Soil nitrite as a source of atmospheric HONO and OH radicals

Emission fluxes of nitrous acid (HONO) from livestock and poultry wastes

Gaseous nitrous acid (HONO) is a critical contributor to daytime hydroxyl radical in the troposphere. Livestock farming has been recognized as an overlooked HONO source, but the lack of detailed flux measurements from livestock and poultry wastes would cause uncertainties in modeling its environmental impacts. Here, based on field flux measurements and laboratory experiments, we observed substantial HONO emissions from the composting of swine feces and chicken manure in the warm season, which might be mainly attributed to nitrification process in livestock and poultry wastes. The HONO emission from chicken manure was found to be much higher than that from swine feces, and the higher NH3 emission but lower N2O and NO emissions from chicken manure were also observed. Considering that the interaction among these nitrogen species during nitrification process, the obviously lower HONO emission from swine feces was likely to be explained by the lack of the total ammonia nitrogen and H+ donors in swine feces. Temperature is also a key factor that influences the HONO emission from livestock wastes. In addition, the total HONO emission from swine feces in China was estimated to be approximately 107.7 Gg-N/yr according to the national swine amounts, which is comparable to the national soil HONO emissions, underscoring its non-negligible contribution to regional air quality. Therefore, effective emission control of HONO from livestock and poultry wastes should be carried out to further improve air quality in China.

Atmospheric Reactive Halogens Reshaped by the Clean Energy Policy and Agricultural Activity in a Rural Area of the North China Plain

Reactive halogen species (RHS) play important roles in air pollution and climate change. Observational evidence has identified coal and biomass burning as an important source of RHS in polluted continental regions, including the North China Plain (NCP). Over the past ten years, the Chinese government has enacted various mitigation measures to control air pollutant emissions, including a clean energy initiative in the NCP. Here we report recent measurements of RHS at an NCP's rural site where extraordinary levels of RHS were observed during the winter of 2017. We show that reactive bromines like BrCl and Br2 largely diminished after the implementation of the clean energy policy, but high levels of reactive chlorine persisted. A surprising finding in the recent field study is a potentially new chlorine source, likely from chlorine-based fertilizers. Moreover, the changes in aerosol acidity and the NO3 production rate led to a large increase in ClNO2 production with an inhibition of Cl2. The high ClNO2 levels (average: 150 pptv, peak: 3.8 ppbv) accounted for 43% of the oxidation of alkanes, increased conventional radicals (OH, HO2, RO2) by 4-8%, and net ozone production by 8-11%. Our study suggests more attention to crop fertilization as a potentially important source of atmospheric chlorine.

Understanding Nitrous Acid (HONO) in the Urban Boundary Layer Using Continuous HONO Measurements at a 450 m Tall Tower in Guangzhou, China

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH) in the urban atmospheric boundary layer. However, most HONO observations so far are on the ground level, while HONO chemistry at higher altitude remains largely unknown. Through one-month observations at a 450 m platform of Canton Tower in Guangzhou, China, we have identified two distinct regimes of nocturnal HONO chemistry. One is dominated by heterogeneous reactions on the ground surface, likely corresponding to the period when the platform was within the stable nocturnal boundary layer. Another regime, occurring in the residual layer, is dominated by in situ formation from oxidation of nitric oxide (NO) by OH. During the daytime, HONO from emissions and heterogeneous sources at the ground undergoes ∼60% loss through photolysis before reaching 450 m. A detailed HONO budget analysis considering chemistry and vertical transport suggests that on average 32% of the observed HONO at 450 m is from OH oxidation of NO, while there remains 51% unidentified. These findings emphasize the increased contribution of NO + OH to the overall HONO budget throughout the urban boundary layer, in contrast to the diminished role of ground-related processes, and warrant future continuous measurements at high altitudes to supplement data at the ground to develop a complete understanding of HONO chemistry in the urban boundary layer.

Emissions of Nitrous Acid, Nitryl Chloride, and Dinitrogen Pentoxide Associated with Automotive Braking

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

Rapid and high-precision cavity-enhanced spectroscopic measurement of HONO and NO2: Application to emissions from heavy-duty diesel vehicles in chassis dynamometer tests and in mobile monitoring

Nitrous acid (HONO) is crucial in atmospheric chemistry as it is a major precursor for hydroxyl radicals (OH), the dominant atmospheric oxidant. Hydroxyl radicals are essential in the formation of secondary air pollutants like ozone and particulate matter. This study presents a newly developed Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS) system for precise and rapid measurements of HONO and nitrogen dioxide (NO2) emissions. The instrument's optical cavity (formed by two mirrors separated by 96 cm and with reflectivity of 0.99955 at 378 nm) resulted in an effective optical path length of 1.4 km. With an integration time of 5 s, the 1σ measurement precisions for HONO and NO2 were 0.19 ppb and 0.48 ppb with overall measurement uncertainties of 10 % and 7 %, respectively. Comparative analysis of the IBBCEAS and a commercial cavity-attenuated phase shift (CAPS) systems under non-emission conditions demonstrated excellent agreement (slope = 1.01 and R2 = 0.98). The instrument was applied to study HONO and NO2 emissions from heavy-duty vehicles in chassis dynamometer tests and mobile monitoring. Chassis dynamometer tests revealed that HONO and NO2 emissions depend strongly on vehicle speed and driving conditions. We find a HONO/NOX ratio of 1.01 × 10-2 across the entire China-World Transient Vehicle Cycle (C-WTVC) driving cycle. Mobile monitoring in urban areas shows emission characteristics similar to those observed in chassis dynamometer tests. Frequent acceleration-deceleration patterns of diesel vehicles under congested traffic conditions lead to higher HONO and NO2 emissions compared to driving under steady speed conditions. Improving traffic flow conditions will help reduce HONO and NO2 emissions.

Enhanced Soil Emissions of Reactive Nitrogen Gases by Fertilization and Their Impacts on Secondary Air Pollution in Eastern China

Nitrogen fertilizer application is accompanied by intense release of multiple reactive nitrogen (Nr) gases such as nitrous acid (HONO), ammonia (NH3), and nitric oxide (NO) from the soil, influencing atmospheric chemistry and air pollution. In current emission inventories, postfertilization soil emissions are poorly characterized due to inaccurate identification of fertilization timing and location. Moreover, pre-existing studies predominantly focus on individual Nr gases, and a comprehensive understanding of simultaneously emitted Nr gases from fertilization and their impacts on air quality is still limited. Here, we developed a novel method to identify the dryland fertilization activity based on satellite and reanalysis data sets. Then, we updated a dynamic soil Nr emissions model (WRF-SoilN-Chem) with lab-derived parametrization and applied it to analyze the time- and space-varying Nr emissions and their effects on air quality. It is estimated that the Nr emissions from a typical fertilization event in the Yangtze River Delta (YRD) region increased ozone (O3) and nitrate concentrations by 2.5 and 18.2%, respectively. HONO and NH3 emissions jointly enhanced nitrate production via gas-particle partitioning. An accurate representation of fertilization and meteorology-emission-chemistry coupled modeling would greatly improve the understanding of the soil Nr emissions and their impacts on regional air pollution.

Increasing soil nitrous acid emissions driven by climate and fertilization change aggravate global ozone pollution

Soil microbial nitrous acid (HONO) production is an important source of atmospheric reactive nitrogen that affects air quality and climate. However, long-term global soil HONO emissions driven by climate change and fertilizer use have not been quantified. Here, we derive the global soil HONO emissions over the past four decades and evaluate their impacts on ozone (O3) and vegetation. Results show that climate change and the increased fertilizer use enhanced soil HONO emissions from 9.4 Tg N in 1980 to 11.5 Tg N in 2016. Chemistry-climate model simulations show that soil HONO emissions increased global surface O3 mixing ratios by 2.5% (up to 29%) and vegetation risk to O3, with increasing impact during 1980s-2016 in low-anthropogenic-emission regions. With future decreasing anthropogenic emissions, the soil HONO impact on air quality and vegetation is expected to increase. We thus recommend consideration of soil HONO emissions in strategies for mitigating global air pollution.

Integrating the updated HONO formation mechanism to better understand urban O3 formation chemistry

As a vital precursor of hydroxyl radicals (OH), atmospheric nitrous acid (HONO) plays a significant role in tropospheric chemistry and the production of secondary pollutants. However, knowledge of its sources remains insufficient. To comprehensively investigate the HONO chemistry in polluted cities and alleviate O3 pollution, based on a comprehensive HONO-related field campaign in Zibo City, on the North China Plain, the parameterized formulas of additional HONO sources were validated in a box model (based on the default MCMv3.3.1) and the resulting chemical impact of HONO was determined. The results showed that the updated mechanism well reproduced the diurnal profile of observed HONO, and budget analysis revealed the predominant role of the NO2 photo-enhanced heterogeneous reaction on the ground surface in HONO formation, with contributions of about 70% during both high-O3 episodes and clean days. Underestimations of 32.1% in net Ox production rate and 28.5% in HOx concentrations were simulated by the default mechanism when HONO was not constrained, while the adoption of the updated mechanism reduced these underestimations to 5.3% and 5.4%, respectively. Additionally, sensitivity tests of NOx emission changes showed that the urban O3 photochemical regime was more inclined to NOx-limited after considering variations in HONO concentrations caused by changes in NOx concentrations. These results can contribute to more precise O3 pollution control strategies in urban areas, especially in regions lacking HONO observations.

Role of Heterogeneous Reactions in the Atmospheric Oxidizing Capacity in Island Environments

The source of nitrous acid (HONO) and its importance in island or marine environments are poorly understood. Herein, based on comprehensive field measurements at a hilltop on Corsica Island, we find an inverse diel variation of HONO with higher concentrations during daytime. Night-time HONO budget analysis indicates significant HONO formation during air mass transport along the hillside. In the daytime, although photosensitized NO2 uptake on the ground and NO + OH make considerable contributions (26% and 5%, respectively), a large part of HONO formation (67%, 320 pptv h-1) still cannot be explained with state-of-the-art parametrization. Nevertheless, photosensitized heterogeneous NO2 reactions are likely to account for the missing source, due to underestimation of the source by typical parametrizations at low NO2 levels. Furthermore, we demonstrate a significant role of HONO formation as a OH primary source at this island site, with a OH production rate exceeding one-fourth of that of O3 photolysis. Our findings underscore a potential role of heterogeneous surface reactions in the oxidizing capacity of the island environments.

Integrating Chemical Mechanisms and Feature Engineering in Machine Learning Models: A Novel Approach to Analyzing HONO Budget

Nitrous acid (HONO) serves as the primary source of OH radicals in the atmosphere, exerting significant impacts on atmospheric secondary pollution. The heterogeneous reactions of NO2 on surfaces and photolysis of particulate nitrate or adsorbed nitric acid are important sources of atmospheric HONO, yet the corresponding kinetic parameters based on laboratory investigations and field observations exhibit considerable variations. In this study, we developed an explainable machine learning model to analyze the HONO budget using two years of summer urban supersite observations. By integrating chemical mechanisms and feature engineering into our machine learning model, we assessed the contributions of different sources to HONO and inferred the kinetic parameters for the primary HONO formation pathways, thereby partially addressing the limitations associated with predetermined rate coefficients. Our findings revealed that the primary source of daytime HONO in the summer was the photolysis of nitric acid adsorbed on both aerosol and ground surfaces, accounting for over 40% of its unknown sources. This was followed by the photoenhanced heterogeneous conversion of NO2 and the photolysis of particulate nitrate. Additionally, we derived the corresponding kinetic parameters, analyzed their influencing factors, and confirmed that machine learning methods hold great potential for the study of the HONO budget.

Isolating Cu-Zn active-sites in Ordered Intermetallics to Enhance Nitrite-to-Ammonia Electroreduction

Electrocatalytic nitrite reduction to the valuable ammonia is a green and sustainable alternative to the conventional Haber-Bosch method for ammonia synthesis, while the activity and selectivity for ammonia production remains poor at low nitrite concentrations. Herein, we report a nanoporous intermetallic single-atom alloy CuZn (np/ISAA-CuZn) catalyst with completely isolated Cu-Zn active-sites, which achieves neutral nitrite reduction reaction with a remarkable NH3 Faradaic efficiency over 95% and the highest energy efficiency of ≈ 59.1% in wide potential range from -0.2 to -0.8 V vs. RHE. The np/ISAA-CuZn electrocatalyst was able to operate stably at 500 mA cm-2 for 220 h under membrane electrode assembly conditions with a stabilized NH3 Faraday efficiency of ~80% and high NO2‒ removal rate of ~100%. A series of in situ experimental studies combined with density functional theory calculations reveal that strong electronic interactions of isolated Cu-Zn active-sites altered the protonation adsorption species, effectively alleviating the protonation barrier of *NO2 and thus greatly facilitating the selective reduction of NO2- into NH3.

Strong upwards transport of HONO in daytime over urban area of Beijing, China

Strong upwards transport of Nitrous acid (HONO) in daytime over urban area of Beijing was observed based on combined observations of HONO, NOx (NO and NO2), nitrate, and PM2.5 at two heights (90 m and 528 m) on the highest building of Beijing (528 m above ground). The mean HONO at the 528 m (0.26 ppb) was lower than that at the 90 m (0.54 ppb), and a clear difference in diurnal variation of HONO between the two heights was observed. HONO at the 90 m showed two peaks in the morning rush hour and mid-night, but decreased sharply in daytime (e.g., from 0.62 ppb at 08:00 to 0.34 at 14:00); while the decreasing trend of HONO in daytime significantly weakened at the 528 m (e.g., from 0.26 ppb at 08:00 to 0.27 at 14:00).With PBL development in the morning, HONO in low layer was upwards transported to the 528 m, which compensated partly HONO loss via photolysis and resulted in a relatively stable concentration at the 528 m in daytime. A positive relationship of the bulk Richardson number (Ri) in 0-500 m with the difference of HONO between the two heights during daytime (08:00-18:00) confirmed the above analyses. HONO budget analysis indicated that a strong unknown HONO source existed at the 528 m in daytime, which was negative correlated to the Ri. These results further confirmed that vertical transport of HONO from low layer was a potential HONO source at the 528 m. Moreover, the contribution of photolysis of particulate nitrate significantly increased at the 528 m. Its contribution in total HONO sources increased from 11.9 % at the 90 m to 16.0 % at the 528 m.

Important yet Overlooked HONO Source from Aqueous-phase Photochemical Oxidation of Nitrophenols

Nitrites (NO2-/HONO), as the primary source of hydroxyl radicals (•OH) in the atmosphere, play a key role in atmospheric chemistry. However, the current understanding of the source of NO2-/HONO is insufficient and therefore hinders the accurate quantification of atmospheric oxidation capacity. Herein, we highlighted an overlooked HONO source by the reaction between nitrophenols (NPs) and •OH in the aqueous phase and provided kinetic data to better evaluate the contribution of this process to atmospheric HONO. Three typical NPs, including 4-nitrophenol (4NP), 2-nitrophenol (2NP), and 4-nitrocatechol (4NC), underwent a denitration process to form aqueous NO2- and gaseous HONO through the •OH oxidation, with the yield of NO2-/HONO varied from 15.0 to 33.5%. According to chemical composition and structure analysis, the reaction pathway, where the ipso addition of •OH to the NO2 group on 4NP generated hydroquinone, can contribute to more than 61.9% of the NO2-/HONO formation. The aqueous photooxidation of NPs may account for HONO in the atmosphere, depending on the specific conditions. The results clearly suggest that the photooxidation of NPs should be considered in the field observation and calculation to better evaluate the HONO budget in the atmosphere.

Exploration of a new approach for detection of nitrite with hydroxyl radical fluorescence probe in aqueous solutions

Nitrite (NO2-) has been widely recognized by the international community as an important substance affecting water quality safety and human health, and the detection of NO2- has always been a hot topic for researchers. Fluorescent probe method is an emerging and ideal way for detecting NO2-. Due to the high dependence of the reported reactive NO2- fluorescent probes on strong acidic systems, using the idea of photochemistry, a fluorescence analysis method for detecting NO2- was proposed in this work to change the necessity of strong acidic solutions in probe detection process. A 365 nm UV-LED lamp was used to irradiate NO2- in aqueous solution to convert it into hydroxyl radicals (HO·), and capture the photodegradation product of NO2- using coumarin-3-carboxylic acid as probe 3-CCA that can react with HO· to generate only one type of strong fluorescent substance. This probe has excellent photostability, selectivity, and anti-interference ability, and can realize the quantitative detection of NO2- (0-15 μM) in pure aqueous solution with pH of 7.4. In addition, its application in actual water samples is also satisfactory, with a recovery rate of (85.91 %-107.30 %). Importantly, we hope that this photolysis strategy can open up the novel thinking to develop suitable fluorescent probes for the analysis and detection of some hardly detected analytes.

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.

HONO chemistry and its impact on the atmospheric oxidizing capacity over the Indo-Gangetic Plain

Chemical processes involving nitrous acid (HONO) play a pivotal role as it is a notable source of hydroxyl (∙OH) radicals, influencing the oxidation capacity of the atmosphere. We conduct a comprehensive investigation into the temporal dynamics of HONO, other gases (nitrogen oxides (NOx), ozone (O3), ammonia (NH3), sulphur dioxide (SO2), and nitric acid (HNO3)), particulate matter (PM2.5), and meteorological parameters using measurements that took place during the Winter Fog Experiment (WiFEx) campaign in Delhi, India, during the winter of 2017-2018. Remarkable day-to-day variations in HONO concentrations are observed, with the peak value reaching 54.5 μg m-3 during a fog event. This coincides with elevated levels of sulfate and nitrate in aerosols, underscoring the significant role of heterogeneous fog chemistry in HONO production. We investigated HONO sources and sinks during fog periods by using a photochemical box model. The model shows that the gas-phased chemistry of HONO predicts concentrations lower by an order of magnitude compared to observations (peaking at 0.60 μg m-3 compared to the average observed value of 7.00 μg m-3). The calculated production rates of HONO from observations for daytime to nighttime peaks are 3.10 μg m-3 h-1 (1.1 × 107 molecules cm3 s-1) and 2.00 μg m-3 h-1 (7.1 × 106 molecules cm3 s-1), respectively. This shows the existence of an undefined heterogeneous reaction pathway for HONO production. At the peak of HONO concentration, we estimated an ∙OH formation rate of 9.4 × 107 molecules cm3 s-1 due to the photolysis of HONO, which is much higher than the production of HONO from the reaction of O1D with H2O. This underscores the predominant role of HONO photolysis as the primary source of ∙OH radicals compared to other pathways and highlights the significant role of HONO chemistry in influencing atmospheric oxidation capacity.

Impact of soil-atmosphere HONO exchange on concentrations of HONO and O3 in the North China Plain

Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH) and plays a vital role in atmospheric photochemistry and nitrogen cycling. Soil emissions have been considered as a potential source of HONO. Lately, the HONO emission via soil-atmosphere exchange (ESA-exchange) from soil nitrite has been validated and quantified through chamber experiments, but has not been assessed in the real atmosphere. We coupled ESA-exchange and the other seven potential sources of HONO (i.e., traffic, indoor and soil bacterial emissions, heterogeneous reactions on ground and aerosol surfaces, nitrate photolysis, and acid displacement) into the Weather Research and Forecasting model with Chemistry (WRF-Chem), and found that diurnal variations of the soil emission flux at the Wangdu site were well simulated. During the non-fertilization period, ESA-exchange contributed ∼28 % and ∼35 % of nighttime and daytime HONO, respectively, and enhanced the net ozone (O3) production rate by ∼8 % across the North China Plain (NCP). During the preintensive/intensive fertilization period, the maximum ESA-Exchange contributions attained ∼70 %/83 % of simulated HONO in the afternoon across the NCP, definitely asserting its dominance in HONO production. ESA-Exchange enhanced the OH production rate via HONO photolysis by ∼3.5/7.0 times, and exhibited an increase rate of ∼13 %/20 % in the net O3 production rate across the NCP. The total enhanced O3 due to the eight potential HONO sources ranged from ∼2 to 20 ppb, and ESA-exchange produced O3 enhancements of ∼1 to 6 ppb over the three periods. Remarkably, the average contribution of ESA-exchange to the total O3 enhancements remained ∼30 %. This study suggests that ESA-exchange should be included in three-dimensional chemical transport models and more field measurements of soil HONO emission fluxes and soil nitrite levels are urgently required.

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.

Rapid hydrolysis of NO2 at High Ionic Strengths of Deliquesced Aerosol Particles

Nitrogen dioxide (NO2) hydrolysis in deliquesced aerosol particles forms nitrous acid and nitrate and thus impacts air quality, climate, and the nitrogen cycle. Traditionally, it is considered to proceed far too slowly in the atmosphere. However, the significance of this process is highly uncertain because kinetic studies have only been made in dilute aqueous solutions but not under high ionic strength conditions of the aerosol particles. Here, we use laboratory experiments, air quality models, and field measurements to examine the effect of the ionic strength on the reaction kinetics of NO2 hydrolysis. We find that high ionic strengths (I) enhance the reaction rate constants (kI) by more than an order of magnitude compared to that at infinite dilution (kI=0), yielding log10(kI/kI=0) = 0.04I or rate enhancement factor = 100.04I. A state-of-the-art air quality model shows that the enhanced NO2 hydrolysis reduces the negative bias in the simulated concentrations of nitrous acid by 28% on average when compared to field observations over the North China Plain. Rapid NO2 hydrolysis also enhances the levels of nitrous acid in other polluted regions such as North India and further promotes atmospheric oxidation capacity. This study highlights the need to evaluate various reaction kinetics of atmospheric aerosols with high ionic strengths.

Intramolecular cascade reaction sensing platform for rapid, specific and ultrasensitive detection of nitrite

Nitrite is a carcinogenic substance in food. Excessive consumption of nitrite severely endangers human health. However, rapid and accurate quantification of nitrite by a simple tool is still very challenging. In this work, we designed a practical sensing platform based on 8-(o-phenylenediamine)-boron dipyrromethene (BDP-OPD) to determine nitrite in food. BDP-OPD can take a specific diazotization-cyclization cascade reaction with nitrite to form boron dipyrromethene (BODIPY), giving rise to a remarkable chromogenic reaction along with high contrast fluorescence turn-on response towards nitrite. BDP-OPD has high sensitivity, rapid response, and good selectivity. Furthermore, a portable smartphone-based fluorescence device integrated with a self-programmed Python program was fabricated, which has been successfully used to determine nitrite in food with the advantages of rapid response, low cost, ease of operation, portability, and satisfactory recoveries (92-112%). The good sensing performance rendered BDP-OPD a promising fluorescence platform for on-site visual detection of nitrite.

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.

Coarse particles compensate for missing daytime sources of nitrous acid and enhance atmospheric oxidation capacity in a coastal atmosphere

Large missing sources of daytime atmospheric nitrous acid (HONO), a vital source of hydroxyl radicals (OH) through its photolysis, frequently exist in global coastal regions. In this study, ambient HONO and relevant species were measured at a coastal site in the Pearl River Delta (PRD), China, during October 2019. Relatively high concentrations (0.32 ± 0.19 ppbv) and daytime peaks at approximately 13:00 of HONO were observed, and HONO photolysis was found to be the dominant (55.5 %) source of the primary OH production. A budget analysis of HONO based on traditional sources suggested large unknown sources during the daytime (66.4 %), which had a significant correlation with the mass of coarse particles (PM2.5-10) and photolysis frequency (J(NO2)). When incorporating photolysis of the abundant nitrate measured in coarse particles with a reasonable enhancement factor relative to fine particles due to favorable aerosol conditions, the missing daytime sources of HONO could be fully compensated by coarse particles serving as the largest source at this coastal site. Our study revealed great potential of coarse particles as a strong daytime HONO source, which has been ignored before but can efficiently promote NOx recycling and thus significantly enhance atmospheric oxidation capacity.

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.

Dominant physical and chemical processes impacting nitrate in Shandong of the North China Plain during winter haze events

Nitrate has been a dominant component of PM2.5 since the stringent emission control measures implemented in China in 2013. Clarifying key physical and chemical processes influencing nitrate concentrations is crucial for eradicating heavy air pollution in China. In this study, we explored dominant processes impacting nitrate concentrations in Shandong of the North China Plain during three haze events from 9 to 25 December 2021, named cases P1 (94.46 (30.85) μg m-3 for PM2.5 (nitrate)), P2 (148.95 (50.12) μg m-3) and P3 (88.03 (29.21) μg m-3), by using the Weather Research and Forecasting/Chemistry model with an integrated process rate analysis scheme and updated heterogeneous hydrolysis of dinitrogen pentoxide on the wet aerosol surface (HET-N2O5) and additional nitrous acid (HONO) sources (AS-HONO). The results showed that nitrate increases in the three cases were attributed to aerosol chemistry, whereas nitrate decreases were due mainly to the vertical mixing process in cases P1 and P2 and to the advection process in case P3. HET-N2O5 (the reaction of OH + NO2) contributed 45 % (51 %) of the HNO3 production rate during the study period. AS-HONO produced a nitrate enhancement of 24 % in case P1, 12 % in case P2 and 19 % in case P3, and a HNO3 production rate enhancement of 0.79- 0.97 (0.18- 0.60) μg m-3 h-1 through the reaction of OH + NO2 (HET-N2O5) in the three cases. This study implies that using suitable parameterization schemes for heterogeneous reactions on aerosol and ground surfaces and nitrate photolysis is vital in simulations of HONO and nitrate, and the MOSAIC module for aerosol water simulations needs to be improved.

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.

Top-down versus bottom-up oxidation of a neonicotinoid pesticide by OH radicals

Emerging contaminants (EC) distributed on surfaces in the environment can be oxidized by gas phase species (top-down) or by oxidants generated by the underlying substrate (bottom-up). One class of EC is the neonicotinoid (NN) pesticides that are widely distributed in air, water, and on plant and soil surfaces as well as on airborne dust and building materials. This study investigates the OH oxidation of the systemic NN pesticide acetamiprid (ACM) at room temperature. ACM on particles and as thin films on solid substrates were oxidized by OH radicals either from the gas phase or from an underlying TiO2 or NaNO2 substrate, and for comparison, in the aqueous phase. The site of OH attack is both the secondary >CH2 group as well as the primary -CH3 group attached to the tertiary amine nitrogen, with the latter dominating. In the case of top-down oxidation of ACM by gas phase OH radicals, addition to the -CN group also occurs. Major products are carbonyls and alcohols, but in the presence of sufficient water, their hydrolyzed products dominate. Kinetics measurements show ACM is more reactive toward gas phase OH radicals than other NN nitroguanidines, with an atmospheric lifetime of a few days. Bottom-up oxidation of ACM on TiO2 exposed to sunlight outdoors (temperatures were above 30 °C) was also shown to occur and is likely to be competitive with top-down oxidation. These findings highlight the different potential oxidation processes for EC and provide key data for assessing their environmental fates and toxicologies.

Primary sources of HONO vary during the daytime: Insights based on a field campaign

Nitrous acid (HONO) is an established precursor of hydroxyl (OH) radical and has significant impacts on the formation of PM2.5 and O3. Despite extensive research on HONO observation in recent years, knowledge regarding its sources and sinks in urban areas remains inadequate. In this study, we monitored the atmospheric concentrations of HONO and related pollutants, including gaseous nitric acid and particulate nitrate, simultaneously at a supersite in downtown Chengdu, a megacity in southwestern China during spring, when was chosen due to its tolerance for both PM2.5 and O3 pollution. Furthermore, we employed the random forest model to fill the missing data of HONO, which exhibited good predictive performance (R2 = 0.96, RMSE = 0.36 ppbv). During this campaign, the average mixing ratio of HONO was measured to be 1.0 ± 0.7 ppbv. Notably, during periods of high O3 and PM2.5 concentrations, the mixing ratio of HONO was >50 % higher compared to the clean period. We developed a comprehensive parameterization scheme for the HONO budget, and it performed well in simulating diurnal variations of HONO. Based on the HONO budget analysis, we identified different mechanisms that dominate HONO formation at different times of the day. Vehicle emissions and NO2 heterogeneous conversions were found to be the primary sources of HONO during nighttime (21.0 %, 30.2 %, respectively, from 18:00 to 7:00 the next day). In the morning (7:00-12:00), NO2 heterogeneous conversions and the reaction of NO with OH became the main sources (35.0 %, 32.2 %, respectively). However, in the afternoon (12:00-18:00), the heterogeneous photolysis of HNO3 on PM2.5 was identified as the most substantial source of HONO (contributing 52.5 %). This study highlights the significant variations in primary HONO sources throughout the day.

Synthesizing evidence for the external cycling of NOx in high- to low-NOx atmospheres

External cycling regenerating nitrogen oxides (NOx ≡ NO + NO2) from their oxidative reservoir, NOz, is proposed to reshape the temporal-spatial distribution of NOx and consequently hydroxyl radical (OH), the most important oxidant in the atmosphere. Here we verify the in situ external cycling of NOx in various environments with nitrous acid (HONO) as an intermediate based on synthesized field evidence collected onboard aircraft platform at daytime. External cycling helps to reconcile stubborn underestimation on observed ratios of HONO/NO2 and NO2/NOz by current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO2 lacking noontime valleys specially observed in low-NOx atmospheres. Perturbation on the budget of HONO and NOx by external cycling is also found to increase as NOx concentration decreases. Consequently, model underestimation of OH observations by up to 41% in low NOx atmospheres is attributed to the omission of external cycling in models.

Nitrite stimulates HONO and NOx but not N2O emissions in Chinese agricultural soils during nitrification

The long-lived greenhouse gas nitrous oxide (N2O) and short-lived reactive nitrogen (Nr) gases such as ammonia (NH3), nitrous acid (HONO), and nitrogen oxides (NOx) are produced and emitted from fertilized soils and play a critical role for climate warming and air quality. However, only few studies have quantified the production and emission potentials for long- and short-lived gaseous nitrogen (N) species simultaneously in agricultural soils. To link the gaseous N species to intermediate N compounds [ammonium (NH4+), hydroxylamine (NH2OH), and nitrite (NO2-)] and estimate their temperature change potential, ex-situ dry-out experiments were conducted with three Chinese agricultural soils. We found that HONO and NOx (NO + NO2) emissions mainly depend on NO2-, while NH3 and N2O emissions are stimulated by NH4+ and NH2OH, respectively. Addition of 3,4-dimethylpyrazole phosphate (DMPP) and acetylene significantly reduced HONO and NOx emissions, while NH3 emissions were significantly enhanced in an alkaline Fluvo-aquic soil. These results suggested that ammonia-oxidizing bacteria (AOB) and complete ammonia-oxidizing bacteria (comammox Nitrospira) dominate HONO and NOx emissions in the alkaline Fluvo-aquic soil, while ammonia-oxidizing archaea (AOA) are dominant in the acidic Mollisol. DMPP effectively mitigated the warming effect in the Fluvo-aquic soil and the Ultisol. In conclusion, our findings highlight NO2- significantly stimulates HONO and NOx emissions from dryland agricultural soils, dominated by nitrification. In addition, subtle differences of soil NH3, N2O, HONO, and NOx emissions indicated different N turnover processes, and should be considered in biogeochemical and atmospheric chemistry models.

Developing a model-ready highly resolved HONO emission inventory in Guangdong using domestic measured emission factors

Nitrous acid (HONO) plays an important role in the budget of hydroxyl radical (OH) in the atmosphere. However, current chemical transport models (CTMs) typically underestimate ambient concentrations of HONO due to a dearth of high resolution primary HONO emission inventories. To address this issue, we have established a highly resolved bottom-up HONO emission inventory for CTMs in Guangdong province, utilizing the best available domestic measured emission factors and newly obtained activity data. Our results indicate that emissions from various sources in 2020, including soil, on-road traffic, non-road traffic, biomass burning, and stationary combustion, were estimated at 21.5, 10.0, 8.2, 2.5, and 0.7 kt, respectively. Notably, the HONO emissions structure differed between the Pearl River Delta (PRD) and the non-PRD regions. Specifically, traffic sources were the dominant contributors (62 %) to HONO emissions in the PRD, whereas soil sources accounted for the majority (65 %) of those in the non-PRD. Among on-road traffic sources, diesel vehicles played a significant role, contributing 99.7 %. Comparisons with previous methods suggest that HONO emissions from diesel vehicles are underestimated by approximately 2.5 times. Higher HONO emissions, dominated by soil emissions, were observed in summer months, particularly in August. Furthermore, diesel vehicle emissions were pronounced at night, likely contributing to the nighttime accumulation of HONO and the morning peak of OH. The emission inventories developed in this study can be directly applied to widely used CTMs, such as CMAQ, CAMx, WRF-Chem, and NAQPMS, to support the simulation of OH formation and secondary air pollution.

Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain

Summertime surface ozone in China has been increasing since 2013 despite the policy-driven reduction in fuel combustion emissions of nitrogen oxides (NOx). Here we examine the role of soil reactive nitrogen (Nr, including NOx and nitrous acid (HONO)) emissions in the 2013-2019 ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NOx emissions and add soil HONO emissions in GEOS-Chem based on observation-constrained parametrization schemes. The model estimates significant daily maximum 8 h average (MDA8) ozone enhancement from soil Nr emissions of 8.0 ppbv over the NCP and 5.5 ppbv over China in June-July 2019. We identify a strong competing effect between combustion and soil Nr sources on ozone production in the NCP region. We find that soil Nr emissions accelerate the 2013-2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil Nr ozone contribution, however, is not primarily driven by weather-induced increases in soil Nr emissions, but by the concurrent decreases in fuel combustion NOx emissions, which enhance ozone production efficiency from soil by pushing ozone production toward a more NOx-sensitive regime. Our results reveal an important indirect effect from fuel combustion NOx emission reduction on ozone trends by increasing ozone production from soil Nr emissions, highlighting the necessity to consider the interaction between anthropogenic and biogenic sources in ozone mitigation in the North China Plain.

Effects of drought and recovery on soil volatile organic compound fluxes in an experimental rainforest

Drought can affect the capacity of soils to emit and consume biogenic volatile organic compounds (VOCs). Here we show the impact of prolonged drought followed by rewetting and recovery on soil VOC fluxes in an experimental rainforest. Under wet conditions the rainforest soil acts as a net VOC sink, in particular for isoprenoids, carbonyls and alcohols. The sink capacity progressively decreases during drought, and at soil moistures below ~19%, the soil becomes a source of several VOCs. Position specific 13C-pyruvate labeling experiments reveal that soil microbes are responsible for the emissions and that the VOC production is higher during drought. Soil rewetting induces a rapid and short abiotic emission peak of carbonyl compounds, and a slow and long biotic emission peak of sulfur-containing compounds. Results show that, the extended drought periods predicted for tropical rainforest regions will strongly affect soil VOC fluxes thereby impacting atmospheric chemistry and climate.

Efficient electrosynthesis of formamide from carbon monoxide and nitrite on a Ru-dispersed Cu nanocluster catalyst

Conversion into high-value-added organic nitrogen compounds through electrochemical C-N coupling reactions under ambient conditions is regarded as a sustainable development strategy to achieve carbon neutrality and high-value utilization of harmful substances. Herein, we report an electrochemical process for selective synthesis of high-valued formamide from carbon monoxide and nitrite with a Ru₁Cu single-atom alloy under ambient conditions, which achieves a high formamide selectivity with Faradaic efficiency of 45.65 ± 0.76% at -0.5 V vs. RHE. In situ X-ray absorption spectroscopy, coupled with in situ Raman spectroscopy and density functional theory calculations results reveal that the adjacent Ru-Cu dual active sites can spontaneously couple *CO and *NH₂ intermediates to realize a critical C-N coupling reaction, enabling high-performance electrosynthesis of formamide. This work offers insight into the high-value formamide electrocatalysis through coupling CO and NO₂^- under ambient conditions, paving the way for the synthesis of more-sustainable and high-value chemical products.

Resolving the Formation Mechanism of HONO via Ammonia-Promoted Photosensitized Conversion of Monomeric NO2 on Urban Glass Surfaces

Understanding the formation processes of nitrous acid (HONO) is crucial due to its role as a primary source of hydroxyl radicals (OH) in the urban atmosphere and its involvement in haze events. In this study, we propose a new pathway for HONO formation via the UVA-light-promoted photosensitized conversion of nitrogen dioxide (NO₂) in the presence of ammonia (NH₃) and polycyclic aromatic hydrocarbons (PAHs, common compounds in urban grime). This new mechanism differs from the traditional mechanism, as it does not require the formation of the NO₂ dimer. Instead, the enhanced electronic interaction between the UVA-light excited triplet state of PAHs and NO₂-H₂O/NO₂-NH₃-H₂O significantly reduces the energy barrier and facilitates the exothermic formation of HONO from monomeric NO₂. Furthermore, the performed experiments confirmed our theoretical findings and revealed that the synergistic effect from light-excited PAHs and NH₃ boosts the HONO formation with determined HONO fluxes of 3.6 × 10¹⁰ molecules cm^-2 s^-1 at 60% relative humidity (RH) higher than any previously reported HONO fluxes. Intriguingly, light-induced NO₂ to HONO conversion yield on authentic urban grime in presence of NH₃ is unprecedented 130% at 60% RH due to the role of NH₃ acting as a hydrogen carrier, facilitating the transfer of hydrogen from H₂O to NO₂. These results show that NH₃-assisted UVA-light-induced NO₂ to HONO conversion on urban surfaces can be a dominant source of HONO in the metropolitan area.

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

In the urban atmosphere, nitrogen oxide (NO_x═NO + NO₂)-related reactions dominate the formation of nitrous acid (HONO). Here, we validated an external cycling route of HONO and NO_x, i.e., formation of HONO resulting from precursors other than NO_x, in the background atmosphere. A chemical budget closure experiment of HONO and NO_x 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 NO_x with HONO as an intermediate tracer. The OH production was doubled as a result of the external cycling. A high HONO/NO_x 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/NO_x 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.

Direct Observation of HONO Emissions from Real-World Residential Natural Gas Heating in China

Atmospheric nitrous acid (HONO) is an important precursor of atmospheric hydroxyl radicals. Vehicle emissions and heterogeneous reactions have been identified as major sources of urban HONO. Here, we report on HONO emissions from residential natural gas (RNG) for water and space heating in urban areas based on in situ measurements. The observed HONO emission factors (EFs) of RNG heating vary between 6.03 and 608 mg·m^-3 NG, which are highly dependent on the thermal load. The highest HONO EFs are observed at a high thermal load via the thermal NO homogeneous reaction. The average HONO EFs of RNG water heating in winter are 1.8 times higher than that in summer due to the increased thermal load caused by the lower inlet water temperatures in winter. The power-based HONO EFs of the traditional RNG heaters are 1085 times and 1.7 times higher than those of gasoline and diesel vehicles that meet the latest emission standards, respectively. It is estimated that the HONO emissions from RNG heaters in a typical Chinese city are gradually close to emissions from on-road vehicles when temperatures decline. These findings highlight that RNG heating is a non-negligible source of urban HONO emissions in China. With the continuous acceleration of coal-to-gas projects and the continuous tightening of NOx emission standards for vehicle exhaust, HONO emissions from RNG heaters will become more prominent in urban areas. Hence, it is urgently needed to upgrade traditional RNG heaters with efficient emission reduction technologies such as frequency-converted blowers, secondary condensers, and low-NOx combustors.

Emissions of Nitric Oxide from Photochemical and Microbial Processes in Coastal Waters of the Yellow and East China Seas

Nitric oxide (NO) is an atmospheric pollutant and climate forcer as well as a key intermediary in the marine nitrogen cycle, but the ocean's NO contribution and production mechanisms remain unclear. Here, high-resolution NO observations were conducted simultaneously in the surface ocean and the lower atmosphere of the Yellow Sea and the East China Sea; moreover, NO production from photolysis and microbial processes was analyzed. The NO sea-air exchange showed uneven distributions (RSD = 349.1%) with an average flux of 5.3 ± 18.5 × 10^-17 mol cm^-2 s^-1. In coastal waters where nitrite photolysis was the predominant source (89.0%), NO concentrations were remarkably higher (84.7%) than the overall average of the study area. The NO from archaeal nitrification accounted for 52.8% of all microbial production (11.0%). We also examined the relationship between gaseous NO and ozone which helped identify sources of atmospheric NO. The sea-to-air flux of NO in coastal waters was narrowed by contaminated air with elevated NO concentrations. These findings indicate that the emissions of NO from coastal waters, mainly controlled by reactive nitrogen inputs, will increase with the reduced terrestrial NO discharge.

Vertical distributions of atmospheric HONO and the corresponding OH radical production by photolysis at the suburb area of Shanghai, China

Nitrous acid (HONO) is considered as one of the main sources of the hydroxyl radical (OH), the most relevant oxidant in the atmosphere. Multi-AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were conducted to obtain the vertical profiles of aerosol and HONO from November 1, 2020 to January 31, 2021 at a suburb site of Shanghai, China. HONO was mainly distributed near the surface, but high values HONO occasionally occurred around 0.7 km, indicating an unaccounted source of daytime HONO at high altitudes. The positive correlation between HONO and aerosols suggested that the photo-enhanced heterogeneous reactions on the aerosol surface were an important source of daytime HONO at high altitudes. To obtain the vertical distribution of OH production by HONO photolysis (P(OH)_HONO), the vertical profiles of photolysis rate of HONO (J_HONO) were calculated by establishing a method of combining observations with empirical relationship based on heterogeneous atmospheric and radiative transfer models. The J_HONO increased approximately linearly with increasing altitudes and the noontime averages value of J_HONO near the ground were 6.68 × 10^-4 s^-1, which was strongly negatively affected by aerosols in the morning and afternoon. The P(OH)_HONO profile varied in different months (November, December, January) that the changes were mainly affected by HONO and J_HONO. P(OH)_HONO was more positively affected by J_HONO at high altitude and noon but greatly influenced by HONO concentrations in the morning and afternoon.

Formation mechanisms and atmospheric implications of summertime nitrous acid (HONO) during clean, ozone pollution and double high-level PM2.5 and O3 pollution periods in Beijing

Nitrous acid (HONO) is a key precursor of the hydroxyl radicals (OH) and has a significant impact on air quality. Nowadays, the source of HONO is still controversial due to its complex formation mechanisms, which is widely explored in extensive field and laboratory studies. In this study, the pollution characteristics and source contribution of HONO under different air quality conditions in summer in Beijing were analyzed. The observation periods were classified as three typical periods: clean, ozone pollution, and double high pollution (co-occurrence of high PM_2.5 and O₃ concentrations). The average concentrations of observed HONO were 0.38 ± 0.35 ppb, 0.21 ± 0.18 ppb, 0.26 ± 0.20 ppb and 0.54 ± 0.45 ppb during the whole, clean, ozone and double high periods, respectively. The elevated HONO levels at night were attributed to vehicle emissions and the RH-dependent heterogeneous conversion of NO₂ to HONO. The average emission ratio (HONO/NO_x) was 0.85 % ± 0.38 %, and the mean value of calculated nocturnal NO₂ to HONO conversion frequency was 0.0076 ± 0.0031 h^-1. Based on daytime HONO budget analysis, the largest potential source of HONO was the homogeneous reaction of NO and OH (0.33 and 0.34 ppb h^-1), followed by the unknown source (0.11 and 0.21 ppb h^-1) during clean and ozone periods, while the unknown source (0.49 ppb h^-1) played the predominant role during double high period. The unknown sources of HONO could be attributed to the photo-enhanced heterogeneous conversion of NO₂ and the photolysis of particulate nitrate. Furthermore, the photolysis of ozone (0.17, 0.34 and 0.44 ppb h^-1) was the major contributor to primary OH during three typical periods. HONO photolysis contributed considerable amounts of primary OH (0.32 ppb h^-1) during double high period. These results are helpful to further understand the linkage between HONO and air quality variation.

Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols

Particulate nitrate ([Formula: see text]) has long been considered a permanent sink for NO_x (NO and NO₂), removing a gaseous pollutant that is central to air quality and that influences the global self-cleansing capacity of the atmosphere. Evidence is emerging that photolysis of [Formula: see text] can recycle HONO and NO_x back to the gas phase with potentially important implications for tropospheric ozone and OH budgets; however, there are substantial discrepancies in "renoxification" photolysis rate constants. Using aircraft and ground-based HONO observations in the remote Atlantic troposphere, we show evidence for renoxification occurring on mixed marine aerosols with an efficiency that increases with relative humidity and decreases with the concentration of [Formula: see text], thus largely reconciling the very large discrepancies in renoxification photolysis rate constants found across multiple laboratory and field studies. Active release of HONO from aerosol has important implications for atmospheric oxidants such as OH and O₃ in both polluted and clean 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 NO_x (=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 NO_x. 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 NO_x analyzers that use Nafion to remove water from sampling lines.

Atmospheric chemistry of nitrous acid and its effects on hydroxyl radical and ozone at the urban area of Beijing in early spring 2021

The atmospheric chemistry of nitrous acid (HONO) has received extensive attention because of its significant contribution to hydroxyl (OH) radicals. Heterogeneous reaction of NO₂ is an important HONO source, and its reaction mechanism is affected by many factors, such as concentration of gaseous NO₂, surface adsorbed water, relative humidity and temperature. Although laboratory studies have confirmed the effect of temperature on heterogeneous reaction of NO₂, there are few field observations reporting about it. We have conducted a field observation in the early spring 2021 when the temperature ranges widely (-0.1-24.7 °C). Concentrations of HONO and related pollutants at the urban area of Beijing are obtained. The hourly averaged HONO concentration reaches 4.87 ppb with a mean value of 1.48 ± 1.09 ppb. Combined with box model and RACM2 mechanism, we found an optimal temperature (∼10 °C) existing for heterogeneous reaction of NO₂ during this measurement. When considering the promotion effect of optimal temperature, the contribution of heterogeneous reaction of NO₂ to HONO can increase by 10%. This result will provide essential information for developing an accurate model of HONO chemistry in the atmosphere especially for certain periods or regions with temperature changing largely. Moreover, heterogeneous reaction of NO₂ is the vital source of HONO, contributing 63-76% to simulated HONO during this measurement. Note that HONO photolysis is the most important formation pathway of OH radicals, and ambient HONO concentration is the obbligato constraint for evaluating atmospheric oxidation by model simulations.

Reactive Uptake of Gas-Phase NO2 by Urban Road Dust in the Dark

Road dust constitutes a prominent source of anthropogenic particulate matter, making its heterogeneous interactions with common atmospheric gas-phase compounds important. Here, we show that three distinct samples of urban road dust-including dust samples collected from city streets in summer and winter, and an urban park in summer-react with NO2 in the dark, forming NO and surface nitrite. The loss of NO2 ranged from ∼2 to 13% of its gas-phase concentration and scaled with its concentration as well as with the mass of the road dust sample. The uptake of NO2 by the winter dust was ∼4 times greater than that seen from summer street dust, which was in turn greater than that by the park dust. The conversion ratio of NO2 → NO ranged from 0.06 to 0.8 NO produced per NO2 lost and was greatest for the summer park dust. Exposure of the summer road dust to NO2 roughly doubles the concentration of inorganic nitrite anion in the dust but does not produce nitrate. The formation of NO and photolabile nitrite products means that heterogeneous NO x reactions occurring on the surface of road dust in the dark could have wide implications for the oxidative potential of urban areas.

The seasonal variations and potential sources of nitrous acid (HONO) in the rural North China Plain

Nitrous acid (HONO), an essential precursor of hydroxyl radicals (OH) in the troposphere, plays an integral role in atmospheric photochemistry. However, potential HONO sources remain unclear, particularly in rural areas, where long-term (including seasonal) measurements are scarce. HONO and related parameters were measured at a rural site in the North China Plain (NCP) during the winter of 2017 and summer and autumn of 2020. The mean HONO level was higher in winter (1.79 ± 1.44 ppbv) than in summer (0.67 ± 0.50 ppbv) and autumn (0.83 ± 0.62 ppbv). Source analysis revealed that the heterogeneous conversion (including photo-enhanced conversion) of NO₂ on the ground surface dominated the daytime HONO production in the three seasons (43.1% in winter, 54.3% in summer, and 62.0% in autumn), and the homogeneous reaction of NO and OH contributed 37.8, 12.2, and 28.4% of the daytime HONO production during winter, summer, and autumn, respectively. In addition, the total contributions of other sources (direct vehicle emissions, particulate nitrate photolysis, NO₂ uptake and its photo-enhanced reaction on the aerosol surface) to daytime HONO production were less than 5% in summer and autumn and 12.0% in winter. Unlike winter and autumn, an additional HONO source was found in summer (0.45 ± 0.21 ppbv h^-1, 31.4% to the daytime HONO formation), which might be attributed to the HONO emission from the fertilized field. Among the primary radical sources (photolysis of HONO, O₃, and formaldehyde), HONO photolysis was dominant, with contributions of 82.6, 49.3, and 63.2% in winter, summer, and autumn, respectively. Our findings may aid in understanding HONO formation in different seasons in rural areas and may highlight the impact of HONO on atmospheric oxidation capacity.

Strong impacts of biomass burning, nitrogen fertilization, and fine particles on gas-phase hydrogen peroxide (H2O2)

Gas-phase hydrogen peroxide (H₂O₂) plays an important role in atmospheric chemistry as an indicator of the atmospheric oxidizing capacity. It is also a vital oxidant of sulfur dioxide (SO₂) in the aqueous phase, resulting in the formation of acid precipitation and sulfate aerosol. However, sources of H₂O₂ are not fully understood especially in polluted areas affected by human activities. In this study, we reported some high H₂O₂ cases observed during one summer and two winter campaigns conducted at a polluted rural site in the North China Plain. Our results showed that agricultural fires led to high H₂O₂ concentrations up to 9 ppb, indicating biomass burning events contributed substantially to primary H₂O₂ emission. In addition, elevated H₂O₂ and O₃ concentrations were measured after fertilization as a consequence of the enhanced atmospheric oxidizing capacity by soil HONO emission. Furthermore, H₂O₂ exhibited unexpectedly high concentration under high NO_x conditions in winter, which are closely related to multiphase reactions in particles involving organic chromophores. Our findings suggest that these special factors (biomass burning, fertilization, and ambient particles), which are not well considered in current models, are significant contributors to H₂O₂ production, thereby affecting the regional atmospheric oxidizing capacity and the global sulfate aerosol formation.

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 (N_r) 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 N_r emissions, but might be impacted by future global change and land management.