Reaction Mechanisms Underlying Unfunctionalized Alkyl Nitrate Hydrolysis in Aqueous Aerosols

Alkyl nitrates (ANs) are both sinks and sources of nitrogen oxide radicals (NOx = NO + NO2) in the atmosphere. Their reactions affect both the nitrogen cycle and ozone formation and therefore air quality and climate. ANs can be emitted to the atmosphere or produced in the gas phase. In either case, they can partition into aqueous aerosols, where they might undergo hydrolysis, producing highly soluble nitrate products, and act as a permanent sink for NOx. The kinetics of AN hydrolysis partly determines the extent of AN contribution to the nitrogen cycle. However, kinetics of many ANs in various aerosols is unknown, and there are conflicting arguments about the effect of acidity and basicity on the hydrolysis process. Using computational methods, this study proposes a mechanism for the reactions of methyl, ethyl, propyl, and butyl nitrates with OH- (hydroxyl ion; basic hydrolysis), water (neutral hydrolysis), and H3O+ (hydronium ion; acidic hydrolysis). Using quantum chemical data and transition state theory, we follow the effect of pH on the contribution of the basic, neutral, and acidic hydrolysis channels, and the rate coefficients of AN hydrolysis over a wide range of pH. Our results show that basic hydrolysis (i.e., AN reaction with OH-) is the most kinetically and thermodynamically favorable reaction among our evaluated reaction schemes. Furthermore, comparison of our kinetics results with experimental data suggests that there is an as yet unknown acidic mechanism responsible for acidic catalysis of AN hydrolysis.Peer reviewe

Observation of isoprene hydroxynitrates in the southeastern United States and implications for the fate of NO_x

Isoprene hydroxynitrates (IN) are tracers of the photochemical oxidation of isoprene in high NO_x environments. Production and loss of IN have a significant influence on the NO_x cycle and tropospheric O_3 chemistry. To better understand IN chemistry, a series of photochemical reaction chamber experiments was conducted to determine the IN yield from isoprene photooxidation at high NO concentrations (> 100 ppt). By combining experimental data and calculated isomer distributions, a total IN yield of 9(+4/−3) % was derived. The result was applied in a zero-dimensional model to simulate production and loss of ambient IN observed in a temperate forest atmosphere, during the Southern Oxidant and Aerosol Study (SOAS) field campaign, from 27 May to 11 July 2013. The 9 % yield was consistent with the observed IN/(MVK+MACR) ratios observed during SOAS. By comparing field observations with model simulations, we identified NO as the limiting factor for ambient IN production during SOAS, but vertical mixing at dawn might also contribute (~ 27 %) to IN dynamics. A close examination of isoprene's oxidation products indicates that its oxidation transitioned from a high-NO dominant chemical regime in the morning into a low-NO dominant regime in the afternoon. A significant amount of IN produced in the morning high NO regime could be oxidized in the low NO regime, and a possible reaction scheme was proposed

Laboratory studies on the production of alpha-pinene-derived organic nitrates and their atmospheric fate

Currently, the formation yields of organic nitrates from the oxidation of biogenic volatile organic compounds, such as α-pinene, is highly uncertain, negatively impacting our knowledge on tropospheric ozone production and the fate of atmospheric NOx. To lower this uncertainty, we quantified the organic nitrate yield from the OH radical oxidation of α-pinene under high NOx conditions. The α-pinene- derived nitrates created in chamber experiments readily partitioned to the aerosol phase and underwent particle phase hydrolysis, indicating that these processes are likely a sink for atmospheric NOx. The hydrolysis of organic nitrates was found to be specific acid-catalyzed and proceeded via unimolecular mechanisms under acidic conditions. The hydrolysis lifetime of a synthesized α-pinene nitrate standard was well within the lifetime of an atmospheric particle. Previously unreported α-pinene oxidation products from chamber studies were identified using mass spectrometry, and, for the first time, individual α-pinene-derived nitrates were identified in a complex mixture. In addition, paper spray ionization was adapted for the direct detection of organosulfates from filter samples and a Raman technique was developed to directly measure the pH of single aerosol particles, for the first time

Determining the dominant degradation mechanisms in Nitrocellulose

Nitrocellulose (NC) is the base component for many modern day propellants and explosives, as well as for everyday items such as printing inks, paint and lacquer coatings. Despite its early beginnings as the first man-made plastic, the decomposition pathways from the bulk material to the products observed from its ambient ageing are still not fully understood. Knowledge of these processes are of critical importance when considering the conservation of NC artefacts, refinement of product formulations, predictions of shelf life and safety improvements. In this study, the dominant degradation pathways of NC were investigated using quantum mechanics (QM) methods to probe the mechanisms leading to the initial cleavage of nitrate groups from the cellulosic backbone. The NC structure was truncated from a polymer chain to monomer, dimer and trimer units. Density functional theory methods (DFT) were used to study the mechanistic detail at individual nitrate sites. Comparison of differently sized units using the quantum theory of atoms in molecules (QTAIM), analysis of the electrostatic potential (ESP) surface and partial charges showed that the most suitable approximation for study of the decomposition reactions was the β-glucopyranose monomer, bi-capped with methoxy groups. The primary thermolytic and hydrolytic denitration routes were explored using transition state (TS) searches and potential energy surface (PES) scans. It was found that the thermolytic behaviour of the NC denitration step matched that of a well studied nitrate ester, pentaerythritol tetranitrate (PETN). The hydrolytic scheme for nitrate cleavage was studied, finding that protonation at the bridging oxygen site was the most likely to lead to denitration. It was not possible to isolate a TS for the hydrolytic reaction, though a number of coordination schemes were tested. Key secondary processes beyond nitrate cleavage were examined to determine the fate of nitrogen in the system and the cause of the transition from a first order reaction rate to autocatalytic decomposition. The energies of reactions in three different decomposition schemes proposed in literature were compared. Ethyl nitrate was used as a test system before extension to the NC monomer. New reaction pathways for decomposition were constructed using the reactions posed in the literature studies. The new schemes revealed that •NO2 was the most likely cause for the experimentally observed autocatalytic rate of degradation

Unprecedented Ambient Sulfur Trioxide (SO3) Detection : Possible Formation Mechanism and Atmospheric Implications

Sulfur trioxide (SO3) is a crucial compound for atmospheric sulfuric acid (H2SO4) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO3 is mainly produced from the photo-oxidation of SO2 by OH radicals. However, the sources of SO3 during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-offlight) mass spectrometer to detect atmospheric SO3 and H2SO4. Our results show the level of SO3 was higher during the winter than during the summer, with high SO3 levels observed especially during the early morning (similar to 05:00 to similar to 08:30) and night (similar to 18:00 to similar to 05:00 the next day). On the basis of analysis of SO2, NOx, black carbon, traffic flow, and atmospheric ions, we suggest SO3 could be formed from the catalytic oxidation of SO2 on the surface of traffic-related black carbon. This previously unidentified SO3 source results in significant H2SO4 formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO3 and formulating policies to mitigate secondary particle formation in Chinese megacities.Peer reviewe

Assessment of effects on vegetation of degradation products from alternative fluorocarbons

Concern with the effects of fluorides on plants has been devoted to that resulting from dry deposition (mainly with reference to gaseous HF and secondarily with particulate forms). The occurrence of precipitation as rain or mist and the presence of dew or free water on the foliage has mainly been considered with respect to their effects on the accumulation of air-borne fluoride and not with fluoride in wet deposition. That is, precipitation has been viewed primarily with respect to its facilitation of the solution and subsequent absorption of deposits by the foliar tissues or its elution of deposited fluoride from foliage. Accordingly, our evaluation of inorganic fluoride from fluorocarbon degradation rests upon a comparison with what is known about the effects of industrial emissions and what could be considered the natural condition

Bases of inorganic and organic chemistry

Stated fundamental theoretical principles of general, inorganic and organic chemistry and analyzed the reactivity of the most important classes of inorganic and organic substances. A multivariate tasks and exercises for classroom and independent work are proposed. For university students full-time and distance learning areas "Chemical Technology and Engineering" "Oil and gas engineering and technology" and others

Interferences in photolytic NO2 measurements: explanation for an apparent missing oxidant?

Measurement of NO2 at low concentrations (tens of ppts) is non-trivial. A variety of techniques exist, with the conversion of NO2 into NO followed by chemiluminescent detection of NO being prevalent. Historically this conversion has used a catalytic approach (molybdenum); however, this has been plagued with interferences. More recently, photolytic conversion based on UV-LED irradiation of a reaction cell has been used. Although this appears to be robust there have been a range of observations in low-NOx environments which have measured higher NO2 concentrations than might be expected from steady-state analysis of simultaneously measured NO, O-3, jNO(2), etc. A range of explanations exist in the literature, most of which focus on an unknown and unmeasured "compound X" that is able to convert NO to NO2 selectively. Here we explore in the laboratory the interference on the photolytic NO2 measurements from the thermal decomposition of peroxyacetyl nitrate (PAN) within the photolysis cell. We find that approximately 5aEuro-% of the PAN decomposes within the instrument, providing a potentially significant interference. We parameterize the decomposition in terms of the temperature of the light source, the ambient temperature, and a mixing timescale (aEuro parts per thousand 0.4aEuro-s for our instrument) and expand the parametric analysis to other atmospheric compounds that decompose readily to NO2 (HO2NO2, N2O5, CH3O2NO2, IONO2, BrONO2, higher PANs). We apply these parameters to the output of a global atmospheric model (GEOS-Chem) to investigate the global impact of this interference on (1) the NO2 measurements and (2) the NO(2)aEuro-:aEuro-NO ratio, i.e. the Leighton relationship. We find that there are significant interferences in cold regions with low NOx concentrations such as the Antarctic, the remote Southern Hemisphere, and the upper troposphere. Although this interference is likely instrument-specific, the thermal decomposition to NO2 within the instrument's photolysis cell could give an at least partial explanation for the anomalously high NO2 that has been reported in remote regions. The interference can be minimized by better instrument characterization, coupled to instrumental designs which reduce the heating within the cell, thus simplifying interpretation of data from remote locations

The effect of ultra violet light on crystal violet leucocyanide

Thesis (M.A.)--Boston University This item was digitized by the Internet Archive