Nitric and nitrous acid formation in plasma-treated water: Decisive role of nitrogen oxides (NOx=1-3).

Nitrogen fixation using low-temperature plasma, particularly in relation to plasma-treated water (PTW) and its chemical and physical properties, has received a renewed research focus. Dissolving highly concentrated nitrogen oxides (NOx = 1-3) generated by air discharge into water results in the formation of two aqueous oxiacids (nitrous and nitric acids; HNOy = 2,3) and their conjugates (nitrate and nitrite ions; NOy-). Nonlinear formation of these species in PTW with respect to plasma conditions has been observed; however, the significance of the time-varying NOx on this nonlinearity has not yet been thoroughly investigated. Here, we demonstrate real-time observations of HNOy/NOy- as well as NOx production in a surface dielectric barrier discharge reactor containing distilled water. Synchronized two optical absorption spectroscopy systems were employed to simultaneously measure gas-phase NOx and liquid-phase HNOy/NOy- in the plasma reactor operated under different oxygen contents of 5, 20, and 50%. Our results showed that reducing the oxygen content in the reactor accelerated the chemical transition from O3 and NO3 to NO1,2, leading to a predominance of nitrite in PTW. Specifically, the NO3-rich period was extended with increasing O2 content, resulting in the production of nitrate-dominant PTW at low pH levels. Our findings highlight the potential for the selective generation of HNOy/NOy- in PTW through the active and passive control of NOx in a plasma reactor. The direct, real-time observation of NOx-HNOy/NOy- conversion presented here has potential for improving the control and optimization of PTW, thereby enhancing its applicability.

Individual evaluation of nitrite and free nitrous acid inhibition on anammox activity.

The anammox reaction simultaneously utilizes ammonia and nitrite as substrates; however, high nitrite concentrations act as strong inhibitors of the reaction. In this study, inhibition by NO2- and free nitrous acid (FNA) was separately evaluated in continuous feeding tests using different biomass carriers. The influent NO2- concentration was increased under pH 7.6, where FNA is less likely to affect anammox activity. A continuous test using polyethylene glycol (PEG) gel carriers containing immobilized anammox bacteria showed that the inhibition ratio was 13% when the NO2--N concentration in the reactor was 350 mg L-1 (FNA ≤0.06 mg L-1). The relationship between NO2- concentration in the reactor and inhibition ratio increased linearly. Evaluation of the inhibitory effect of FNA by increasing the influent NO2- concentration at pH 6.4, where FNA is easily formed, demonstrated that the relationship between FNA and inhibition ratio could be fitted to a sigmoid curve, and the 50% inhibitory concentration (IC50) of FNA was 0.88 mg L-1. A similar test performed using polyvinyl alcohol carriers containing anammox bacteria on their surface showed the same trend as the PEG gel carriers, with the IC50 for FNA at 0.70 mg L-1. These results indicate that the inhibitory effect of FNA on anammox activity was greater than that of NO2-. The evaluation of these two factors helped identify important operational indicators of the stable application of anammox processes.

Quantifying HONO Production from Nitrate Photolysis in a Polluted Atmosphere.

The photolysis of particulate nitrate (pNO3-) has been suggested to be an important source of nitrous acid (HONO) in the troposphere. However, determining the photolysis rate constant of pNO3- (jpNO3-) suffers from high uncertainty. Prior laboratory measurements of jpNO3- using aerosol filters have been complicated by the "shadow effect"─a phenomenon of light extinction within aerosol layers that potentially skews these measurements. We developed a method to correct the shadow effect on the photolysis rate constant of pNO3- for HONO production (jpNO3- â†’ HONO) using aerosol filters with identical chemical compositions but different aerosol loadings. We applied the method to quantify jpNO3- â†’ HONO over the North China Plain (NCP) during the winter haze period. After correcting for the shadow effect, the normalized average jpNO3- â†’ HONO at 5 °C increased from 5.89 × 10-6 s-1 to 1.72 × 10-5 s-1. The jpNO3- â†’ HONO decreased with increasing pH and nitrate proportions in PM2.5 and had no correlation with nitrate concentrations. A parametrization for jpNO3- â†’ HONO was developed for model simulation of HONO production in NCP and similar environments.

Dynamic pH regulation drives Nitrosomonas for high-rate stable acidic partial nitritation.

How to intensify the ammonia oxidation rate (AOR) is still a bottleneck impeding the technology development for the innovative acidic partial nitritation because the eosinophilic ammonia-oxidizing bacteria (AOB), such as Nitrosoglobus or Nitrosospira, were inhibited by the high-level free nitrous acid (FNA) accumulation in acidic environments. In this study, an innovative approach of dynamic acidic pH regulation control strategy was proposed to realize high-rate acidic partial nitritation driven by common AOB genus Nitrosomonas. The acidic partial nitrification process was carried out in a laboratory-scale sequencing batch moving bed biofilm reactor (SBMBBR) for long-term (700 days) to track the effect of dynamic acidic pH on nitrifying bacterial activity. The results indicated that the influent NH4+-N concentration was about 100 mg/L, the nitrite accumulation ratio was exceeding 90%, and the maximum AOR can reach 14.5 ± 2.6 mg N L-1h-1. Although the half-saturation inhibition constant of NOB (KI_FNA(AOB)) reached 0.37 ± 0.10 mg HNO2N/L and showed extreme adaptability in FNA, the inactivation effect of FNA (6.1 mg HNO2N/L) for NOB was much greater than that of AOB, with inactivation rates of 0.61 ± 0.08 h-1 and 0.06 ± 0.01 h-1, respectively. The effluent pH was gradually reduced to 4.5 by ammonia oxidation process and the periodic FNA concentration reached 6.5 mg HNO2N/L to inactivate nitrite-oxidizing bacteria (NOB) without negatively affecting Nitrosomonas during long-term operation. This result provides new insights for the future implementation of high-rate stabilized acidic partial nitritation by Nitrosomonas.

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.

Observation-Based Diagnostics of Reactive Nitrogen Recycling through HONO Heterogenous Production: Divergent Implications for Ozone Production and Emission Control.

Understanding of nitrous acid (HONO) production is crucial to photochemical studies, especially in polluted environments like eastern China. In-situ measurements of gaseous and particulate compositions were conducted at a rural coastal site during the 2018 spring Ozone Photochemistry and Export from China Experiment (OPECE). This data set was applied to investigate the recycling of reactive nitrogen through daytime heterogeneous HONO production. Although HONO levels increase during agricultural burning, analysis of the observation data does not indicate more efficient HONO production by agricultural burning aerosols than other anthropogenic aerosols. Box and 1-D modeling analyses reveal the intrinsic relationships between nitrogen dioxide (NO2), particulate nitrate (pNO3), and nitric acid (HNO3), resulting in comparable agreement between observed and simulated HONO concentrations with any one of the three heterogeneous HONO production mechanisms, photosensitized NO2 conversion on aerosols, photolysis of pNO3, and conversion from HNO3. This finding underscores the uncertainties in the mechanistic understanding and quantitative parametrizations of daytime heterogeneous HONO production pathways. Furthermore, the implications for reactive nitrogen recycling, ozone (O3) production, and O3 control strategies vary greatly depending on the HONO production mechanism. On a regional scale, the conversion of HONO from pNO3 can drastically enhance O3 production, while the conversion from NO2 can reduce O3 sensitivity to NOx changes in polluted eastern China.

The observation of atmospheric HONO by wet-rotating-denuder ion chromatograph in a coastal city: Performance and influencing factors.

Due to the significance of atmospheric HONO as a reservoir for radicals and the presence of substantial unknown sources of HONO, there is a pressing need for accurate and consistent measurement of its concentration. In this study, we compared the measurements obtained from the monitor for aerosols and gases in ambient air (MARGA) based on wet chemical method with those from the incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) based on optical method to assess the suitability of the MARGA instrument for accurate HONO detection. The diurnal patterns obtained by the two instruments are similar, with peaks at 8 a.m. and lows at 5 p.m. Over the course of the observation period, it was often observed that HONO concentrations recorded by the MARGA instrument consistently exceeded those obtained through the IBBCEAS technique, accounting for approximately 91.33% of the total observation time. Throughout the entire observation period, the R2 value between the two instruments was 0.49, indicating relatively good correlation. However, with a slope of only 0.27, it suggests poor agreement between the two instruments. Furthermore, the R2 and slopes between the two instruments vary with the seasons and day-night. The larger the quartile values of NO2, NH3, and BC, the greater the slopes of both MARGA and IBBCEAS instruments, and the higher the concentrations of NO2, NH3, and BC (indicator of semivolatile oxidizable hydrocarbons), the greater the differences between the two instruments, all indicating that NH3 may promote the reaction of NO2 with semivolatile oxidizable hydrocarbons to produce HONO. The O3 with its strong oxidizing properties may cause underestimation in the MARGA instrument by oxidizing NO2- to NO3- in the absorbing solution. It is challenging to derive a universal correction formula due to the interference of various chemical substances. Hence, MARGA should not be used for HONO research in the future.

Potential of free nitrous acid (FNA) for sludge treatment and resource recovery from waste activated sludge: A review.

The escalating production of waste activated sludge (WAS) presents significant challenges to wastewater treatment plants (WWTPs). Free nitrous acid (FNA), known for its biocidal effect, has gained a growing focus on sludge dewatering, sludge reduction, and resource recovery from WAS due to its eco-friendly and cost-effective properties. Nevertheless, there have been no attempts made to systematically summarize or critically analyze the application of FNA in enhancing treatment and resource utilization of sludge. In this paper, we provided an overview of the current understanding regarding the application potential and influencing factors of FNA in sludge treatment, with a specific focus on enhancing sludge dewatering efficiency and reducing volume. To foster resource development from sludge, various techniques based on FNA have recently been proposed, which were comprehensively reviewed with the corresponding mechanisms meticulously discussed. The results showed that the chemical oxidation and interaction with microorganisms of FNA played the core role in improving resource utilization. Furthermore, current challenges and future prospects of the FNA-based applications were outlined. It is expected that this review can refine the theoretical framework of FNA-based processes, providing a theoretical foundation and technical guidance for the large-scale demonstration of FNA.

Impacts of free nitrous acid on stabilizing food waste and sewage sludge for anaerobic digestion.

This work investigated the effectiveness of free nitrous acid (FNA) in enhancing organic waste solubilization to improve biogas production in anaerobic digestion (AD). The results indicated that FNA pretreatment can enhance soluble organic content and control H2S odor in tested organic wastes, including food waste, sewage sludge, and their combination. However, a significant decrease (>50 %) in FNA concentration was found in the reactors, possibly due to denitrifier-driven NO2- consumption. Biochemical methane potential (BMP) tests showed a 25 ± 8 % enhancement in CH4 production in the reactors fed with mixed substrate pretreated with 2.9 mg FNA-N/L. However, the presence of NO2- (325.6-2368.0 mg N/L) in some BMP reactors, due to carryover from FNA pretreatment, adversely affected CH4 production (>55 %) and prolonged lag time (>4.2 times). These findings are valuable for researchers and practitioners in waste management, offering insights for implementing FNA pretreatment to enhance the biodegradability of organic wastes in AD.

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.

Favipiravir biotransformation by a side-stream partial nitritation sludge: Transformation mechanisms, pathways and toxicity evaluation.

Information on biotransformation of antivirals in the side-stream partial nitritation (PN) process was limited. In this study, a side-stream PN sludge was adopted to investigate favipiravir biotransformation under controlled ammonium and pH levels. Results showed that free nitrous acid (FNA) was an important factor that inhibited ammonia oxidation and the cometabolic biodegradation of favipiravir induced by ammonia oxidizing bacteria (AOB). The removal efficiency of favipiravir reached 12.6% and 35.0% within 6 days at the average FNA concentrations of 0.07 and 0.02 mg-N L-1, respectively. AOB-induced cometabolism was the sole contributing mechanism to favipiravir removal, excluding AOB-induced metabolism and heterotrophic bacteria-induced biodegradation. The growth of Escherichia coli was inhibited by favipiravir, while the AOB-induced cometabolism facilitated the alleviation of the antimicrobial activities with the formed transformation products. The biotransformation pathways were proposed based on the roughly identified structures of transformation products, which mainly involved hydroxylation, nitration, dehydrogenation and covalent bond breaking under enzymatic conditions. The findings would provide insights on enriching AOB abundance and enhancing AOB-induced cometabolism under FNA stress when targeting higher removal of antivirals during the side-stream wastewater treatment processes.

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx.

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.

Low alkalinity, free ammonia, and free nitrous acid cooperatively stabilize partial nitrification under excessive aeration condition.

In this study, a lab-scale sequencing batch reactor (SBR) was established to assess the feasibility of maintaining partial nitrification in treating high-ammonium wastewater under the condition of excessive aeration (1 L/min, 1289 min). Results showed that the average ammonia (NH4+-N) removal efficiency (ARE) was 93.4 ± 2.7% and the average nitrite accumulation ratio (NAR) was 90.7 ± 2.8% during 15-50 d. In a typical cycle, free ammonia (FA) and free nitrous acid (FNA) alternately inhibited the activity of nitrite oxidizing bacteria (NOB), while low alkalinity limited further nitrification at even excessive aeration. Metagenomic analysis revealed that the relative abundance of Nitrosomonas, as the ammonia oxidizing bacteria (AOB), was up to 1.61%, while NOB were not detected. Taken together, this study indicated partial nitrification was almost not affected by excessive aeration, demonstrating its robustness depending on the cooperative strategy of the low alkalinity limitation and the inhibition of FA and FNA.

Modeling Free Nitrous Acid Inhibition on the Removal of Nitrogen and Atenolol during Sidestream Partial Nitritation Processes.

Sidestream serves as an important reservoir collecting pharmaceuticals from sludge. However, the knowledge on sidestream pharmaceutical removal is still insufficient. In this work, atenolol biodegradation during sidestream partial nitritation (PN) processes characterized by high free nitrous acid (FNA) accumulation was modeled. To describe the FNA inhibition on ammonia oxidation and atenolol removal, Vadivelu-type and Hellinga-type inhibition kinetics were introduced into the model framework. Four inhibitory parameters along with four biodegradation kinetic parameters were calibrated and validated separately with eight sets of batch experimental data and 60 days' PN reactor operational data. The developed model could accurately reproduce the dynamics of nitrogen and atenolol. The model prediction further revealed that atenolol biodegradation efficiencies by ammonia-oxidizing bacteria (AOB)-induced cometabolism, AOB-induced metabolism, and heterotrophic bacteria-induced biodegradation were 0, ∼ 60, and ∼35% in the absence of ammonium and FNA; ∼ 14, ∼ 29, and ∼28% at 0.03 mg-N L-1 FNA; and 7, 15, and 5% at 0.19 mg-N L-1 FNA. Model simulation showed that the nitritation efficiency of ∼99% and atenolol removal efficiency of 57.5% in the PN process could be achieved simultaneously by controlling pH at 8.5, while 89.2% total nitrogen and 57.1% atenolol were removed to the maximum at pH of 7.0 in PN coupling with the anammox process. The pH-based operational strategy to regulate FNA levels was mathematically demonstrated to be effective for achieving the simultaneous removal of nitrogen and atenolol in PN-based sidestream processes.

Revisiting the Ultraviolet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget.

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultraviolet (UV) absorption cross section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. In this study, we performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometry and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultrapure standard HONO source were utilized for laboratory intercomparisons. Results show a consistent ∼22% negative bias in measurements of the IBBCEAS compared with a SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation of the absolute UV absorption cross sections through careful analysis of possible uncertainties. We quantified the absorption cross section of gaseous HONO (360-390 nm) utilizing a custom-built IBBCEAS instrument, and the results were found to be 22-34% lower than the previously published absorption cross sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross section of HONO and the re-evaluation of the previously reported HONO budgets.

Limitations of a biokinetic model to predict the seasonal variations of nitrous oxide emissions from a full-scale wastewater treatment plant.

A biokinetic model based on BioWin's Activated Sludge Digestion Model (ASDM) coupled with a nitrous oxide (N2O) model was setup and calibrated for a full-scale wastewater treatment plant (WWTP) Amsterdam West, in the Netherlands. The model was calibrated using one year of continuous data to predict the seasonal variations of N2O emissions in the gaseous phase. This, according to our best knowledge, is the most complete full-scale data set used to date for this purpose. The results obtained suggest that the currently available biokinetic model predicted the winter, summer, and autumn N2O emissions well but failed to satisfactorily simulate the spring peak. During the calibration process, it was found that the nitrifier denitrification pathway could explain the observed emissions during all seasons while a combination of the nitrifier denitrification and incomplete heterotrophic denitrification pathways seemed to be dominant during the emissions peak observed during the spring season. Specifically, kinetic parameters related to free nitrous acid (FNA) displayed significant sensitivity leading to increased N2O production. The obtained values of two kinetic parameters, i.e., the FNA half-saturation during ammonia oxidising bacteria (AOB) denitrification and the FNA inhibition concentration related to heterotrophic denitrification, suggested a strong influence of the FNA bulk concentration on the N2O emissions and the observed seasonal variations. Based on the suboptimal performance and limitations of the biokinetic model, further research is needed to better understand the biochemical processes behind the seasonal peak and the influence of FNA.

Atmospheric HONO emissions in China: Unraveling the spatiotemporal patterns and their key influencing factors.

Nitrous acid (HONO) can be photolyzed to produce hydroxyl radicals (OH) in the atmosphere. OH plays a critical role in the formation of secondary pollutants like ozone (O3) and secondary organic aerosols (SOA) via various oxidation reactions. Despite the abundance of recent HONO studies, research on national HONO emissions in China remains relatively limited. Therefore, this study employed a "wetting-drying" model and bottom-up approach to develop a high-resolution gridded inventory of HONO emissions for mainland China using multiple data. We used the Monte Carlo method to estimate the uncertainty in HONO emissions. In addition, the primary sources of HONO emissions were identified and their spatiotemporal distribution and main influencing factors were studied. The results indicated that the total HONO emissions in mainland China in 2016 were 0.77 Tg N (R50: 0.28-1.42 Tg N), with soil (0.42 Tg N) and fertilization (0.26 Tg N) as the primary sources, jointly contributing to over 87% of the total. Notably, the North China Plain (NCP) had the highest HONO emission density (3.51 kg N/ha/yr). Seasonal HONO emissions followed the order: summer (0.38 kg N/ha) > spring (0.19 kg N/ha) > autumn (0.17 kg N/ha) > winter (0.06 kg N/ha). Moreover, HONO emissions were strongly correlated with fertilization, cropland, temperature, and precipitation. This study provides vital scientific groundwork for the atmospheric nitrogen cycle and the formation of secondary pollutants.

Unveiling the effect of O2 on the photochemical reaction of NO2 with polycyclic aromatic hydrocarbons.

The photochemical reaction of NO2 with organics may be a source of atmospheric HONO during the daytime. Here, the conversion of NO2 to HONO on polycyclic aromatic hydrocarbons (PAHs) under solar irradiation under aerobic and anaerobic conditions was investigated using a flow tube reactor coupled to a NOx analyzer. O2 played an inhibition role in NO2 uptake and HONO formation on PAHs, as shown by 7%-45% and 15%-52% decrease in NO2 uptake coefficient (γ) and HONO yield (YHONO), respectively. The negative effect of O2 on the reaction between NO2 and PAHs should be attributed to three reasons. First, O2 could compete with NO2 for the available sites on PAHs. Second, the quenching of the triple excited state of PAHs (3PAHs*) by O2 inhibited the NO2 uptake. Third, NO3- formed under aerobic conditions reduced the conversion efficiency of NO2 to HONO. The environmental implications suggested that the NO2 uptake on PAHs could contribute to a HONO source strength of 10-120 ppt h-1 in the atmosphere.

Quantifying the Nitrogen Sources and Secondary Formation of Ambient HONO with a Stable Isotopic Method.

Nitrous acid (HONO) is a reactive gas that plays an important role in atmospheric chemistry. However, accurately quantifying its direct emissions and secondary formation in the atmosphere as well as attributing it to specific nitrogen sources remains a significant challenge. In this study, we developed a novel method using stable nitrogen and oxygen isotopes (δ15N; δ18O) for apportioning ambient HONO in an urban area in North China. The results show that secondary formation was the dominant HONO formation processes during both day and night, with the NO2 heterogeneous reaction contributing 59.0 ± 14.6% in daytime and 64.4 ± 10.8% at nighttime. A Bayesian simulation demonstrated that the average contributions of coal combustion, biomass burning, vehicle exhaust, and soil emissions to HONO were 22.2 ± 13.1, 26.0 ± 5.7, 28.6 ± 6.7, and 23.2 ± 8.1%, respectively. We propose that the isotopic method presents a promising approach for identifying nitrogen sources and the secondary formation of HONO, which could contribute to mitigating HONO and its adverse effects on air quality.