Reaction Of NO Research Articles

Progress in quantitative research on the relationship between atmospheric oxidation and air quality

Atmospheric oxidizing capacity (AOC) is an essential driving force of troposphere chemistry and self-cleaning, but the definition of AOC and its quantitative representation remain uncertain. Driven by national demand for air pollution control in recent years, Chinese scholars have carried out studies on theories of atmospheric chemistry and have made considerable progress in AOC research. This paper will give a brief review of these developments. First, AOC indexes were established that represent apparent atmospheric oxidizing ability (AOIe) and potential atmospheric oxidizing ability (AOIp) based on aspects of macrothermodynamics and microdynamics, respectively. A closed study refined the quantitative contributions of heterogeneous chemistry to AOC in Beijing, and these AOC methods were further applied in Beijing-Tianjin-Hebei and key areas across the country. In addition, the detection of ground or vertical profiles for atmospheric OH·, HO2·, NO3· radicals and reservoir molecules can now be obtained with domestic instruments in diverse environments. Moreover, laboratory smoke chamber simulations revealed heterogeneous processes involving reactions of O3 and NO2, which are typical oxidants in the surface/interface atmosphere, and the evolutionary and budgetary implications of atmospheric oxidants reacting under multispecies, multiphase and multi-interface conditions were obtained. Finally, based on the GRAPES-CUACE adjoint model improved by Chinese scholars, simulations of key substances affecting atmospheric oxidation and secondary organic and inorganic aerosol formation have been optimized. Normalized numerical simulations of AOIe and AOIp were performed, and regional coordination of AOC was adjusted. An optimized plan for controlling O3 and PM2.5 was analyzed by scenario simulation.

A diurnal story of Δ17O(rm{NO}_{3}^{-}) in urban Nanjing and its implication for nitrate aerosol formation

Inorganic nitrate production is critical in atmospheric chemistry that reflects the oxidation capacity and the acidity of the atmosphere. Here we use the oxygen anomaly of nitrate (Δ17O(rm{NO}_{3}^{-})) in high-time-resolved (3 h) aerosols to explore the chemical mechanisms of nitrate evolution in fine particles during the winter in Nanjing, a megacity of China. The continuous Δ17O(rm{NO}_{3}^{-}) observation suggested the dominance of nocturnal chemistry (NO3 + HC/H2O and N2O5 + H2O/Cl−) in nitrate formation in the wintertime. Significant diurnal variations of nitrate formation pathways were found. The contribution of nocturnal chemistry increased at night and peaked (72%) at midnight. Particularly, nocturnal pathways became more important for the formation of nitrate in the process of air pollution aggravation. In contrast, the contribution of daytime chemistry (NO2 + OH/H2O) increased with the sunrise and showed a highest fraction (48%) around noon. The hydrolysis of N2O5 on particle surfaces played an important role in the daytime nitrate production on haze days. In addition, the reaction of NO2 with OH radicals was found to dominate the nitrate production after nitrate chemistry was reset by the precipitation events. These results suggest the importance of high-time-resolved observations of Δ17O(rm{NO}_{3}^{-}) for exploring dynamic variations in reactive nitrogen chemistry.

Oxidative Damage of Aliphatic Amino Acid Residues by the Environmental Pollutant NO3·: Impact of Water on the Reactivity.

The rate of oxidative damage of aliphatic amino acids and dipeptides by the environmental pollutant nitrate radical (NO3·) in an aqueous acidic environment was studied by laser flash photolysis. The reactivity dropped by a factor of about four for amino acid residues with secondary amide bonds and by a factor of up to nearly 20 for amino acid residues with tertiary amide bonds, compared with that in acetonitrile. According to density functional theory studies, the lower reactivity is due to protonation of the amide moiety, whereas in neutral water, hydrogen bonding with the amide should have little impact on the absolute reaction rate compared with that in acetonitrile. This finding can be rationalized by the high reactivity and broad reaction pattern of NO3·. Although hydrogen bonding involving the amide group raises the energies associated with some electron transfer processes, alternative low-energy pathways remain available so that the overall reaction rate is barely affected. The undiminished high reactivity of NO3· toward aliphatic amino acid residues in a neutral aqueous environment highlights the health-damaging potential of exposure to the combined air pollutants nitrogen dioxide (NO2·) and ozone (O3).

Molecular simulation study of the stabilization process of NEPE propellant

In this reported study, the density functional theory (DFT) was used at the (U)B3LYP/6-311G(d,p) level to investigate the stabilization process of the nitrate ester plasticized polyether propellant (NEPE). Molecular simulations were conducted of the reaction that generates NO2, the autocatalytic and aging reaction triggered by the NO2, and the nitrogen dioxide absorption reaction of the stabilizers during the propellent stabilization process. These simulations were derived using the transition-state theory (TST) and variational transition-state theory (VTST). The simulation results suggested that the stabilization of the NEPE propellant consisted of three stages. First, heat and NO2 were generated during the denitrification reaction of nitroglycerine (NG) and 1,2,4-butanetriol trinitrate(BTTN) in the NEPE propellant. Second, nitroso products were generated by the reactions of N-Methyl-4-nitroaniline (MNA) and 2-nitrodiphenylamine (2NDPA) with NO2. Third, the stabilizers were exhausted and the autocatalytic reaction of NG and BTTN and the aging reaction of polyethylene glycol (PEG) were triggered by the heat and NO2 generated in the first stage. By comparing the energy barriers of the various reactions, it was found that the NO2 generated from the denitrification reaction significantly reduced the reaction energy barrier to 105.56–126.32 kJ/mol, also increased the reaction rate constant, and decreased the thermal stability and energetic properties of the NEPE propellant. In addition, the NO2 also weakened the mechanical properties of the NEPE propellant by attacking the –CH2 groups and the O atoms in the PEG molecular chain. The energy barriers of the reactions of MNA and 2NDPA with NO2 (94.61–133.61 kJ/mol) were lower than those of the autocatalytic and decomposition reactions of NG, BTTN, and the aging reactions of PEG (160.30–279.46 kJ/mol). This indicated that, by eliminating NO2, the stabilizer in the NEPE propellant can effectively prevent NO2 from reacting with the NG, BTTN, and PEG in the NEPE propellant. Consequently, this would help maintain the energy and mechanical properties of the NEPE propellant, thereby improving its thermal stability.

Secondary PM<sub>2.5</sub> decreases significantly less than NO<sub>2</sub> emission reductions during COVID lockdown in Germany

Abstract. This study estimates the influence of anthropogenic emission reductions on the concentration of particulate matter with a diameter smaller than 2.5 µm (PM2.5) during the 2020 lockdown period in German metropolitan areas. After accounting for meteorological effects, PM2.5 concentrations during the spring 2020 lockdown period were 5 % lower compared to the same time period in 2019. However, during the 2020 pre-lockdown period (winter), PM2.5 concentrations with meteorology accounted for were 19 % lower than in 2019. Meanwhile, NO2 concentrations with meteorology accounted for dropped by 23 % during the 2020 lockdown period compared to an only 9 % drop for the 2020 pre-lockdown period, both compared to 2019. SO2 and CO concentrations with meteorology accounted for show no significant changes during the 2020 lockdown period compared to 2019. GEOS-Chem (GC) simulations with a COVID-19 emission reduction scenario based on the observations (23 % reduction in anthropogenic NOx emission with unchanged anthropogenic volatile organic compounds (VOCs) and SO2) are consistent with the small reductions of PM2.5 during the lockdown and are used to identify the underlying drivers for this. Due to being in a NOx-saturated ozone production regime, GC OH radical and O3 concentrations increased (15 % and 9 %, respectively) during the lockdown compared to a business-as-usual (BAU, no lockdown) scenario. Ox (equal to NO2+O3) analysis implies that the increase in ozone at nighttime is solely due to reduced NO titration. The increased O3 results in increased NO3 radical concentrations, primarily during the night, despite the large reductions in NO2. Thus, the oxidative capacity of the atmosphere is increased in all three important oxidants, OH, O3, and NO3. PM nitrate formation from gas-phase nitric acid (HNO3) is decreased during the lockdown as the increased OH concentration cannot compensate for the strong reductions in NO2, resulting in decreased daytime HNO3 formation from the OH + NO2 reaction. However, nighttime formation of PM nitrate from N2O5 hydrolysis is relatively unchanged. This results from the fact that increased nighttime O3 results in significantly increased NO3, which roughly balances the effect of the strong NO2 reductions on N2O5 formation. Ultimately, the only small observed decrease in lockdown PM2.5 concentrations can be explained by the large contribution of nighttime PM nitrate formation, generally enhanced sulfate formation, and slightly decreased ammonium. This study also suggests that high PM2.5 episodes in early spring are linked to high atmospheric ammonia concentrations combined with favorable meteorological conditions of low temperature and low boundary layer height. Northwest Germany is a hot-spot of NH3 emissions, primarily emitted from livestock farming and intensive agricultural activities (fertilizer application), with high NH3 concentrations in the early spring and summer months. Based on our findings, we suggest that appropriate NOx and VOC emission controls are required to limit ozone, and that should also help reduce PM2.5. Regulation of NH3 emissions, primarily from agricultural sectors, could result in significant reductions in PM2.5 pollution.

Fate of the nitrate radical at the summit of a semi-rural mountain site in Germany assessed with direct reactivity measurements

Abstract. The reactivity of NO3 plays an important role in modifying the fate of reactive nitrogen species at nighttime. High reactivity (e.g. towards unsaturated volatile organic compounds – VOCs) can lead to formation of organic nitrates and secondary organic aerosol, whereas low reactivity opens the possibility of heterogeneous NOx losses via the formation and uptake of N2O5 to particles. We present direct NO3 reactivity measurements (kNO3) that quantify the VOC-induced losses of NO3 during the TO2021 campaign at the summit of the Kleiner Feldberg mountain (825 m, Germany) in July 2021. kNO3 was on average ∼0.035 s−1 during the daytime, ∼0.015 s−1 for almost half of the nights and below the detection limit of 0.006 s−1 for the other half, which may be linked to sampling from above the nocturnal surface layer. NO3 reactivities derived from VOC measurements and the corresponding rate coefficient were in good agreement with kNO3, with monoterpenes representing 84 % of the total reactivity. The fractional contribution F of kNO3 to the overall NO3 loss rate (which includes an additional reaction of NO3 with NO and photolysis) were on average ∼16 % during the daytime and ∼50 %–60 % during the nighttime. The relatively low nighttime value of F is related to the presence of several tens of parts per trillion by volume (pptv) of NO on several nights. NO3 mixing ratios were not measured, but steady-state calculations resulted in nighttime values between <1 and 12 pptv. A comparison of results from TO2021 with direct measurements of NO3 during previous campaigns between 2008 and 2015 at this site revealed that NO3 loss rates were remarkably high during TO2021, while NO3 production rates were low. We observed NO mixing ratios of up to 80 pptv at night, which has implications for the cycling of reactive nitrogen at this site. With O3 present at levels of mostly 25 to 60 ppbv (parts per billion by volume), NO is oxidized to NO2 on a timescale of a few minutes. We find that maintaining NO mixing ratios of, e.g., 40 pptv requires a ground-level NO emission rate of 0.33 pptv s−1 (into a shallow surface layer of 10 m depth). This in turn requires a rapid deposition of NO2 to the surface (vdNO2∼0.15 cm s−1) to reduce nocturnal NO2 levels to match the observations.

Laminar flame speeds and ignition delay times for isopropyl nitrate and propane blends

Laminar flame speed and ignition delay time measurements are presented for blends of isopropyl-nitrate and propane. The laminar flame speeds were measured in a spherical bomb, with the majority of the experiments performed at 373 K and 1 bar, with equivalence ratios between 0.6 <ϕ< 2.1; the fuel composition was 0%, 10%, and 100% isopropyl-nitrate. Additional experiments were performed at 300 K for pure propane as a fuel, and at 0.5 bar for pure isopropyl-nitrate as fuel. The ignition delay measurements were performed in a shock tube, with reflected shock temperatures between 1050 K and 1530 K and a nominal reflected shock pressure of 20 bar. Equivalence ratios of 0.5, 1.0, and 1.5 were considered for fuel blends with 0% and 1% isopropyl-nitrate in propane; additional experiments were performed with 10% isopropyl-nitrate at ϕ = 1. In comparison with the pure propane flames, the pure isopropyl-nitrate have a similar peak flame speed (SL0=60.3 cm/s at ϕ=1.09 for isopropyl-nitrate versus SL0=57.9 cm/s at ϕ=1.10 for propane), but the nitrate is considerably faster under fuel lean conditions, and it exhibits a much broader domain. The ignition delay times for 1% isopropyl-nitrate in propane are indistinguishable from the pure propane experiments for all three equivalence ratios. The 10% isopropyl-nitrate in propane mixture, in contrast, is significantly more reactive. Transition state theory calculations were performed for select RH + NO2 reactions. These calculations were added to a recently developed mechanism for isopropyl-nitrate and propane. Detailed simulations of the experiments were performed in Cantera. The agreement between the models and experiments is excellent. The largest discrepancies occur for the ignition delay times for 10% isopropyl-nitrate at T5<1150 K and for the flame speeds for ϕ>1.8, which suggests that further work needs to be done under these conditions. To the best of our knowledge, these data represent the first published results on the laminar flame speeds of any alkyl nitrate.

An experimental study of the reactivity of terpinolene and &amp;lt;i&amp;gt;β&amp;lt;/i&amp;gt;-caryophyllene with the nitrate radical

Abstract. Biogenic volatile organic compounds (BVOCs) are intensely emitted by forests and crops into the atmosphere. They can rapidly react with the nitrate radical (NO3) during the nighttime to form a number of functionalized products. Among them, organic nitrates (ONs) have been shown to behave as reservoirs of reactive nitrogen and consequently influence the ozone budget and secondary organic aerosols (SOAs), which are known to have a direct and indirect effect on the radiative balance and thus on climate. Nevertheless, BVOC + NO3 reactions remain poorly understood. Thus, the primary purpose of this study is to furnish new kinetic and mechanistic data for one monoterpene (C10H16), terpinolene, and one sesquiterpene (C15H24), β-caryophyllene, using simulation chamber experiments. These two compounds have been chosen in order to complete the few experimental data existing in the literature. Rate constants have been measured using both relative and absolute methods. They have been measured to be (6.0 ± 3.8) ×10-11 and (1.8 ± 1.4) ×10-11 cm3 molec.−1 s−1 for terpinolene and β-caryophyllene respectively. Mechanistic studies have also been conducted in order to identify and quantify the main reaction products. Total organic nitrates and SOA yields have been determined. Both terpenes appear to be major ON precursors in both gas and particle phases with formation yields of 69 % for terpinolene and 79 % for β-caryophyllene respectively. They are also major SOA precursors, with maximum SOA yields of around 60 % for terpinolene and 90 % for β-caryophyllene. In order to support these observations, chemical analyses of the gas-phase products were performed at the molecular scale using a proton transfer reaction–time-of-flight–mass spectrometer (PTR-ToF-MS) and FTIR. Detected products allowed proposing chemical mechanisms and providing explanations through peroxy and alkoxy reaction pathways.

Study on the Catalytic Decomposition Reaction of N2O on MgO (100) in SO2 and CO Environments

To study the role of MgO in the reduction of N2O in circulating fluidized bed boilers, density functional theory was used to evaluate heterogeneous decomposition. The interference of SO2 and CO on N2O was considered. N2O on MgO (100) is a two-step process that includes O transfer and surface recovery processes. The O transfer process is the rate-determining step with barrier energy of 1.601 eV, while for the Langmuir–Hinshelwood and Eley–Rideal surface recovery mechanisms, the barrier energies are 0.840 eV and 1.502 eV, respectively. SO2 has a stronger interaction with the surface-active O site than that of N2O. SO2 will occupy the active site and hinder N2O decomposition. CO cannot improve the catalysis of MgO (100) for N2O because O transfer is the rate-determining step. Compared with homogeneous reduction by CO, MgO has a limited catalytic effect on N2O, where the barrier energy decreases from 1.691 eV to 1.601 eV.

Reaction of nitrous oxide and ammonia molecules at 4H-SiC/SiO[formula omitted] interface: An ab initio study

The reactions of N2O and NH3 molecules at the interface between 4H-SiC and SiO2 during post oxidation annealing (POA) are theoretically investigated using ab initio calculations. We find that the reactions of N2O molecule at (0001) and (0001¯) interfaces result in the desorption of carbon oxide molecules in addition to Si–N bond formation. For the reaction of NH3 molecule, various atomic configurations are formed depending on the plane orientation. Furthermore, the energy barriers for the N2O and NH3 reactions are found to be higher than that of the NO reaction owing to different atomic configurations at the transition state structure. The calculated results shed some insights for understanding POA process at 4H-SiC/SiO2 interfaces.

Inorganic Ions Enhance the Number of Product Compounds through Heterogeneous Processing of Gaseous NO2 on an Aqueous Layer of Acetosyringone.

Methoxyphenols represent important pollutants that can participate in the formation of secondary organic aerosols (SOAs) through chemical reactions with atmospheric oxidants. In this study, we determine the influence of ionic strength, pH, and temperature on the heterogeneous reaction of NO2 with an aqueous film consisting of acetosyringone (ACS), as a proxy for methoxyphenols. The uptake coefficient of NO2 (50 ppb) on ACS (1 × 10-5 mol L-1) is γ = (9.3 ± 0.09) × 10-8 at pH 5, and increases by one order of magnitude to γ = (8.6 ± 0.5) × 10-7 at pH 11. The lifetime of ACS due to its reaction with NO2 is largely affected by the presence of nitrate ions and sulfate ions encountered in aqueous aerosols. The analysis performed by membrane inlet single-photon ionization-time-of-flight mass spectrometry (MI-SPI-TOFMS) reveals an increase in the number of product compounds and a change of their chemical composition upon addition of nitrate ions and sulfate ions to the aqueous thin layer consisting of ACS. These outcomes indicate that inorganic ions can play an important role during the heterogeneous oxidation processes in aqueous aerosol particles.

DFT Study on the Combined Catalytic Removal of N2O, NO, and NO2 over Binuclear Cu-ZSM-5

The large amount of nitrogen oxides (N2O, NO, NO2, etc.) contained in the flue gas of industrial adipic acid production will seriously damage the environment. A designed binuclear Cu-ZSM-5 catalyst can be applied to decompose N2O and reduce NO and NO2, purifying the air environment. Using the density functional theory method, the catalytic decomposition mechanisms of N2O, NOX-NH3-SCR, and NOX-assisted N2O decomposition is simulated over the Cu-ZSM-5 model. The results indicate that N2O can be catalytically decomposed over the binuclear Cu active site in the sinusoidal channel. The speed-limiting step is the second N2O molecule activation process. After the decomposition of the first N2O molecule, a stable extra-frame [Cu-O-Cu]2+ structure will generate. The subsequent discussion proved that the NOX-NH3-SCR reaction can be realized over the [Cu-O-Cu]2+ active site. In addition, it proved that the decomposition reaction of NO and NO2 can be carried out over the [Cu-O-Cu]2+ active site, and NO can greatly reduce the energy barrier for the conversion of the active site from [Cu-O-Cu]2+ to the binuclear Cu form, while NO2 can be slightly reduced. Through discussion, it is found that the binuclear Cu-ZSM-5 can realize the combined removal of N2O and NOX from adipic acid flue gas, hoping to provide a theoretical basis for the development of a dual-functional catalyst.

Gas phase degradation of trifluoromethyl peroxynitrate and trifluoromethyl nitrate in presence of water vapour

Trifluoromethyl peroxynitrate (CF3O2NO2) and trifluoromethyl nitrate (CF3ONO2) could be formed in the degradation of chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons and hydrofluoroethers, from the reaction of CF3Ox radicals and NO2. In the present work, the role of gas-phase water in the degradation of CF3O2NO2 and CF3ONO2 was evaluated for the first time. The study has included experiments with and without water, theoretical calculation and kinetics mechanism modelling.Concentration of fluorinated species in different experimental systems He, He/H2O, N2, and N2/H2O were monitored by infrared spectroscopy. Results indicated that water vapour does not affect the peroxynitrate stability. On the other hand, nitrate degrades quickly in presence of water forming HNO3, CF2O and HF, while, in absence of water, the products are NO2 and CF2O.Kinetics mechanism was proposed and modelled to corroborate the experimental results. Theoretical calculations show that the formation of Van der Waals complexes CF3O2NO2·(H2O)n, CF3ONO2·(H2O)n is unfavourable. Mechanism for CF3ONO2 degradation in the presence and absence of water are discussed.

The impact of chlorine chemistry combined with heterogeneous N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; reactions on air quality in China

Abstract. The heterogeneous reaction of N2O5 on Cl-containing aerosols (heterogeneous N2O5 + Cl chemistry) plays a key role in chlorine activation, NOx recycling, and consequently O3 and PM2.5 formation. In this study, we use the GEOS-Chem model with additional anthropogenic and biomass burning chlorine emissions combined with updated parameterizations for the heterogeneous N2O5 + Cl chemistry (i.e., the uptake coefficient of N2O5 (γN2O5) and the ClNO2 yield (φClNO2)) to investigate the impacts of chlorine chemistry on air quality in China, the role of the heterogeneous N2O5 + Cl chemistry, and the sensitivity of air pollution formation to chlorine emissions and parameterizations for γN2O5 and φClNO2. The model simulations are evaluated against multiple observational datasets across China and show significant improvement in reproducing observations of particulate chloride, N2O5, and ClNO2 when including anthropogenic chlorine emissions and updates to the parameterization of the heterogeneous N2O5 + Cl chemistry relative to the default model. The simulations show that total tropospheric chlorine chemistry could increase annual mean maximum daily 8 h average (MDA8) O3 by up to 4.5 ppbv but decrease PM2.5 by up to 7.9 µg m−3 in China, 83 % and 90 % of which could be attributed to the effect of the heterogeneous N2O5 + Cl chemistry. The heterogeneous uptake of N2O5 on chloride-containing aerosol surfaces is an important loss pathway of N2O5 as well as an important source of O3 and hence is particularly useful in elucidating the commonly seen ozone underestimations relative to observations. The importance of chlorine chemistry largely depends on both chlorine emissions and the parameterizations for the heterogeneous N2O5 + Cl chemistry. With the additional chlorine emissions, the simulations show that annual MDA8 O3 in China could be increased by up to 3.5 ppbv. The corresponding effect on PM2.5 concentrations varies largely with regions, with an increase of up to 4.5 µg m−3 in the North China Plain but a decrease of up to 3.7 µg m−3 in the Sichuan Basin. On the other hand, even with the same chlorine emissions, the effects on MDA8 O3 and PM2.5 in China could differ by 48 % and 27 %, respectively, between different parameterizations.

OH-initiated atmospheric degradation of hydroxyalkyl hydroperoxides: mechanism, kinetics, and structure–activity relationship

Abstract. Hydroxyalkyl hydroperoxides (HHPs), formed in the reactions of Criegee intermediates (CIs) with water vapor, play essential roles in the formation of secondary organic aerosol (SOA) under atmospheric conditions. However, the transformation mechanisms for the OH-initiated oxidation of HHPs remain incompletely understood. Herein, the quantum chemical and kinetics modeling methods are applied to explore the mechanisms of the OH-initiated oxidation of the distinct HHPs (HOCH2OOH, HOCH(CH3)OOH, and HOC(CH3)2OOH) formed from the reactions of CH2OO, anti-CH3CHOO, and (CH3)2COO with water vapor. The calculations show that the dominant pathway is H-abstraction from the -OOH group in the initiation reactions of the OH radical with HOCH2OOH and HOC(CH3)2OOH. H-abstraction from the -CH group is competitive with that from the -OOH group in the reaction of the OH radical with HOCH(CH3)OOH. The barrier of H-abstraction from the -OOH group slightly increases when the number of methyl groups increase. In pristine environments, the self-reaction of the RO2 radical initially produces a tetroxide intermediate via oxygen-to-oxygen coupling, and then it decomposes into propagation and termination products through asymmetric two-step O–O bond scission, in which the rate-limiting step is the first O–O bond cleavage. The barrier height of the reactions of distinct RO2 radicals with the HO2 radical is not affected by the number of methyl substitutions. In urban environments, the reaction with O2 to form formic acid and the HO2 radical is the dominant removal pathway for the HOCH2O radical formed from the reaction of the HOCH2OO radical with NO. The β-site C–C bond scission is the dominant pathway in the dissociation of the HOCH(CH3)O and HOC(CH3)2O radicals formed from the reactions of NO with HOCH(CH3)OO and HOC(CH3)2OO radicals. These new findings deepen our understanding of the photochemical oxidation of hydroperoxides under realistic atmospheric conditions.

Effect of Different Combustion Processes on Atmospheric Nitrous Acid Formation Mechanisms: A Winter Comparative Observation in Urban, Suburban and Rural Areas of the North China Plain.

Atmospheric nitrous acid (HONO) is a dominant precursor of hydroxyl (OH) radicals, and its formation mechanisms are still controversial. Few studies have simultaneously explored effects of different combustion processes on HONO sources. Hereby, synchronous HONO measurement in urban (BJ), suburban (XH) and rural (DBT) areas with different combustion processes is performed in the North China Plain in winter. A box model is utilized to analyze HONO formation mechanisms. HONO concentration is the highest at the DBT site (2.51 ± 1.90 ppb), followed by the XH (2.18 ± 1.95 ppb) and BJ (1.17 ± 1.20 ppb) sites. Vehicle exhaust and coal combustion significantly contribute to nocturnal HONO at urban and rural sites, respectively. During a stagnant pollution period, the NO+OH reaction and combustion emissions are more crucial to HONO in urban and rural areas; meanwhile, the heterogeneous reaction of NO2 is more significant in suburban areas. Moreover, the production rate of OH from HONO photolysis is about 2 orders of magnitude higher than that from ozone photolysis. Consequently, vehicle exhaust and coal combustion can effectively emit HONO, further causing environmental pollution and health risks. It is necessary to expand the implementation of the clean energy transition policy in China, especially in areas with substantial coal combustion.

Interpretation of NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;–N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; observation via steady state in high-aerosol air mass: the impact of equilibrium coefficient in ambient conditions

Abstract. Steady-state approximation for interpreting NO3 and N2O5 has large uncertainty under complicated ambient conditions and could even produce incorrect results unconsciously. To provide an assessment and solution to the dilemma, we formulate datasets based on in situ observations to reassess the applicability of the method. In most of steady-state cases, we find a prominent discrepancy between Keq (equilibrium coefficient for reversible reactions of NO3 and N2O5) and correspondingly simulated [N2O5]/[NO2]×[NO3], especially under high-aerosol conditions in winter. This gap reveals that the accuracy of Keq has a critical impact on the steady-state analysis in polluted regions. In addition, the accuracy of γ (N2O5) derived by steady-state fit depends closely on the reactivity of NO3 (kNO3) and N2O5(kN2O5). Based on a complete set of simulations, air mass of kNO3 less than 0.01 s−1 with high aerosol and temperature higher than 10 ∘C is suggested to be the best suited for steady-state analysis of NO3–N2O5 chemistry. Instead of confirming the validity of steady state by numerical modeling for every case, this work directly provides appropriate concentration ranges for accurate steady-state approximation, with implications for choosing suited methods to interpret nighttime chemistry in high-aerosol air mass.

Reaction mechanism and kinetics of the important but neglected reaction of Hg with NO2 at low temperature

Reaction of Hg and NO2 significantly contributes to mercury removal in CO2 compression and purification unit. However, the reaction mechanism remains unclear and the product species is difficult to collect and identify experimentally. To gain insight into this reaction, we studied it using quantum chemical and kinetic calculations. A two-step reaction mechanism of Hg and NO2 was investigated. Initially, Hg could react with NO2 without a barrier to form two isomeric HgNO2 complexes (i.e. HgO2N and HgNO2) that achieve equilibrium very quickly. Then both HgNO2 isomers can add another NO2 to form a variety of HgN2O4 compounds, among which the most stable is trans-syn, syn-Hg(ONO)2. Kinetics were investigated using variational transition state theory (VTST) and RRKM theory to calculate the rate constants with respect to both temperature and pressure. The only effective reaction pathway is the one described above leading to trans-syn, syn-Hg(ONO)2. The temperature- and pressure-dependent rate constants of these relevant reactions were presented. The calculated third-order rate constant is 7.4 × 10-35 cm6 molecule-2 s−1 at 298 K and 1 atm, which is of the same order of magnitude as the experimental value. This reaction pathway shows a negative temperature dependence from 250 to 400 K, which is in accordance with the existing experimental results. The gas phase reaction mechanism of Hg with NO2 could well explain the observations in the transformation of mercury in the CO2 compression and purification unit.

Nocturnal atmospheric chemistry of NO3 and N2O5 over Changzhou in the Yangtze River Delta in China

Comprehensive observations of the nocturnal atmospheric oxidation of NO3 and N2O5 were conducted at a suburban site in Changzhou in the YRD using cavity ring-down spectroscopy (CRDS) from 27 May to 24 June, 2019. High concentrations of NO3 precursors were observed, and the nocturnal production rate of NO3 was determined to be 1.7 ± 1.2 ppbv/hr. However, the nighttime NO3 and N2O5 concentrations were relatively low, with maximum values of 17.7 and 304.7 pptv, respectively, illustrating the rapid loss of NO3 and N2O5. It was found that NO3 dominated the nighttime atmospheric oxidation, accounting for 50.7%, while O3 and OH only contributed 34.1% and 15.2%, respectively. For the reactions of NO3 with volatile organic compounds (VOCs), styrene was found to account for 60.3%, highlighting its dominant role in the NO3 reactivity. In general, the contributions of the reactions between NO3 and VOCs and the N2O5 uptake to NO3 loss were found to be about 39.5% and 60.5%, respectively, indicating that N2O5 uptake also played an important role in the loss of NO3 and N2O5, especially under the high humidity conditions in China. The formation of nitrate at night mainly originated from N2O5 uptake, and the maximum production rate of NO3− reached 6.5 ppbv/hr. The average NOx consumption rate via NO3 and N2O5 chemistry was found to be 0.4 ppbv/h, accounting for 47.9% of the total NOx removal. The predominant roles of NO3 and N2O5 in nitrate formation and NOx removal in the YRD region was highlighted in this study.

Formation mechanisms of nitrous acid (HONO) during the haze and non-haze periods in Beijing, China

As an important precursor of hydroxyl radical (OH), nitrous acid (HONO) plays a significant role in atmospheric chemistry. Here, an observation of HONO and relevant air pollutants in an urban site of Beijing from 14 to 28 April, 2017 was performed. Two distinct peaks of HONO concentrations occurred during the observation. In contrast, the concentration of particulate matter in the first period (period Ⅰ) was significantly higher than that in the second period (period Ⅱ). Comparing to HONO sources in the two periods, we found that the direct vehicle emission was an essential source of the ambient HONO during both periods at night, especially in period Ⅱ. The heterogeneous reaction of NO2 was the dominant source in period Ⅰ, while the homogeneous reaction of NO with OH was more critical source at night in period Ⅱ. In the daytime, the heterogeneous reaction of NO2 was a significant source and was confirmed by the good correlation coefficients (R2) between the unknown sources (Punknown) with NO2, PM2.5, NO2 × PM2.5 in period Ⅰ. Moreover, when solar radiation and OH radicals were considered to explore unknown sources in the daytime, the enhanced correlation of Punknown with photolysis rate of NO2 and OH (JNO2 × OH) were 0.93 in period Ⅰ, 0.95 in period Ⅱ. These excellent correlation coefficients suggested that the unknown sources released HONO highly related to the solar radiation and the variation of OH radicals.