Calibration Method for N2O5, ClNO2, and HONO with Iodide Chemical Ionization Mass Spectrometry (I-CIMS)

Vol. 116, No. 7 EnvironewsOpen AccessAir Pollution: Salt Mist Is the Right Seasoning for Ozone Carol Potera Carol Potera Search for more papers by this author Published:1 July 2008https://doi.org/10.1289/ehp.116-a288Cited by:2AboutSectionsPDF ToolsDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InReddit Shipping ports face a newly discovered air pollution problem—the production of the ozone precursor nitryl chloride. Nitryl chloride was detected for the first time in the lowest part of the Earth’s atmosphere by a team from the National Oceanic and Atmospheric Administration (NOAA) that was monitoring air quality in Galveston Bay to understand why nearby Houston, Texas, has one of the worst air pollution problems in the nation. Salts in ocean mists were thought to be relatively inert until the connection to ozone was uncovered. “People never before thought that nitryl chloride was important,” says James Roberts, a NOAA research chemist and the team’s coordinator.In the summer of 2006, the researchers used chemical ionization mass spectrometry to detect trace levels of airborne chemicals, including nitryl chloride. They found that when ship exhaust plumes rich in nitrogen oxides (NOx) meet ocean air at night, unexpectedly high levels of nitryl chloride form due to the NOx species dinitrogen pentoxide combining with chloride in sea mist. After the sun comes up, this buildup of photoactive nitryl chloride splits into chlorine atoms and nitrogen dioxide. These compounds then accelerate the production of ozone, a key component of smog.The amount of nitryl chloride measured by Roberts and colleagues—as high as 650 ppt by volume, or about 15% of total reactive nitrogen species from ship exhaust—is much greater than that estimated by standard air pollution models, which have taken into account neither the heightened presence of nitryl chloride around ports nor its importance in forming ozone. “This preliminary study indicates that nitryl chloride chemistry could make a significant contribution—up to ten to thirty percent—to ozone production during the morning hours in Houston,” says Roberts. These results were published in the May 2008 issue of Nature Geoscience.The study findings point to the need to control NOx emissions from fossil fuel combustion in coastal cities, says Roberts. He adds that one solution may be to require docked ships to use local electrical power, rather than burning diesel fuel, to generate electricity. Some ports have begun to pursue this course of action [see “Ports in a Storm,” EHP 114:A222–A231 (2006)]. The 10-year $750-million Middle Harbor Redevelopment Project proposed by the Port of Long Beach (California) would, among other pollution mitigation strategies, provide shoreside electricity for docked vessels.About half the world’s population lives near coastlines where industrial pollution meets ocean air, Roberts says, so nitryl chloride could play a major role in air quality worldwide. The same reaction likely occurs inland, where chloride-containing aerosols drive the chemical reaction. Inland sources of chloride include natural soil salts such as calcium chloride and de-icing compounds spread on winter roads.“We don’t know how widespread nitryl chloride is as a source of ozone pollution,” says Roberts. The health consequences of nitryl chloride itself are unknown. As for ozone, an April 2008 report by the National Research Council, Estimating Mortality Risk Reduction and Economic Benefits from Controlling Ozone Air Pollution, links even short-term exposure with premature death.“This is the first field measurement of nitryl chloride, and it’s very exciting,” says Barbara Finlayson-Pitts, a professor of chemistry at the University of California, Irvine. Experiments in her laboratory 20 years ago first showed that mixing sodium chloride with dinitrogen pentoxide in the dark generated nitryl chloride. “The data will be extremely useful for further development and application of air quality models for coastal urban areas,” she says.The NOx in ship exhaust combines with chloride in sea mist to produce a potent ozone precursorFiguresReferencesRelatedDetailsCited by Jiang J and Li D (2016) Theoretical analysis and experimental confirmation of exhaust temperature control for diesel vehicle NOx emissions reduction, Applied Energy, 10.1016/j.apenergy.2016.04.096, 174, (232-244), Online publication date: 1-Jul-2016. Fleer M and Verkuijl B (2014) Optimization of the use of a chiral bio-based building block for the manufacture of DHPPA, a key intermediate for propionate herbicides, Green Chemistry, 10.1039/C4GC00797B, 16:8, (3993) Vol. 116, No. 7 July 2008Metrics About Article Metrics Publication History Originally published1 July 2008Published in print1 July 2008 Financial disclosuresPDF download License information EHP is an open-access journal published with support from the National Institute of Environmental Health Sciences, National Institutes of Health. All content is public domain unless otherwise noted. Note to readers with disabilities EHP strives to ensure that all journal content is accessible to all readers. However, some figures and Supplemental Material published in EHP articles may not conform to 508 standards due to the complexity of the information being presented. If you need assistance accessing journal content, please contact [email protected]. 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High field multinuclear n.m.r. spectroscopy can be applied with advantage to the study of inorganic solutes in 100% nitric acid and mixtures with dinitrogen tetraoxide or pentaoxide. The behaviour of phosphorus haloand oxo-compounds in high density acid, HDA (44 wt. % dinitrogen tetraoxide in pure nitric acid) reveals the hydrolytic character of this type of medium and in 100% nitric acid, indicates the latter to be more highly protonating than its Hammett acidity function would suggest. In the present lecture, attention is focussed on5olutions 9 vanadium and aluminium compounds, especially in relation to V and Al n.m.r. studies, and the results emphasise that metal chemistry occurring in nitric acid solutions can be much richer than might be inferred solely from a knowledge of isolable solid products. INTRODUCTION Pure (100%) nitric acid is in some respects a unique solvent system (ref. 1,2). The extensive and unusual self-dissociation ( 1) 2HNO3NO2 + NO3 + H20 . (1) (concentration of each dissociation product O.251mo11kg I) accounts for its exceptionally high electrical conductivity (K = 3.72 x 10 ohm cm at 25°C) and also for the presence of unprotonated but solvated water molecules which have a profound effect on chemistry in nitric acid solutions. Physical properties of 100% nitric acid and its solutions with dinitrogen tetraoxide (red fuming nitric acids, which are important oxidiser components of liquid rocket propeilants) and dinitrogen pentaoxide (nitric oleums) have been extensively studied (ref. 1,2) but little information was available hitherto on chemistry in these liquids. Our long-standing interest in these solvent media has been further stimulated by involvement with technological problems arising from the corrosion of metals by red fuming nitric acids when used as liquid rocket propellant oxidisers. In rocket engines, these liquids are in contact with stainless steels or aluminium alloys and contain a small amount (ca. 0.7 wt.%) of hydrogen fluoride, added to inhibit metal corrosion. Many other compounds have been tested as inhibitors, one of the most successful alternatives to hydrogen fluoride being phosphorus pentafluoride, and this has led to an interest in both metallic and non-metallic solutes in nitric acid and its dinitrogen tetraoxide and pentaoxide solutions. We are using high field multinuclear n.m.r. spectroscopy to characterise (sometimes novel) chemical species generated and stabilised in these solvent media and these species, in turn, help to elucidate the nature of the media themselves; both solvent and solute magnetic nuclei may be exploitable. NON-METAL COMPOUNDS AS SOLUTES The behaviour of phosphorus pentafluoride, difluorophosphoric acid and phosphoric oxide in a mixture of nitrogr tetraoxide (44 wt.%) and nitric acid (high density acid', HDA), studied by F and P n.m.r. spectroscopy, reveals the hydrolytic nature of this medium (ref. 3). The hydrolysis of all three solutes is, however, time—dependent and incomplete (except at low concentrations), equilibrium mixtures of fluorophosphate species (from phosphorus pentafluoride or difluorophosphoric acid) or phosphoric acids (from phosphoric oxide) being ultimately obtained. Preliminary results indicate that the reaction of phosphorus pentafluoride with 100% nitric acid is similar to that with HDA. The hydrolysis of phosphoric oxide in 100% nitric acid is more extensive and rapid than in HDA, although similar species are observed in both media. These hydrolytic reactions of phosphorus compounds suggest that solvation of the wat mocules 100% nitric acid or HDA limits their usual chemical reactivity (ref. 2). 0, F and P n.m.r. spectroscopy has been used (ref4) to investigate the protonating ability of 100% HNO3 towards POCl, POBr3, HPO2F2 and PO3F ; the results show the acid to be more strongly protonating than indicated by its

As worldwide trends move toward replacing combustion transportation modes with electric vehicles, characterizing non-tailpipe emissions, such as those from brake wear, becomes increasingly important. Nitrous acid (HONO), nitryl chloride (ClNO2), and dinitrogen pentoxide (N2O5) are important sources of radical oxidants (e.g., •OH, •Cl, •NO3) and nitrogen oxides (NOx) in the atmosphere, driving the chemistry that leads to air quality degradation. Discrepancies between measurements and model predictions indicate that there are significant unknown sources of these species, particularly HONO, where the contributions of different formation processes have been controversial since the first ambient observations in the 1970s. We report the generation of these reactive nitrogen species during automotive braking using chemical ionization mass spectrometry configured with iodide reagent ion. Substantial HONO levels are observed from ceramic and semi-metallic brake pads, and smaller quantities of ClNO2 and N2O5 were also detected. We propose that HONO is formed in the hot plume emanating from the brake rotor via abstraction by NO2 of allylic and aldehyde hydrogen atoms found in the complex mixture of volatile organic compounds emitted simultaneously. These results suggest that emissions from automotive braking must be taken into account in urban oxidation chemistry.

Abstract. Dinitrogen pentoxide (N2O5) and nitryl chloride (ClNO2) are key species in nocturnal tropospheric chemistry and have significant effects on particulate nitrate formation and the following day's photochemistry through chlorine radical production and NOx recycling upon photolysis of ClNO2. To better understand the roles of N2O5 and ClNO2 in the high-aerosol-loading environment of northern China, an intensive field study was carried out at a high-altitude site (Mt. Tai, 1465 m a.s.l.) in the North China Plain (NCP) during the summer of 2014. Elevated ClNO2 plumes were frequently observed in the nocturnal residual layer with a maximum mixing ratio of 2.1 ppbv (1 min), whilst N2O5 was typically present at very low levels (< 30 pptv), indicating fast heterogeneous N2O5 hydrolysis. Combined analyses of chemical characteristics and backward trajectories indicated that the ClNO2-laden air was caused by the transport of NOx-rich plumes from the coal-fired industry and power plants in the NCP. The heterogeneous N2O5 uptake coefficient (γ) and ClNO2 yield (ϕ) were estimated from steady-state analysis and observed growth rate of ClNO2. The derived γ and ϕ exhibited high variability, with means of 0.061 ± 0.025 and 0.28 ± 0.24, respectively. These values are higher than those derived from previous laboratory and field studies in other regions and cannot be well characterized by model parameterizations. Fast heterogeneous N2O5 reactions dominated the nocturnal NOx loss in the residual layer over this region and contributed to substantial nitrate formation of up to 17 µg m−3. The estimated nocturnal nitrate formation rates ranged from 0.2 to 4.8 µg m−3 h−1 in various plumes, with a mean of 2.2 ± 1.4 µg m−3 h−1. The results demonstrate the significance of heterogeneous N2O5 reactivity and chlorine activation in the NCP, and their unique and universal roles in fine aerosol formation and NOx transformation, and thus their potential impacts on regional haze pollution in northern China.

<strong class="journal-contentHeaderColor">Abstract.</strong> We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO<span class="inline-formula">₂</span>), nitrous acid (HONO), total reactive odd nitrogen (measured both as the total (NO<span class="inline-formula">_<i>y</i></span>) and from the sum of individually measured species (<span class="inline-formula">Σ</span>NO<span class="inline-formula">_<i>y</i></span>)), and carbon monoxide (CO) in the troposphere during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. By targeting smoke from summertime wildfires, prescribed fires, and agricultural burns across the continental United States, FIREX-AQ provided a unique opportunity to investigate measurement accuracy in concentrated plumes where hundreds of species coexist. Here, we compare NO measurements by chemiluminescence (CL) and laser-induced fluorescence (LIF); NO<span class="inline-formula">₂</span> measurements by CL, LIF, and cavity-enhanced spectroscopy (CES); HONO measurements by CES and iodide-adduct chemical ionization mass spectrometry (CIMS); and CO measurements by tunable diode laser absorption spectrometry (TDLAS) and integrated cavity output spectroscopy (ICOS). Additionally, total NO<span class="inline-formula">_<i>y</i></span> measurements using the CL instrument were compared with <span class="inline-formula">Σ</span>NO<span class="inline-formula">_<i>y</i></span> (<span class="inline-formula">=</span> NO <span class="inline-formula">+</span> NO<span class="inline-formula">₂</span> <span class="inline-formula">+</span> HONO <span class="inline-formula">+</span> nitric acid (HNO<span class="inline-formula">₃</span>) <span class="inline-formula">+</span> acyl peroxy nitrates (APNs) <span class="inline-formula">+</span> submicrometer particulate nitrate (<span class="inline-formula"><i>p</i></span>NO<span class="inline-formula">₃</span>)). Other NO<span class="inline-formula">_<i>y</i></span> species were not included in <span class="inline-formula">Σ</span>NO<span class="inline-formula">_<i>y</i></span> as they either contributed minimally to it (e.g., C<span class="inline-formula">₁</span>–C<span class="inline-formula">₅</span> alkyl nitrates, nitryl chloride (ClNO<span class="inline-formula">₂</span>), dinitrogen pentoxide (N<span class="inline-formula">₂</span>O<span class="inline-formula">₅</span>)) or were not measured during FIREX-AQ (e.g., higher oxidized alkyl nitrates, nitrate (NO<span class="inline-formula">₃</span>), non-acyl peroxynitrates, coarse-mode aerosol nitrate). The aircraft instrument intercomparisons demonstrate the following points: (1) NO measurements by CL and LIF agreed well within instrument uncertainties but with potentially reduced time response for the CL instrument; (2) NO<span class="inline-formula">₂</span> measurements by LIF and CES agreed well within instrument uncertainties, but CL NO<span class="inline-formula">₂</span> was on average 10 % higher; (3) CES and CIMS HONO measurements were highly correlated in each fire plume transect, but the correlation slope of CES vs. CIMS for all 1 Hz data during FIREX-AQ was 1.8, which we attribute to a reduction in the CIMS sensitivity to HONO in high-temperature environments; (4) NO<span class="inline-formula">_<i>y</i></span> budget closure was demonstrated for all flights within the combined instrument uncertainties of 25 %. However, we used a fluid dynamic flow model to estimate that average <span class="inline-formula"><i>p</i></span>NO<span class="inline-formula">₃</span> sampling fraction through the NO<span class="inline-formula">_<i>y</i></span> inlet in smoke was variable from one flight to another and ranged between 0.36 and 0.99, meaning that approximately 0 %–24 % on average of the total measured NO<span class="inline-formula">_<i>y</i></span> in smoke may have been unaccounted for and may be due to unmeasured species such as organic nitrates; (5) CO measurements by ICOS and TDLAS agreed well within combined instrument uncertainties, but with a systematic offset that averaged 2.87 ppbv; and (6) integrating smoke plumes followed by fitting the integrated values of each plume improved the correlation between independent measurements.

Abstract. We present a comparison of fast-response instruments installed onboard the NASA DC-8 aircraft that measured nitrogen oxides (NO and NO2), nitrous acid (HONO), total reactive odd nitrogen (measured both as the total (NOy) and from the sum of individually measured species (ΣNOy)), and carbon monoxide (CO) in the troposphere during the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign. By targeting smoke from summertime wildfires, prescribed fires, and agricultural burns across the continental United States, FIREX-AQ provided a unique opportunity to investigate measurement accuracy in concentrated plumes where hundreds of species coexist. Here, we compare NO measurements by chemiluminescence (CL) and laser-induced fluorescence (LIF); NO2 measurements by CL, LIF, and cavity-enhanced spectroscopy (CES); HONO measurements by CES and iodide-adduct chemical ionization mass spectrometry (CIMS); and CO measurements by tunable diode laser absorption spectrometry (TDLAS) and integrated cavity output spectroscopy (ICOS). Additionally, total NOy measurements using the CL instrument were compared with ΣNOy (= NO + NO2 + HONO + nitric acid (HNO3) + acyl peroxy nitrates (APNs) + submicrometer particulate nitrate (pNO3)). Other NOy species were not included in ΣNOy as they either contributed minimally to it (e.g., C1–C5 alkyl nitrates, nitryl chloride (ClNO2), dinitrogen pentoxide (N2O5)) or were not measured during FIREX-AQ (e.g., higher oxidized alkyl nitrates, nitrate (NO3), non-acyl peroxynitrates, coarse-mode aerosol nitrate). The aircraft instrument intercomparisons demonstrate the following points: (1) NO measurements by CL and LIF agreed well within instrument uncertainties but with potentially reduced time response for the CL instrument; (2) NO2 measurements by LIF and CES agreed well within instrument uncertainties, but CL NO2 was on average 10 % higher; (3) CES and CIMS HONO measurements were highly correlated in each fire plume transect, but the correlation slope of CES vs. CIMS for all 1 Hz data during FIREX-AQ was 1.8, which we attribute to a reduction in the CIMS sensitivity to HONO in high-temperature environments; (4) NOy budget closure was demonstrated for all flights within the combined instrument uncertainties of 25 %. However, we used a fluid dynamic flow model to estimate that average pNO3 sampling fraction through the NOy inlet in smoke was variable from one flight to another and ranged between 0.36 and 0.99, meaning that approximately 0 %–24 % on average of the total measured NOy in smoke may have been unaccounted for and may be due to unmeasured species such as organic nitrates; (5) CO measurements by ICOS and TDLAS agreed well within combined instrument uncertainties, but with a systematic offset that averaged 2.87 ppbv; and (6) integrating smoke plumes followed by fitting the integrated values of each plume improved the correlation between independent measurements.

When low-temperature thin films of either ionic or covalent dinitrogen pentaoxide, N2O5, are exposed to gaseous HCl and water, the only products observed in the solid phase by reflection−absorption infrared spectroscopy (RAIRS) are molecular nitric acid and the oxonium ion. Nitryl chloride, ClNO2, is not detectable. When dinitrogen pentaoxide is co-deposited with hydrogen chloride and water at 85 K and annealed to 140 K, the resultant RAIR spectra indicate that the film is composed of H3O+Cl-, N2O5, HNO3, and D2h-N2O4. When nitryl chloride is co-deposited with either water or HCl/water mixtures, infrared spectra indicative of solid D2h-N2O4 are measured, as well as peaks corresponding to nitrate ions and cis-ClONO (chlorine nitrite). Reaction between ClNO2 and its isomer, cis-ClONO, is proposed as an explanation for the formation of dinitrogen tetraoxide in both systems. The proposed reaction mechanism for this hydrolysis is extended to the N2O5/H2O/HCl deposits in order to explain the lack of observable ClNO2 in such thin films.

Conductance measurements in the system water – nitric acid – nitrogen pentoxide have been made over a range of temperatures. The results are in accordance with the findings of Veley and Manley (12) as regards (1) the presence of a minimum in the specific conductance at about 96% by weight nitric acid (2) the existence of a negative temperature coefficient of specific conductance for solutions rich in nitric acid. As reported by Berl and Saenger (1) the specific conductance of solutions of nitrogen pentoxide in nitric acid at first increases with increasing concentration of the former, ultimately attaining a maximum and then decreasing with further increase in the nitrogen pentoxide content. Some viscosity and density data for nitric acid – nitrogen pentoxide mixtures are included.

Abstract. Here we present a field measurement of ClNO2 (nitryl chloride) and N2O5 (dinitrogen pentoxide) by a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) with the Filter Inlet for Gas and AEROsols (FIGAERO) at a regional site in the Pearl River Delta during a photochemical pollution season from 26 September to 17 November 2019. Three patterns of air masses are sampled during this campaign, including the dominating air masses from the north and northeast urban regions (Type A), the southeast coast (Type B), and the South China Sea (Type C). The concentration of ClNO2 and N2O5 was observed to be much higher in Type A and B than in Type C, indicating that the urban nighttime chemistry is more active than the background marine regions. The N2O5 uptake coefficient and ClNO2 production yield were estimated based on the field measurement, and the performance of the previously derived parameterizations was assessed. The nighttime ClNO2 correlated with particulate chloride and the mass concentration of fine particles (most likely due to aerosol surface area) suggested that the ClNO2 formation was limited by the N2O5 uptake at this site. By examining the relationship between particulate chloride and other species, we implied that anthropogenic emissions (e.g., biomass burning) rather than sea salt particles dominate the origin of particulate chloride, although the site was only about 100 km away from the ocean. A box model with detailed chlorine chemistry is used to investigate the impacts of ClNO2 chemistry on atmospheric oxidation. Model simulations showed that the chlorine radical liberated by ClNO2 photolysis during the next day had a slight increase in concentrations of OH, HO2, and RO2 radicals, as well as minor contributions to RO2 radical and O3 formation (&lt; 5 %, on daytime average), in all the three types of air masses. Relatively high contributions were observed in Type A and B. The overall low contributions of ClNO2 to atmospheric oxidation are consistent with those reported recently from wintertime observations in China (including Shanghai, Beijing, Wangdu, and Mt. Tai). This may be attributed to the following: (1) relatively low particle mass concentration limited ClNO2 formation; (2) other reactions channels, like nitrous acid (HONO), oxygenated volatile organic compounds (OVOCs, including formaldehyde), and ozone photolysis had a more significant radical formation rate during the ozone pollution episodes and weakened the ClNO2 contribution indirectly. The results provided scientific insights into the role of nighttime chemistry in photochemical pollution under various scenarios in coastal areas.

SUMMARY1. Graham states that when nitric acid of specific gravity greater than 1·4 acts on copper oxide a basic nitrate is obtained. We were unable to obtain a basic salt under these conditions.2. Copper oxide and approximately 100 per cent. nitric acid yield copper nitrate trihydrate, nitrogen peroxide, and oxygen. The equation for the reaction is probably CuO + 6HNO3 = Cu(NO3)2, 3H2O + 4NO2 + O2.3. The only basic nitrate of copper appears to be Cu(NO3)2·3Cu(OH)2. The product obtained by heating the trihydrate to 100° has this composition, and is not Cu(NO3)2·2Cu(OH)2, as stated by Graham.4. Dehydration of copper nitrate does not yield anhydrous copper nitrate, but results in decomposition, with formation of basic copper nitrate, whether the dehydration is performed at ordinary or higher temperatures.5. Copper oxide does not interact with nitric anhydride.6. The only hydrates of copper nitrate appear to be the trihydrate and hexahydrate. The transition temperature is 24·65° (±0·05) corr.7. Concentrated nitric acid dehydrates the hexahydrate, while dilute nitric acid hydrates the trihydrate. It was not found possible to determine the concentration of nitric acid, which would be in equilibrium with both hydrates.

Part I. The investigation of the kinetics of the decomposition of the liquid phase of nitric acid has indicated that the reaction probably proceeds by way of the unimolecular decomposition of dinitrogen pentoxide which exists in the pure acid. The effects of several inorganic additives in suppressing the rate of decomposition are in agreement with the postulate involving dinitrogen pentoxide. The magnitude of the inhibition of the decomposition by various additives has been found to be insufficient to prevent the eventual attainment of high equilibrium pressures resulting from decomposition when nitric acid is stored in closed containers. The solubility of oxygen in fuming nitric acid has been determined for oxygen pressures up to 21 atmospheres in the temperature range between 35[degrees] and 70[degrees]C. Under these conditions, the solubility of oxygen was found to increase with an increase in temperature. Part II. The electrolytic conductance of the system nitric acid--nitrogen dioxide--water in the liquid phase was measured at 0[degrees]C and a pressure of 1 atmosphere for compositions containing more than 0.80 weight fraction nitric acid. The conductance of the associated binary systems nitric acid--water and nitric acid--nitrogen dioxide has been measured over the entire range of compositions from 0 to 1.00 weight fraction nitric acid. The conductance measurements offer additional proof of the self-ionization of pure nitric acid and of the ionization of nitrogen dioxide dissolved in nitric acid.

Conductance measurements have been made with ammonium nitrate in water – nitric acid – nitrogen pentoxide mixtures at various temperatures and over a wide range of concentrations. For all solutions except those rich in nitrogen pentoxide the variation of the specific conductance with the concentration of ammonium nitrate is similar to that observed for many electrolytes in a variety of solvents. With nitrogen-pentoxide-rich solvents however the specific conductance of ammonium nitrate is observed to decrease with increasing salt concentration. This is regarded as indicative of an interaction between the salt and one or more of the components of the nitric acid – nitrogen pentoxide system. The variation of conductance with temperature is normal in all cases. It is shown that ammonium nitrate confers a considerable degree of thermal stability on anhydrous nitric acid.

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Activated phagocytic cells generate reactive nitrogen species, including nitryl chloride and peroxynitrite, for host defense against invading pathogens. It has been proposed that these reactive nitrogen species may cause DNA damage and thus contribute to the multistage carcinogenesis process associated with chronic infections and inflammation. Previous studies showed that peroxynitrite reacted with guanine, 2'-deoxyguanosine, or DNA forming 8-nitroguanine. We herein report formation of 8-nitroxanthine as the major nitration product in reactions of 2'-deoxyguanosine or calf thymus DNA with nitryl chloride produced by mixing nitrite with hypochlorous acid, and 8-nitroguanine was a minor product in these reactions. 8-Nitroxanthine was characterized by its NMR and laser desorption ionization mass spectra and by deamination of 8-nitroguanine. Formation of 8-nitroxanthine was also detected by xanthine reaction with various reactive nitrogen species, including nitryl chloride, peroxynitrite, nitronium tetrafluoroborate, and heated nitric and nitrous acid. The identity of 8-nitroxanthine in nitryl chloride-treated dG and DNA was confirmed by co-injection with synthetic 8-nitroxanthine and by its reduction to 8-aminoxanthine. Levels of 8-nitroxanthine and 8-nitroguanine in these reactions were quantified by reversed-phase HPLC with photodiode array detection. Once formed, 8-nitroxanthine was spontaneously removed from DNA with a half-life of 2 h at 37 degrees C and pH 7.4. Therefore, 8-nitroxanthine might be an important DNA lesion derived from reactive nitrogen species in vivo.

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is an important pathway in nitrate formation; however, its formation rate and relative contribution to total particulate nitrate () are highly variable. Here we report that nocturnal formation via N2O5 hydrolysis is dependent on the regime defined by the ratio of NO2 to O3. Nocturnal formation via N2O5 hydrolysis is suppressed in an O3‐limited regime but enhanced in a NO2‐limited regime. The results have crucial implications for effective control of nitrate pollution in the future. An exclusive decrease in NO2 will decrease nocturnal formation in a NO2‐limited regime but may be less effective or even increase nocturnal formation in an O3‐limited regime.