylallylamine reactions with ozone were too fast (>10-18 cm3 molecule-1 s-1) to be measured, implying lifetimes of less than 5 days. A multiphase kinetics model (KM-GAP) was used to probe the gas-solid kinetics of 1-dimethylamino-2-nitroethylene, yielding a rate constant for the surface reaction of 1.8 × 10-9 cm2 molecule-1 s-1 and in the bulk 1× 10-16 cm3 molecule-1 s-1. These results show that a nitro group attached to the C═C lowers the gas phase rate constant by 2-3 orders of magnitude compared to the simple alkenes, while amino groups have the opposite effect. The presence of both groups provides counterbalancing effects. Products with deleterious health effects including dimethylformamide and formaldehyde were identified by FTIR. The identified products differentiate whether the initial site of ozone attack is C═C and/or the amino group. This study provides a basis for predicting the environmental fates of emerging contaminants and shows that both the toxicity of both the parent compounds and the products should be taken into account in assessing their environmental impacts.

Unexpected formation of oxygen-free products and nitrous acid from the ozonolysis of the neonicotinoid nitenpyram.

The neonicotinoid nitenpyram (NPM) is a multifunctional nitroenamine [(R1N)(R2N)C=CHNO2] pesticide. As a nitroalkene, it is structurally similar to other emerging contaminants such as the pharmaceuticals ranitidine and nizatidine. Because ozone is a common atmospheric oxidant, such compounds may be oxidized on contact with air to form new products that have different toxicity compared to the parent compounds. Here we show that oxidation of thin solid films of NPM by gas-phase ozone produces unexpected products, the majority of which do not contain oxygen, despite the highly oxidizing reactant. A further surprising finding is the formation of gas-phase nitrous acid (HONO), a species known to be a major photolytic source of the highly reactive hydroxyl radical in air. The results of application of a kinetic multilayer model show that reaction was not restricted to the surface layers but, at sufficiently high ozone concentrations, occurred throughout the film. The rate constant derived for the O3-NPM reaction is 1 × 10-18 cm3⋅s-1, and the diffusion coefficient of ozone in the thin film is 9 × 10-10 cm2⋅s-1 These findings highlight the unique chemistry of multifunctional nitroenamines and demonstrate that known chemical mechanisms for individual moieties in such compounds cannot be extrapolated from simple alkenes. This is critical for guiding assessments of the environmental fates and impacts of pesticides and pharmaceuticals, and for providing guidance in designing better future alternatives.

Photochemistry of Solid Films of the Neonicotinoid Nitenpyram.

The environmental fates of nitenpyram (NPM), a widely used neonicotinoid insecticide, are not well-known. A thin solid film of NPM deposited on a germanium attenuated total reflectance (ATR) crystal was exposed to radiation from a low-pressure mercury lamp at 254 nm, or from broadband low pressure mercury photolysis lamps centered at 350 or 313 nm. The loss during photolysis was followed in time using FTIR. The photolysis quantum yields (ϕ), defined as the number of NPM molecules lost per photon absorbed, were determined to be (9.4 ± 1.5) × 10-4 at 350 nm, (1.0 ± 0.3) × 10-3 at 313 nm, and (1.2 ± 0.4) × 10-2 at 254 nm (±2σ). Imines, one with a carbonyl group, were detected as surface-bound products and gaseous N2O was generated in low (11%) yield. The UV-vis absorption spectra of NPM in water was different from that in acetonitrile, dichloromethane, and methanol, or in a thin solid film. The photolytic lifetime of solid NPM at a solar zenith angle at 35° is calculated to be 36 min, while that for NPM in water is 269 min, assuming that the quantum yield is the same as in the solid. Thus, there may be a significant sensitivity to the medium for photolytic degradation and the lifetime of NPM in the environment.

Knudsen cell studies of the uptake of gaseous ammonia and amines onto C3-C7 solid dicarboxylic acids.

While atmospheric particles affect health, visibility and climate, the details governing their formation and growth are poorly understood on a molecular level. A simple model system for understanding the interactions between the gas and particle phases is the reaction of bases with acids, both of which are common constituents of atmospheric particles. In the present study, uptake coefficients for the reactions of gas phase ammonia, methylamine, ethylamine, dimethylamine and trimethylamine with a series of solid dicarboxylic acids (diacids) were measured at 296 ± 1 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. The uptake coefficients (γ) for a given amine follow an odd-even trend in carbon number of the diacid, and are larger for the odd carbon diacids. Values range from γ = 0.4 for ethylamine on malonic acid (C3) to less than ∼10-6 for ammonia and all amines on adipic (C6) and pimelic (C7) acids. Basicity or structure of the amines/ammonia alone do not explain the effect of the base on uptake. The crystal structures of the diacids also play a key role, which is especially evident for malonic acid (C3). Evaporation of aqueous mixtures of amines/ammonia with odd carbon diacids show the formation of ionic liquids (ILs) or in some cases, metastable ILs that revert back to a stable solid salt upon complete evaporation of water. The trends with amine and diacid structure provide insight into the mechanisms of uptake and molecular interactions that control it, including the formation of ionic liquid layers in some cases. The diversity in the kinetics and mechanisms involved in this relatively simple model system illustrate the challenges in accurately representing such processes in atmospheric models.

Kinetics, mechanisms and ionic liquids in the uptake of n-butylamine onto low molecular weight dicarboxylic acids.

Atmospheric particles adversely affect visibility, health, and climate, yet the kinetics and mechanisms of particle formation and growth are poorly understood. Multiphase reactions between amines and dicarboxylic acids (diacids) have been suggested to contribute. In this study, the reactions of n-butylamine (BA) with solid C3-C8 diacids were studied at 296 ± 1 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Uptake coefficients for amines on the diacids with known geometric surface areas were measured at initial amine concentrations from (3-50) × 1011 cm-3. Uptake coefficients ranged from 0.7 ± 0.1 (2σ) for malonic acid (C3) to <10-6 for suberic acid (C8), show an odd-even carbon number effect, and decrease with increasing chain length within each series. Butylaminium salts formed from evaporation of aqueous solutions of BA with C3, C5 and C7 diacids (as well as C8) were viscous liquids, suggesting that ionic liquids (ILs) form on the surface during the reactions of gas phase amine with the odd carbon diacids. Predictions from the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB) were quantitatively consistent with uptake occurring via dissolution of the underlying diacid into the IL layer and reaction with amine taken up from the gas phase. The butylaminium salts formed from the C4 and C6 diacids were solids, and their uptake coefficients were smaller. These experiments and kinetic modeling demonstrate the unexpected formation of ILs in a gas-solid reaction, and suggest that ILs should be considered under some circumstances in atmospheric processes.

Correction: New particle formation and growth from methanesulfonic acid, trimethylamine and water.

Correction for 'New particle formation and growth from methanesulfonic acid, trimethylamine and water' by Haihan Chen et al., Phys. Chem. Chem. Phys., 2015, 17, 13699-13709.

New particle formation and growth from methanesulfonic acid, trimethylamine and water.

New particle formation from gas-to-particle conversion represents a dominant source of atmospheric particles and affects radiative forcing, climate and human health. The species involved in new particle formation and the underlying mechanisms remain uncertain. Although sulfuric acid is commonly recognized as driving new particle formation, increasing evidence suggests the involvement of other species. Here we study particle formation and growth from methanesulfonic acid, trimethylamine and water at reaction times from 2.3 to 32 s where particles are 2-10 nm in diameter using a newly designed and tested flow system. The flow system has multiple inlets to facilitate changing the mixing sequence of gaseous precursors. The relative humidity and precursor concentrations, as well as the mixing sequence, are varied to explore their effects on particle formation and growth in order to provide insight into the important mechanistic steps. We show that water is involved in the formation of initial clusters, greatly enhancing their formation as well as growth into detectable size ranges. A kinetics box model is developed that quantitatively reproduces the experimental data under various conditions. Although the proposed scheme is not definitive, it suggests that incorporating such mechanisms into atmospheric models may be feasible in the near future.

Simplified mechanism for new particle formation from methanesulfonic acid, amines, and water via experiments and ab initio calculations.

Airborne particles affect human health and significantly influence visibility and climate. A major fraction of these particles result from the reactions of gaseous precursors to generate low-volatility products such as sulfuric acid and high-molecular weight organics that nucleate to form new particles. Ammonia and, more recently, amines, both of which are ubiquitous in the environment, have also been recognized as important contributors. However, accurately predicting new particle formation in both laboratory systems and in air has been problematic. During the oxidation of organosulfur compounds, gas-phase methanesulfonic acid is formed simultaneously with sulfuric acid, and both are found in particles in coastal regions as well as inland. We show here that: (i) Amines form particles on reaction with methanesulfonic acid, (ii) water vapor is required, and (iii) particle formation can be quantitatively reproduced by a semiempirical kinetics model supported by insights from quantum chemical calculations of likely intermediate clusters. Such an approach may be more broadly applicable in models of outdoor, indoor, and industrial settings where particles are formed, and where accurate modeling is essential for predicting their impact on health, visibility, and climate.

Nonequilibrium atmospheric secondary organic aerosol formation and growth.

Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO(3) radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO(3) reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.

Surprising formation of p-cymene in the oxidation of α-pinene in air by the atmospheric oxidants OH, O3, and NO3.

Anthropogenic sources release into the troposphere a wide range of volatile organic compounds (VOCs) including aromatic hydrocarbons, whose major sources are believed to be combustion and the evaporation of fossil fuels. An important question is whether there are other sources of aromatics in air. We report here the formation of p-cymene [1-methyl-4-(1-methylethyl) benzene, C6H4(CH3)(C3H7)] from the oxidation of α-pinene by OH, O3, and NO3 at 1 atm in air and 298 K at low (<5%) and high (70%) relative humidities (RH). Loss of α-pinene and the generation of p-cymene were measured using GC-MS. The fractional yields of p-cymene relative to the loss of α-pinene, Δ [p-cymeme]/Δ [α-pinene], were measured to range from (1.6±0.2)×10(-5) for the O3 reaction to (3.0±0.3)×10(-4) for the NO3 reaction in the absence of added water vapor. The yields for the OH and O3 reactions increased by a factor of 4-8 at 70% RH (uncertainties are ±2s). The highest yields at 70% RH for the OH and O3 reactions, ∼15 times higher than for dry conditions, were observed if the walls of the Teflon reaction chamber had been previously exposed to H2SO4 formed from the OH oxidation of SO2. Possible mechanisms of the conversion of α-pinene to p-cymene and the potential importance in the atmosphere are discussed.

Identification of organic nitrates in the NO3 radical initiated oxidation of alpha-pinene by atmospheric pressure chemical ionization mass spectrometry.

The gas-phase reactions of nitrate radicals (NO3) with biogenic organic compounds are a major sink for these organics during night-time. These reactions form secondary organic aerosols, including organic nitrates that can undergo long-range transport, releasing NOx downwind. We report here studies of the reaction of NO3 with alpha-pinene at 1 atm in dry synthetic air (relative humidity approximately 3%) and at 298 K using atmospheric pressure chemical ionization triple quadrupole mass spectrometry (APCI-MS) to identify gaseous and particulate products. The emphasis is on the identification of individual organic nitrates in the particle phase that were obtained by passing the product mixture through a denuder to remove gas-phase reactants and products prior to entering the source region of the mass spectrometer. Filter extracts were also analyzed by GC-MS and by APCI time-of-flight mass spectrometry (APCI-ToF-MS) with methanol as the proton source. In addition to pinonaldehyde and pinonic acid, five organic nitrates were identified in the particles as well as in the gas phase: 3-oxopinane-2-nitrate, 2-hydroxypinane-3-nitrate, pinonaldehyde-PAN, norpinonaldehyde-PAN, and (3-acetyl-2,2-dimethyl-3-nitrooxycyclobutyl)acetaldehyde. Furthermore, there was an additional first-generation organic nitrate product tentatively identified as a carbonyl hydroxynitrate with a molecular mass of 229. These studies suggest that a variety of organic nitrates would partition between the gas phase and particles in the atmosphere, and serve as a reservoir for NOx.

Comparison of FTIR and particle mass spectrometry for the measurement of particulate organic nitrates.

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH(4)NO(3), NaNO(3), and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO(3) radical reactions at 22 degrees C and 1 atm in air with alpha- and beta-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the alpha-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z 30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [C(x)H(y)N(z)O(a)](+) were identified using HR-ToF-AMS. The NO(+)/NO(2)(+) ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the alpha- and beta-pinene, 3-carene, and limonene reactions, approximately 5 for the isoprene reaction, 2.4 for NH(4)NO(3) and 80 for NaNO(3). The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO(2)/C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO(+)/NO(2)(+) ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH(4)NO(3) or NaNO(3). However, unique identification of specific organic nitrates is not possible with either method.

Nitrate ion photochemistry at interfaces: a new mechanism for oxidation of alpha-pinene.

The photooxidation of 0.6-0.9 ppm alpha-pinene in the presence of a deliquesced thin film of NaNO(3), and for comparison increasing concentrations of NO(2), was studied in a 100 L Teflon(R) chamber at relative humidities from 72-88% and temperatures from 296-304 K. The loss of alpha-pinene and the formation of gaseous products were followed with time using proton transfer mass spectrometry. The yields of gas phase products were smaller in the NaNO(3) experiments than in NO(2) experiments. In addition, pinonic acid, pinic acid, trans-sobrerol and other unidentified products were detected in the extracts of the wall washings only for the NaNO(3) photolysis. These data indicate enhanced loss of alpha-pinene at the NaNO(3) thin film during photolysis. Supporting the experimental results are molecular dynamics simulations which predict that alpha-pinene has an affinity for the surface of the deliquesced nitrate thin film, enhancing the opportunity for oxidation of the impinging organic gas during the nitrate photolysis. This new mechanism of oxidation of organics may be partially responsible for the correlation between nitrate and the organic component of particles observed in many field studies, and may also contribute to the missing source of SOA needed to reconcile model predictions and field measurements. In addition, photolysis of nitrate on surfaces in the boundary layer may lead to oxidation of co-adsorbed organics.

A new approach to determining gas-particle reaction probabilities and application to the heterogeneous reaction of deliquesced sodium chloride particles with gas-phase hydroxyl radicals.

The reaction kinetics for gaseous hydroxyl radicals (OH) with deliquesced sodium chloride particles (NaCl(aq)) were investigated using a novel experimental approach. The technique utilizes the exposure of substrate-deposited aerosol particles to reactive gases followed by chemical analysis of the particles using computer-controlled scanning electron microscopy with energy-dispersive analysis of X-rays (CCSEM/EDX) capability. Experiments were performed at room temperature and atmospheric pressure with deliquesced NaCl particles in the micron size range at 70-80% RH and with OH concentrations in the range of 1 to 7 x 10(9) cm(-3). The apparent, pseudo first-order rate constant for the reaction was determined from measurements of changes in the chloride concentration of individual particles upon reaction with OH as a function of the particle loading on the substrate. Quantitative treatment of the data using a model that incorporates both diffusion and reaction kinetics yields a lower limit to the net reaction probability of gamma(net) > or = 0.1, with an overall uncertainty of a factor of 2.