Excess NO2 Research Articles

Production of gas phase NO2 and halogens from the photolysis of thin water films containing nitrate, chloride and bromide ions at room temperature

Nitrate and halide ions coexist in particles generated in marine regions, around alkaline dry lakes, and in the Arctic snowpack. Although the photochemistry of nitrate ions in bulk aqueous solution is well known, there is recent evidence that it may be more efficient at liquid-gas interfaces, and that the presence of other ions in solution may enhance interfacial reactivity. This study examines the 311 nm photolysis of thin aqueous films of ternary halide-nitrate salt mixtures (NaCl-NaBr-NaNO3) deposited on the walls of a Teflon chamber at 298 K. The films were generated by nebulizing aqueous 0.25 M NaNO3 solutions which had NaCl and NaBr added to vary the mole fraction of halide ions. Molar ratios of chloride to bromide ions were chosen to be 0.25, 1.0, or 4.0. The subsequent generation of gas phase NO2 and reactive halogen gases (Br2, BrCl and Cl2) were monitored with time. The rate of gas phase NO2 formation was shown to be enhanced by the addition of the halide ions to thin films containing only aqueous NaNO3. At [Cl(-)]/[Br(-)] ≤ 1.0, the NO2 enhancement was similar to that observed for binary NaBr-NaNO3 mixtures, while with excess chloride NO2 enhancement was similar to that observed for binary NaCl-NaNO3 mixtures. Molecular dynamics simulations predict that the halide ions draw nitrate ions closer to the interface where a less complete solvent shell allows more efficient escape of NO2 to the gas phase, and that bromide ions are more effective in bringing nitrate ions closer to the surface. The combination of theory and experiments suggests that under atmospheric conditions where nitrate ion photochemistry plays a role, the impact of other species such as halide ions should be taken into account in predicting the impacts of nitrate ion photochemistry.

Spatially resolving SCR reactions over a Fe/zeolite catalyst

There have been several recent reports regarding the spatial resolution of gas species in monolith-supported automobile exhaust catalysts, with examples including characterization of DOCs (diesel oxidation catalysts) and LNTs (lean NOx traps). However, spatially resolving gas concentrations in NH3-SCR (selective catalytic reduction) applications is limited due to the difficulty in ppm-level NH3 detection in the presence of percent levels of water and N2 using mass spectrometry. In this study, a gas-phase FT-IR analyzer was used for the first time to spatially resolve gas concentrations in a monolith-supported SCR catalyst. The reactions analyzed include standard SCR, fast SCR and SCR with NO2. The results show that the three SCR reactions proceed at significantly different rates, especially at temperatures below 300°C, and can be correlated to the amount of catalyst used. For example, the catalyst lengths needed to achieve 80% NOx conversion at 300°C are 2.4, 1.2 and 0.5cm for conditions that meet the standard SCR, NO2-SCR and fast SCR reaction stoichiometries, respectively. For the standard SCR reaction, kinetic analysis, and spatially resolved NO oxidation and SCR results consistently indicate that the rate-determining step is NO oxidation. NH3 has an inhibition effect, as it suppresses NO oxidation by competitive adsorption on the active sites. At 300°C, the outlet NOx conversion is not limited by the reaction kinetics, but by insufficient NH3 supply, since part of the NH3 is oxidized by O2. Compared with 300°C, higher NOx conversions are attained at 400 or 500°C, which is due to significantly enhanced NO oxidation, and the resulting increase in NH3 reacting with NOx via SCR rather than O2 via NH3 oxidation. For NO2-SCR, a considerable amount of N2O was formed at 250°C but decreased with increasing temperature. The decreased N2O is due to improved selectivity in the NO2-SCR to N2, as well as N2O decomposition at the back part of the catalyst at high temperature. Finally, different SCR reaction patterns were identified when testing with NO:NO2=3:1 and 1:3. For NO:NO2=3:1, the SCR reactions proceed in series, namely through the fast reaction first, followed by standard SCR. The fast SCR and NO2-SCR reactions proceed in parallel for NO:NO2=1:3. The results indicate that if NO2 is the limiting reactant, fast SCR dominates, but if excess NO2 is available, the NO2 SCR reaction can proceed in parallel.

Biogeochemical controls and isotopic signatures of nitrous oxide production by a marine ammonia-oxidizing bacterium

Abstract. Nitrous oxide (N2O) is a trace gas that contributes to the greenhouse effect and stratospheric ozone depletion. The N2O yield from nitrification (moles N2O-N produced per mole ammonium-N consumed) has been used to estimate marine N2O production rates from measured nitrification rates and global estimates of oceanic export production. However, the N2O yield from nitrification is not constant. Previous culture-based measurements indicate that N2O yield increases as oxygen (O2) concentration decreases and as nitrite (NO2−) concentration increases. Here, we have measured yields of N2O from cultures of the marine β-proteobacterium Nitrosomonas marina C-113a as they grew on low-ammonium (50 μM) media. These yields, which were typically between 4 × 10−4 and 7 × 10−4 for cultures with cell densities between 2 × 102 and 2.1 × 104 cells ml−1, were lower than previous reports for ammonia-oxidizing bacteria. The observed impact of O2 concentration on yield was also smaller than previously reported under all conditions except at high starting cell densities (1.5 × 106 cells ml−1), where 160-fold higher yields were observed at 0.5% O2 (5.1 μM dissolved O2) compared with 20% O2 (203 μM dissolved O2). At lower cell densities (2 × 102 and 2.1 × 104 cells ml−1), cultures grown under 0.5% O2 had yields that were only 1.25- to 1.73-fold higher than cultures grown under 20% O2. Thus, previously reported many-fold increases in N2O yield with dropping O2 could be reproduced only at cell densities that far exceeded those of ammonia oxidizers in the ocean. The presence of excess NO2− (up to 1 mM) in the growth medium also increased N2O yields by an average of 70% to 87% depending on O2 concentration. We made stable isotopic measurements on N2O from these cultures to identify the biochemical mechanisms behind variations in N2O yield. Based on measurements of δ15Nbulk, site preference (SP = δ15Nα−δ15Nβ), and δ18O of N2O (δ18O-N2O), we estimate that nitrifier-denitrification produced between 11% and 26% of N2O from cultures grown under 20% O2 and 43% to 87% under 0.5% O2. We also demonstrate that a positive correlation between SP and δ18O-N2O is expected when nitrifying bacteria produce N2O. A positive relationship between SP and δ18O-N2O has been observed in environmental N2O datasets, but until now, explanations for the observation invoked only denitrification. Such interpretations may overestimate the role of heterotrophic denitrification and underestimate the role of ammonia oxidation in environmental N2O production.

Potential Ozone Impacts of Excess NO2 Emissions from Diesel Particulate Filters for On- and Off-Road Diesel Engines

This study considers potential impacts of increased use of diesel oxidation catalysts (DOCs) and catalyzed diesel particulate filters (DPFs) on ozone formation in the Dallas/Fort Worth (DFW) area. There is concern that excess nitrogen dioxide (NO2) emissions from vehicles equipped with these devices could increase ambient ozone levels. The approach involved developing two scenarios for use of these devices, quantifying excess NO2 emissions in each scenario, and using a photochemical model to estimate the resulting ozone changes. In the “maximum penetration” scenario, DOC/DPF devices in a 2009 fleet of heavy-duty on-road trucks, school buses, and construction equipment were significantly increased by accelerating turnover of these vehicles and equipment to models that would require DOCs/DPFs. In the “realistic” scenario, current fractional usage of these devices was assessed for 2009. For both scenarios, excess NO2 emissions from DOCs/DPFs were estimated using U.S. Environmental Protection Agency’s MOBILE6 and NONROAD emissions inventory modeling tools. The emissions analyses were used to adjust the DFW photochemical modeling emissions inventories and the Comprehensive Air Quality Model with extensions air quality model was rerun for the DFW area to determine the impact of these two scenarios on ozone formation. The maximum penetration scenario, which showed an overall reduction in oxides of nitrogen (NOx) because of the accelerated turnover of equipment to cleaner models, resulted in a net decrease in daily maximum 8-hr ozone of 4–5 parts per billion (ppb) despite the increase in NO2 emissions. The realistic scenario resulted in a small increase in daily maximum 8-hr ozone of less than 1 ppb for the DFW area. It was concluded that the excess NO2 emissions from DOC/DPF devices result in very small ozone impacts, particularly for the realistic scenario, in the DFW area. There are noticeable decreases in ozone for the maximum penetration scenario because NOx reductions associated with DOC/DPFs (i.e., accelerated fleet turnover) exert more influence than excess NO2.

Effect of subacute dietary nitrate on production traits and plasma analytes in Suffolk ewes

Elevated dietary nitrate (NO3−) is associated with production losses in ruminant livestock, resulting in substantial economic losses incurred by producers. Severe drought, fertilization practices and poorly maintained pastures increase the risk of elevated NO3− intake among cattle and sheep. Nitrate is metabolized to nitrite (NO2−) in the rumen and further reduced to ammonia. Ruminants consuming high dietary NO3− vary in ability to efficiently reduce excess NO2− to ammonia. This leads to methemoglobin formation and ultimately NO3− toxicity signs. Variation in individual tolerance to elevated dietary NO3− can be partially attributed to rate and duration of exposure, rate of elimination, metabolism, species and dose. Our objectives were to confirm and quantify variation in individual tolerance to subacute levels of dietary NO3−, and determine if individuals could be identified as highly or lowly tolerant to elevated dietary NO3− based on production traits, plasma analytes and(or) signs of subacute NO3− toxicity. Purebred Suffolk ewes were administered supplement mixed with tap water (control; n = 8) or potassium nitrate (NO3− treated; 300 mg NO3−/kg BW daily; n = 47) for 8 days. Coefficients of variation (CV) indicated that supplement intake was more variable in NO3− treated ewes (CV = 59.3%) than in control ewes (CV = 13.6%). Among NO3− treated ewes, six ewes highly tolerant and six ewes lowly tolerant to elevated dietary NO3− were identified based on individual performance, NO3− treated supplement intake, and signs of toxicity. Supplement intake was lower (P < 0.0001) in NO3− treated ewes than in control ewes, indicating elevated dietary NO3− influences feed intake. Supplement intake differed (P < 0.0001) between control, highly tolerant and lowly tolerant ewes. Supplement intake of highly and lowly tolerant ewes was 82% and 23%, respectively, of the control ewes’ intake. Weight change and plasma concentrations of NO2−, cortisol, glucose and retinol were not different (P ⩾ 0.38) among control, highly tolerant and lowly tolerant ewes. Plasma urea nitrogen (PUN) levels were not different (P = 0.25) between control and lowly tolerant ewes, but were lower (P = 0.02) in highly tolerant ewes than in control ewes. Furthermore, PUN and NO3− treated supplement intake were highly correlated (0.71; P < 0.0001) in lowly tolerant ewes. These results confirm and quantify variation in response to subacute levels of dietary NO3− and indicate that individuals can be identified as highly or lowly tolerant to elevated dietary NO3− based on their performance and NO3− toxicity signs.

Nitration of Simple Aromatics with NO2 under Air Atmosphere in the Presence of Novel Brønsted Acidic Ionic Liquids

Aromatics nitrate with NO2/air catalyzed by novel Brønsted acidic ionic liquids (ILs) without any volatile chlorinated organic solvent under mild conditions. The ILs employed were caprolactam based, [Caprolactam]X (X−=pTSO−, BSO−, BF4 −, NO3 −), which are of relatively lower cost and lower toxicity than traditional imidazolium‐based ILs. The nitration reactions were carried out at −15 to −0°C first, then at room temperature for a longer time with a little excessive NO2 (ca. 1.4 eqv.) for moderate yield (for toluene). The IL could be reused four times.

Lightning-produced NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; observed by two ground-based UV-visible spectrometers at Vanscoy, Saskatchewan in August 2004

Abstract. Ground-based measurements of ozone and NO2 differential slant columns by the SAOZ (Système d'Analyse par Observations Zénithales) and UT-GBS (University of Toronto Ground-Based Spectrometer) instruments during the MANTRA 2004 field campaign are presented herein. During the afternoon of 28 August, a thunderstorm passed over the instruments, which were installed at Vanscoy, Saskatchewan (52° N, 107° W). Enhanced differential slant columns of ozone and NO2 were observed by both instruments during the storm, with maximum values of two and 25 times the expected clear sky columns, respectively. The enhanced ozone differential slant columns are primarily due to the longer path traversed by the solar radiation caused by multiple scattering inside the thick cloud layer associated with the thunderstorm. The enhanced NO2 columns are partly attributed to NOx production by lightning. Two new methods are used to separate the NO2 enhancements into contributions from the longer path length and production by lightning. Combining the observed excess NO2 with lightning flash data from the Canadian Lightning Detection Network and Environment Canada Doppler radar measurements, the production of NO2 molecules per lightning flash is determined. Using these two methods, the best estimate of the production rate is found to be (7.88±2.52)×1026 molecules NO2/flash from the UT-GBS and (6.81±2.17)×1026 molecules NO2/flash from SAOZ. These results are consistent with the range of previous estimates reported in the literature.

Heterogeneous Chemistry of the NO3 Free Radical and N2O5 on Decane Flame Soot at Ambient Temperature: Reaction Products and Kinetics

The interaction of NO3 free radical and N2O5 with laboratory flame soot was investigated in a Knudsen flow reactor at T = 298 K equipped with beam-sampling mass spectrometry and in situ REMPI detection of NO2 and NO. Decane (C10H22) has been used as a fuel in a co-flow device for the generation of gray and black soot from a rich and a lean diffusion flame, respectively. The gas-phase reaction products of NO3 reacting with gray soot were NO, N2O5, HONO, and HNO3 with HONO being absent on black soot. The major loss of NO3 is adsorption on gray and black soot at yields of 65 and 59%, respectively, and the main gas-phase reaction product is N2O5 owing to heterogeneous recombination of NO3 with NO2 and NO according to NO3 + {C} --> NO + products. HONO was quantitatively accounted for by the interaction of NO2 with gray soot in agreement with previous work. Product N2O5 was generated through heterogeneous recombination of NO3 with excess NO2, and the small quantity of HNO3 was explained by heterogeneous hydrolysis of N2O5. The reaction products of N2O5 on both types of soot were equimolar amounts of NO and NO2, which suggest the reaction N2O5 + {C} --> N2O3(ads) + products with N2O3(ads) decomposing into NO + NO2. The initial and steady-state uptake coefficients gamma 0 and gamma ss of both NO3 and N2O5 based on the geometric surface area continuously increase with decreasing concentration at a concentration threshold for both types of soot. gamma ss of NO3 extrapolated to [NO3] --> 0 is independent of the type of soot and is 0.33 +/- 0.06 whereas gamma ss for [N2O5] --> 0 is (2.7 +/- 1.0) x 10(-2) and (5.2 +/- 0.2) x 10(-2) for gray and black soot, respectively. Above the concentration threshold of both NO3 and N2O5, gamma ss is independent of concentration with gamma ss(NO3) = 5.0 x 10(-2) and gamma ss(N2O5) = 5.0 x 10(-3). The inverse concentration dependence of gamma below the concentration threshold reveals a complex reaction mechanism for both NO3 and N2O5. The atmospheric significance of these results is briefly discussed.

Thermal Decomposition of ClOOCl

ClOOCl was prepared in situ in a temperature controlled photoreactor (v = 420 L) by photolyzing OClO/N2 mixtures in the wavelength range 300-500 nm at temperatures between 242 and 261 K and total pressures between 2 and 480 mbar. After switching off the lights, excess NO2 was added, and IR and UV spectra were monitored simultaneously as a function of time. By spectral stripping of all other known UV absorbers (in particular, other chlorine oxides and chlorine nitrate), we determined rate constants k-1 of the reaction ClOOCl (+M) --> ClO + ClO (+M) from the first-order decay of the residual UV absorption of ClOOCl at 246 and 255 nm. k-1,0 = [N2] x 7.6 x 10(-9) exp[(-53.6 +/- 6.0) kJ mol(-1)/RT] cm3 molecule(-1) s(-1) (2sigma) was derived for the low-pressure limiting rate constant. Application of Troe's expression for the limiting low-pressure rate constants of unimolecular decomposition reactions leads to E0 = Delta(r)H0(0)(ClOOCl-->ClO+ClO) = 66.4 +/- 3.0 kJ mol(-1). k-1,0 started to fall off from the pressure proportional low pressure behavior at p approximately 30 mbar; however, reliable extrapolation to the high pressure limit was not possible. The decomposition rate constants of ClOOCl were directly measured for the first time, and they are higher, depending on temperature and pressure, by factors between 1.5 and 4.2 as compared to experimental data on k-1 by Nickolaisen et al. [J. Phys. Chem. 1994, 98, 155] which were derived from the approach of ClO to thermal equilibrium with its dimer ClOOCl. Combination of the present dissociation rate constants with recommended temperature and pressure dependent data on the reverse reaction (k1) demonstrate inconsistencies between the dissociation and recombination rate constants. Summarizing laboratory data on k1 and k-1 above 250 K and field measurements on the ClO + ClO <= => ClOOCl equilibrium in the nighttime polar stratosphere close to 200 K, the expression Kc = k1/k-1 = 3.0 x 10(-27) exp(8433 K/T) cm3 molecule(-1) is derived for the temperature range 200-300 K.

A kinetics and mechanistic study of the OH and NO2 initiated oxidation of cyclohexa-1,3-diene in the gas phase

The kinetics and products of the OH and NO2-initiated oxidation of cyclohexa-1,3-diene have been investigated at 296 K and 700 Torr using long path FTIR spectroscopy. Relative rate methods were employed using the photolysis of cyclohexa-1,3-diene/CH3ONO/NO/air mixtures to measure kappa(OH + cyclohexa-1,3-diene) = (1.68 +/- 0.43) x 10(-10) cm3 molecule(-1) s(-1). From the pseudo-first order decay of cyclohexa-1,3-diene in the presence of excess NO2, a value of kappa(NO2 + cyclohexa-1,3-diene) = (1.75 +/- 0.15) x 10(-18) cm3 molecule)-1) s(-1) was derived. An upper limit of kappa < or = 7 x 10(-21) cm3 molecule(-1) s(-1) was established for the reaction of NO with cyclohexa-1,3-diene. Benzene was observed as a product of both the OH and NO2 initiated oxidation, providing evidence of H atom abstraction in both reactions. Assuming the reaction of cyclohexadienyl radicals (C6H7) with O2 produces benzene as the sole organic product, the results are consistent with abstraction channel branching ratios of (8.1 +/- 0.2)% and (1.5 +/- 0.4)%, respectively. The results also indicate that C6H7 reacts with NO2, with a relative rate coefficient kappa(C6H7 + NO2)/kappa(C6H7 +O2) = (1.8 +/- 0.5) x 10(5), and that this partially forms benzene, with a branching ratio of (27 +/- 7)%. The stoichiometry and products of the NO2 reaction were investigated in the absence of O2, in the presence of O2, and in the presence of O2 and NO. Reaction mechanisms consistent with the observations are presented. In the presence of NO and O2, the NO2-initiated chemistry leads to NO-to-NO2 conversion, and the formation of HOx radicals in significant yield, (0.79 +/- 0.05), such that cyclohexa-1,3-diene removal occurs by reaction with both NO2 and OH. HCOOH was detected as a product in this system, providing evidence for significant formation of stabilised C6 alpha-hydroxyperoxy radicals from the OH-initiated chemistry, and their subsequent reaction with NO. An estimate of ca. 500-1000 s(-1) is made for their decomposition rate, based on the [NO]-dependence of the HCOOH yields. The implications of the results are discussed within the context of the atmospheric chemistry of conjugated dienes.

Reactions of nitrogen oxides with heme models. Characterization of NO and NO2 dissociation from Fe(TPP)(NO2)(NO) by flash photolysis and rapid dilution techniques: Fe(TPP)(NO2) as an unstable intermediate.

Described are studies directed toward elucidating the controversial chemistry relating to the solution phase reactions of nitric oxide with the iron(II) porphyrin complex Fe(TPP)(NO) (1, TPP = meso-tetraphenylporphinato2-). The only reaction observable with clean NO is the formation of the diamagnetic dinitrosyl species Fe(TPP)(NO)2 (2), and this is seen only at low temperatures (K(1) < 3 M(-1) at ambient temperature). However, 1 does readily react reversibly with N2O3 in the presence of excess NO to give the nitro nitrosyl complex Fe(TPP)(NO2)(NO) (3), suggesting that previous claims that 1 promotes NO disproportionation to give 3 may have been compromised by traces of air in the nitric oxide sources. It is also noted that 3 undergoes reversible loss of NO to give the elusive nitro species Fe(TPP)(NO2) (4), which has been implicated as a powerful oxygen atom transfer agent in reactions with various substrates. Furthermore, in the presence of excess NO2, the latter undergoes oxidation to the stable nitrato analogue Fe(TPP)(NO3) (5). Owing to such reactivity of Fe(TPP)(NO2), flash photolysis and stopped-flow kinetics rather than static techniques were necessary for the accurate measurement of dissociation equilibria characteristic of Fe(TPP)(NO2)(NO) in 298 K toluene solution. Flash photolysis of 3 resulted in competitive NO2 and NO dissociation to give Fe(TPP)(NO) and Fe(TPP)(NO2), respectively. The rate constant for the reaction of 1 with N2O3 to generate Fe(TPP)(NO2)(NO) was determined to be 1.8 x 10(6) M(-1) s(-1), and that for the NO reaction with 4 was similarly determined to be 4.2 x 10(5) M(-1) s(-1). Stopped-flow rapid dilution techniques were used to determine the rate constant for NO dissociation from 3 as 2.6 s(-1). The rapid dilution experiments also demonstrated that Fe(TPP)(NO2) readily undergoes further oxidation to give Fe(TPP)(NO3). The mechanistic implications of these observations are discussed, and it is suggested that NO2 liberated spontaneously from Fe(P)(NO2) may play a role in an important oxidative process involving this elusive species.

The Effect of an Oxidation Precatalyst on the NOx Reduction by Ammonia SCR

The performances of oxidation and SCR catalysts were investigated individually and as a combined system for the removal of NOx by ammonia. Experiments were carried out with synthetic gas mixtures matching the composition of diesel exhaust. Introducing an oxidation catalyst upstream of the SCR catalyst enhances the removal of NOx at low temperatures. The beneficial effects of combining the two catalysts are the oxidation of unburned hydrocarbons that inhibit the SCR reaction and the conversion of NO to NO2 on the oxidation catalyst. NO2 fractions up to 50% of total NOx enhance the NOx conversion at temperatures below 300 °C. However, excess NO2 should be avoided because the reaction of NO2 with ammonia is slow, resulting in a decreased NOx conversion. Above 350 °C, the NOx conversion essentially does not depend on the ratio NO2/NOx or on the presence of hydrocarbons in the feed. A simple mathematical prediction of the NOx conversion for the combined system is also possible. The parameters used in this calculation are the ratio NO2/NOx behind the oxidation catalyst and the experimentally determined conversions of pure NO or NO2 over the SCR catalyst.

Influence of environmental parameters on the accuracy of nitrogen dioxide passive diffusion tubes for ambient measurement.

Two studies at three sites in the UK provided confirmation that systematic positive bias in NO2 diffusion tube measurement occurred because of changes to "within-tube" chemistry, rather than eddy diffusion at the mouth of the tube. In the first study in Cambridge, UK, sampler overestimation for 1 and 2 week exposures was compared to corresponding time-averaged monitor measurements (NO-NO2-NOx, O3) and weather variables. Noninearity between sampler and monitor NO2 measurements was interpreted in terms of spatial and temporal variations in relative and absolute availability of NO, NO2 and O3 at the site. A maximum overestimation occurred for an exposure mean NO2/NOx approximately 0.5. The separate contributions of reduced NO2 photolysis and eddy diffusion were compared in Study II using samplers of two materials, acrylic and quartz, and of different lengths (40, 55, 71 and 120 mm) at three sites: Norwich background, Cambridge intermediate, London kerbside. For compared sites, NO2 measured by acrylic samplers was significantly higher than for equivalent quartz samplers. For quartz samplers [NO2]mean was only just above the monitor at Norwich and London; sampler/monitor NO2 = 1.04 (P = 0.59) and 1.01(P = 0.76), respectively. For acrylic samplers the order of [NO2]mean was 40 mm > 120 mm > 71 mm > or = 55 mm. Excepting 40 mm samplers, this accords with a chemical bias where co-diffusing NO and 03 molecules in longer tubes have more time to react to form excess NO2. Bias in 40 mm samplers is discussed. Eddy diffusion is negligible for standard samplers because [NO2]mean was equivalent for 55 mm and 71 mm acrylic samplers and close to monitor NO2 for 71 mm quartz tubes. Both studies showed that sampler accuracy was dependent on location. Significantly, overestimation was greatest (approximately 3-4 ppb) where the NO2 annual mean was approximately 20 ppb, close to the UK and EU air quality standard of 21 ppb.

C6, C7, and C8 perfluoroalkyl-substituted phosphinic acids.

Reaction of red phosphorus with RfI in a 1:2 molar ratio at 230 degrees C led to the formation of a mixture of (Rf)2PI and (Rf)PI2 (Rf = C6F13, C7F15, C8F17) in about a 70:30 ratio, respectively. These mixtures were separated by vacuum distillation. (Rf)2PI (Rf = C6F13, C7F15) are yellow liquids whereas (C8F17)2PI is a yellow solid. Oxidation of (Rf)2PI with excess NO2 led to (Rf)2P(O)OH (Rf = C6F13, C7F15, C8F17) in > 90% isolated yields after aqueous hydrolysis of the anhydride intermediates. These highly fluorinated phosphinic acids are white solids with sharp melting points and are highly soluble in methyl sulfoxide (DMSO) and 1,1,2-trichlorotrifluoroethane. However, solubility in chloroform and methylene dichloride is low. These perfluoroalkylphosphinic acids were characterized by IR, NMR (1H, 19F, and 31P), and mass spectra and elemental analysis.

Evidence for formation of a PAN analogue of pinonic structure and investigation of its thermal stability

The first evidence and laboratory study of a peroxyacetyl nitrate (PAN) analogue produced by the photooxidation of a terpene, α‐pinene, is presented. This PAN analogue, assigned to 3‐acetyl‐2,2‐dimethyl‐cyclobutane‐acetyl peroxynitrate and referred to as “α‐pinonyl peroxynitrate” (αP‐PAN) was synthesized in the gas phase from the radical (OH, Cl, Br, or NO3) initiated oxidation of pinonaldehyde (3‐acetyl‐2,2‐dimethyl‐cyclobutyl‐ethanal) in the presence of excess NO2 and evidenced by Fourier transform‐infrared (FT‐IR) spectroscopy. Another reaction channel producing PAN was also observed for some of the radical initiators. Of particular atmospheric interest, the experiments with OH radicals demonstrated that αP‐PAN is the main product of pinonaldehyde under NOx‐rich conditions with a yield of (81.3±16)%, while an upper limit of the PAN yield for this reaction is around 8%. The further photooxidation of αP‐PAN was also observed to produce PAN directly. The thermal stability of αP‐PAN was studied between 303 and 281 K. The rate constant of thermal dissociation was found to be k−1 = 10(9.25±0.33) × exp [−(72.0±1.9)/RT] where the activation energy is in kJ mol−1. Distortions of the kinetic profiles attributed to aerosol formation were observed and led to large errors in the above estimation of k−1. Within the uncertainties, the observed thermal stability of αP‐PAN is comparable to that of PAN. The tropospheric importance of pinonaldehyde and of αP‐PAN are discussed.

Differential inhibition of nitrification by three metabolic inhibitors

In a soil slurry the effects of two uncouplers of oxidative phosphorylation, DNP (2,4 dinitrophenol) and CCCP (carbonyl cyanide m‐chlorophenyl hydrazone), and one inhibitor of terminal oxidase (KCN) on biological oxidation (nitrification) of NH4 + and NO2 ‐ to NO3 ‐ were studied. DNP did not affect, KCN reduced, and CCCP fully inhibited the oxidation of both NH4 + and NO2 ‐ to NO3 ‐Addition of DNP to soil slurry resulted in formation of excess NO3 ‐ (over that of the control) which was stoichiometrically close to the amount of nitrogen in the added DNP. It is suggested that the ineffectiveness of DNP as nitrification inhibitor was partly due to its rapid hydrolysis and formation of excess NO2 ‐which was in turn nitrified to NO3 ‐ It is concluded that in comparison to DNP, CCCP and KCN introduce no artefacts into the experimental cultures and are thus better suited for studies involving inhibition of biological nitrification than DNP.

ChemInform Abstract: Slow Electron Transfer Between Main‐Group Species: Oxidation of Nitrite by Chlorine Dioxide.

AbstractClO2 reacts with excess NO2‐ in aqueous media to yield ClO2‐ and NO3‐.

Chlorine nitrate: Kinetics of formation by ClO+NO2+M and of reaction with OH

Kinetic studies by the discharge–flow technique are reported. For ClONO2 formation from T=250–365 K, ClO decays in excess NO2 are monitored by conversion of ClO to Cl followed by resonance fluorescence detection at 134.7 nm. For OH reaction over T=246–387 K, pseudo-first-order OH decays are observed directly by OH-resonance fluorescence. The recombination reaction (1) is near its low pressure, third-order limit with k1= (0.83±0.12) ×10−31 cm6 s−1 at 300 K for M=He and (1.52±0.23) ×10−31 cm6 s−1 at 297 K for M=N2. The k1,He temperature dependence corresponds to T−3.15. These results are in good agreement with those of other studies. A temperature- and pressure-dependent, empirical rate constant expression for stratospheric modeling calculations is proposed. The rate constant k2 for the OH reaction is found to be (1.19±0.10) ×10−12 exp[(−333 ±22)/T] cm3 s−1, i.e., (3.93±0.11) ×10−13 at 295 K. The results are compared with other measurements and their significance for stratospheric processes is discussed.

Gas phase chain reaction of H2O2+ NO2+ CO

Reactions in gas phase mixtures of H2O2 vapour, NO2 and CO at 292±2 K have been studied by measurement of the variation of yield of the CO2 product. It has been shown that the mechanism is the chain reaction sequence H2O2+ NO2→ OH + HNO3(1), OH + CO → CO2+ H (2), H + NO → OH + NO (3), OH + NO2(+M)→ HNO3(+M)(4) provided that excess NO2 is present.An estimate of k4/k2 is obtained which is rationalised in terms of reaction (4) being between second and third order for M = CO at pressures of 8 and 13.3 kN m–2.Addition of n-butane to the mixture induces the reaction OH + n-C4H10→ H2O + n-C4H9. (8).From the resultant decrease in the kinetic chain length a ratio of k8/k2= 14.8 ± 0.9 is deduced, in good agreement with ratios of available absolute values of k8 and k2.It is considered that the H2O2+ NO2+ CO + substrate reaction system provides a useful non-photolytic source of OH + substrate rate constant data relative to k2, of wider application than photolytic sources so far developed.

Mechanism of Peroxyacetyl Nitrate Formation

It is proposed that peroxyacetyl nitrate and its homologues are formed in polluted air by a reaction between acylate (or aroylate) radicals and nitrogen trioxide. The proposed reaction accounts for the delayed formation of peroxyacetyl nitrate upon the irradiation of low concentrations of hydrocarbons and nitrogen oxides in air. It also explains why peroxyacetyl nitrate is not formed in photooxidations at high reactant concentrations. It accounts for the formation of peroxyacetyl nitrate in the dark reaction of acetaldehyde, oxygen, and N2O5; and, finally, it explains the inhibiting effect that excess NO2 has on that dark reaction. The proposed reaction could yield either the accepted peroxyacetyl nitrate molecular structure or a molecular structure called acetyl pernitrate that does not include an oxygen-oxygen bond. The chemical properties of the molecule can be reconciled with either formulation.