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 cm³ 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 cm³ 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

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 m³ 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 cm³ molec^–1 s^–1 against OH, translating to an atmospheric lifetime of ∼1 day. <i>N</i>-methylformamide, <i>N</i>-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 cm³ 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 cm³ 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 <i>N</i>-methylformamide, respectively

Gas Phase Oxidation of Nicotine by OH Radicals: Kinetics, Mechanisms, and Formation of HNCO

Cigarette smoke is recognized as having harmful health effects for the smoker and for people breathing second-hand smoke. In an atmospheric chemistry context, however, little is known about the fate of organic nitrogen compounds present in cigarette smoke. Indeed, the atmospheric oxidation of nicotine, a major nitrogen-containing component of cigarette smoke, by OH radicals has yet to be investigated. We measured the first rate coefficient between OH and nicotine to be (8.38 ± 0.28) × 10^–11 cm³ molecule^–1 s^–1 at 298 ± 3 K. We use an online proton-transfer-reaction mass spectrometer (PTR-MS) to quantify nicotine’s oxidation products, including formamide and isocyanic acid (HNCO). We present the first evidence that HNCO is formed from nicotine’s gas phase oxidation, and we highlight the potential for this toxic molecule to be an indoor air pollutant after smoking has ended. Mechanistic pathways for the oxidation of nicotine by OH radicals were investigated by theoretical calculations at the M06-2X level of theory, and we find that there are many competitive H-abstraction sites on nicotine. Our findings suggest that the atmospheric removal of nicotine by OH radicals may compete with surface deposition and air exchange and may be a source of HNCO in indoor air

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

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