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Field studies suggest that NO3− photolysis may play a more significant role than previously thought. In this study, we concurrently measured HONO, NO2, and NO2− in situ to gain a deeper understanding of the photogenerated HONO transfer to air and to better constrain the rate constants of NO3− photolysis. The presence of fatty acids (e.g., nonanoic acid, NA), which are naturally present in the environment, significantly increases the production of photogenerated HONO and NO2. With an increase in oxygen percentage, the release rate of photoinduced HONO slowed, while the release rate of NO2 accelerated. The measured J N O 3 − value averaged 1.65 × 10−5 s−1, which is two orders of magnitude higher than values reported in similar systems. The HONO transfer rate from the solutions increased from 2.3 × 10−4 s−1 to 5.6 × 10−4 s−1 as the NA concentration increased from 0.1 to 20 mM. This can be attributed to the accumulation of NO2− induced by NA at the interface. Within this interfacial region, NO2− in the solutions becomes more prone to transfer into gaseous HONO, suggesting that photogenerated NO2− hosted in atmospheric droplets may serve as a temporary reservoir of atmospheric HONO without illumination, influencing the atmospheric oxidizing capacity in the region for hours. Therefore, simultaneous measurements of both gas and particle phase photoproducts are recommended to better constrain the rate constants of NO3− photolysis, thereby enhancing the accuracy of predicting the photochemical production of HONO in the atmosphere. [ABSTRACT FROM AUTHOR]

This study explores the efficiency and applicability of a HONO collection system that incorporates an ultrasonic nozzle and spray chamber for the measurement of ambient air. The system demonstrates (1) a remarkable efficiency of 97.7% across two serial stages, (2) lower detection limits of 0.15 ppbv for HONO, and (3) an absence of interference from NO2 or OH radicals. Practical ambient monitoring with the HONO collection system revealed typical diurnal variations in HONO, O3, and HNO3 concentrations, aligning with photolysis dynamics. Notably, HONO concentrations peaked at 0.37 ppb during nighttime and decreased to 0.27 ppb by midday. O3 demonstrated an inverse relationship with HONO, especially during ozone depletion phases, with r2 values of 0.94, 0.81, and 0.52 across various intervals. The HONO/NOx ratio during periods of enhanced HONO suggested the presence of additional formation mechanisms beyond heterogeneous NOx reactions. Moreover, ozone levels often fell below 20 ppb, indicating a consistent inverse correlation with HONO, thereby reaffirming further mechanisms of HONO formation beyond heterogeneous NOx reactions. The real-time atmospheric chemical reactions involving HONO, monitored concurrently with O3 and NOx, were effectively validated by the HONO collection system employed in this investigation. [ABSTRACT FROM AUTHOR]

The lack of vertical observation of reactive nitrogen oxides in agricultural areas has posed a significant challenge in fully understanding their sources and impacts on atmospheric oxidation. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were conducted in the agricultural regions of the North China Plain (NCP) during the summer of 2019 to measure the vertical distributions of aerosols, nitrogen dioxide (NO2), and nitrous acid (HONO). This study aimed at revealing the spatiotemporal distribution, sources, and environmental effects of reactive nitrogen oxides in the NCP agricultural areas. Our findings indicated that the vertical profiles of aerosols and NO2 exhibited a near-Gaussian distribution, with distinct peak times occurring between 8:00–10:00 and 16:00–18:00. HONO reached its maximum concentration near the surface around 8:00 in the morning and decreased exponentially with altitude. After sunrise, the concentration of HONO rapidly decreased due to photolysis. Additionally, the potential source contribution function (PSCF) was used to evaluate the potential sources of air pollutants. The results indicated that the main potential pollution sources of aerosols were located in the southern part of the Hebei, Shanxi, Shandong, and Jiangsu provinces, while the potential pollution sources of NO2 were concentrated in the Beijing–Tianjin–Hebei region. At altitudes exceeding 500 m, the heterogeneous reactions of NO2 on aerosol surfaces were identified as one of the important contributors to the formation of HONO. Furthermore, we discussed the production rate of hydroxyl radicals (OH) from HONO photolysis. It was found that the production rate of OH from HONO photolysis decreased with altitude, with peaks occurring in the morning and late afternoon. This pattern was consistent with the variations in HONO concentration, indicating that HONO was the main contributor to OH production in the agricultural regions of the NCP. This study provides a new perspective on the sources of active nitrogen in agricultural regions and their contribution to atmospheric oxidation capacity from a vertical perspective. [ABSTRACT FROM AUTHOR]

Field observations have confirmed that halide ions are widely distributed among aerosols from the marine boundary layer and on the surfaces of ice and snow in polar regions. Consequently, the coexistence of halide ions may play a more significant role in nitrate photolysis than previously thought. In this study, we simultaneously measured HONO, NO2, and NO2- in situ to gain a deeper understanding of the coexisting system, including the photogenerated nitrogen products and the effects on nitrate photolysis rates due to enhanced aqueous nitrite and HONO transfer rates by halides. The presence of halides significantly increased the photogenerated nitrogen products across various molar ratios ([X-]/[NO3-]) at pH 3.5. By eliminating oxygen flux, the transformation of the primary photogenerated products was affected, resulting in higher concentrations of N(III) as both HONO and NO2-. Experiments involving OH scavengers indicated that the attack from-OH initiated by halides leads to side reactions that enhance nitrate photolysis. Both theoretical calculations and nitrate actinometry were used to determine the photolysis rate of nitrate solutions, which together indicated that the presence of halides enhances nitrate photolysis. A newly developed model was used to determine the HONO transfer rate, finding that the presence of halides ([X-]/[NO3-] = 0.2) enhanced the evaporation of N(III) in solution by factors of 0.68, 0.95, and 1.27 for Cl-, Br-, and I-, respectively. To our knowledge, this is the first determination of halide effects on the mass transfer of HONO. The enhanced nitrate photolysis rate can be attributed to the differential surface effects of halides and parallel reactions initiated via -OH stemming from nitrate photolysis, with varying rates leading to different quantities of nitrogenous products. Additionally, simultaneous measurements of photoproducts in both gas and condensed phases are recommended to better constrain the rate constants of NO3- photolysis. [ABSTRACT FROM AUTHOR]

Tobacco-specific nitrosamines (TSNAs) are emitted during smoking and form indoors by nitrosation of nicotine. Two of them, N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), are human carcinogens with No Significant Risk Levels (NSRLs) of 500 and 14 ng day-1, respectively. Another TSNA, 4-(methylnitrosamino)-4-(3-pyridyl) butanal (NNA), shows genotoxic and mutagenic activity in vitro. Here, we present additional evidence of genotoxicity of NNA, an assessment of TSNA dermal uptake, and predicted exposure risks through different pathways. Dermal uptake was investigated by evaluating the penetration of NNK and nicotine through mice skin. Comparable mouse urine metabolite profiles suggested that both compounds were absorbed and metabolized via similar mechanisms. We then investigated the effects of skin constituents on the reaction of adsorbed nicotine with nitrous acid (epidermal chemistry). Higher TSNA concentrations were formed on cellulose and cotton substrates that were precoated with human skin oils and sweat compared to clean substrates. These results were combined with reported air, dust, and surface concentrations to assess NNK intake. Five different exposure pathways exceeded the NSRL under realistic scenarios, including inhalation, dust ingestion, direct dermal contact, gas-to-skin deposition, and epidermal nitrosation of nicotine. These results illustrate potential long-term health risks for nonsmokers in homes contaminated with thirdhand tobacco smoke.

Solar radiation triggers atmospheric nitrous acid (HONO) photolysis, producing OH radicals, thereby accelerating photochemical reactions, leading to severe secondary pollution formation. Missing daytime sources were detected in the extensive HONO budget studies carried out in the past. In the rural North China Plain, some studies attributed those to soil emissions and more recent studies to dew evaporation. To investigate the contributions of these two processes to HONO temporal variations and unknown production rates in rural areas, HONO and related field observations obtained at the Gucheng Agricultural and Ecological Meteorological Station during spring and autumn were thoroughly analyzed. Morning peaks in HONO frequently occurred simultaneously with those of ammonia (NH3) and water vapor both during spring and autumn, which were mostly caused by dew and guttation water evaporation. In spring, the unknown HONO production rate revealed pronounced afternoon peaks exceeding those in the morning. In autumn, however, the afternoon peak was barely detectable compared to the morning peak. The unknown afternoon HONO production rates were attributed to soil emissions due to their good relationship to soil temperatures, while NH3 soil emissions were not as distinctive as dew emissions. Overall, the relative daytime contribution of dew emissions was higher during autumn, while soil emissions dominated during spring. Nevertheless, dew emission remained the most dominant contributor to morning time HONO emissions in both seasons, thus being responsible for the initiation of daytime OH radical formation and activation of photochemical reactions, while soil emissions further maintained HONO and associated OH radial formation rates at a high level, especially during spring. Future studies need to thoroughly investigate the influencing factors of dew and soil emissions and establish their relationship to HONO emission rates, form reasonable parameterizations for regional and global models, and improve current underestimations in modeled atmospheric oxidation capacity. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NOX) in the atmosphere cause oxidation reactions with photochemical radicals and volatile organic compounds, leading to the accumulation of ozone (O3). NOX constitutes a significant portion of the NOy composition, with nitrous acid (HONO) and nitric acid (HNO3) following. HONO plays a crucial role in the reaction cycle of NOX and hydrogen oxides. The majority of HNO3 reduction mechanisms result from aerosolization through heterogeneous reactions, having adverse effects on humans and plants by increasing secondary aerosol concentrations in the atmosphere. The investigation of the formation and conversion mechanisms of HONO and HNO3 is important; however, research in this area is currently lacking. In this study, we observed HONO, HNO3, and their precursor gases were observed in the atmosphere using parallel-plate diffusion scrubber-ion chromatography. A 0-D box model simulated the compositional distribution of NOy in the atmosphere. The formation reactions and conversion mechanisms of HONO and HNO3 were quantified using reaction equations and reaction coefficients. Among the various mechanisms, dominant mechanisms were identified, suggesting their importance. According to the calculation results, the produce of HONO was predominantly attributed to heterogeneous reactions, excluding an unknown source. The sink processes were mainly governed by photolysis during daytime and reactions with OH radicals during nighttime. HNO3 showed dominance in its production from N2O5, and in its conversion mechanisms primarily involving aerosolization and deposition. [ABSTRACT FROM AUTHOR]

This study explores the efficiency and applicability of a HONO collection system that incorporates an ultrasonic nozzle and spray chamber for the measurement of ambient air. The system demonstrates (1) a remarkable efficiency of 97.7% across two serial stages, (2) lower detection limits of 0.15 ppbv for HONO, and (3) an absence of interference from NO2 or OH radicals. Practical ambient monitoring with the HONO collection system revealed typical diurnal variations in HONO, O3, and HNO3 concentrations, aligning with photolysis dynamics. Notably, HONO concentrations peaked at 0.37 ppb during nighttime and decreased to 0.27 ppb by midday. O3 demonstrated an inverse relationship with HONO, especially during ozone depletion phases, with r2 values of 0.94, 0.81, and 0.52 across various intervals. The HONO/NOx ratio during periods of enhanced HONO suggested the presence of additional formation mechanisms beyond heterogeneous NOx reactions. Moreover, ozone levels often fell below 20 ppb, indicating a consistent inverse correlation with HONO, thereby reaffirming further mechanisms of HONO formation beyond heterogeneous NOx reactions. The real-time atmospheric chemical reactions involving HONO, monitored concurrently with O3 and NOx, were effectively validated by the HONO collection system employed in this investigation.

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. [ABSTRACT FROM AUTHOR]

Daytime HONO photolysis is an important source of atmospheric hydroxyl radicals (OH). Knowledge of HONO formation chemistry under typical haze conditions, however, is still limited. In the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain in 2018, we investigated the wintertime HONO formation and its atmospheric implications at a rural site Gucheng. Three different episodes based on atmospheric aerosol loading levels were classified: clean periods (CPs), moderately polluted periods (MPPs) and severely polluted periods (SPPs). Correlation analysis revealed that HONO formation via heterogeneous conversion of N O 2 was more efficient on aerosol surfaces than on ground, highlighting the important role of aerosols in promoting HONO formation. Daytime HONO budget analysis indicated a large missing source (with an average production rate of 0.66 ± 0.26, 0.97 ± 0.47 and 1.45 ± 0.55 ppbV/hr for CPs, MPPs and SPPs, respectively), which strongly correlated with photo-enhanced reactions (N O 2 heterogeneous reaction and particulate nitrate photolysis). Average OH formation derived from HONO photolysis reached up to (0.92 ± 0.71), (1.75 ± 1.26) and (1.82 ± 1.47) ppbV/hr in CPs, MPPs and SPPs respectively, much higher than that from O 3 photolysis (i.e., (0.004 ± 0.004), (0.006 ± 0.007) and (0.0035 ± 0.0034) ppbV/hr). Such high OH production rates could markedly regulate the atmospheric oxidation capacity and hence promote the formation of secondary aerosols and pollutants. [ABSTRACT FROM AUTHOR]

Chemical processing of reactive nitrogen species, especially of NOx (= NO + NO2) and nitrous acid (HONO), determines the photochemical ozone production and oxidation capacity in the troposphere. However, sources of HONO and NOx in the remote marine atmosphere are still poorly understood. In this work, the multiphase chemistry mechanism CAPRAM in the model framework SPACCIM was used to study HONO formation at Cape Verde (CVAO) in October 2017, adopted with the input of current parameterizations for various HONO sources. Three simulations were performed that adequately reproduced ambient HONO levels and its diurnal pattern. The model performance for NOx and O3 improves significantly when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds with high correlation coefficients up to 0.93, 0.56, and 0.89 for NO, NO2, and O3, respectively. Photocatalytic conversion of the adsorbed HNO3 on dust is modeled to be the predominant contributor for daytime HONO at CVAO, that is, accounting for about 62% of the chemical formation rate at noontime. In contrast, the ocean‐surface‐mediated conversion of NO2 to HONO and other discussed pathways are less important. The average OH levels at midday (9:00–16:00) modeled for cluster trajectory 1, 2, and 3 are 5.2, 5.1, and 5.2 × 106 molecules cm−3, respectively. Main OH formation is driven by O3 photolysis with a contribution of 74.6% to the total source rate, while HONO photolysis is negligible (∼1.8%). In summary, this study highlights the key role of dust aerosols for HONO formation and NOx cycling at CVAO and possibly in other dust‐affected regions, urgently calling for further investigations using field and model studies. Plain Language Summary: Chemical processing of NOx (= NO + NO2) and nitrous acid (HONO) is important for the tropospheric O3 budget and oxidation capacity. However, the sources of HONO and cycling of NOx in the remote marine atmosphere are still poorly explored. A detailed multiphase chemistry model simulation showed a better performance of HONO, NOx and O3 when considering dust‐surface‐photocatalytic conversions of reactive nitrogen compounds, especially the photocatalytic conversion of the adsorbed HNO3 on dust. The simulations demonstrated that OH formation is mainly driven by the O3 photolysis, while HONO photolysis is a negligible OH radical source due to its low concentration levels at Cape Verde. The study highlights the key role of dust aerosols for HONO and NOx chemistry in the remote marine boundary layer. Key Points: The sources of HONO and NOx at Cape Verde are well modeled with CAPRAMPhotocatalytic conversion of adsorbed HNO3 on dust is the predominant contributor for daytime HONOPhotolysis of O3 is the prevailing source of OH radical at Cape Verde, while HONO photolysis is a negligible OH radical source [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is a crucial precursor of tropospheric hydroxyl radicals but its sources are not fully understood. Soil is recognized as an important HONO source, but the lack of measurements of soil-atmosphere HONO exchange flux (FHONO) has led to uncertainties in modeling its atmospheric impacts and understanding the reactive nitrogen budget. To address this, we conduct long-period FHONO measurements over agricultural fields under fertilized (FHONO-NP) and non-fertilized (FHONO-CK) treatments. Our results show that nitrogen fertilizer use causes a remarkable increase in FHONO-NP and it exhibits distinct diurnal variations, with an average noontime peak of 152 ng N m-2 s-1. The average FHONO-NP within three weeks after fertilization is 97.7 ± 8.6 ng N m-2 s-1, around two orders of magnitude higher than before fertilization, revealing the remarkable promotion effect of nitrogen fertilizer on HONO emissions. We also discuss other factors that influence soil HONO emissions, such as meteorological parameters and soil properties/nutrients. Additionally, we estimate the HONO emission factor of 0.68 ± 0.07% relative to the applied nitrogen during the whole growing season of summer maize. Accordingly, the fertilizer-induced soil HONO emission is estimated to be 0.06 and 0.16 Tg N yr-1 in the North China Plain (NCP) and China, respectively, representing a significant reactive nitrogen source. Furthermore, our observations reveal that soil emissions sustain a high level of daytime HONO, enhancing the atmospheric oxidizing capacity and aggravating O3 pollution in the NCP. Our results indicate that in order to effectively mitigate regional air pollution, future policies should consider reactive nitrogen emissions from agricultural soils. [ABSTRACT FROM AUTHOR]

Solar radiation triggers atmospheric nitrous acid (HONO) photolysis, producing OH radicals, thereby accelerating photochemical reactions, leading to severe secondary pollution formation. Missing daytime sources were detected in the extensive HONO budget studies carried out in the past. In the rural North China Plain, some studies attributed those to soil emissions and more recent studies to dew evaporation. To investigate the contributions of these two processes to HONO temporal variations and unknown production rates in rural areas, HONO and related field observations obtained at the Gucheng Agricultural and Ecological Meteorological Station during spring and autumn were thoroughly analyzed. Morning peaks in HONO frequently occurred simultaneously with those of ammonia (NH3) and water vapor both during spring and autumn, which were mostly caused by dew and guttation water evaporation. In spring, the unknown HONO production rate revealed pronounced afternoon peaks exceeding those in the morning. In autumn, however, the afternoon peak was barely detectable compared to the morning peak. The unknown afternoon HONO production rates were attributed to soil emissions due to their good relationship to soil temperatures, while NH3 soil emissions were not as distinctive as dew emissions. Overall, the relative daytime contribution of dew emissions was higher during autumn, while soil emissions dominated during spring. Nevertheless, dew emission remained the most dominant contributor to morning time HONO emissions in both seasons, thus being responsible for the initiation of daytime OH radical formation and activation of photochemical reactions, while soil emissions further maintained HONO and associated OH radial formation rates at a high level, especially during spring. Future studies need to thoroughly investigate the influencing factors of dew and soil emissions and establish their relationship to HONO emission rates, form reasonable parameterizations for regional and global models, and improve current underestimations in modeled atmospheric oxidation capacity.

Depending on the IO approaches the configuration of HONO, two different H-extraction pathways (IO + cis-HONO and IO + trans-HONO) were located at the CCSD(T)//ωB97X-D level. With the introduction of single water molecule, nine pathways were investigated for the IO + HONO reaction. The computed results manifested that the barriers of Pathway W11, Pathway W12, Pathway W21A, Pathway W21B, Pathway W22B, and Pathway W23 are reduced by 0.39, 3.05, 8.14, 12.63, 13.13, and 10.02 kcal/mol, respectively. The rate coefficients of the reaction of IO + cis-HONO and IO + trans-HONO at 298.15 K in the existence of water are 5.98 × 10−13 and 4.93 × 10−11 cm−3 molecule−1 s−1, which are lower than the corresponding reactions in the absence of water (1.14 × 10−14 and 1.39 × 10−19 cm−3 molecule−1 s−1). To further comprehend the influence of H2O on the IO + HONO reaction, the effective rate coefficients were computed through taking account on the water concentration. The effective rate coefficients of the IO + trans-HONO reaction are much larger than the IO + trans-HONO reaction in the absence of water, 'as water molecule could cause the inhibition of the IO + cis-HONO reaction. In contrast to the IO + HONO reaction with water-free, the feasible reaction is the IO + trans-HONO instead of the IO + cis-HONO reaction. The current investigation proved that water possesses positive influence on the IO + trans-HONO reaction, which could devote to the degradation of HONO. [ABSTRACT FROM AUTHOR]

Soil can release HONO, affecting atmospheric oxidation and tropospheric chemistry processes through the production of hydroxyl radicals (OH) and nitric oxide (NO) via photolysis. However, there is limited field observation of HONO flux, hindering a comprehensive understanding of its emission mechanisms. In this study, a dual open-top chambers system combined with a long path absorption photometer (LOPAP) was employed to measure HONO flux from paddy fields in the Pearl River Delta area (PRD), showing good reproducibility and effectivity to mitigate greenhouse effects. The average HONO flux was 0.77 ± 0.72 ng N m−2 s−1, displaying a diurnal pattern of higher fluxes during the day and lower fluxes at night, similar to previous flux observations. A strong linear correlation between the HONO flux with the product of NO 2 and solar radiation (R2 = 0.90) suggests that surface reactions involving NO 2 and sunlight may dominate HONO production from the paddy fields, surpassing microbial activity within the soil. Given the diversity of field environments, more field observations are needed for assessing emission rates of HONO and understanding underlying mechanisms. [Display omitted] • A system based on dynamic chamber and long path absorption photometer method to measure soil HONO emission was developed. • The parallelism between different chambers was tested under laboratory and field measurement conditions. • The average HONO flux was 0.77 ± 0.72 ng N m−2 s−1, displaying a diurnal pattern of higher fluxes during the day and lower fluxes at night. • Surface reactions involving NO 2 and sunlight may dominate HONO production from the paddy fields. [ABSTRACT FROM AUTHOR]

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 O 3 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 NO 2 photo-enhanced heterogeneous reaction on the ground surface in HONO formation, with contributions of about 70% during both high-O 3 episodes and clean days. Underestimations of 32.1% in net O x production rate and 28.5% in HO x 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 NO x emission changes showed that the urban O 3 photochemical regime was more inclined to NO x -limited after considering variations in HONO concentrations caused by changes in NO x concentrations. These results can contribute to more precise O 3 pollution control strategies in urban areas, especially in regions lacking HONO observations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Elsevier Ltd. All rights reserved.)

Reactive nitrogen (Nr) emissions significantly affect air quality and the nitrogen cycle in ecosystems. Heavy-duty diesel vehicles (HDDVs), as major sources of these emissions, exhibit complex emission characteristics because of the combined effects of different driving conditions and aftertreatment technologies. This study first investigated the emission factors (EFs) of Nr species, including NO, NO 2 , HONO, N 2 O, and NH 3 , from HDDVs under different emission standards (China IV/V/VI) and cheating strategies, with a particular focus on the impact of selective catalytic reduction (SCR) systems. Vehicles employing water injection cheating present NO, NO 2 , and HONO EFs that are consistent with the China III standards, significantly undermining the effectiveness of Nr emission control. The evolution of SCR technology in China IV, V, and VI standards has generally led to substantial reductions in NO, NO 2 , and HONO emissions, yet the integration of ammonia slip catalysts (ASC) systems in China VI vehicles presents new challenges. While ASCs have successfully reduced NH 3 slip to an average of 17 ± 12 mg/km, they have also caused a 6-13-fold increase in N 2 O emissions compared with those of China IV and V vehicles, reaching levels of 205 ± 85 mg/km. Additionally, China VI vehicles exhibit a marked increase in the HONO/NOx ratio, which increases from 0.9% in China V to 4.6%. These increases are attributed to high-temperature oxidation of NH 3 within the ASC catalyst, leading to undesirable byproducts. The temporal dynamics of Nr emissions under real-world driving conditions further reveal that the effectiveness of aftertreatment technologies and their selectivity toward byproducts vary depending on the driving mode. This variability underscores the need for further optimization of the SCR and ASC technologies to balance the control of all the reactive nitrogen species effectively.

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 NO 2 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 NO 2 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.

As primary emissions have been declining in recent decades in China, secondary pollution, like ozone (O 3) and secondary aerosols are main pollutants deteriorating air quality. The chemistry of reactive nitrogen (Nr) plays a crucial role in the chemical transformation of secondary aerosols and O 3 , and its representation in three-dimensional (3-D) regional/global chemical transport models is vital for quantitatively understanding Nr's role in air pollution as well as future emission control. This study reviews the parameterizations of the Nr modeling in chemical transport models (CTMs) and their environmental implications over the past decade in China. A number of Nr species are present in the atmosphere. Given the significant impact of Nr on air quality, we have identified four research topics within the broader field of Nr research. These Nr topics include the multiphase chemistry of inorganic nitrates (which involves NO, NO 2 , NH 3 , HNO 3 , NO 3 radicals, N 2 O 5 , ClNO 2 and nitrate), organic nitrates (which involves NO, NO 3 , RONO 2 and HNO 3), HONO (which involves NO, NO 2 , HONO, and nitrate), and reduced‑nitrogen (which involves NH 3 and amines). Regional CTM modeling of inorganic nitrates mainly focuses on homogeneous NO 2 oxidation and heterogeneous N 2 O 5 hydrolysis, which contribute to over 80% of inorganic nitrate formation in China. During hazy days, the contribution of heterogeneous uptake of N 2 O 5 can be amplified. By contrast, organic nitrates modeling, including their gas-phase reactions, gas-particle conversion, and hydrolysis, is poorly explored in China, despite their importance for secondary organic aerosol (SOA) formation. HONO sources and their impacts on atmospheric oxidizing capacity (AOC) are an emerging research topic. Parameterizations of major HONO sources were presented, including the homogeneous pathway, direct emissions, heterogeneous uptake of NO 2 on solid surfaces, and nitrate photolysis. The heterogeneous uptake of NO 2 is the predominant pathway of HONO formation. HONO affects AOC by regulating the OH budget, particularly during the early morning when O 3 photochemistry is not efficient. For NH 3 and amines, current CTMs fail to accurately represent their sources in urban regions due to the lack of non-agricultural emissions. Although amines do not significantly influence chemical cycles, they show strong potential in inducing new particle formation. Overall, further work is needed to improve Nr modeling in China by developing parameterizations suitable for the Chinese atmospheric conditions, establishing accurate sector-based emission inventories for NH 3 and amines, and realizing effective modeling of organic nitrates in the future. • Reactive nitrogen plays a vital role in the formation of secondary air pollution. • Parameterizations of reactive nitrogen in chemical transport models are summarized. • Numerous studies have been conducted in China to model inorganic nitrates and HONO. • Organic nitrates are rarely simulated due to their high computational complexity. • More attention has been paid to amines due to their importance in particle nucleation. [ABSTRACT FROM AUTHOR]

Radicals and their precursors play a central role in the chemical transformations occurring in indoor air and on indoor surfaces. Such species include OH, HO2, peroxy radicals, nitrous acid, reactive chlorine species, NO3, N2O5, Criegee intermediates, and glyoxal and methylglyoxal. Recent advances on instrumental analysis and modeling studies have demonstrated the need for a wider range of measurements of radical species and their precursors in indoor air. This work reviews measurement techniques and provides considerations for indoor measurements of several radicals and their precursors. Techniques to determine the actinic flux are also presented owing to the relevance of photolytically-initiated processes indoors. This review is also intended to provide pointers for those wanting to learn more about measurements of radicals indoors. [ABSTRACT FROM AUTHOR]

The ISO standard 22197-1:2016 used for the evaluation of the photocatalytic nitric oxide removal has a main drawback, which allows only the decrease of nitric oxide to be determined specifically. The remaining amount, expressed as "NO2", is considered as a sum of HNO3, HONO NO2, and other nitrogen-containing species, which can be potentially formed during the photocatalytic reaction. Therefore, we developed a new methodology combining our custom-made analyzers, which can accurately determine the true NO2 and HONO species, with the conventional NO one. Their function was validated via a photocatalytic experiment in which 100 ppbv of either NO or NO2 dispersed in air passed over (3 L min−1) an Aeroxide© TiO2 P25 surface. The gas-phase analysis was complemented with the spectrophotometric determination of nitrates ( NO 3 −)   and/or nitrites ( NO 2 −)   deposited on the P25 layer. Importantly, an almost perfect mass balance (94%) of the photocatalytic NOx abatement was achieved. The use of custom-made analyzers enables to obtain (i) no interference, (ii) high sensitivity, (iii) good linearity in the relevant concentration range, (iv) rapid response, and (v) long-term stability. Therefore, our approach enables to reveal the reaction complexity and is highly recommended for the photocatalytic NOx testing. [ABSTRACT FROM AUTHOR]

Carcinogenic nitrosamines have been widely studied due to their risk to human health. However, the universality and evolutionary processes of their generation, particularly concerning their secondary sources, remain unclear at present. We demonstrated through laboratory flow tube experiments that corresponding nitrosamines were generated from heterogeneous reactions of nitrous acid (HONO) with five structurally diverse amines commonly found indoors, including diphenylamine (DPhA), dibenzylamine (DBzA), dioctylamine (DOtA), N -(1,3-dimethylbutyl)- N '-phenyl- p -phenylenediamine (6PPD), and N -phenyl-1-naphthylamine (PANA). The heterogeneous reaction rate constants of DBzA and DOtA with HONO (∼70 ppb) were 1.21 × 10 -3 and 2.13 × 10 -3 min -1 at 30% relative humidity (RH), resulting in a lifetime of 13.8 and 7.8 h. As compared to higher RH (∼80%), more nitrosamines were produced from the reaction of HONO with surface-sorbed DBzA, DOtA, 6PPD, and PANA at lower RH (30%), with product yields ranging from <0.1% to 0.5%. Furthermore, we observed the formation of nitroso-6PPDs and nitro-6PPDs during room air exposure of 6PPD in a genuine indoor environment, in addition to various other transformation products indicative of reactions of 6PPD with HONO, NOx, and ozone indoors. This study confirmed the universality of the heterogeneous reaction of surface-sorbed amine with HONO as a source of nitrosamines indoors.

Soot, primarily composed of elemental carbon (EC) and organic carbon (OC), is ubiquitous in PM 2.5 . In the atmosphere, the heterogeneous interaction between NO 2 and soot is not only an important pathway driving soot aging but also of central importance to nitrous acid (HONO) formation. It is commonly believed that the surface redox reaction between reductive OC and NO 2 dominates the night aging of soot and the conversion of NO 2 to HONO. However, completely differing from the currently popular explanation, we find here that the redox reaction between EC and NO 2 can also drive the conversion of NO 2 to HONO during soot aging. By combining in situ experiments with density functional theory (DFT) calculations, we proposed that the surface carbon vacancy defects on graphite/graphene-like EC should be a type of potential primary adsorption and reactive sites inducing the heterogeneous reduction of NO 2 . We suggested a new mechanism that NO 2 is reduced to form HONO on surface vacancy defects through the splitting of H 2 O molecules, and the carbon atoms adjacent to surface vacancy are simultaneously oxidized to form hydroxyl-functionalized EC. This novel finding provides insights into the chemical mechanism driving the NO 2 -to-HONO conversion and rapid soot aging, which expands our knowledge of the heterogeneous chemistry of soot in the atmosphere.

We present OH observations from Nitrogen, Aerosol Composition, and Halogens on a Tall Tower 2011 (NACHTT-11) held at the Boulder Atmospheric Observatory in Weld County, Colorado. Average OH levels at noon were ~ 2.7 × 106 molecules cm-3 at 2 m above ground level. Nitrous acid (HONO) photolysis was the dominant OH source (80.4%) during this campaign, while alkene ozonolysis (4.9%) and ozone photolysis (14.7%) were smaller contributions to OH production. To evaluate recycling sources of OH from HO2 and RO2, an observationally constrained University of Washington Chemical Mechanism (UWCM) box model (version 2.1) was employed to simulate ambient OH levels over several scenarios. For the base run, not constrained by observed HONO, the model significantly underestimated OH by a factor of 5.3 in the morning (9:00–11:00) and by a factor of 3.2 in the afternoon (13:00–15:00). The results suggest that known chemistry cannot constrain HONO and, subsequently, OH during the observational period. When HONO is constrained in the model by observations (< 50 m), the discrepancy between observation and model simulation improves to a factor of 1.3 in the morning and a factor 1.1 in the afternoon, within the 35% estimated instrumental uncertainty. However, the model produces both a morning and afternoon maximum in OH, in contrast to the observations, which show strong evidence for morning OH production but no distinct morning maximum. Two additional OH sources were also considered, although they do not improve the differences in modeled and measured temporal OH profiles. First, the impact of daytime HONO gradients near the ground surface (< 20 m) was evaluated. Strong HONO gradients were observed between 06:00 and 09:00 MST (mountain standard time), especially within 20 m of the surface. When constrained to HONO observed below 20 m (rather than 50 m), the model produced an even larger morning OH maximum, in contrast to the observations. Second, Cl atoms from ClNO2 photolysis producing RO2 from reaction with alkanes, while significant, produced steady state Cl atom levels (~ 103 atoms cm3) that were too low to significantly perturb measured OH through reactions of organic peroxy radicals produced from Cl reactions with volatile organic compounds.

• The ratio between OH production from HONO and from (HONO+O3) lager than 90% before 12:00. • The source of HONO from combustion emission is ~26%. • NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity was a dominant factor for the secondary produce of HONO. • HONO mainly distributed near the surface and localized, and NO 2 mainly transmitted from north at ~ 0.6 km. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) observations were operated from 02 to 21 December 2018 in Leshan, southwest China, to measure HONO, NO 2 and aerosol extinction vertical distributions, and these were the first MAX-DOAS measurement results in Sichuan Basin. During the measurement period, characteristic ranges for surface concentration were found to be 0.26–4.58 km−1 and averaged at 0.93 km−1 for aerosol extinction, 0.49 to 35.2 ppb and averaged at 4.57 ppb for NO 2 and 0.03 to 7.38 ppb and averaged at 1.05 ppb for HONO. Moreover, vertical profiles of aerosol, NO 2 and HONO were retrieved from MAX-DOAS measurements using the Heidelberg Profile (HEIPRO) algorithm. By analysing the vertical gradients of pollutants and meteorological information, we found that aerosol and HONO are strongly localised, while NO 2 is mainly transmitted from the north direction (city center direction). Nitrogen oxides such as HONO and NO 2 are important for the production of hydroxyl radical (OH) and oxidative capacity in the troposphere. In this study, the averaged value of OH production rate from HONO is about 0.63 ppb/hr and maximum value of ratio between OH production from HONO and from (HONO+O 3) is > 93% before12:00 in Leshan. In addition, combustion emission contributes to 26% for the source of HONO in Leshan, and we found that more NO 2 being converted to HONO under the conditions with high aerosol extinction coefficient and high relative humidity is also a dominant factor for the secondary produce of HONO. [Display omitted] [ABSTRACT FROM AUTHOR]

Heterogeneous surface reactions play a key role in the chemistry of the indoor environment because of the large indoor surface‐to‐volume ratio. The presence of photocatalytic material in indoor paints may allow photochemical reactions to occur at wavelengths of light that are present indoors. One such potential reaction is the heterogeneous photooxidation of NO to HONO. NO(g) is commonly found indoors, originating from combustion sources, ventilation and infiltration of outdoor air. We studied the interaction of NO(g) with painted surfaces illuminated with indoor fluorescent and incandescent lighting. There is a loss of NO(g) to painted surfaces in the dark at both 0 and 50% RH. At 50% RH, there is a re‐release of some of that NO(g) under illumination. The same behavior is observed for illumination of different colored paints. This is in contrast to what is seen with TiO2 as the substrate, where photoenhanced uptake of NO(g) and formation of NO2(g) are observed. We hypothesize that the loss of NO(g) is due to adsorption and diffusion into the paint. The re‐release of NO under illumination is thought to be due to photooxidation of NO to HONO on the painted surface at higher relative humidities and subsequent HONO photolysis. [ABSTRACT FROM AUTHOR]

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 (NO 2 ), particulate nitrate (pNO 3 ), and nitric acid (HNO 3 ), resulting in comparable agreement between observed and simulated HONO concentrations with any one of the three heterogeneous HONO production mechanisms, photosensitized NO 2 conversion on aerosols, photolysis of pNO 3 , and conversion from HNO 3 . 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 (O 3 ) production, and O 3 control strategies vary greatly depending on the HONO production mechanism. On a regional scale, the conversion of HONO from pNO 3 can drastically enhance O 3 production, while the conversion from NO 2 can reduce O 3 sensitivity to NOx changes in polluted eastern China.

Existing studies suggest various potential daytime sources of atmospheric nitrous acid (HONO), including photolysis surface reactions and photo-enhanced NO2 conversion on organic surfaces. However, the understanding of daytime HONO sources is still inadequate. In this study, we report the HONO formation on asphalt surfaces under various NO2, VOCs (toluene and hexane), and UV irradiance conditions using a continuous flow chamber. Although no HONO formation was found without light exposure, the light threshold for HONO formation on the asphalt surface was very low, with a total UV (TUV) of 0.7 W m−2. HONO formation on the asphalt surface was linearly dependent on NO2 up to 300 ppb in the presence of VOCs, but no HONO formation was observed with humified air and NO2. HONO production was saturated at high hydrocarbon concentrations and light intensities. The calculated first-order NO2 conversion rate to HONO on the asphalt surface was 1.2 × 10−4 s −1. The observed mean HONO emission flux was 1.3 × 109 molecules cm−2 s −1 with a similar range of those on other urban covered surfaces. The calculated vertical HONO profile using the measured HONO emission flux and 1-D steady state model revealed that the asphalt surface may account for 13% of daytime HONO in the elevated on-road pollutant concentrations in Seoul. However, we show that its HONO contribution could be much higher on real-life road surfaces directly exposed to much higher NO2 emissions from vehicle exhaust. [ABSTRACT FROM AUTHOR]

The heterogeneous degradation of nitrogen dioxide (NO 2) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NO x analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO 2 , NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO 2 varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO 2 degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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., NaNO 3 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 ( P HONO ) 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 P HONO /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.3 /malonic acid (MA) and NaNO 3 /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.

We coupled the heterogeneous hydrolysis of N 2 O 5 into the newly updated Weather Research and Forecasting model with Chemistry (WRF-Chem) to reveal the relative importance of the hydrolysis of N 2 O 5 and additional nitrous acid (HONO) sources for the formation of nitrate during high PM 2.5 events in the North China Plain (NCP) in four seasons. The results showed that additional HONO sources produced the largest nitrate concentrations in winter and negligible nitrates in summer, leading to a 10% enhancement of a PM 2.5 peak in southern Beijing and a 15% enhancement in southeastern Hebei in winter. In contrast, the hydrolysis of N 2 O 5 produced high nitrate in summer and low nitrate in winter, with the largest contribution of nearly 20% for a PM 2.5 peak in southeastern Hebei in summer. During PM 2.5 explosive growth events, the additional HONO sources played a key role in nitrate increases in southern Beijing and southwestern Hebei in winter, whereas the hydrolysis of N 2 O 5 contributed the most to a rapid increase in nitrate in other seasons. HONO photolysis produced more hydroxyl radicals, which were >1.5 μg m−3 h−1 in the early explosive stage and led to a rapid nitrate increase at the southwestern Hebei sites in winter, while the heterogeneous reaction of N 2 O 5 contributed greatly to a significant increase in nitrate in summer. The above results suggest that the additional HONO sources and the heterogeneous hydrolysis of N 2 O 5 contributed the most to nitrate formation in NCP in winter and summer, respectively. Unlabelled Image • The heterogeneous hydrolysis of N 2 O 5 was coupled into the newly updated WRF-Chem model. • The relative importance of the hydrolysis of N 2 O 5 and additional HONO sources in the formation of nitrate during high PM 2.5 events was assessed. • The additional HONO sources produced the largest nitrate concentration in winter. • The hydrolysis of N 2 O 5 produced high nitrate in summer. • HONO photolysis contributed most to rapid nitrate increases in the PM 2.5 explosive growth in winter. [ABSTRACT FROM AUTHOR]

The Weather Research and Forecasting/Chemistry (WRF-Chem) model updated with six potential HONO sources (i.e., traffic, soil, biomass burning and indoor emissions, and heterogeneous reactions on aerosol and ground surfaces) was used to quantify the impact of the six potential HONO sources on the production and loss rates of OH and HO 2 radicals and the concentrations of secondary organic aerosol (SOA) in the Beijing-Tianjin-Heibei (BTH) region of China during a winter heavy haze period of Nov. 29–Dec. 3, 2017. The updated WRF-Chem model well simulated the observed HONO concentrations at the Wangdu site, especially in the daytime, and well reproduced the observed diurnal variations of regional-mean O 3 in the BTH region. The traffic emission source was an important HONO source during nighttime but not significant during daytime, heterogeneous reactions on ground/aerosol surfaces were important during nighttime and daytime. We found that the six potential HONO sources led to a significant enhancement in the dominant production and loss rates of HO x on the wintertime heavy haze and nonhaze days (particularly on the heavy haze day), an enhancement of 5–25 μg m−3 (75–200%) in the ground SOA in the studied heavy haze event, and an enhancement of 2–15 μg m−3 in the meridional-mean SOA on the heavy haze day, demonstrating that the six potential HONO sources accelerate the HO x cycles and aggravate haze events. HONO was the key precursor of primary OH in the BTH region in the studied wintertime period, and the photolysis of HONO produced a daytime mean OH production rate of 2.59 ppb h−1 on the heavy haze day, much higher than that of 0.58 ppb h−1 on the nonhaze day. Anthropogenic SOA dominated in the BTH region in the studied wintertime period, and its main precursors were xylenes (42%), BIGENE (31%) and toluene (21%). Unlabelled Image • HONO photolysis produced much more OH on the heavy haze day than on the nonhaze day. • The six HONO sources accelerated dominant production and loss rates of HOx. • The six HONO sources produced 75–200% enhancements in the ground SOA. • ASOA dominated the BTH region and mainly derived from xylenes, BIGENE and toluene. [ABSTRACT FROM AUTHOR]

Abstract Episodic haze is frequently observed in Zhengzhou, China. Such haze typically contains secondary inorganic aerosols. In this paper, we explore the formation mechanisms of sulfate, nitrate, and ammonium (SNA) in Zhengzhou from January 3 to 25, 2018 based on the results of a series of online instruments and a size-segregated filter sampler. Our results document the remarkable contributions of SNA to winter haze episodes in Zhengzhou, where they account for about 50% of PM 2.5 mass concentration. SNA were mainly concentrated in droplet-mode particles, which increased remarkably with the aggravation of the haze episode. In addition, KNO 3 and NaNO 3 were formed in droplet-mode particles and coarse-mode particles respectively with increasing PM 2.5 concentration. The atmosphere during the observation period was ammonia-rich, and the aerosol was acidic under high PM 2.5 concentration. Homogeneous reactions dominated the formation of nitrate. HONO photolysis played a more important role in the origin of OH radicals when O 3 decreased during haze episode. Under high relative humidity (RH), nitrate formation was influenced by heterogeneous hydrolysis reactions of N 2 O 5. Sulfates were mainly formed through aqueous-phase reactions, especially when the RH was higher than 60%. Under these conditions, there were amounts of liquid water content existed in aerosols. Finally, we observed enhanced conversion of SO 2 and NO 2 during snowfall periods. This effect may be attributable to the higher RH and O 3 levels despite the unfavorable effects of wet deposition and low concentrations of gaseous precursors. Graphical abstract Unlabelled Image Highlights • Droplet-mode nitrates and sulfates increased remarkably with the aggravation of the haze episode. • HONO photolysis became more important for the origin of OH when O 3 decreased during haze episode. • Sulfates were mainly formed through aqueous-phase reactions, especially when the RH was higher than 60%. • Higher RH and O 3 levels may favor the conversation of SO 2 and NO 2 during periods of snowfall. [ABSTRACT FROM AUTHOR]

Abstract HONO (nitrous acid) is a crucial precursor for tropospheric OH radicals, and its sources are not well understood. In the past decade, soil was proven to be a potential source for HONO. However, more field measurements of soil HONO emission flux are needed to explore the mechanism and its impact on regional air quality. Here, we developed a system based on twin open-top chambers (OTCs) and wet chemical methods to measure HONO emission flux from agricultural soil in the North China Plain (NCP). The performance of the OTC system was tested under laboratory and field measurement conditions. The results showed that the system could reflect the strength (>90%) and variation of gas emission with an average residence time of 4–5 min. The greenhouse effect and chemical reaction interference in the chamber was proven to have no significant influence on the HONO flux measurement. Field measurement revealed that agricultural soil before fertilization was an important source of HONO. The emission flux showed radiation-dependent or temperature-dependent variation, with a peak of 3.21 ng m−2 s−1 at noontime that could account for approximately 67 pptv h−1 of the missing HONO source under an assumed mixing layer height of 300 m. Fertilization substantially accelerated HONO emission, which was rationally attributed to biological processes including nitrification. Considering the high fertilization rate in the NCP and other similar regions in China, HONO emission from agricultural soil likely has enormous impact on regional photochemistry and air quality, suggesting that more research should be conducted on this aspect. Graphical abstract Unlabelled Image Highlights • A system based on open-top chamber method and wet chemical method to measure soil HONO emission was developed. • The performance of the OTCs system was tested in laboratory and field measurement conditions. • Soil HONO emission fluxes in NCP showed radiation-dependent or temperature-dependent variation. • Fertilization process substantially accelerated HONO emission which was rationally attributed to biological process including nitrification. [ABSTRACT FROM AUTHOR]

In recent years, high fine particulate (PM2.5) pollution episodes with high ozone (O3) levels have been observed in Shanghai from time to time. However, their occurrence and characteristics remain poorly understood. Meanwhile, as a major precursor of tropospheric hydroxyl radical (OH) that initiates the formation of hydroperoxyl and organic peroxy radicals, HONO would inevitably affect the formation of O3, but its role in the formation of O3 during the double high-level PM2.5 and O3 pollution episodes remains unclear. In this study, the characteristics of the double high pollution episodes and the role of HONO in O3 formation in these episodes were investigated based on field observation in urban Shanghai from 2014 to 2016. Results showed that high PM2.5 pollution and high O3 pollution could occur simultaneously. The cases with data of double high O3 and PM2.5 concentrations accounted for about 1.0% of the whole sampling period. During the double high pollution episodes, there still existed active photochemical processes, while the active photochemical processes at high PM2.5 concentration were conductive to the production and accumulation of O3 under a VOC-limited regime and a calm atmospheric condition including high temperature, moderately high relative humidity, and low wind speed, which in turn enhanced the conversions of SO2 and NO2 and the formation and accumulation of secondary sulfate and nitrate aerosols and further promoted the increase of PM2.5 concentration and the deterioration of air pollution. Further analysis indicated that the daytime HONO concentration could be strongly negatively correlated with O3 concentration in most of the double high pollution episodes, revealing the dominant role of HONO in O3 formation during these pollution episodes. This study provides important field measurement-based evidence for understanding the significant contribution of daytime HONO to O3 formation, and helps to clarify the formation and coexistence mechanisms of the double high-level O3 and PM2.5 pollution episodes.

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 (O 3) 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 (R 50 : 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. [Display omitted] • The total HONO emissions in mainland China were 0.77 Tg N in 2016. • HONO emissions peaked in July and reached their lowest point in January. • North China Plain had highest HONO emissions density (3.51 kg N/ha/yr). • HONO emissions positively correlated with fertilization, cropland, and temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • 30 well-resolved trans-HONO absorption lines determined with a single cw-QCL. • The line positions and effective line strengths of the ν3 fundamental band of trans-HONO near 1280 cm−1. • The maximum measurement uncertainty of 7.64% in the line strengths. • The measurement precision of the line positions is better than 5.56 * 10−3 cm−1. • Broad applications in gas monitoring and analysis of the sources and sinks of atmospheric HONO. The measurement of the line positions and effective line strengths of the ν 3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm−1 (7.8 µm) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8–1282.2 cm−1 were determined using the HONO line strength at 1280.3841 cm−1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10−3 cm−1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO. [ABSTRACT FROM AUTHOR]

Existing studies suggest various potential daytime sources of atmospheric nitrous acid (HONO), including photolysis surface reactions and photo-enhanced NO2 conversion on organic surfaces. However, the understanding of daytime HONO sources is still inadequate. In this study, we report the HONO formation on asphalt surfaces under various NO2, VOCs (toluene and hexane), and UV irradiance conditions using a continuous flow chamber. Although no HONO formation was found without light exposure, the light threshold for HONO formation on the asphalt surface was very low, with a total UV (TUV) of 0.7 W m−2. HONO formation on the asphalt surface was linearly dependent on NO2 up to 300 ppb in the presence of VOCs, but no HONO formation was observed with humified air and NO2. HONO production was saturated at high hydrocarbon concentrations and light intensities. The calculated first-order NO2 conversion rate to HONO on the asphalt surface was 1.2 × 10−4 s −1. The observed mean HONO emission flux was 1.3 × 109 molecules cm−2 s −1 with a similar range of those on other urban covered surfaces. The calculated vertical HONO profile using the measured HONO emission flux and 1-D steady state model revealed that the asphalt surface may account for 13% of daytime HONO in the elevated on-road pollutant concentrations in Seoul. However, we show that its HONO contribution could be much higher on real-life road surfaces directly exposed to much higher NO2 emissions from vehicle exhaust.