The surface reaction of NO2 and H2O vapor to emit HONO into the gas phase was studied in the evacuable and bakeable photochemical chamber under the irradiation of UV-visible light (? 290 nm). Kinetic analysis of the NO, NO2, and HONO with the aid of computer modeling strongly suggested that the formation of HONO by the surface reaction is photoenhanced. When a linear regression was assumed, the photoenhancement factor defined by {(k′21/k21) ? 1} was expressed as (6.8 ± 2.5)k1 under our experimental conditions, where k1 is the primary photolysis rate of NO2, and k21, k′21 are the second-order-equivalent rate constants of the HONO formation reaction in dark and under irradiation, respectively. The discussion was made that this photocatalitic enhancement of HONO formation would explain the nature of the extra OH radical flux in the smog chamber experiments, which has been discussed as “unknown radical source” and has still been unexplained by the surface dark reaction of NO2 and H2O to emit HONO. 相似文献
The formation of nitrous acid (HONO) in the dark from initial concentrations of NO2 of 0.1–20 ppm in air, and the concurrent disappearance of NO2, were monitored quantitatively by UV differential optical absorption spectroscopy in two different environmental chambers of ca.4300- and 5800-L volume (both with surface/volume ratios of 3.4 m?1). In these environmental chambers the initial HONO formation rate was first order in the NO2 concentration and increased with the water vapor concentration. However, the HONO formation rate was independent of the NO concentration and relatively insensitive to temperature. The initial pseudo-first-order consumption rate of NO2 was (2.8 ± 1.2) × 10?4 min?1 in the 5800-L Teflon-coated evacuable chamber and (1.6 ± 0.5) × 10?4 min?1 in a 4300-L all-Teflon reaction chamber at ca.300 K and ca.50% RH. The initial HONO yields were ca.40–50% of the NO2 reacted in the evacuable chamber and ca.10–30% in the all-Teflon chamber. Nitric oxide formation was observed during the later stages of the reaction in the evacuable chamber, but ca.50% of the nitrogen could not be accounted for, and gas phase HNO3 was not detected. The implications of these data concerning radical sources in environmental chamber irradiations of NOx? organic-air mixtures, and of HONO formation in polluted atmospheres, are discussed. 相似文献
Data from several smog chamber reaction vessels have been analyzed in an attempt to elucidate the chemical species which are responsible for chamber specific background phenomena, and the nature of the processes which determine the heterogeneous interactions of those species. There is good evidence for the emission of a compound which yields both NOx, and free radicals (probably HONO) and emissions of reactive organics (e.g. HCHO) may also be deduced. Total integrated chamber emission of these compounds may be as high as 20 to 60 ppb during a typical smog chamber experiment. In addition to the direct emission of these contaminants, the surface reaction of NO2 and H2O to HONO is examined. In some cases this reaction may have as great an effect on a smog chamber experiment as the emission of trace contaminants. Overall chamber perturbations to gas phase chemistry have been estimated for several experiments and were found to be less than 20 percent in the majority of cases, although higher perturbations were found in experiments involving compounds of low reactivity such as butane. 相似文献
Nitrous acid (HONO) has been observed to build in the atmosphere of cities during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urban atmospheres, such as modeling of urban O3 formation. This paper reviews the available information on measurements of HONO in the atmosphere and suggest mechanisms of HONO formation. The most extensive atmospheric measurement databases are used to investigate the relations between HONO and potential precursors. Based on these analyses, the nighttime HONO concentrations are found to correlate best with the product of NO, NO2 and H2O concentrations, or possibly the NO, NO2, H2O, and aerosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N2O3 with moist aerosols (or other surfaces) to form two HONO molecules. Theoretical considerations of the equilibrium constant for N2O3 formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in urban atmospheres. For photochemical modeling purposes, a relation is derived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO2 + H2O → 2 HONO with a rate constant of 1.68 x 10-17 e6348/T (ppm-2 min-1). This rate constant is based on an analysis of ambient measurements of HONO, NO, NO2 and H2O, with a temperature dependence from the equilibrium constant for formation of N2O3. Photochemical grid modeling is used to investigate the effects of this relation on simulated HONO and O3 concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the proposed relation results in HONO concentrations consistent with ambient measurements. Furthermore, the relation represents a conservative modeling approach because HONO production is effectively confined to the model surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived. 相似文献
The photolysis of nitrous acid (HONO) is an important reaction of atmospheric chemistry due to the fact that it can be the source of OH radical in the troposphere. Despite its role as a radical precursor, the chemical mechanisms leading to HONO formation are not well understood. It is commonly assumed that HONO formation is due to both homogeneous and heterogeneous processes involving NOx (mixture of NO and NO2) in which the kinetic and mechanistic details are still under investigation. In this discussion, we would like to highlight the formation of HONO from NO2 and nitric acid (HNO3) in the presence of organic particulate. We understood that in the real case, many parameters can influence the reaction mechanism; however, this is just an effort to have a better understanding of the study of HONO formation in the atmospheric process. 相似文献
The decay of pernitric acid (HO2NO2) in the presence of excess nitric oxide has been studied in a 5800-liter, Teflon-lined chamber over the temperature range 254 to 283 K at 1 atm pressure of N2 by Fourier transform infrared spectroscopy. A heterogeneous reaction of NO2 and H2O2 was used to generate HO2NO2 with less than 20% HNO3 and less than 5% NO2 present as impurities. The HO2NO2 had lifetimes of 5 to 20 h in our chamber, presumably determined by heterogeneous loss to the walls. Two paths have been proposed for the reaction of NO2 with HO2: (1), NO2 + HO2 → HONO + O2 (2). In this study the ratio k1/k2 was calculated to be greater than 103 throughout the temperature range studied. The homogeneous unimolecular decay of the HO2NO2, reaction (3), was investigated by adding excess NO in order to remove HO2 by reaction (4). (3), HO2 + NO → NO2 + OH (4). The rate constant k3 was determined to be 1.4 × 1014 exp(?20700 ± 500/RT)s?1. The thermal decomposition lifetimes of HO2O2 at 1 atm total pressure calculated from k3 are 12 s at 298 K, 5 min at 273 K and 1 month at 220 K. Implications of these results for the role of pernitric acid in the lower and upper atmosphere are discussed. 相似文献
Hydrogen peroxide formation in the photooxidation of CO? NOx, ethene? NOx, and propene? NOx mixtures has been determined in the TVA 31 cubic meter smog chamber under the following conditions: [NOx] ca. 22–46 ppb; ethene = 0.22–1.1 ppm, [propene] = 0.12–0.97 ppm; [H2O] ca. 8 × 10?3 ppm. Ethene, propene, NO, NOx, PAN, HCHO, and CH3CHO were also monitored. Computer modeling was performed using the gas phase ethene and propene mechanism of the Regional Acid Deposition Model. There is good agreement between the model predicted and observed H2O2 concentrations. However, to successfully model all the propene? NOx experimental results, organic nitrate formation from the reaction of peroxy radicals with NO must be included in the mechanism. 相似文献
The reaction of phenols with nitrite (nitrous acid HONO, or its conjugated base, NO2?) is of importance in stomach fluids (low pH) and in atmospheric hydrometeors (mild acid and basic pH). The initial reaction associated with the oxidation/nitration of 4‐substitued phenols promoted by HONO/NO2 depends on the pH of the solution. At low pH, the initial step involves the reaction between HONO and phenol, whereas at basic conditions this involves an electron transfer from the phenoxy anion to nitrogen dioxide (NO2) producing the nitrite anion. The rate of both processes is determined by the donor capacity of the substituent at the 4‐position of the phenol, and the data obtained at pH 2.3 follow a linear Hammett‐type correlation with a slope equal to –1.23. The partition of the gaseous intermediates (NO and NO2) makes the rate of HONO‐mediated oxidation dependent on their gas–liquid distribution. At low pH, the main process is phenol oxidation, even in oxygen‐free conditions, and the presence of any 4‐substituted phenol decreases the rate of HONO auto‐oxidation. 相似文献
Nitrous acid(HONO),as a primary precursor of OH radicals,has been considered one of the most important nitrogencontaining species in the atmosphere.Up to 30%of primary OH radical production is attributed to the photolysis of HONO.However,the major HONO formation mechanisms are still under discussion.During the Campaigns of Air Quality Research in Beijing and Surrounding Region(CAREBeijing2006)campaign,comprehensive measurements were carried out in the megacity Beijing,where the chemical budget of HONO was fully constrained.The average diurnal HONO concentration varied from 0.33 to 1.2 ppbv.The net OH production rate from HONO,POH(HONO)net,was on average(from 05:00 to 19:00)7.1×106 molecule/(cm3 s),2.7 times higher than from O3 photolysis.This production rate demonstrates the important role of HONO in the atmospheric chemistry of megacity Beijing.An unknown HONO source(Punknown)with an average of 7.3×106molecule/(cm3 s)was derived from the budget analysis during daytime.Punknown provided four times more HONO than the reaction of NO with OH did.The diurnal variation of Punknown showed an apparent photo-enhanced feature with a maximum around 12:00,which was consistent with previous studies at forest and rural sites.Laboratory studies proposed new mechanisms to recruit NO2 and J(NO2)in order to explain a photo-enhancement of of Punknown.In this study,these mechanisms were validated against the observation-constraint Punknown.The reaction of exited NO2 accounted for only 6%of Punknown,and Punknown poorly correlated with[NO2](R=0.26)and J(NO2)[NO2](R=0.35).These results challenged the role of NO2 as a major precursor of the missing HONO source. 相似文献
The state of ruthenium in conjugated phases upon extraction of trans-[Ru(15NO)(15NO2)4(OH)]2? complex with tri-n-octylphosphine oxide (TOPO) in the presence of Zn2+ and subsequent back extraction with H15NO3 and NH3(concd.) solutions was studied by 15N NMR. Binuclear complexes [Ru(NO)(NO2)5?n(μ-NO2)n?1(μ-OH)Zn(TOPO)n] and [Ru(NO)(NO2)4?n(ONO)(μ-NO2)n?1(μ-OH)Zn(TOPO)n], where n = 2, 3, are predominant forms in extract. Kinetic restrictions for ruthenium extraction with TOPO solution in hexane and its back extraction with aqueous solutions of nitric acid and ammonia are eliminated in the absence of direct coordination of extractant to ruthenium. fac-Dinitronitrosyl forms [Ru(NO)(H2O)3(NO2)2]+, [Ru(NO)(H2O)2(NO2)2(NO3)]0 (3 and 6 M HNO3) and [Ru(NO)(H2O)(NO2)2(NO3)2]? (6 M HNO3) prevail in nitric acid back extracts. Equilibrium constant at ambient temperature (0.05 ± 0.01) was assessed for the coordination of second nitrate ion to nitrosylruthenium dinitronitrato complex. Complex species [Ru(NO)(NO2)4(OH)]2? and [Ru(NO)(NO2)3(ONO)(OH)]2? prevail in ammonia back extract. 相似文献
Rate coefficients, k1, for the reaction OH + HONO → H2O + NO2, have been measured over the temperature range 298 to 373 K. The OH radicals were produced by 266 nm laser photolysis of O3 in the presence of a large excess of H2O vapor. The temporal profiles of OH were measured under pseudo-first-order conditions, in an excess of HONO, using time resolved laser induced fluorescence. The measured rate coefficient exhibits a slight negative temperature dependence, with k1 = (2.8 ± 1.3) × 10?12 exp((260 ± 140)/T) cm3 molecule?1 s?1. The measured values of k1 are compared with previous determinations and the atmospheric implications of our findings are discussed. 相似文献
The reaction scheme of thermal decomposition for four zinc hydroxynitrates was investigated by means of differential scanning calorimetry, thermogravimetry, mass spectrometry, and radiocrystallography. The thermal transformation of Zn(OH)(NO3) · H2O and of Zn3(OH)4(NO3)2 involves the formation of gaseous water and nitric acid from an actual chemical reaction. This reaction is not observed for Zn5(OH)8(NO3)2 · 2H2O and Zn5(OH)8(NO3)2. These results show that the formation of gaseous nitric acid molecules inside the solids is specific to hydroxynitrates of divalent metals M, whose lamellar crystalline structure is characterized by a stacking of hexagonal close-packed layers of formula MX2+m, where m = 0 or 1 and X = OH?, H2O, or NO?3. 相似文献
The kinetics of the reaction between nitric oxide and chlorine have been investigated in both carbon tetrachloride and glacial acetic acid. The nitric oxide-oxygen reaction has been investigated in carbon tetrachloride. The appearance of product, NOCl or NO2, was monitored spectrophotometrically at a wavelength of 475 nm for NOCl and 343 nm for NO2. These measurements were performed using an Amino-Morrow stopped-flow apparatus equipped with a Beckman D U monochromator. The data for both the NO? Cl2 and NO? O2 systems could be fitted to the third-order integrated equation and the calculated rate constants were 2.75 × 103 M?2 s?1 and 2.79 × 106 M2 s?1, respectively, at 25.1°C. There was a noted increase in rate constants on changing the solvent from carbon tetrachloride to acetic acid. The likelihood of a termolecular encounter is inherent in the mechanism, however, no real evidence to substantiate either a direct termolecular or a series of two bimolecular steps has been obtained, although a ?7 kcal for ΔH0 would support the latter. 相似文献
This study investigated the reactive dissolution of nitric oxide (NO) and nitrogen dioxide (NO2) mixtures in deionized water. The dissolution study was carried out in a flat surface type gas–liquid reaction chamber utilizing a gas flow-pattern resembling plasma jets which are often used in biomedical applications. The concentration of NO and NO2 in the gas mixtures was varied in a broad range by oxidizing up to 800 ppm of nitric oxide in Ar carrier gas with variable amount of ozone. The production of nitrite (NO2?) and nitrate (NO3?) in the water was proportional to treatment time up to 50 min. The concentration of NO3? was a power function of gas phase NO2 while the concentration of NO2? increased approximately linearly with gas phase NO2. The formation of NO2? and NO3? could be described by reactions between dissolved NO2 and NO in the water while the production rate was determined by diffusion-limited mass transport of nitrogen oxides to the bulk of the liquid. At higher NO2 concentrations, the formation of dinitrogen tetraoxide (N2O4) increased the formation rate of NO2? and NO3?. The identified mass transport limitation by diffusion suggests that convection of water created by the gas jet is insufficient and dissolution of nitrogen oxides can be increased by additional mixing. In respect of practical applications, the ratio of NO2? /NO3? in water could be varied from 0.8 to 5.3 with treatment time and gas phase NO2 and NO concentrations.
The reaction H2O+(2B)+NO2(2A) → H2O(1A) + NO2+(1Σ) occurs at near the collision rate constant 1.2 × 10?9 cm3 s?1, in spite of the fact that the reactants produce both a singlet and a triplet state and the products correlate only with the singlet state. This would be expected to yield a statistical weight factor of to be multiplied by the collision rate constant to obtain the maximum charge-tranfer rate constant. The triplet products of the charge transfer are clearly endothermic. The singlet—triplet intersection has not been identified but the available information about the singlet and triplet states of the intermediate protonated nitric acid molecule is discussed. Four other examples of apparent “spin violation” charge-transfer reactions have been noted H2O+ + NO, N2O+ + NO.CO+ + NO and CH4+ + O2. 相似文献
Flash photolysis of CH3CHO and H2CO in the presence of NO has been investigated by the intracavity laser spectroscopy technique. The decay of HNO formed by the reaction HCO + NO → HNO + CO was studied at NO pressures of 6.8–380 torr. At low NO pressure HNO was found to decay by the reaction HNO + HNO → N2O + H2O. The rate constant of this reaction was determined to be k1 = (1.5 ± 0.8) × 10?15 cm3/s. At high NO pressure the reaction HNO + NO → products was more important, and its rate constant was measured to be k2 = (5 ± 1.5) × 10?19 cm3/s. NO2 was detected as one of the products of this reaction. Alternative mechanisms for this reaction are discussed. 相似文献