Reaction Of HNO3 Research Articles

Effect of Boron Clusters on the Ignition Reaction of HNO3 and Dicynanamide-Based Ionic Liquids.

Many ionic liquids containing the dicynamide anion (DCA-, formula N(CN)2-) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA- and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA- or DCAH with borane clusters B10H14 or B9H14- is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

Communication: Charge transfer dominates over proton transfer in the reaction of nitric acid with gas-phase hydrated electrons.

The reaction of HNO3 with hydrated electrons (H2O)n − (n = 35–65) in the gas phase was studied using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and ab initio molecular dynamics simulations. Kinetic analysis of the experimental data shows that OH−(H2O)m is formed primarily via a reaction of the hydrated electron with HNO3 inside the cluster, while proton transfer is not observed and NO3 −(H2O)m is just a secondary product. The reaction enthalpy was determined using nanocalorimetry, revealing a quite exothermic charge transfer with −241 ± 69 kJ mol−1. Ab initio molecular dynamics simulations indicate that proton transfer is an allowed reaction pathway, but the overall thermochemistry favors charge transfer.

Hidden complexities in the reaction of H2O2 and HNO revealed by ab initio quantum chemical investigations.

Nitroxyl (HNO) and hydrogen peroxide have both been implicated in a variety of reactions relevant to environmental and physiological processes and may contribute to a unique, unexplored, pathway for the production of nitrous acid (HONO) in soil. To investigate the potential for this reaction, we report an in-depth investigation of the reaction pathway of H2O2 and HNO forming HONO and water. We find the breaking of the peroxide bond and a coupled proton transfer in the first step leads to hydrogen nitryl (HNO2) and an endogenous water, with an extrapolated NEVPT2 (multireference perturbation theory) barrier of 29.3 kcal mol-1. The first transition state is shown to possess diradical character linking the far peroxide oxygen to the bridging, reacting, peroxide oxygen. The energy of this first step, when calculated using hybrid density functional theory, is shown to depend heavily on the amount of Hartree-Fock exchange in the functional, with higher amounts leading to a higher barrier and more diradical character. Additionally, high amounts of spin contamination cause CCSD(T) to significantly overestimate the TS1 barrier with a value of 36.2 kcal mol-1 when using the stable UHF wavefunction as the reference wavefunction. However, when using the restricted Hartree-Fock reference wavefunction, the TS1 CCSD(T) energy is lowered to yield a barrier of 31.2 kcal mol-1, in much better agreement with the NEVPT2 result. The second step in the reaction is the isomerization of HNO2 to trans-HONO through a Grotthuss-like mechanism accepting a proton from and donating a proton to the endogenous water. This new mechanism for the isomerization of HNO2 is shown to have an NEVPT2 barrier of 23.3 kcal mol-1, much lower than previous unimolecular estimates not including an explicit water. Finally, inclusion of an additional explicit water is shown to lower the HNO2 isomerization barrier even further.

Selenols are resistant to irreversible modification by HNO

The discovery of nitric oxide (NO) as an endogenously generated signaling species in mammalian cells has spawned a vast interest in the study of the chemical biology of nitrogen oxides. Of these, nitroxyl (azanone, HNO) has gained much attention for its potential role as a therapeutic for cardiovascular disease. Known targets of HNO include hemes/heme proteins and thiols/thiol-containing proteins. Recently, due to their roles in redox signaling and cellular defense, selenols and selenoproteins have also been speculated to be additional potential targets of HNO. Indeed, as determined in the current work, selenols are targeted by HNO. Such reactions appear to result only in formation of diselenide products, which can be easily reverted back to the free selenol. This characteristic is distinct from the reaction of HNO with thiols/thiolproteins. These findings suggest that, unlike thiolproteins, selenoproteins are resistant to irreversible oxidative modification, support that Nature may have chosen to use selenium instead of sulfur in certain biological systems for its enhanced resistance to electrophilic and oxidative modification.

168 - Sulfomics and Nitromics: Characterizations of S- and N-Based Speciation in Reactions of S-nitroso-glutathione (GSNO) with Hydrogen Sulfide (H2S)

The discovery of nitric oxide (NO) as an endogenously generated signaling species in mammalian cells has spawned a vast interest in the study of the chemical biology of nitrogen oxides. Of these, nitroxyl (azanone, HNO) has gained much attention for its potential role as a therapeutic for cardiovascular disease. Known targets of HNO include hemes/heme proteins and thiols/thiol-containing proteins. Recently, due to their roles in redox signaling and cellular defense, selenols and selenoproteins have also been speculated to be additional potential targets of HNO. Indeed, as determined in the current work, selenols are targeted by HNO. Such reactions appear to result only in formation of diselenide products, which can be easily reverted back to the free selenol. This characteristic is distinct from the reaction of HNO with thiols/thiolproteins. These findings suggest that, unlike thiolproteins, selenoproteins are resistant to irreversible oxidative modification, support that Nature may have chosen to use selenium instead of sulfur in certain biological systems for its enhanced resistance to electrophilic and oxidative modification.

Influence of crustal dust and sea spray supermicron particle concentrations and acidity on inorganic NO<sub>3</sub><sup>−</sup> aerosol during the 2013 Southern Oxidant and Aerosol Study

Abstract. Inorganic aerosol composition was measured in the southeastern United States, a region that exhibits high aerosol mass loading during the summer, as part of the 2013 Southern Oxidant and Aerosol Study (SOAS) campaign. Measurements using a Monitor for AeRosols and GAses (MARGA) revealed two periods of high aerosol nitrate (NO3−) concentrations during the campaign. These periods of high nitrate were correlated with increased concentrations of supermicron crustal and sea spray aerosol species, particularly Na+ and Ca2+, and with a shift towards aerosol with larger (1 to 2.5 μm) diameters. We suggest this nitrate aerosol forms by multiphase reactions of HNO3 and particles, reactions that are facilitated by transport of crustal dust and sea spray aerosol from a source within the United States. The observed high aerosol acidity prevents the formation of NH4NO3, the inorganic nitrogen species often dominant in fine-mode aerosol at higher pH. Calculation of the rate of the heterogeneous uptake of HNO3 on mineral aerosol supports the conclusion that aerosol NO3− is produced primarily by this process, and is likely limited by the availability of mineral cation-containing aerosol surface area. Modeling of NO3− and HNO3 by thermodynamic equilibrium models (ISORROPIA II and E-AIM) reveals the importance of including mineral cations in the southeastern United States to accurately balance ion species and predict gas–aerosol phase partitioning.

Effect of relative humidity on HCl formation from the reaction of H2SO4 and HNO3 with NaCl particles

Short path gas cell Fourier transform infrared spectroscopy was used to analyze the production of hydrochloric acid (HCl) gas from the reaction of sodium chloride (NaCl) particles and sulfuric acid (H2SO4) or nitric acid (HNO3) at a range of relative humidity (2–95 %). HCl was produced from both acids, and reached equilibrium in 10 min for both H2SO4 and HNO3 acids. The equilibrium is thought to be a result of two competing reactions which produce HCl and sequester it back into the NaCl particles. The HCl production from H2SO4 was instantaneous, but its production from HNO3 involved an induction period of a few minutes. The equilibrium HCl conditions observed here have implications in influencing the pH of aerosol particles, and consequently the acidity of precipitation in the atmosphere.

Theoretical Study on the Dynamics of the Reaction of HNO((1)A') with HO2((2)A″).

We used stochastic one-dimensional chemical master equation (CME) simulation to gain insight into the dynamics of the reaction of HNO((1)A') with HO2((2)A″). The reaction takes place over a multiwell, multichannel potential energy surface that is based on the computations at the CBS-QB3 level of theory. The calculated multipath potential energy surface consists of three potential wells and three van der Waals complexes. In solving the master equation, the Lennard-Jones potential is used to model the collision between the collider gases. The fractional population of different intermediates and products in the early stages of the reaction is examined to determine the role of the energized intermediates and van der Waals complexes on the kinetics of the title reaction. The major products of the title reaction at lower temperatures are OH, HNO2, HNOH, and O2(X(3)Σg(-)). The temperature- and pressure-dependence of the reaction over a wide range of temperature (300-3000 K) and pressure (0.1-2000 Torr) are studied. No sign of pressure dependence was being observed for the title reaction over the stated range of pressure. The calculated rate constants from the CME simulation are compared with those obtained from the RRKM-SSA method that is based on strong collision assumption. Our results indicate that the strong collision assumption increases the calculated rate constant for the formation of the main products (HNO2 + OH) by a factor of 2 at 300 K and 1 atm pressure, compared to the results of CME simulation, although the results are in good agreement at higher temperatures.

Exploring Possible Missing Sinks of Nitrate and Its Precursors in Current Air Quality Models —A Case Simulation in the Pearl River Delta, China, Using an Observation-Based Box Model

Nitrate and its precursor (gaseous HNO3) in China are generally overestimated by current chemical transport models in comparison to in-situ observations. In this study, we used an observation-based box model and in-situ measurements at a rural site in southern China to investigate possible missing sinks of nitrate. We found that a heterogeneous reaction of HNO3 to NOx on soot can better balance the NOx/HNO3 chemistry in models and improve model performance with regard to nitrate, particularly where an additional HONO source (the heterogeneous reaction of NO2 on soot) is incorporated into current models. Through a series of sensitivity simulations, the uptake coefficients of heterogeneous reactions were suggested to be 3.0 × 10−3 and 1 × 10−4 for HNO3 and HONO over southern China in fall, respectively. A 3-D simulation with the suggested uptake coefficients further confirmed that heterogeneous reactions significantly decreased the nitrate concentrations (PNO3) in southern China, by up to 10 μg m−3 (50%-80% of simulated PNO3) in polluted regions (e.g., the Yangtze River). In contrast to nitrate and HNO3, NOx concentrations in China were enhanced, which partly explained the underestimation of NO2 in current models compared to satellite observations.

Secondary Formation of Sulfate and Nitrate during a Haze Episode in Megacity Beijing, China

A heavy haze episode that occurred in Beijing from 20 September to 27 September, 2011 was observed to explore the secondary processes of the haze episode. During the haze episode, the relatively stable synoptic conditions and regional transport from polluted areas in the south and southwest of Beijing favored the formation of haze. Significant increases of PM2.5/PM10 ratio was observed during haze period, which implied that the haze was caused by fine particles. Additionally, the presence of secondary inorganic pollutants (SO42–, NO3– and NH4+) sharply increased during the haze episode, which indicated that secondary processes significantly strengthened the haze episode. The sulfur oxidation ratio (SOR) sharply increased from a non-haze episode with a highest value of 0.11 to a haze episode with a highest value of 0.62. Low correlations between SOR and O3 and the temperature were found, whereas a high correlation between SOR and RH was found during the haze episode, which implied that sulfate was mainly produced by the aqueous-phase oxidation of SO2 rather than the gas-phase conversion of SO2 to sulfate in haze episode in Beijing. Furthermore, a fine linear relationship between SOR and the surface area (dS) of particles smaller than 1 µm confirmed the heterogeneous processes of sulfate formation in haze episode. The nitrogen oxidation ratio (NOR) also sharply increased from a non-haze episode with a highest value of 0.03 to a haze episode with a highest value of 0.26, which indicated more intense secondary formation of nitrate in haze episode. Nitrate was found to be mainly produced by a homogenous reaction under ammonium-rich conditions. Higher RH in haze episode reduced the thermodynamic equilibrium constant Ke’, and favored the thermodynamic equilibrium reaction of HNO3(g) + NH3(g) ↔ NH4NO3(s, aq) to formed nitrate, which might help explain the enhanced homogenous production of nitrate in haze episode. In addition, a good empirical fit (R2 = 0.70) between NOR and dS was found, which indicated that the particle surface area significantly contributed to the intense homogeneous production of nitrate in haze episode.

308nm photochemical reaction of gaseous HNO3 and benzene on α-Fe2O3 surfaces

Abstract The 308 nm photochemical reactions of nitric acid (HNO3) and benzene in the gas phase and on α-Fe2O3 surface at 298 K was investigated by using Fourier transform infrared spectroscopy (FT-IR) combined with high performance liquid chromatography (HPLC). The concentration and yield of HONO and p-nitrophenol (p-NP) had been examined as a function of reaction time, benzene initial concentration and relative humidity on photochemical reaction. The results showed that gaseous HNO3 did not directly react with benzene in the dark, and p-NP was formed irradiation under 308 nm UV light. When HNO3 initial concentration was 400 Pa and benzene was 300 Pa, the illumination time was 100 min, the concentration of p-NP produced from the photochemical reaction of HNO3 and benzene on α-Fe2O3 surface was about 3.08 times higher than that in the gas phase. In the meantime, while reaction time was 40 min and relative humidity was 70%, the concentration of HONO and p-NP formed on α-Fe2O3 surface were about 3.55 and 2.51 times higher than those in the gas phase, and the yield of p-NP was 3.74% and 2.99%, respectively. Surfaces effect played a leading role in photochemical reaction of HNO3 and benzene on α-Fe2O3 surface.

Aerial observation of nitrogen compounds over the East China Sea in 2009 and 2010

Aerial observations of atmospheric pollutants were made over the East China Sea to analyze the transport of air pollutants, especially nitrogen oxides, from East Asia. Three flights each were conducted in October 2009 (autumn) and December 2010 (winter). Onboard measurements of gaseous total odd nitrogen species, gaseous nitric acid (HNO3(g)), O3, SO2, CO and black carbon were made and particles were collected on filters for ionic and metal component analyses. The NO3−(p)/T.NO3 (T.NO3 = HNO3(g) + NO3−(p); NO3−(p) indicates particulate nitrate) ratios were less than 0.5 in most cases. Exceptions were 17 October and 11 December, when high concentrations of dust particles (Kosa) were transported. The average T.NO3/NOy (NOy indicates total odd nitrogen species) ratio in winter was 0.59, and that in autumn was 0.71. In addition, positive and negative correlations between NOy − T.NO3 and ozone were observed in autumn and winter, respectively. These results indicate that the main components of NOy − T.NO3 are NOx and its descendant photochemical products, such as peroxyacyl nitrates and alkyl nitrates, in winter and autumn, respectively. Ratios of Na+ to Cl− + NO3− in particles were close to the seawater ratio for observations in both autumn and winter, save for the Kosa events. The main NO3−(p) formation process was the reaction of HNO3(g) with sea salt aerosols during autumn and winter. On the other hand, NO3−(p) was generated by the reaction of HNO3(g) with dust particles and sea salt during the Kosa events. The fraction of NH4NO3 in NO3−(p) was very small.

Atmospheric formation of the NO3 radical from gas-phase reaction of HNO3 acid with the NH2 radical: proton-coupled electron-transfer versus hydrogen atom transfer mechanisms.

The gas-phase reaction of nitric acid with the amidogen radical under atmospheric conditions has been investigated using quantum mechanical (QCISD and CCSD(T)) and DFT (B3LYP, BH&HLYP, M05, M05-2X, and M06-2X) calculations with the 6-311+G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and extrapolation to the CBS basis sets. The reaction begins with the barrierless formation of a hydrogen-bonded complex, which can undergo two different reaction pathways, in addition to the decomposition back to the reactants. The lowest energy barrier pathway involves a proton-coupled electron-transfer mechanism, whereas the highest energy barrier pathway takes place through a hydrogen atom transfer mechanism. The performance of the different DFT functionals in predicting both the geometries and relative energies of the stationary points investigated has been analyzed.

Nitrate-to-Nitrite-to-Nitric Oxide Conversion Modulated by Nitrate-Containing {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC)

Nitrosylation of high-spin [Fe(κ(2)-O(2)NO)(4)](2-) (1) yields {Fe(NO)}(7) mononitrosyl iron complex (MNIC) [(κ(2)-O(2)NO)(κ(1)-ONO(2))(3)Fe(NO)](2-) (2) displaying an S = 3/2 axial electron paramagnetic resonance (EPR) spectrum (g(⊥) = 3.988 and g(∥) = 2.000). The thermally unstable nitrate-containing {Fe(NO)(2)}(9) dinitrosyl iron complex (DNIC) [(κ(1)-ONO(2))(2)Fe(NO)(2)](-) (3) was exclusively obtained from reaction of HNO(3) and [(OAc)(2)Fe(NO)(2)](-) and was characterized by IR, UV-vis, EPR, superconducting quantum interference device (SQUID), X-ray absorption spectroscopy (XAS), and single-crystal X-ray diffraction (XRD). In contrast to {Fe(NO)(2)}(9) DNIC [(ONO)(2)Fe(NO)(2)](-) constructed by two monodentate O-bound nitrito ligands, the weak interaction between Fe(1) and the distal oxygens O(5)/O(7) of nitrato-coordinated ligands (Fe(1)···O(5) and Fe(1)···O(7) distances of 2.582(2) and 2.583(2) Å, respectively) may play important roles in stabilizing DNIC 3. Transformation of nitrate-containing DNIC 3 into N-bound nitro {Fe(NO)}(6) [(NO)(κ(1)-NO(2))Fe(S(2)CNEt(2))(2)] (7) triggered by bis(diethylthiocarbamoyl) disulfide ((S(2)CNEt(2))(2)) implicates that nitrate-to-nitrite conversion may occur via the intramolecular association of the coordinated nitrate and the adjacent polarized NO-coordinate ligand (nitrosonium) of the proposed {Fe(NO)(2)}(7) intermediate [(NO)(2)(κ(1)-ONO(2))Fe(S(2)CNEt(2))(2)] (A) yielding {Fe(NO)}(7) [(NO)Fe(S(2)CNEt(2))(2)] (6) along with the release of N(2)O(4) (·NO(2)) and the subsequent binding of ·NO(2) to complex 6. The N-bound nitro {Fe(NO)}(6) complex 7 undergoes Me(2)S-promoted O-atom transfer facilitated by imidazole to give {Fe(NO)}(7) complex 6 accompanied by release of nitric oxide. This result demonstrates that nitrate-containing DNIC 3 acts as an active center to modulate nitrate-to-nitrite-to-nitric oxide conversion.

Nitroxyl, Redox Switches, Cardiac Myofilaments, and Heart Failure

Heart failure (HF) remains to be the leading cause of death in developed countries, and the prevalence of HF worldwide continues to escalate.1 Despite enormous efforts made to improve our understanding of the molecular mechanisms of HF and to develop effective therapies, HF remains to be a therapeutic challenge. One hallmark of HF is altered cardiac myofilament responsiveness to intracellular Ca2+, resulting in depressed contraction. Although increasing intracellular Ca2+ is a possible strategy for strengthening contraction, currently available inotropic agents, such as β-agonists and phosphodiesterase inhibitors, which raise intracellular cAMP levels can lead to deleterious side effects when given as a long-term therapy.2 Recently, promising inotropic agents targeting novel mechanisms have been developed, including (1) nitroxyl (HNO) released from HNO donors, (2) istaroxime, an inhibitor of Na+/K+-ATPase and an activator of the sarcoplasmic reticulum Ca2+ pump (SERCA), (3) cardiac myosin activator, and (4) ryanodine receptor stabilizer.2 In this issue, Gao et al3 report that HNO can increase myofilament Ca2+ responsiveness by inducing disulfide bonds between key cysteine residues in the myofilament, suggesting that HNO donors may improve cardiac function in patients with HF without the possible deleterious consequences of continued Ca2+ manipulation (Figure). Figure. Schematic representation of nitroxyl (HNO)-induced disulfide effects on cardiac myofilaments . A , Chemical reactions showing (1) the release of HNO from nitrosocyclohexyl acetate (NCA) by hydrolysis; (2) reaction of HNO with thiols on cysteine forming N-hydroxysulfenamide intermediates, which leads to sulfinamide or disulfide formation in the presence of an additional free thiol. B, Schematic drawing showing that disulfide bond formation between key cysteines in actin and tropomyosin and in myosin light chain 1 (MLC1) and myosin heavy chain (MHC) increases maximum force and Ca2+ sensitivity. Modified with permission from Moss …

Surface-Catalyzed Chlorine and Nitrogen Activation: Mechanisms for the Heterogeneous Formation of ClNO, NO, NO2, HONO, and N2O from HNO3 and HCl on Aluminum Oxide Particle Surfaces

It is well-known that chlorine active species (e.g., Cl(2), ClONO(2), ClONO) can form from heterogeneous reactions between nitrogen oxides and hydrogen chloride on aerosol particle surfaces in the stratosphere. However, less is known about these reactions in the troposphere. In this study, a potential new heterogeneous pathway involving reaction of gaseous HCl and HNO(3) on aluminum oxide particle surfaces, a proxy for mineral dust in the troposphere, is proposed. We combine transmission Fourier transform infrared spectroscopy with X-ray photoelectron spectroscopy to investigate changes in the composition of both gas-phase and surface-bound species during the reaction under different environmental conditions of relative humidity and simulated solar radiation. Exposure of surface nitrate-coated aluminum oxide particles, from prereaction with nitric acid, to gaseous HCl yields several gas-phase products, including ClNO, NO(2), and HNO(3), under dry (RH < 1%) conditions. Under humid more conditions (RH > 20%), NO and N(2)O are the only gas products observed. The experimental data suggest that, in the presence of adsorbed water, ClNO is hydrolyzed on the particle surface to yield NO and NO(2), potentially via a HONO intermediate. NO(2) undergoes further hydrolysis via a surface-mediated process, resulting in N(2)O as an additional nitrogen-containing product. In the presence of broad-band irradiation (λ > 300 nm) gas-phase products can undergo photochemistry, e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particle mineralogy (Al(2)O(3) vs CaCO(3)) and the presence of other coadsorbed gases (e.g., NH(3)). These newly identified reaction pathways discussed here involve continuous production of active ozone-depleting chlorine and nitrogen species from stable sinks such as gas-phase HCl and HNO(3) as a result of heterogeneous surface reactions. Given that aluminosilicates represent a major fraction of mineral dust aerosol, aluminum oxide can be used as a model system to begin to understand various aspects of possible reactions on mineral dust aerosol surfaces.

Rapid and Selective Nitroxyl (HNO) Trapping by Phosphines: Kinetics and New Aqueous Ligations for HNO Detection and Quantitation

Recent studies distinguish the biological and pharmacological effects of nitroxyl (HNO) from its oxidized/deprotonated product nitric oxide (·NO), but the lack of HNO detection methods limits the understanding its in vivo mechanisms and the identification of endogenous sources. We previously demonstrated that reaction of HNO with triarylphosphines provides aza-ylides and HNO-derived amides, which may serve as stable HNO biomarkers. We now report a kinetic analysis for the trapping of HNO by phosphines, ligations of enzyme-generated HNO, and compatibility studies illustrating the selectivity of phosphines for HNO over other physiologically relevant nitrogen oxides. Quantification of HNO using phosphines is demonstrated using an HPLC-based assay and ligations of phosphine carbamates generate HNO-derived ureas. These results further demonstrate the potential of phosphine probes for reliable biological detection and quantification of HNO.

First Principles Study of the Ignition Mechanism for Hypergolic Bipropellants: N,N,N′,N′-Tetramethylethylenediamine (TMEDA) and N,N,N′,N′-Tetramethylmethylenediamine (TMMDA) with Nitric Acid

We report quantum mechanics calculations (B3LYP flavor of density functional theory) to determine the chemical reaction mechanism underlying the hypergolic reaction of pure HNO(3) with N,N,N',N'-tetramethylethylenediamine (TMEDA) and N,N,N',N'-tetramethylmethylenediamine (TMMDA). TMEDA and TMMDA are dimethyl amines linked by two CH(2) groups or one CH(2) group, respectively, but ignite very differently with HNO(3). We explain this dramatic difference in terms of the role that N lone-pair electrons play in activating adjacent chemical bonds. We identify two key atomistic level factors that affect the ignition delay: (1) The exothermicity for formation of the dinitrate salt from TMEDA or TMMDA. With only a single CH(2) group between basic amines, the diprotonation of TMMDA results in much stronger electrostatic repulsion, reducing the heat of dinitrate salt formation by 6.3 kcal/mol. (2) The reaction of NO(2) with TMEDA or TMMDA, which is the step that releases the heat and reactive species required to propagate the reaction. Two factors of TMEDA promote the kinetics by providing routes with low barriers to oxidize the C: (a) formation of a stable intermediate with a C-C double bond and (b) the lower bond energy for breaking the C-C single bond (by 18 kcal/mol comparing to alkane) between two amines. Both factors would decrease the ignition delay for TMEDA versus TMMDA. The same factors also explain the shorter ignition delay of 1,4-dimethylpiperazine (DMPipZ) versus 1,3,5-trimethylhexahydro-1,3,5-triazine (TMTZ). These results indicate that TMEDA and DMPipZ are excellent green replacements for hydrazines as the fuel in bipropellants.

Heterogeneous Photochemistry of Trace Atmospheric Gases with Components of Mineral Dust Aerosol

Mineral dust aerosol is known to provide a reactive surface in the troposphere for heterogeneous chemistry to occur. Certain components of mineral dust aerosol, such as semiconductor metal oxides, can act as chromophores that initiate chemical reactions, while adsorbed organic and inorganic species may also be photoactive. However, relatively little is known about the impact of heterogeneous photochemistry of mineral dust aerosol in the atmosphere. In this study, we investigate the heterogeneous photochemistry of trace atmospheric gases including HNO(3) and O(3) with components of mineral dust aerosol using an environmental aerosol chamber that incorporates a solar simulator. For reaction of HNO(3) with aluminum oxide, broadband irradiation initiates photoreactions to form gaseous NO and NO(2). A complex dynamic balance between surface adsorbed nitrate and gaseous nitrogen oxide products including NO and NO(2) is observed. For heterogeneous photoreactions of O(3), iron oxide shows catalytic decompositions toward O(3) while aluminum oxide is deactivated by ozone exposure. Furthermore, the role of relative humidity, and, thus, adsorbed water, on heterogeneous photochemistry has been explored. The atmospheric implications of these results are discussed.

Evidence of high PM2.5 strong acidity in ammonia-rich atmosphere of Guangzhou, China: Transition in pathways of ambient ammonia to form aerosol ammonium at [NH4+]/[SO42–] = 1.5

In this study, 24-h PM2.5 samples were collected using Harvard Honeycomb denuder/filter-pack system during different seasons in 2006 and 2007 at an urban site in Guangzhou, China. The particles collected in this study were generally acidic (average strong acidity ([H+]) ~70nmol m−3). Interestingly, aerosol sulfate was not fully neutralized in the ammonia-rich atmosphere (NH3~30ppb) and even when NH4+]/[SO42−] was larger than 2. Consequently, strong acidity ([H+]) as high as 170nmol m−3 was observed in these samples. The kinetic rate of neutralization of acidity (acidic sulfate) by ambient ammonia was significantly higher than the rate of formation of ammonium nitrate involving HNO3 and NH3 for [NH4+]/[SO42−]≤1.5 and much lower for NH4+]/[SO42−]>1.5. Therefore, higher nitrate principally formed via homogeneous gas phase reactions involving ammonia and nitric acid were observed for [NH4+]/[SO42−]>1.5. However, little nitrate, probably formed via heterogeneous processes e.g. reaction of HNO3 with sea salt or crustal species, was observed for [NH4+]/[SO42−]≤1.5. These demonstrate a clear transition in the pathways of ambient ammonia to form aerosol ammonium at [NH4+]/[SO42−]=1.5 and evidently explain the observed high acidity due to the unneutralized sulfate in the ammonium-rich aerosol (NH4+]/[SO42−]>1.5). In fact, the measured acidity was almost similar to the excess acid defined as the acid that remains at [NH4+]/[SO42−]=1.5 due to the un-neutralized fraction of sulfate ([H+]=0.5[SO42−]). The presence of high excess acid and ammonium nitrate significantly lowered the deliquescence relative humidity of ammonium sulfate (from 80% to 40%) in the ammonium-rich samples.