Accurately Predicting Spatiotemporal Variations of Near-Surface Nitrous Acid (HONO) Based on a Deep Learning Approach.

Gaseous nitrous acid (HONO) is identified as a critical precursor of hydroxyl radicals (OH), influencing atmospheric oxidation capacity and the formation of secondary pollutants. However, large uncertainties persist regarding its formation and elimination mechanisms, impeding accurate simulation of HONO levels using chemical models. In this study, a deep neural network (DNN) model was established based on routine air quality data (O3, NO2, CO, and PM2.5) and meteorological parameters (temperature, relative humidity, solar zenith angle, and season) collected from four typical megacity clusters in China. The model exhibited robust performance on both the train sets [slope = 1.0, r2 = 0.94, root mean squared error (RMSE) = 0.29 ppbv] and two independent test sets (slope = 1.0, r2 = 0.79, and RMSE = 0.39 ppbv), demonstrated excellent capability in reproducing the spatiotemporal variations of HONO, and outperformed an observation-constrained box model incorporated with newly proposed HONO formation mechanisms. Nitrogen dioxide (NO2) was identified as the most impactful features for HONO prediction using the SHapely Additive exPlanation (SHAP) approach, highlighting the importance of NO2 conversion in HONO formation. The DNN model was further employed to predict the future change of HONO levels in different NOx abatement scenarios, which is expected to decrease 27-44% in summer as the result of 30-50% NOx reduction. These results suggest a dual effect brought by abatement of NOx emissions, leading to not only reduction of O3 and nitrate precursors but also decrease in HONO levels and hence primary radical production rates (PROx). In summary, this study demonstrates the feasibility of using deep learning approach to predict HONO concentrations, offering a promising supplement to traditional chemical models. Additionally, stringent NOx abatement would be beneficial for collaborative alleviation of O3 and secondary PM2.5.

Reducing Soil-Emitted Nitrous Acid as a Feasible Strategy for Tackling Ozone Pollution.

Severe ozone (O3) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NOx) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions (FHONO) enhanced by nitrogen fertilizer application at an agricultural site. The observed FHONO significantly improves model performance in predicting atmospheric HONO and leads to regional O3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NOx, by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O3 pollution and climate change.

Phase State Regulates Photochemical HONO Production from NaNO3/Dicarboxylic Acid Mixtures.

Field observations of daytime HONO source strengths have not been well explained by laboratory measurements and model predictions up until now. More efforts are urgently needed to fill the knowledge gaps concerning how environmental factors, especially relative humidity (RH), affect particulate nitrate photolysis. In this work, two critical attributes for atmospheric particles, i.e., phase state and bulk-phase acidity, both influenced by ambient RH, were focused to illuminate the key regulators for reactive nitrogen production from typical internally mixed systems, i.e., NaNO3 and dicarboxylic acid (DCA) mixtures. The dissolution of only few oxalic acid (OA) crystals resulted in a remarkable 50-fold increase in HONO production compared to pure nitrate photolysis at 85% RH. Furthermore, the HONO production rates (PHONO) increased by about 1 order of magnitude as RH rose from <5% to 95%, initially exhibiting an almost linear dependence on the amount of surface absorbed water and subsequently showing a substantial increase in PHONO once nitrate deliquescence occurred at approximately 75% RH. NaNO3/malonic acid (MA) and NaNO3/succinic acid (SA) mixtures exhibited similar phase state effects on the photochemical HONO production. These results offer a new perspective on how aerosol physicochemical properties influence particulate nitrate photolysis in the atmosphere.

Validating HONO as an Intermediate Tracer of the External Cycling of Reactive Nitrogen in the Background Atmosphere.

In the urban atmosphere, nitrogen oxide (NOx═NO + NO2)-related reactions dominate the formation of nitrous acid (HONO). Here, we validated an external cycling route of HONO and NOx, i.e., formation of HONO resulting from precursors other than NOx, in the background atmosphere. A chemical budget closure experiment of HONO and NOx was conducted at a background site on the Tibetan Plateau and provided direct evidence of the external cycling. An external daytime HONO source of 100 pptv h-1 was determined. Both soil emissions and photolysis of nitrate on ambient surfaces constituted likely candidate mechanisms characterizing this external source. The external source dominated the chemical production of NOx with HONO as an intermediate tracer. The OH production was doubled as a result of the external cycling. A high HONO/NOx ratio (0.31 ± 0.06) during the daytime was deduced as a sufficient condition for the external cycling. Literature review suggested the prevalence of high HONO/NOx ratios in various background environments, e.g., polar regions, pristine mountains, and forests. Our analysis validates the prevalence of external cycling in general background atmosphere and highlights the promotional role of external cycling regarding the atmospheric oxidative capacity.

Measurements of Hydroxyl Radical Concentrations during Indoor Cooking Events: Evidence of an Unmeasured Photolytic Source of Radicals.

The hydroxyl radical (OH) is the dominant oxidant in the outdoor environment, controlling the lifetimes of volatile organic compounds (VOCs) and contributing to the growth of secondary organic aerosols. Despite its importance outdoors, there have been relatively few measurements of the OH radical in indoor environments. During the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign, elevated concentrations of OH were observed near a window during cooking events, in addition to elevated mixing ratios of nitrous acid (HONO), VOCs, and nitrogen oxides (NOX). Particularly high concentrations were measured during the preparation of a traditional American Thanksgiving dinner, which required the use of a gas stove and oven almost continually for 6 h. A zero-dimensional chemical model underpredicted the measured OH concentrations even during periods when direct sunlight illuminated the area near the window, which increases the rate of OH production by photolysis of HONO. Interferences with measurements of nitrogen dioxide (NO2) and ozone (O3) suggest that unmeasured photolytic VOCs were emitted during cooking events. The addition of a VOC that photolyzes to produce peroxy radicals (RO2), similar to pyruvic acid, into the model results in better agreement with the OH measurements. These results highlight our incomplete understanding of the nature of oxidation in indoor environments.

HONO Measurement by Catalytic Conversion to NO on Nafion Surfaces.

A selective catalytic converter has been developed to quantify nitrous acid (HONO), a photochemical precursor to NO and OH radicals that drives the formation of ozone and other pollutants in the troposphere. The converter is made from a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) that was found to convert HONO to NO with unity yield under specific conditions. When coupled to a commercially available NOx (=NO + NO2) chemiluminescence (CL) analyzer, the system measures HONO with a limit of detection as low as 64 parts-per-trillion (ppt) (1 min average) in addition to NOx. The converter is selective for HONO when tested against other common gas-phase reactive nitrogen species, although loss of O3 on Nafion is a potential interference. The sensitivity and selectivity of this method allow for accurate measurement of atmospherically relevant concentrations of HONO. This was demonstrated by good agreement between HONO measurements made with the Nafion-CL method and those made with chemical ionization mass spectrometry in a simulation chamber and in indoor air. The observed reactivity of HONO on Nafion also has significant implications for the accuracy of CL NOx analyzers that use Nafion to remove water from sampling lines.

Discovery of a Missing Link: First Observation of the HONO-Water Complex.

The still unexplained daytime HONO concentration in the Earth's atmosphere and the impact of water on the HONO chemistry have been a mystery for decades. Several pathways and many modeling methods have failed to reproduce the atmospheric measurements. We reveal in this study the first spectroscopic observation and characterization of the complex of HONO with water observed through its rotational signature. Under the experimental conditions, HONO-water is stable, particularly straightforward to form, and features intense absorption signals. This could explain both the influence of water on the HONO chemistry and the missing HONO sources, as well as the missing contribution of many other molecules of atmospheric relevance that skew the accuracy of field measurements and the full account of partitioning species in the atmosphere.

Reactive Nitrogen Hotspots Related to Microscale Heterogeneity in Biological Soil Crusts.

Biocrusts covering drylands account for major fractions of terrestrial biological nitrogen fixation and release large amounts of gaseous reactive nitrogen (Nr) as nitrous acid (HONO) and nitric oxide (NO). Recent investigations suggested that aerobic and anaerobic microbial nitrogen transformations occur simultaneously upon desiccation of biocrusts, but the spatio-temporal distribution of seemingly contradictory processes remained unclear. Here, we explore small-scale gradients in chemical concentrations related to structural characteristics and organism distribution. X-ray microtomography and fluorescence microscopy revealed mixed pore size structures, where photoautotrophs and cyanobacterial polysaccharides clustered irregularly in the uppermost millimeter. Microsensor measurements showed strong gradients of pH, oxygen, and nitrite, nitrate, and ammonium ion concentrations at micrometer scales in both vertical and lateral directions. Initial oxygen saturation was mostly low (∼30%) at full water holding capacity, suggesting widely anoxic conditions, and increased rapidly upon desiccation. Nitrite concentrations (∼6 to 800 µM) and pH values (∼6.5 to 9.5) were highest around 70% WHC. During further desiccation they decreased, while emissions of HONO and NO increased, reaching maximum values around 20% WHC. Our results illustrate simultaneous, spatially separated aerobic and anaerobic nitrogen transformations, which are critical for Nr emissions, but might be impacted by future global change and land management.

Gas-Phase Nitrous Acid (HONO) Is Controlled by Surface Interactions of Adsorbed Nitrite (NO2-) on Common Indoor Material Surfaces.

Nitrous acid (HONO) is a household pollutant exhibiting adverse health effects and a major source of indoor OH radicals under a variety of lighting conditions. The present study focuses on gas-phase HONO and condensed-phase nitrite and nitrate formation on indoor surface thin films following heterogeneous hydrolysis of NO2, in the presence and absence of light, and nitrate (NO3-) photochemistry. These thin films are composed of common building materials including zeolite, kaolinite, painted walls, and cement. Gas-phase HONO is measured using an incoherent broadband cavity-enhanced ultraviolet absorption spectrometer (IBBCEAS), whereby condensed-phase products, adsorbed nitrite and nitrate, are quantified using ion chromatography. All of the surface materials used in this study can store nitrogen oxides as nitrate, but only thin films of zeolite and cement can act as condensed-phase nitrite reservoirs. For both the photo-enhanced heterogeneous hydrolysis of NO2 and nitrate photochemistry, the amount of HONO produced depends on the material surface. For zeolite and cement, little HONO is produced, whereas HONO is the major product from kaolinite and painted wall surfaces. An important result of this study is that surface interactions of adsorbed nitrite are key to HONO formation, and the stronger the interaction of nitrite with the surface, the less gas-phase HONO produced.

The influence of a single water molecule on the reaction of BrO + HONO.

Quantum chemical computations and transition state theory are employed to systematically research the influence of a single molecule water on the BrO + HONO reaction. Two distinct reactions, namely BrO + trans-HONO and BrO + cis-HONO are explored for the reaction in the absence of water, which is mainly decided by the configuration of HONO. With introduction a single water molecule to the reaction, the rate coefficient of the channel starting from BrO + cis-HONO and BrO + trans-HONO are 2.43 × 10-19 and 5.22 × 10-22 cm3 molecule-1 s-1, which is larger than the reaction in the absence of water. For further comprehend the impact of water on the BrO + HONO reaction, it is necessary to compute the effective rate coefficient by taking into account the concentration of water. The water-assisted effective rate coefficients for the BrO + HONO reaction are smaller than that the reaction in the absence of water. The reaction of BrO with cis-HONO is feasible both in absence and existence of water.

Obtaining Hexoses from Chitosan through Depolymerization with Nitrous Acid.

BACKGROUND AND OBJECTIVE: Residues from shrimp farming have a great potential for sugar production and the production of derivatives for the low-carbon chemical industry. Obtainment of bioactives from chitosan has been extensively investigated using different methodologies. The purpose of this work was to study the chitosan depolymerization reaction aiming at the production of monomers without the use of additional enzymes or mineral acids. MATERIALS AND METHODS: In this work, we systematically study the effect of sodium nitrite concentration and reaction conditions (pH and temperature ranges) with acetic acid as the solvent on the chitosan depolymerization reaction aiming at the production of monomers, specifically 2,5- anhydromannose, without the use of additional enzymes or mineral acids. RESULTS: The results indicate that only a small range of reaction conditions and nitrite concentrations allow for obtaining the monomer, while in most combinations of these parameters, oligomers are obtained. We found that the temperature decisively affects the reaction yield, with the attainment of 2,5-anhydromannose favored at lower temperatures. CONCLUSION: The method proved to be simple and easy to perform allowing to obtain 2,5- anhydromannose with the use of low-cost reagents. This monomer can be converted into several derivatives for industrial application (5-Hydroxymethylfurfural, ethanol, etc.).

Reducing-End Functionalization of 2,5-Anhydro-d-mannofuranose-Linked Chitooligosaccharides by Dioxyamine: Synthesis and Characterization.

The nitrous acid depolymerization of chitosan enables the synthesis of singular chitosan oligosaccharides (COS) since their reducing-end unit is composed of 2,5-anhydro-d-mannofuranose (amf). In the present study, we describe a chemical method for the reducing-end conjugation of COS-amf by the commercially available dioxyamine O,O'-1,3-propanediylbishydroxylamine in high mass yields. The chemical structure of resulting dioxyamine-linked COS-amf synthesized by both oximation and reductive amination ways were fully characterized by 1H- and 13C-NMR spectroscopies and MALDI-TOF mass spectrometry. The coupling of chemically attractive linkers such as dioxyamines at the reducing end of COS-amf forms a relevant strategy for the development of advanced functional COS-based conjugates.

Oligosaccharides mapping of nitrous acid degraded heparin through UHPLC-HILIC/WAX-MS.

Building blocks characterization is a significant approach for understanding the molecular structure of heparin and its derivatives. Nitrous acid (HONO) depolymerization of heparin generates oligosaccharides that maintain the epimerization conformation on C5 of the uronic acids, reflecting the authentic structure of the parental chain. HONO treatment at pH 1.5 selectively cleaves the bond between N-sulfated glucosamine and hexuronic acid, resulting mainly disaccharides, as well as tetra-, tri-, and mono-saccharides. The tetrasaccharides are derived from the structure of N-acetylated domains while tri-, and mono-saccharides are derived from the reducing or the non-reducing end of the heparin chain. The resulted oligosaccharides were separated and analyzed using a UHPLC-HILIC/WAX-MS method. We succeeded in the identification of 19 tetrasaccharides, 19 trisaccharides and 4 monosaccharides species, majority of which is structurally characterized. By comparing the theoretical possibilities and actual occurrence of the well-characterized tetrasaccharides, we demonstrated that the biosynthesis of heparin is a systematic process.

Parametrization and Molecular Dynamics Simulations of Nitrogen Oxyanions and Oxyacids for Applications in Atmospheric and Biomolecular Sciences.

Nitrogen oxyanions and oxyacids are important agents in atmospheric chemistry and medical biology. Although their chemical behavior in solution is relatively well understood, they may behave very differently at the water/air interface of atmospheric aerosols or at the membrane/water interface of cells. Here, we developed a fully classical model for molecular dynamics simulations of NO3-, NO2-, HNO3, and HNO2 in the framework of the GROMOS 53A6 and 54A7 force field versions. The model successfully accounted for the poorly structured solvation shell and ion pairing tendency of NO3-. Accurate pure-liquid properties and hydration free energies were obtained for the oxyacids. Simulations at the water/air interface showed a local enrichment of HNO3 and depletion of NO3-. The effect was discussed in light of earlier spectroscopic data and ab initio calculations, suggesting that HNO3 behaves as a weaker acid at the surface of water. Our model will hopefully allow for efficient and accurate simulations of nitrogen oxyanions and oxyacids in solution and at microheterogeneous interface environments.

Using nitrous acid-modified de Man, Rogosa, and Sharpe medium to selectively isolate and culture lactic acid bacteria from dairy foods.

Nitrous acid was used to modify traditional de Man, Rogosa, Sharpe medium to evaluate whether the addition of sodium nitrite to MRS medium could improve the rate of growth and density of various lactic acid bacteria and nontarget species. Yogurt and Cheddar cheese were inoculated with individual bacterial species followed by the recovery and enumeration of the species using the pour plate method to compare the sensitivity between nitrous acid-modified MRS (mMRS) and traditional MRS. Lactobacillus delbrueckii ssp. bulgaricus were recovered at significantly higher counts from cheese in nitrous acid mMRS than MRS, whereas no significant difference was observed for other species and food systems. Growth curves were also generated for multiple lactic acid bacteria as well as nonstarters in both mMRS and MRS to measure the selectivity of nitrous acid mMRS. The selectivity evaluation of nitrous acid mMRS demonstrated that 5 of the tested lactic acid bacterial species (Bifidobacterium longum, Streptococcus salivarius, Lactococcus lactis, Lactobacillus acidophilus, and Lactobacillus delbrueckii ssp. bulgaricus) grew to significantly higher densities more rapidly in mMRS broth than in traditional MRS. Nontarget bacteria Enterococcus faecalis and Bacillus cereus revealed a more prolific growth rate and higher optical density readings in traditional MRS compared with mMRS. It was determined that nitrous acid mMRS is a viable alternative medium for culturing selected lactic acid bacteria, and offers an improved formulation of MRS for use in standard evaluation methods and optimization of probiotic and other dairy cultures.

[Effect of Free Nitrous Acid on the Activity of Nitrifying Bacteria in Different Sludge Concentrations Under Anoxic Conditions].

This study investigated the inhibitory effect of free nitrous acid (FNA) on the activity of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) under anoxic conditions with different mixed liquid suspended solids (MLSS). Sequencing batch reactors were used to study the changes in the activity of AOB and NOB in nitrifying activated sludge based on four different MLSS (8398, 11254, 15998, and 19637 mg·L-1), after treatment, under anoxic conditions with FNA (at an initial concentration of 1.3 mg·L-1) for 48 h. The results showed that the pH increased by approximately 0.9, but the concentration of NO2--N did not decrease significantly. With over-aeration, the concentration of NH4+-N gradually degraded to 0 mg·L-1, and the removal rate of NH4+-N gradually increased to a maximum of 4.4-6.8 mg·(L·h)-1 which time used was shorter with the increase of the inhibition MLSS. The nitrite accumulation rate was more than 92% when the sludge concentration was 8398, 11254, 15998, and 19637 mg·L-1 and with over-aeration for 0-396 h, 0-396 h, 0-372 h, and 0-168 h, respectively. When aerated for 468 h, 468 h, 444 h, and 264 h, the NO2--N concentration and NAR decreased to 0, and NO3--N concentrations increased to their highest with the values of 42.6, 49.9, 42.9, and 47.9 mg·L-1 respectively.

Insight into the distribution of amino groups along the chain of chemically deacetylated hyaluronan.

Deacetylated hyaluronan (daHA) containing reactive free amino groups is an important intermediate for further modification. Comparing direct and indirect NMR and HPLC to characterize the degree of HA deacetylation (DD), direct NMR approach using area ratio of anomeric CH and CH-NH2 groups was the most precise one. To describe the substitution pattern, daHA was selectively cleaved by nitrous acid generated in situ or hyaluronan lyase from Streptococcus pneumoniae. The resulting oligomers were identified by LC-ESI-MS. The experimental distribution of these oligomers was compared with theoretically expected random oligomer distribution. Independently on the starting HA molecular weight and deacetylation conditions, the experimental data differed from the random distribution model and suggested that deacetylation of certain N-acetyl-d-glucosamine had reduced the probability of deacetylation at the neighbouring disaccharide. This phenomenon was explained by conformational changes of HA caused by intra- and intermolecular interactions between positively charged amino and negatively charged carboxylic groups.

Microscale pH variations during drying of soils and desert biocrusts affect HONO and NH3 emissions.

Microscale interactions in soil may give rise to highly localised conditions that disproportionally affect soil nitrogen transformations. We report mechanistic modelling of coupled biotic and abiotic processes during drying of soil surfaces and biocrusts. The model links localised microbial activity with pH variations within thin aqueous films that jointly enhance emissions of nitrous acid (HONO) and ammonia (NH3) during soil drying well above what would be predicted from mean hydration conditions and bulk soil pH. We compared model predictions with case studies in which reactive nitrogen gaseous fluxes from drying biocrusts were measured. Soil and biocrust drying rates affect HONO and NH3 emission dynamics. Additionally, we predict strong effects of atmospheric NH3 levels on reactive nitrogen gas losses. Laboratory measurements confirm the onset of microscale pH localisation and highlight the critical role of micro-environments in the resulting biogeochemical fluxes from terrestrial ecosystems.

A single water molecule accelerating the atmospheric reaction of HONO with ClO.

The role of a single water molecule on the atmospheric reaction of HONO + ClO is systematically investigated employing quantum chemical calculation combined with harmonic transition state theory. Two reaction pathways, cis-HONO + ClO and trans-HONO + ClO, are identified for the naked reaction, which depends on the configurations of HONO. When adding a single water molecule to this reaction, the rate constants of cis-HONO + ClO and trans-HONO + ClO pathways are 7.97 × 10-21 and 2.29 × 10-17 cm3 molecule-1 s-1, respectively, larger than the corresponding naked reaction. To further understand the role of water on the HONO + ClO reaction, it is necessary to calculate the effective rate constant by considering the concentration of water. It shows that the effective rate constants of water-assisted cis-HONO + ClO pathway are much smaller than those of the naked reaction, whereas the presence of water accelerates the trans-HONO + ClO at room temperature. This study demonstrates that water has a positive role in the pathway of trans-HONO + ClO by modifying the stabilities of reactant complexes and transition states through the hydrogen bond formation, which contributes to the sink of atmospheric HONO. In addition, the kinetic branching ratio indicates that the favorable reaction is the trans-HONO + ClO instead of the cis-HONO + ClO pathway, in contrast to the naked reaction. These results reveal the importance of water in the evaluation of the fate of active species in the atmosphere. Graphical Abstract.

Photochemical reaction kinetics and mechanistic investigations of nitrous acid with sulfamethazine in tropospheric water.

Nitrous acid (HONO) is an important atmospheric pollutant that can strongly absorb ultraviolet irradiation in the region of 300-400 nm, as previously reported. Since the solar irradiance that reaches the surface of the earth has wavelengths greater than 290 nm, the photodissociation of HONO is considered the major method of hydroxyl radical formation in the troposphere. Thus, the photoinduced chemical reactivity of HONO is important. The present work investigated the reaction mechanism and kinetic parameters of HONO and sulfamethazine by using a laser flash photolysis technique and liquid chromatography-mass spectrometry. The results indicated that the sulfamethazine degradation rate was influenced by the HONO concentration and the initial concentration of sulfamethazine. Hydroxyl radicals derived from the photolysis of HONO attacked the aromatic ring of sulfamethazine to form sulfamethazine-OH adducts with a second-order rate constant of (3.8 ± 0.3) × 109 L mol-1 s-1. This intermediate would then react with HO· and oxygen molecules. The reaction rate constants of sulfamethazine-OH adducts with oxygen are (1.3 ± 0.1) × 107 L mol-1 s-1. The generation of sulfanilic acid and pyrimidine implies that the breaking down of S-N bonds of sulfamethazine and its HO adducts probably occur at the same time.