Formation of Nitric Acid in the Gas-Phase HO2 + NO Reaction: Effects of Temperature and Water Vapor

Yields of HONO2 and HOONO Products from the Reaction of HO2 and NO Using Pulsed Laser Photolysis and Mid-Infrared Cavity-Ringdown Spectroscopy

The reaction of HO₂ with NO is one of the most important steps in radical cycling throughout the stratosphere and troposphere. Previous literature experimental work revealed a small yield of nitric acid (HONO₂) directly from HO₂ + NO. Atmospheric models previously treated HO₂ + NO as radical recycling, but inclusion of this terminating step had large effects on atmospheric oxidative capacity and the concentrations of HONO₂ and ozone (O₃), among others. Here, the yield of HONO₂, φ_HONO2, from the reaction of HO₂ + NO was investigated in a flow tube reactor using mid-IR pulsed-cavity ringdown spectroscopy. HO₂, produced by pulsed laser photolysis of Cl₂ in the presence of methanol, reacted with NO in a buffer gas mixture of N₂ and CO between 300 and 700 Torr at 278 and 300 K. HONO₂ and its weakly bound isomer HOONO were directly detected by their v₁ absorption bands in the mid-IR region. CO was used to suppress HONO₂ produced from OH + NO₂ and exploit a chemical amplification scheme, converting OH back to HO₂. Under the experimental conditions described here, no evidence for the formation of either HONO₂ or HOONO was observed from HO₂ + NO. Using a comprehensive chemical model, constrained by observed secondary reaction products, all HONO₂ detected in the system could be accounted for by OH + NO₂. At 700 ± 14 Torr and 300 ± 3 K, φ_HONO2 = 0.00 ± 0.11% (2σ) with an upper limit of 0.11%. If all of the observed HONO₂ was attributed to the HO₂ + NO reaction, φ_HONO2 = 0.13 ± 0.07% with an upper limit of 0.20%. At 278 ± 2 K and 718 ± 14 Torr, we determine an upper limit, φ_HONO2 ≤ 0.37%. Our measurements are significantly lower than those previously reported, lying outside of the uncertainty of the current experimental and recommended literature values.

Perspectives on the Competition between the Electrochemical Water and N2 Oxidation on a TiO2(110) Electrode

The electrochemical nitrogen oxidation reaction (NOR) has recently drawn attention due to promising experimental and theoretical results. It provides an alternative, environmentally friendly route to directly synthesize nitrate from N₂(g). There is to date a limited number of investigations focused on the electrochemical NOR. Herein, we present a detailed computational study on the kinetics of both the NOR and the competing oxygen evolution reaction (OER) on the TiO₂(110) electrode under ambient conditions. The use of grand canonical density functional theory in combination with the linearized Poisson-Boltzmann equation allows a continuous tuning of the explicitly applied electrical potential. We find that the OER may either promote or suppress the NOR on TiO₂(110) depending on reaction conditions. The detailed atomistic insights provided on the mechanisms of these competing processes make possible further developments toward a direct electrochemical NOR process.

Monitoring of endogenous nitric oxide exhaled by pig lungs during ex-vivo lung perfusion

In the context of organ shortage for transplantation, new criteria for better organ evaluation should be investigated. Ex-Vivo Lung Perfusion (EVLP) allows extra-corporal lung re-conditioning and evaluation, under controlled parameters of the organ reperfusion and mechanical ventilation. This work reports on the interest of exhaled gas analysis during the EVLP procedure. After a one-hour cold ischemia, the endogenous gas production by an isolated lung of nitric oxide and carbon monoxide is simultaneously monitored in real time. The exhaled gas is analysed with two very sensitive and selective laser spectrometers developed upon the technique of optical-feedback cavity-enhanced absorption spectroscopy. Exhaled gas concentration measured for an ex-vivo lung is compared to the corresponding production by the whole living pig, measured before euthanasia. On-line measurements of the fraction of nitric oxide in exhaled gas (FENO) in isolated lungs are reported here for the first time, allowing to resolve the respiratory cycles. In this study, performed on 9 animals, FENO by isolated lungs range from 3.3 to 10.6 ppb with a median value of 4.4 ppb. Pairing ex-vivo lung and pig measurements allows to demonstrate a systematic increase of FENO in the ex-vivo lung as compared to the living animal, by a factor of 3 ± 1.2. Measurements of the fraction of carbon monoxide in exhaled gas (FECO) confirm levels recorded during previous studies driven to evaluate FECO as a potential marker of ischemia reperfusion injuries. FECO production by ex-vivo lungs ranges from 0.31 to 2.3 ppm with a median value of 0.8 ppm. As expected, these FECO values are lower than the production by the corresponding whole pig body, by a factor of 6.9 ± 2.7.

Past and present anthropic environmental stress reflect high susceptibility of natural freshwater ecosystems in Romania

The concentrations of twelve heavy metals and trace elements (Cr, Mn, Co, Ni, Cu, As, Cd, Pb, Hg, Zn, Fe, and Al) in bed sediment and river freshwater that received sewage discharge, industrial wastewater inputs and mining residue were discussed. Spatial distribution, intra-annual trends and diffuse flux in 2019 in the middle and lower reaches of Olt River Basin (ORB) were investigated using inductively coupled mass spectrometry (ICP-MS) and atomic absorption spectroscopy (AAS). We applied correlation and principal component analysis (PCA) to quantify metal distribution relationship within environmental factors (pH, air temperature) and organic matter existing in the ORB. Moreover, the ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁷Pb isotope ratios analysis was employed to conclude the possible origin of the contamination. PCA analysis categorized metal presence in the four-component model, which explains 91% (May), 92% (July) and 93% (September) of the variance and indicates the potential origins of pollutants. The HCA and correlation analysis emphasized the relationship between trace elements, heavy metals in water and sediments and physicochemical characteristics of water. It was observed a high discrepancy in metal distribution between riverbed sediments and water body. In September, correlation indices highlighted sparse positive relationship with trace elements in water and mainly negative correlation values with trace elements from sediments. The origin of pollutants in sediments and water appear to be both natural and human-related activities. In all seasons increased the total exchangeable concentration of Ni, Cu and Zn in the sediments downstream sewage treatment plants and upstream of dams. The consideration of environmental factors and physicochemical characteristics of water is required to develop strategies for pollution management, assessment and mitigation in the actual condition of climate change. This study evaluated the heavy metals pollution in the Olt River Basin over three periods in 2019 under human-induced changes.

The production of formaldehyde and hydroxyacetone in methacrolein photooxidation: New insights into mechanism and effects of water vapor

Methacrolein (MACR) is an abundant multifunctional carbonyl compound with high reactivity in the atmosphere. In this study, we investigated the hydroxyl radical initiated oxidation of MACR at various NO/MACR ratios (0 to 4.04) and relative humidities (<3% to 80%) using a flow tube. Meanwhile, a box model based on the Master Chemical Mechanism was performed to test our current understanding of the mechanism. In contrast to the reasonable predictions for hydroxyacetone production, the modeled yields of formaldehyde (HCHO) were twice higher than the experimental results. The discrepancy was ascribed to the existence of unconsidered non-HCHO forming channels in the chemistry of CH₃C(CH₂)OO, which account for approx. 50%. In addition, the production of hydroxyacetone and HCHO were affected by water vapor as well as the initial NO/MACR ratio. The yields of HCHO were higher under humid conditions than that under dry condition. The yields of hydroxyacetone were higher under humid conditions at low-NO_x level, while lower at high-NO_x level. The reasonable explanation for the lower hydroxyacetone yield under humid conditions at high-NO_x level is that water vapor promotes the production of methacrolein nitrate in the reaction of HOCH₂C(CH₃)(OO)CHO with NO due to the peroxy radical-water complex formation, which was evidenced by calculational results. And the minimum equilibrium constant of this water complex formation was estimated to be 1.89×10^-18cm³/molecule. These results provide new insights into the MACR oxidation mechanism and the effects of water vapor.

Nitrogen Oxides in Early Earth's Atmosphere as Electron Acceptors for Life's Emergence

We quantify the amount of nitrogen oxides (NOx) produced through lightning and photochemical processes in the Hadean atmosphere to be available in the Hadean ocean for the emergence of life. Atmospherically generated nitrate (NO₃^-) and nitrite (NO₂^-) are the most attractive high-potential electron acceptors for pulling and enabling crucial redox reactions of autotrophic metabolic pathways at submarine alkaline hydrothermal vents. The Hadean atmosphere, dominated by CO₂ and N₂, will produce nitric oxide (NO) when shocked by lightning. Photochemical reactions involving NO and H₂O vapor will then produce acids such as HNO, HNO₂, HNO₃, and HO₂NO₂ that rain into the ocean. There, they dissociate into or react to form nitrate and nitrite. We present new calculations based on a novel combination of early-Earth global climate model and photochemical modeling, and we predict the flux of NOx to the Hadean ocean. In our 0.1-, 1-, and 10-bar pCO₂ models, we calculate the NOx delivery to be 2.4 × 10⁵, 6.5 × 10⁸, and 1.9 × 10⁸ molecules cm^-2 s^-1. After only tens of thousands to tens of millions of years, these NOx fluxes are expected to produce sufficient (micromolar) ocean concentrations of high-potential electron acceptors for the emergence of life. Key Words: Nitrogen oxides-Nitrate-Nitrite-Photochemistry-Lightning-Emergence of life. Astrobiology 17, 975-983.

Measurement of tropospheric RO2 and HO2 radicals by a laser-induced fluorescence instrument

A new method (ROxLIF) for the measurement of atmospheric peroxy radicals (HO(2) and RO(2)) was developed using a two-step chemical conversion scheme and laser-induced fluorescence (LIF) for radical detection. Ambient air is sampled into a differentially pumped flow reactor, in which atmospheric RO(x) radicals (=RO(2)+RO+HO(2)+OH) are chemically converted to HO(2) by a large excess of NO and CO at reduced pressures (ROx mode). When only CO is added as a reagent, the sum of atmospheric HO(2)+OH is converted to HO(2) (HOx mode). At the reactor outlet, part of the air flow is transferred into a low-pressure detection chamber, where the HO(2) is further converted by reaction with NO to OH, which is then detected with high sensitivity by LIF at 308 nm. The ROxLIF technique has been implemented in an existing LIF instrument that is also capable of measuring atmospheric OH. From the concurrent measurements of RO(x), HO(x) and OH, concentrations of HO(2) and RO(2) can be determined. The system is calibrated using the quantitative photolysis of water vapor at 185 nm as a radical source. Addition of CO or hydrocarbons to the calibration gas yields well-defined concentrations of HO(2) or RO(2), respectively, providing an estimated accuracy for the calibration of about 20%. The ROxLIF technique is extremely sensitive and has detection limits (signal-to-noise ratio=2) of about 0.1 pptv of HO(2) or RO(2) at a time resolution of 1 min. The paper describes the technique and its calibration, discusses the chemistry in the conversion reactor and possible interferences, and gives an example of ambient air measurements to demonstrate the performance of the new technique.

HNO3 Forming Channel of the HO2 + NO Reaction as a Function of Pressure and Temperature in the Ranges of 72−600 Torr and 223−323 K

A high-pressure turbulent flow reactor coupled with a chemical ionization mass-spectrometer was used to determine the branching ratio of the HO(2) + NO reaction: HO(2) + NO --> OH + NO(2) (1a), HO(2) + NO --> HNO(3) (1b). The branching ratio, beta = k(1b)/k(1a), was derived from the measurements of "chemically amplified" concentrations of the NO(2) and HNO(3) products in the presence of O(2) and CO. The pressure and temperature dependence of beta was determined in the pressure range of 72-600 Torr of N(2) carrier gas between 323 and 223 K. At each pressure, the branching ratio was found to increase with the decrease of temperature, the increase becoming less pronounced with the increase of pressure. In the 298-223 K range, the data could be fitted by the expression: beta(T,P) = (530 +/- 10)/T(K) + (6.4 +/- 1.3) x 10(-4)P(Torr) - (1.73 +/- 0.07), giving beta approximately 0.5% near the Earth's surface (298 K, 760 Torr) and 0.8% in the tropopause region (220 K, 200 Torr). The atmospheric implication of these results is briefly discussed.

Vibrational overtone spectrum of matrix isolated cis, cis-HOONO

Cis, cis-peroxynitrous acid is known to be an intermediate in atmospheric reactions between OH and NO2 as well as HOO and NO. The infrared absorption spectra of matrix-isolated cc-HOONO and cc-DOONO in argon have been observed in the range of 500-8000 cm-1. Besides the seven fundamental vibrational modes that have been assigned earlier for this molecule [Zhang et al., J. Chem. Phys. 124, 084305 (2006)], more than 50 of the overtone and combination bands have been observed for cc-HOONO and cc-DOONO. Ab initio CCSD(T)/atomic natural orbital anharmonic force field calculations were used to help guide the assignments. Based on this study of the vibrational overtone transitions of cis, cis-HOONO that go as high as 8000 cm-1 and the earlier paper on the vibrational fundamentals, we conclude that the CCSD(T)/ANO anharmonic frequencies seem to correct to +/-35 cm-1. The success of the theoretically predicted anharmonic frequencies {upsilon} in assigning overtone spectra of HOONO up to 8000 cm-1 suggests that the CCSD(T)/ANO method is producing a reliable potential energy surface for this reactive molecule.

Measurement of trace atmospheric species by chemical ionization mass spectrometry: speciation of reactive nitrogen and future directions

Chemical ionization mass spectrometry (CIMS) has proven to be a powerful method for sensitive, fast time response (t approximately 1 sec) measurements of various atmospheric compounds with limits of detection (LOD) of the order of tens of pptv and lower. The rapid time response of CIMS is particularly well suited for airborne measurements and its application has largely grown out of airborne measurements in the stratosphere and upper troposphere. This work reviews some of the advances in CIMS technology that have occurred in the past decade. In particular, CIMS methods for selective measurement of reactive nitrogen species (e.g., HNO3, HO2NO2, PAN, and NH3) in the lower atmosphere (altitudes approximately 0-8 km) are described. In addition, recent developments in CIMS technology for the selective measurement of gas-phase hydroperoxides and aerosol chemical composition are briefly described.

Temperature dependence of pentyl nitrate formation from the reaction of pentyl peroxy radicals with NO

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary congruent with tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past.

Mechanism of the OH-Initiated Oxidation of Glycolaldehyde over the Temperature Range 233−296 K

The mechanism of the gas-phase OH-initiated oxidation of glycolaldehyde (HOCH(2)CHO) was studied in the 233-296 K temperature range using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. In the presence of O2, formaldehyde, CO2, formic acid, and glyoxal were observed at room temperature with the yields of 80, 34, 18, and 14%, respectively. Decrease of temperature to 233 K led to significant changes in the yields of the stable products: those of formaldehyde and glyoxal decreased to 50 and 4%, respectively, whereas that of formic acid increased to 52%. It was also found that the OH + glycolaldehyde + O2 reaction proceeds with considerable reformation of OH radicals (by 25% at 296 K). The observed product yields are explained by a mechanism including formation of short-lived intermediate adducts of the primary radicals with O2. The implication of the obtained results for the HOx budget in the upper troposphere is discussed.

Constraining the Mechanism and Kinetics of OH + NO2 and HO2 + NO Using the Multiple-Well Master Equation

Several recent experimental studies have provided substantial new constraints for the mechanisms on the HNO3 potential energy surface. These include observations of biexponential OH decay over short time scales from OH + NO2, which constrain key properties of the short-lived HOONO intermediate, observations of both conformers of the HOONO intermediate itself, isotopic scrambling data for 18OH + NO2, and observations of HONO2 production from the HO2 + NO reaction. We combine all of these recent data in a master-equation simulation of the system. This simulation is initialized with computational values for both stable species (wells) and transition states, but parameters are then adjusted to fit the observations. All parameters are kept within limits defined by experimental and theoretical uncertainty, and all converge away from their bounds. The primary fitting is carried out on the OH kinetic data-we first fit the biexponential kinetics, then address the isotopic scrambling. Isotopic scrambling is shown to be rapid but not complete at low pressure, while at least two parameter sets are shown to be consistent with the biexponential data. Of these two parameter sets, one is far more consistent with recent observations of trans-HOONO decay, isotopic scrambling, and HONO2 production from HO2 + NO. This we regard as the most probable potential energy surface for the reaction. On this PES, cis-trans isomerization for HOONO is slow but isomerization of trans-HOONO to HONO2 is rapid. This has significant implications for observed HOONO behavior and also HONO2 formation in the atmosphere from both HO2 + NO and OH + NO2.

Mechanism of the OH-Initiated Oxidation of Hydroxyacetone over the Temperature Range 236−298 K

The mechanism of the gas-phase reaction of OH radicals with hydroxyacetone (CH3C(O)CH2OH) was studied at 200 Torr over the temperature range 236-298 K in a turbulent flow reactor coupled to a chemical ionization mass-spectrometer. The product yields and kinetics were measured in the presence of O2 to simulate the atmospheric conditions. The major stable product at all temperatures is methylglyoxal. However, its yield decreases from 82% at 298 K to 49% at 236 K. Conversely, the yields of formic and acetic acids increase from about 8% to about 20%. Other observed products were formaldehyde, CO2 and peroxy radicals HO2 and CH3C(O)O2. A partial re-formation of OH radicals (by approximately 10% at 298 K) was found in the OH + hydroxyacetone + O2 chemical system along with a noticeable inverse secondary kinetic isotope effect (k(OH)/k(OD) = 0.78 +/- 0.10 at 298 K). The observed product yields are explained by the increasing role of the complex formed between the primary radical CH3C(O)CHOH and O2 at low temperature. The rate constant of the reaction CH3C(O)CHOH + O2 --> CH3C(O)CHO + HO2 at 298 K, (3.0 +/- 0.6) x 10(-12) cm3 molecule(-1) s(-1), was estimated by computer simulation of the concentration-time profiles of the CH3C(O)CHO product. The detailed mechanism of the OH-initiated oxidation of hydroxyacetone can help to better describe the atmospheric oxidation of isoprene, in particular, in the upper troposphere.

Infrared absorption spectra of matrix-isolated cis, cis-HOONO and its ab initio CCSD(T) anharmonic vibrational bands

The infrared absorption spectra of matrix-isolated cis, cis-peroxynitrous acid (HOONO and DOONO) in argon have been observed. Six of the nine fundamental vibrational modes for cis, cis-HOONO have been assigned definitively, and one tentatively. Coupled-cluster, ab initio anharmonic force field calculations were used to help guide some of the assignments. The experimental matrix frequencies (cm(-1)) for cis, cis-HOONO are (a' modes) nu1 = 3303+/-1, nu2 = 1600.6+/-0.6, nu3 = 1392+/-1, nu4 = 922.8+/-0.5, nu5 = 789.7+/-0.4, nu6 = 617+/-1; and (a" mode) nu8 = 462+/-1. The fundamentals for the deuterated isotopomer, cis, cis-DOONO, are (a' modes) nu1 = 2447.2+/-0.6, nu2 = 1595.7+/-0.7, nu3 = 1089.1+/-0.4, nu4 = 888.1+/-0.4, nu5 = 786.6+/-0.5, nu6 = 613.9+/-0.9; and (a" mode) nu8 = 456.5+/-0.5.

Quasiclassical Trajectory Simulations of OH(v) + NO2 → HONO2* → OH(v‘) + NO2: Capture and Vibrational Deactivation Rate Constants

Quasiclassical trajectory calculations are used to investigate the dynamics of the OH(v) + NO(2) --> HONO(2) --> OH(v') + NO(2) recombination/dissociation reaction on an analytic potential energy surface (PES) that gives good agreement with the known structure and vibrational frequencies of nitric acid. The calculated recombination rate constants depend only weakly on temperature and on the initial vibrational energy level of OH(v). The magnitude of the recombination rate constant is sensitive to the potential function describing the newly formed bond and to the switching functions in the PES that attenuate inter-mode interactions at long range. The lifetime of the nascent excited HONO(2) depends strongly not only on its internal energy but also on the identity of the initial state, in disagreement with statistical theory. This disagreement is probably due to the effects of slow intramolecular vibrational energy redistribution (IVR) from the initially excited OH stretching mode. The vibrational energy distribution of product OH(v') radicals is different from statistical distributions, a result consistent with the effects of slow IVR. Nonetheless, the trajectory results predict that vibrational deactivation of OH(v) via the HONO(2) transient complex is approximately 90% efficient, almost independent of initial OH(v) vibrational level, in qualitative agreement with recent experiments. Tests are also carried out using the HONO(2) PES, but assuming the weaker O-O bond strength found in HOONO (peroxynitrous acid). In this case, the predicted vibrational deactivation efficiencies are significantly lower and depend strongly on the initial vibrational state of OH(v), in disagreement with experiments. This disagreement suggests that the actual HOONO PES may contain more inter-mode coupling than found in the present model PES, which is based on HONO(2). For nitric acid, the measured vibrational deactivation rate constant is a useful proxy for the recombination rate, but IVR randomization of energy is not complete, suggesting that the efficacy of the proxy method must be evaluated on a case-by-case basis.