Imidacloprid (IMD) is the most widely used neonicotinoid insecticide found on environmental surfaces and in water. Analysis of surface-bound IMD photolysis products was performed using attenuated total reflectance Fourier transfer infrared (ATR-FTIR) analysis, electrospray ionization (ESI-MS), direct analysis in real time mass spectrometry (DART-MS), and transmission FTIR for gas-phase products. Photolysis quantum yields (ϕ) for loss of IMD were determined to be (1.6 ± 0.6) × 10–3 (1s) at 305 nm and (8.5 ± 2.1) × 10–3 (1s) at 254 nm. The major product is the imidacloprid urea derivative (IMD-UR, 84% yield), with smaller amounts of the desnitro-imidacloprid (DN-IMD, 16% yield) product, and gaseous nitrous oxide (N2O). Theoretical calculations show that the first step of the main mechanism is the photodissociation of NO2, which then recombines with the ground electronic state of IMD radical to form IMD-UR and N2O in a thermally driven process. The photolytic lifetime of IMD at a solar zenith angle of 35° is calculated to be 16 h, indicating the significant reaction of IMD over the course of a day. Desnitro-imidacloprid has been identified by others as having increased binding to target receptors compared to IMD, suggesting that photolysis on environmental surfaces increases toxicity.

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NOx analyzer was used to study the kinetics of NH3 uptake and NOx formation from photochemistry initiated on irradiated (λ > 290 nm) TiO2 surfaces under atmospherically relevant conditions. The speciation of NH3 on TiO2 surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH3 concentration as expected for reactions proceeding via a Langmuir–Hinshelwood mechanism. The mechanism of NOx formation is shown to be humidity dependent: Water-catalyzed reactions promote NOx formation up to a relative humidity of 50%. Less NOx is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH4+, and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH2 photoproduct and O2 in the presence of H2O supports the experimental conclusion that NOx formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH2 + O2 reaction and the reaction of NH2 + O2 in small water clusters were carried out. Solvation of NH2OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH3 on TiO2 surfaces represents a source of atmospheric NOx, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH3 on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.

Atmospheric particles influence visibility, health and climate but the mechanisms of their formation from initial clusters and their growth to detectable particles remain largely unknown. Previous studies show that reactions of methanesulfonic acid (MSA) with ammonia and amines form particles, a process which is enhanced by water. We report here results from a combined experimental-theoretical investigation of the effect of oxalic acid (OxA) on particle formation and growth from the reaction of MSA with trimethylamine (TMA) in the absence and presence of water. The gas phase reactants were mixed in an aerosol flow reactor (1 atm, 294 K). Particle number concentrations and size distributions were measured as a function of reaction time from 0.8–12 s. The interaction of OxA with TMA with and without water does not lead to significant particle formation. When OxA is present during the reaction of MSA with TMA, there is little change (≤2 times more) in the particle number concentration but particles are larger compared to the base case of MSA with TMA alone. However, the presence of water with MSA and TMA overwhelms the effect of OxA so that OxA has no significant impact on particle number concentration or size. Results of these experiments suggest the MSA hydrate is important for particle formation and growth of the four component OxA–MSA–TMA–H2O system. These results are compared to earlier studies of the effect of OxA on the MSA–methylamine reaction and interpreted based on theoretically calculated properties of small clusters of the components.

New particle formation (NPF) from gaseous precursors as a significant source of aerosol needs to be better understood to accurately predict the impacts on visibility, climate change, and human health. While ternary nucleation of sulfuric acid, amines/NH3, and water is recognized as a significant driver for NPF, increasing evidence suggests a contribution from methanesulfonic acid (MSA) and amines under certain conditions. Here we report the formation of particles 2.5–10 nm in diameter from the reactions of MSA with methylamine (MA), dimethylamine (DMA), and NH3 at reaction times of 2.3–7.8 s in a flow reactor and compare these particles with those previously reported to be formed from reaction with trimethylamine (TMA). The effects of water vapor and concentrations of gaseous precursors on the particle number concentration and particle size were studied. The presence of water significantly enhances particle formation and growth. Under similar experimental conditions, particle number concentrations decrease in the order MA ≫ TMA ≈ DMA ≫ NH3, where NH3 is 2–3 orders of magnitude less efficient than DMA. Quantum chemical calculations of likely intermediate clusters were carried out to provide insights into the role of water and the different capacities of amines/NH3 in particle formation. Both gas-phase basicity and hydrogen-bonding capacity of amines/NH3 contribute to the potential for particles to form and grow. Our results indicate that, although amines typically have concentrations 1–3 orders of magnitude lower than that of NH3 in the atmosphere, they still play an important role in driving NPF.

Aerosol particles are ubiquitous in the atmosphere and have been shown to impact the Earth’s climate, reduce visibility, and adversely affect human health. Modeling the evolution of aerosol systems requires an understanding of the species and mechanisms involved in particle growth, including the complex interactions between particle- and gas-phase species. Here we report studies of displacement of amines (methylamine, dimethylamine, or trimethylamine) in methanesulfonate salt particles by exposure to a different gas-phase amine, using a single particle mass spectrometer, SPLAT II. The variation of the displacement with the nature of the amine suggests that behavior is dependent on water in or on the particles. Small clusters of methanesulfonic acid with amines are used as a model in quantum chemical calculations to identify key structural elements that are expected to influence water uptake, and hence the efficiency of displacement by gas-phase molecules in the aminium salts. Such molecular-level understanding of the processes affecting the ability of gas-phase amines to displace particle-phase aminium species is important for modeling the growth of particles and their impacts in the atmosphere.

Investigations of reaction pathways between a proton and cellobiose (CB), a glucose disaccharide of importance, were carried out in cis and trans CB using Ab Initio Molecular Dynamics (AIMD) simulations starting from optimized configurations where the proton is initially placed near groups with affinity for it. Near and above 300 K, protonated CB (H+CB) undergoes several transient reactions including charge transfer to the sugar backbone, water formation and dehydration, ring breaking and glycosidic bond breaking events as well as mutarotation and ring puckering events, all on a 10 ps timescale. cis H+CB is energetically favoured over trans H+CB in vacuo, with an energy gap larger than for the neutral CB.

Simulations show that photodissociation of methyl hydroperoxide, CH3OOH, on water clusters produces a surprisingly wide range of products on a subpicosecond time scale, pointing to the possibility of complex photodegradation pathways for organic peroxides on aerosols and water droplets. Dynamics are computed at several excitation energies at 50 K using a semiempirical PM3 potential surface. CH3OOH is found to prefer the exterior of the cluster, with the CH3O group sticking out and the OH group immersed within the cluster. At atmospherically relevant photodissociation wavelengths the OH and CH3O photofragments remain at the surface of the cluster or embedded within it. However, none of the 25 completed trajectories carried out at the atmospherically relevant photodissociation energies led to recombination of OH and CH3O to form CH3OOH. Within the limited statistics of the available trajectories the predicted yield for the recombination is zero. Instead, various reactions involving the initial fragments and water promptly form a wide range of stable molecular products such as CH2O, H2O, H2, CO, CH3OH, and H2O2.

Calculations were performed to determine the structures, energetics, and spectroscopy of the atmospherically relevant complexes (HNO3)·(NO2), (HNO3)·(N2O4), (NO3−)·(NO2), and (NO3−)·(N2O4). The binding energies indicate that three of the four complexes are quite stable, with the most stable (NO3−)·(N2O4) possessing binding energy of almost −14 kcal mol−1. Vibrational frequencies were calculated for use in detecting the complexes by infrared and Raman spectroscopy. An ATR-FTIR experiment showed features at 1632 and 1602 cm−1 that are attributed to NO2 complexed to NO3− and HNO3, respectively. The electronic states of (HNO3)·(N2O4) and (NO3−)·(N2O4) were investigated using an excited state method and it was determined that both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. Evidence for the existence of (NO3−)·(N2O4) was obtained from UV/vis absorption spectra of N2O4 in concentrated HNO3, which show a band at 320 nm that is blue shifted by 20 nm relative to what is observed for N2O4 dissolved in organic solvents. Finally, hydrogen transfer reactions within the (HNO3)·(NO2) and (HNO3)·(N2O4) complexes leading to the formation of HONO, were investigated. In both systems the calculated potential profiles rule out a thermal mechanism, but indicate the reaction could take place following the absorption of visible radiation. We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and field studies.

Ammonia is the most abundant reduced nitrogen species in the atmosphere and an important precursor in the industrial-scale production of nitric acid. A coated-wall flow tube coupled to a chemiluminescence NOx analyzer was used to study the kinetics of NH3 uptake and NOx formation from photochemistry initiated on irradiated (λ > 290 nm) TiO2 surfaces under atmospherically relevant conditions. The speciation of NH3 on TiO2 surfaces in the presence of surface-adsorbed water was determined using diffuse reflection infrared Fourier transform spectroscopy. The uptake kinetics exhibit an inverse dependence on NH3 concentration as expected for reactions proceeding via a Langmuir–Hinshelwood mechanism. The mechanism of NOx formation is shown to be humidity dependent: Water-catalyzed reactions promote NOx formation up to a relative humidity of 50%. Less NOx is formed above 50%, where increasing amounts of adsorbed water may hinder access to reactive sites, promote formation of unreactive NH4+, and reduce oxidant levels due to higher OH radical recombination rates. A theoretical study of the reaction between the NH2 photoproduct and O2 in the presence of H2O supports the experimental conclusion that NOx formation is catalyzed by water. Calculations at the MP2 and CCSD(T) level on the bare NH2 + O2 reaction and the reaction of NH2 + O2 in small water clusters were carried out. Solvation of NH2OO and NHOOH intermediates likely facilitates isomerization via proton transfer along water wires, such that the steps leading ultimately to NO are exothermic. These results show that photooxidation of low levels of NH3 on TiO2 surfaces represents a source of atmospheric NOx, which is a precursor to ozone. The proposed mechanism may be broadly applicable to dissociative chemisorption of NH3 on other metal oxide surfaces encountered in rural and urban environments and employed in pollution control applications (selective catalytic oxidation/reduction) and during some industrial processes.

Investigations of reaction pathways between a proton and cellobiose (CB), a glucose disaccharide of importance, were carried out in cis and trans CB using Ab Initio Molecular Dynamics (AIMD) simulations starting from optimized configurations where the proton is initially placed near groups with affinity for it. Near and above 300 K, protonated CB (H+CB) undergoes several transient reactions including charge transfer to the sugar backbone, water formation and dehydration, ring breaking and glycosidic bond breaking events as well as mutarotation and ring puckering events, all on a 10 ps timescale. cis H+CB is energetically favoured over trans H+CB in vacuo, with an energy gap larger than for the neutral CB.

Calculations were performed to determine the structures, energetics, and spectroscopy of the atmospherically relevant complexes (HNO3)·(NO2), (HNO3)·(N2O4), (NO3−)·(NO2), and (NO3−)·(N2O4). The binding energies indicate that three of the four complexes are quite stable, with the most stable (NO3−)·(N2O4) possessing binding energy of almost −14 kcal mol−1. Vibrational frequencies were calculated for use in detecting the complexes by infrared and Raman spectroscopy. An ATR-FTIR experiment showed features at 1632 and 1602 cm−1 that are attributed to NO2 complexed to NO3− and HNO3, respectively. The electronic states of (HNO3)·(N2O4) and (NO3−)·(N2O4) were investigated using an excited state method and it was determined that both complexes possess one low-lying excited state that is accessible through absorption of visible radiation. Evidence for the existence of (NO3−)·(N2O4) was obtained from UV/vis absorption spectra of N2O4 in concentrated HNO3, which show a band at 320 nm that is blue shifted by 20 nm relative to what is observed for N2O4 dissolved in organic solvents. Finally, hydrogen transfer reactions within the (HNO3)·(NO2) and (HNO3)·(N2O4) complexes leading to the formation of HONO, were investigated. In both systems the calculated potential profiles rule out a thermal mechanism, but indicate the reaction could take place following the absorption of visible radiation. We propose that these complexes are potentially important in the thermal and photochemical production of HONO observed in previous laboratory and field studies.