Exploring Conditions for Ultrafine Particle Formation from Oxidation of Cigarette Smoke in Indoor Environments.

Cigarette smoke is an important source of particles and gases in the indoor environment. In this work, aging of side-stream cigarette smoke was studied in an environmental chamber via exposure to ozone (O3), hydroxyl radicals (OH) and indoor fluorescent lights. Aerosol mass concentrations increased by 13-18% upon exposure to 15 ppb O3 and by 8-42% upon exposure to 0.45 ppt OH. Ultrafine particle (UFP) formation was observed during all ozone experiments, regardless of the primary smoke aerosol concentration (185-1950 µg m-3). During OH oxidation, however, UFP formed only when the primary particle concentration was relatively low (<130 µg m-3) and the OH concentration was high (∼1.1 × 107 molecules cm-3). Online aerosol composition measurements show that oxygen- and nitrogen- containing species were formed during oxidation. Gas phase oxidation of NO to NO2 occurred during fluorescent light exposure, but neither primary particle growth nor UFP formation were observed. Overall, exposure of cigarette smoke to ozone will likely lead to UFP formation in indoor environments. On the other hand, UPF formation via OH oxidation will only occur when OH concentrations are high (∼107 molecules cm-3), and is therefore less likely to have an impact on indoor aerosol associated with cigarette smoke.

Gas-Phase Mechanisms of the Reactions of Reduced Organic Nitrogen Compounds with OH Radicals.

Research on the fate of reduced organic nitrogen compounds in the atmosphere has gained momentum since the identification of their crucial role in particle nucleation and the scale up of carbon capture and storage technology which employs amine-based solvents. Reduced organic nitrogen compounds have strikingly different lifetimes against OH radicals, from hours for amines to days for amides to years for isocyanates, highlighting unique functional group reactivity. In this work, we use ab initio methods to investigate the gas-phase mechanisms governing the reactions of amines, amides, isocyanates and carbamates with OH radicals. We determine that N-H abstraction is only a viable mechanistic pathway for amines and we identify a reactive pathway in amides, the formyl C-H abstraction, not currently considered in structure-activity relationship (SAR) models. We then use our acquired mechanistic knowledge and tabulated literature experimental rate coefficients to calculate SAR factors for reduced organic nitrogen compounds. These proposed SAR factors are an improvement over existing SAR models because they predict the experimental rate coefficients of amines, amides, isocyanates, isothiocyanates, carbamates and thiocarbamates with OH radicals within a factor of 2, but more importantly because they are based on a sound fundamental mechanistic understanding of their reactivity.

Research highlights: laboratory studies of the formation and transformation of atmospheric organic aerosols.

Atmospheric particles are emitted from a variety of anthropogenic and natural precursors and have direct impacts on climate, by scattering solar irradiation and nucleating clouds, and on health, by causing oxidative stress in the lungs when inhaled. They may also form from gaseous precursors, creating complex mixtures of organic and inorganic material. The chemical composition and the physical properties of aerosols will evolve during their one-week lifetime which will consequently change their impact on climate and health. The heterogeneity of aerosols is difficult to model and thus atmospheric aerosol research strives to characterize the mechanisms involved in nucleating and transforming particles in the atmosphere. Recent advances in four laboratory studies of aerosol formation and aging are highlighted here.

Experimental and theoretical understanding of the gas phase oxidation of atmospheric amides with OH radicals: kinetics, products, and mechanisms.

Atmospheric amides have primary and secondary sources and are present in ambient air at low pptv levels. To better assess the fate of amides in the atmosphere, the room temperature (298 ± 3 K) rate coefficients of five different amides with OH radicals were determined in a 1 m(3) smog chamber using online proton-transfer-reaction mass spectrometry (PTR-MS). Formamide, the simplest amide, has a rate coefficient of (4.44 ± 0.46) × 10(-12) cm(3) molec(-1) s(-1) against OH, translating to an atmospheric lifetime of ∼1 day. N-methylformamide, N-methylacetamide and propanamide, alkyl versions of formamide, have rate coefficients of (10.1 ± 0.6) × 10(-12), (5.42 ± 0.19) × 10(-12), and (1.78 ± 0.43) × 10(-12) cm(3) molec(-1) s(-1), respectively. Acetamide was also investigated, but due to its slow oxidation kinetics, we report a range of (0.4-1.1) × 10(-12) cm(3) molec(-1) s(-1) for its rate coefficient with OH radicals. Oxidation products were monitored and quantified and their time traces were fitted using a simple kinetic box model. To further probe the mechanism, ab initio calculations are used to identify the initial radical products of the amide reactions with OH. Our results indicate that N-H abstractions are negligible in all cases, in contrast to what is predicted by structure-activity relationships. Instead, the reactions proceed via C-H abstraction from alkyl groups and from formyl C(O)-H bonds when available. The latter process leads to radicals that can readily react with O2 to form isocyanates, explaining the detection of toxic compounds such as isocyanic acid (HNCO) and methyl isocyanate (CH3NCO). These contaminants of significant interest are primary oxidation products in the photochemical oxidation of formamide and N-methylformamide, respectively.

Gas phase oxidation of monoethanolamine (MEA) with OH radical and ozone: kinetics, products, and particles.

Monoethanolamine (MEA) is currently the benchmark solvent in carbon capture and storage (CCS), a technology aimed at reducing CO2 emissions in large combustion industries. To accurately assess the environmental impact of CCS, a sound understanding of the fate of MEA in the atmosphere is necessary. Relative and absolute rate kinetic experiments were conducted in a smog chamber using online proton transfer reaction mass spectrometry (PTR-MS) to follow the decay of MEA. The room temperature (295 ± 3K) kinetics of oxidation with hydroxyl radicals from light and dark sources yield an average value of (7.02 ± 0.46) × 10(-11) cm(3) molec(-1) s(-1), in good agreement with previously published data. For the first time, the rate coefficient for MEA with ozone was measured: (1.09 ± 0.05) × 10(-18) cm(3) molec(-1) s(-1). An investigation into the oxidation products was also conducted using online chemical ionization mass spectrometry (CI-TOFMS) where formamide, isocyanic acid as well as higher order products including cyclic amines were detected. Significant particle numbers and mass loadings were observed during the MEA oxidation experiments and accounted for over 15% of the fate of MEA-derived nitrogen.

Aerobic and electrochemical oxidative cross-dehydrogenative-coupling (CDC) reaction in an imidazolium-based ionic liquid.

The ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF(4)] has demonstrated high efficiency when applied as a solvent in the oxidative nitro-Mannich carbon--carbon bond formation. The copper-catalyzed cross-dehydrogenative coupling (CDC) between N-phenyltetrahydroisoquinoline and nitromethane in [BMIm][BF(4)] occurred with high yield under the described reaction conditions. Both the ionic liquid and copper catalyst were recycled nine times with almost no lost of activity. The electrochemical behavior of the tertiary amine substrate and beta-nitroamine product was investigated employing [BMIm][BF(4)] as electrolyte solvent. The potentiostatic electrolysis in ionic liquid afforded the desired product with a high yield. This result and the cyclic voltammetric investigation provide a better understanding of the reaction mechanism, which involves radical and iminium cation intermediates.

Copper-catalyzed oxidative coupling of benzylic C-H bonds with 1,3-dicarbonyl compounds.

A copper-catalyzed oxidative coupling of benzylic C-H bonds with 1,3-dicarbonyl compounds is described. The reaction utilizes an inexpensive copper catalyst-oxidant system that is suitable for the coupling of a range of benzylic C-H bonds with various 1,3-dicarbonyl compounds. Kinetic isotope studies support a mechanism involving a benzylic hydrogen abstraction.