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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.

Nitrous acid (HONO) is an important source of atmospheric hydroxyl radical (OH). However, HONO abundance in the marine boundary layer remain largely unknown. Here, ship-based measurements were performed to characterize the origins of HONO from ship plumes, marine atmosphere, and continental emissions over South China Sea (SCS) during September 2021. The results showed that the HONO concentrations were measured at substantial levels (1.0 ± 0.8 ppbv) in polluted plumes due to generally high NO x concentrations (52.3 ± 52.5 ppbv). Comparably, much lower HONO concentrations (0.086 ± 0.102 ppbv) were observed for marine atmosphere. During nighttime, the heterogeneous conversion of NO 2 was the predominant source of HONO and occurred mainly on the sea surfaces through NO 2 deposition. The HONO yield from this deposition (the proportion of NO 2 that was converted to HONO) was 0.08 and 0.06 for the marine atmosphere and continental emissions, respectively. In contrast, daytime known HONO formation of marine atmospheric origins was mainly attributed to homogeneous OH + NO reaction, although the contribution of heterogeneous NO 2 conversion might not be negligible. Approximately half of HONO sources during daytime are unknown, which were likely from photo-enhanced NO 2 conversation on the sea surfaces. Our results showed that for marine atmosphere and ship plumes, the daily contributions of HONO photolysis to OH radical formation were about 20.8 % and 72.2 %, respectively, while the contributions from ozone photolysis were 79.2 % and 27.8 %. An average HONO concentration of 0.17 ppbv was measured in close shore regions when air masses originated from mainland China, with the contributions from HONO and ozone to OH radical of 21.4 % and 78.6 % respectively, similar to those for the marine atmosphere. This study suggests that HONO in the SCS has various sources (e.g., marine atmosphere, ship plumes, and continental emissions) and makes significant contributions to the OH abundance which affects the oxidation capacity in this area., 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 © 2024 Elsevier B.V. All rights reserved.)

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.

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 (NO x ), ozone (O 3 ), ammonia (NH 3 ), sulphur dioxide (SO 2 ), and nitric acid (HNO 3 )), particulate matter (PM 2.5 ), 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 × 10 7 molecules cm 3 s -1 ) and 2.00 μg m -3  h -1 (7.1 × 10 6 molecules cm 3 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 × 10 7 molecules cm 3 s -1 due to the photolysis of HONO, which is much higher than the production of HONO from the reaction of O 1 D with H 2 O. 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., Competing Interests: Declaration of competing interest The authors declare no competing financial interest., (Copyright © 2024 Elsevier B.V. All rights reserved.)

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.

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.

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 (NH 3 ) 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 NH 3 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.

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 use the Community Multiscale Air Quality (CMAQv5.4) model to examine the potential impact of particulate nitrate (pNO 3 - ) photolysis on air quality over the Northern Hemisphere. We estimate the photolysis frequency of pNO 3 - by scaling the photolysis frequency of nitric acid (HNO 3 ) with an enhancement factor that varies between 10 and 100 depending on pNO 3 produces gaseous nitrous acid (HONO) and nitrogen dioxide (NO - ) over seawater thereby increasing atmospheric HONO and NO 3 mixing ratios. HONO subsequently undergoes photolysis, producing hydroxyl radicals (OH). The increase in NO - photolysis to quantify the range of impacts on tropospheric composition. The photolysis of pNO 3 - produces gaseous nitrous acid (HONO) and nitrogen dioxide (NO 2 ) over seawater thereby increasing atmospheric HONO and NO 2 mixing ratios. HONO subsequently undergoes photolysis, producing hydroxyl radicals (OH). The increase in NO 2 and OH alters atmospheric chemistry and enhances the atmospheric ozone (O 3 ) mixing ratio over seawater, which is subsequently transported to downwind continental regions. Seasonal mean model O 3 vertical column densities without pNO 3 - photolysis are lower than the Ozone Monitoring Instrument (OMI) retrievals, while the column densities with the pNO 3 - photolysis agree better with the OMI retrievals of tropospheric O 3 burden. We compare model O 3 mixing ratios with available surface observed data from the U.S., Japan, the Tropospheric Ozone Assessment Report - Phase II, and OpenAQ; and find that the model without pNO 3 - photolysis underestimates the observed data in winter and spring seasons and the model with pNO 3 - photolysis improves the comparison in both seasons, largely rectifying the pronounced underestimation in spring. Compared to measurements from the western U.S., model O 3 mixing ratios with pNO 3 - photolysis agree better with observed data in all months due to the persistent underestimation of O 3 without pNO 3 - photolysis. Compared to the ozonesonde measurements, model O 3 mixing ratios with pNO 3 - photolysis also agree better with observed data than the model O 3 without pNO 3 - photolysis., 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., (Published by Elsevier B.V.)

The insufficient study on vertical observations of main atmospheric reactive nitrogen oxides (NO 2 and HONO) posed a great challenge to evaluate their intertransport between urban and agricultural areas, and to further learn the atmospheric nitrogen chemistry and the atmospheric oxidation capacity at high altitudes. A stereoscopic measurement campaign (satellite remote sensing, hyperspectral unmanned aerial vehicle (UAV) remote sensing and MAX-DOAS observation) was performed in a typical inland city Hefei and its surrounding agricultural fields from June to October 2022. Average aerosol vertical profiles exhibited a Gaussian shape above 100 m with maximum values of 0.67 km -1 and 0.55 km -1 at 300-400 m layer at Anhui University (AHU) and Changfeng (CF), respectively. The distinct layered structure was mainly attributed to regional transport. Average H 2 O and NO 2 vertical profiles all showed a Gaussian shape and an exponential shape at AHU and CF, respectively. Moreover, the diurnal evolution of H 2 O profiles performed one peak and bi-peak patterns at AHU and CF, respectively, whereas the diurnal evolution of NO 2 at two stations all exhibited bi-peak patterns attributed to vehicle emissions. Average HONO vertical profiles showed an exponential shape and a Gaussian shape at AHU and CF, respectively. Higher HONO (> 0.05 ppb) above 1.0 km at 14:00-16:00 was observed at CF. The transport flux analysis showed that the northern transport flux always larger than southern transport flux for aerosol and H 2 O. The maximum northern transport fluxes appeared at 300 m and surface for aerosol and H 2 O, respectively. It indicated that surrounding agricultural fields was an important source of atmospheric H 2 O of city. The southern transport flux was larger than northern transport flux for NO 2 , with a maximum net transport flux of 9.20 ppb m s -1 at 100 m. It demonstrated that NO 2 transported from urban areas was an important source of NO 2 in agricultural fields. For HONO, the southern transport flux was larger than northern transport flux under 100 m, whereas it was opposite above 100 m. It indicated that the HONO distributed at high altitudes at agricultural fields had potential to enhance the atmospheric oxidation capacity of urban area. The net horizontal transport fluxes of HONO of our defined cropland were 5.25 μg m -2  s -1 and -3.65 μg m -2  s -1 during non-fertilization and fertilization periods, respectively. It indicated that the cropland could obviously export HONO to surrounding atmosphere during the fertilization period. Deducing the contribution of direct emission, heterogeneous process was a major source of HONO at urban and agricultural areas. The average surface conversion rate of NO 2 -to-HONO (C HONO ) was 0.01467 h -1 , and this value decreased with the increase of height at urban station. While average surface C HONO was 0.0322 h -1 at agricultural fields, which was ~1.2-2.8 times higher than that at urban area. The C HONO at agricultural fields significantly increased with the increase of height. The average C HONO at 1.0 km was ~2.0-3.6 times higher than that at surface. That suggested that the heterogeneous process was the main HONO source at high altitudes at CF, and this process obviously correlated with aerosol and H 2 O. The higher OH production from HONO (P(OH) HONO ) occurred at 0-200 m and 100-400 m with averaged values of 0.31 ppb h -1 and 0.39 ppb h -1 at AHU and CF, respectively. The high P(OH) HONO above 1.0 km at CF from September to October was strongly correlated with high O 3 (> 80 ppb). This study emphasized the importance of the stereoscopic of HONO on the analysis of its distribution, evolution, source and atmospheric oxidizing contribution., Competing Interests: Declaration of competing interest The authors declare that they have no competing interests., (Copyright © 2023. Published by Elsevier B.V.)

The unknown daytime source of HONO has been extensively investigated due to unexplained atmospheric oxidation capacity and current modelling bias, especially during cold seasons. In this study, abrupt morning increases in atmospheric HONO at a rural site in the North China Plain (NCP) were observed almost on daily basis, which were closely linked to simultaneous rises in atmospheric water vapor content and NH 3 concentrations. Dew and guttation water formation was frequently observed on wheat leaves, from which water samples were taken and chemically analyzed for the first time. Results confirmed that such natural processes likely governed the daily nighttime deposition and daytime release of HONO and NH 3 , which have not been considered in the numerous HONO budget studies investigating its large missing daytime source in the NCP. The dissolved HONO and NH 3 in leaf surface water droplets reached 1.4 and 23 mg L -1 during the morning on average, resulting in averaged atmospheric HONO and NH 3 increases of 0.89 ± 0.61 and 43.7 ± 29.3 ppb during morning hours, with relative increases of 186 ± 212 % and 233 ± 252 %, respectively. The high atmospheric oxidation capacity contained within HONO was stored in near surface liquid water (such as dew, guttation and soil surface water) during nighttime, which prevented its atmospheric dispersion after sunset and protected it from photodissociation during early morning hours. HONO was released in a blast during later hours with stronger solar radiation, which triggered and then accelerated daytime photochemistry through the rapid photolysis of HONO and subsequent OH production, especially under high RH conditions, forming severe secondary gaseous and particulate pollution. Results of this study demonstrate that global ecosystems might play significant roles in atmospheric photochemistry through nighttime dew formation and guttation processes., 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 © 2023. Published by Elsevier B.V.)

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., 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 © 2023 Elsevier Ltd. All rights reserved.)

A severe haze pollution occurred in Guangzhou from January 14 to 16, 2021, during which the mass concentration of PM 2.5 ranged from 76 to 243 μg m -3 . This level of pollution was rarely observed in recent years considering the improved air quality in Guangzhou. Therefore, it is crucial to comprehensively study the formation mechanisms of this severe haze pollution to prevent its reoccurrence. During the haze period, the concentrations of NO and NO 2 sharply increased by 7.4 and 3.8 times, respectively, and total volatile organic compounds (TVOCs) increased 7 times, suggesting enhanced primary emissions from vehicles due to stagnant meteorological conditions. Nitrate concentration (43 ± 20 μg m -3 ) increased 6.7 times and became the dominant species in PM 2.5 during the haze period. Notably, gaseous NH 3 , HONO and HNO 3 also exhibited a sharp increase, suggesting the important role of nitrate chemistry in the evolution of haze pollution. The simulation results from chemical box model revealed that the OH + NO 2 reaction was the dominant formation pathway for nitrate production (82 %) during the haze period. The net production rate of RO x radicals (including OH, HO 2 and RO 2 ) was 4.4 times higher during the haze period (5.8 ppb h -1 ) compared to the pre-haze period (1.3 ppb h -1 ). This was mainly attributed to the enhanced HONO and OVOCs photolysis, which increased from 0.6 ppb h -1 to 3.1 ppb h -1 and 0.4 ppb h -1 to 2.1 ppb h -1 , respectively. Furthermore, the sensitivity tests demonstrated the reductions in VOCs and NO x were both beneficial for controlling nitrate production by influencing OH production and N 2 O 5 uptake rate. These findings provide insights into the formation mechanisms of nitrate production during severe haze pollution and suggest that joint mitigation of PM 2.5 and O 3 can be achieved through the control of VOCs emission., Competing Interests: Declaration of competing interest The authors declare that they have no conflict of interest., (Copyright © 2023. Published by Elsevier B.V.)

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., 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 © 2023 Elsevier B.V. All rights reserved.)

The photochemical reaction of NO 2 with organics may be a source of atmospheric HONO during the daytime. Here, the conversion of NO 2 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 NO x analyzer. O 2 played an inhibition role in NO 2 uptake and HONO formation on PAHs, as shown by 7%-45% and 15%-52% decrease in NO 2 uptake coefficient (γ) and HONO yield (Y HONO ), respectively. The negative effect of O 2 on the reaction between NO 2 and PAHs should be attributed to three reasons. First, O 2 could compete with NO 2 for the available sites on PAHs. Second, the quenching of the triple excited state of PAHs ( 3 PAHs*) by O 2 inhibited the NO 2 uptake. Third, NO 3 - to HONO. The environmental implications suggested that the NO 2 uptake on PAHs could contribute to a HONO source strength of 10-120 ppt h 2 uptake on PAHs could contribute to a HONO source strength of 10-120 ppt h -1 in the atmosphere., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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., 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 © 2023 Elsevier B.V. All rights reserved.)

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 NO 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 (NO 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no conflict of interest., (Copyright © 2022. Published by Elsevier B.V.)

Two ozone (O 3 ) processes, summer episode dominated by local production and autumn episode dominated by regional transport, were chosen to investigate the role of HONO in different pollution processes. Meteorological conditions, diurnal variation of O 3 , potential source contribution factor (PSCF) analysis, concentration weighted trajectory (CWT) models, and the distribution of the eight-hour maximum values of O 3 on mainland China all prove that summer O 3 was mainly locally generated while autumn O 3 episode was more susceptible to regional transport. The gaps between observations and simulation results with the default HONO chemistry in Master Chemical Mechanism (MCM) of Observation Based Model (OBM) were higher in summer episode (0.58 ppb) than autumn episode (0.37 ppb). Although we implemented nine additional sources in the model to revise the HONO chemistry, the simulated values were still lower than the observed values. HONO promoted O 3 production by accelerating the reaction of HO 2  + NO and RO 2  + NO, and promoted loss of O 3 by the reaction of OH + NO 2 and RO 2  + NO 2 . The net production rate of O 3 with HONO constraint increased by 28.50 % in summer and 22.43 % in autumn, which also indicated that HONO played more important role in the O 3 production in summer. The difference of NOx of daily RIR between the cases with and without HONO constraint was higher in summer O 3 episode (0.15 %/%) than that in autumn O 3 episode (0.09 %/%), the same as to VOCs with -0.20 %/% in summer O 3 episode and - 0.14 %/% in autumn O 3 episode, which indicated that the presence or absence of the HONO constraint has a greater impact on the RIR value in the case of dominant local generation. In brief, the O 3 sensitivity would be more favorable for VOCs without HONO constrained in the model, which would inevitably mislead policy makers to develop efficient policies to control O 3 pollution., 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 © 2023 Elsevier B.V. All rights reserved.)

Photosensitized renoxification of HNO 3 is found to produce HONO in an unexpectedly high yield, which has been considered an important source for atmospheric HONO. Conventionally, the production of HONO is ascribed to the secondary photolysis of the primarily formed NO 2 . In this study, by using humic acid (HA) as a model environmental photosensitizer, we provide evidence of the direct formation of NO 2 in its electronic excited state (NO 2 *) as a key intermediate during the photosensitizing renoxification of HNO 3 . Moreover, the high HONO yield originates from the heterogeneous reaction of the primarily formed NO 2 * with the co-adsorbed water molecules on HA. Such a mechanism is supported by the increase of the product selectivity of HONO with relative humidity. Further luminescence measurements demonstrate clearly the occurrence of an electronic excited state (NO 2 *) from photolysis of adsorbed HNO 3 on HA. This work deepens our understanding of the formation of atmospheric HONO and gives insight into the transformation of RNS.

Atmospheric HONO acts as a major source for OH radicals in polluted areas, playing an important role in formation of secondary pollutants. However, the atmospheric HONO sources remain unclear. Here we propose that the heterogeneous reaction of NO 2 on aerosols during aging processes acts as the dominant source for nocturnal HONO. Based on the nocturnal variations of HONO and related species in Tai'an city of China, we firstly developed new method to estimate the localized HONO dry deposition velocity (v(HONO)). The estimated v(HONO) of 0-0.077 m/s was in a good agreement with the reported ranges. Additionally, we set up a parametrization to reflect the HONO formation from the aged air parcels based on the variation of HONO/NO 2 ratio. The detailed variation of nocturnal HONO could be well reproduced by a complete budget calculation coupled with above parameterizations, with the difference between the observed and calculated HONO levels being <5 %. The results also revealed the average contribution of HONO formation from aged air parcels to atmospheric HONO could achieve to be ~63 % in average., 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 © 2023 Elsevier B.V. All rights reserved.)

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., 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 © 2022 Elsevier B.V. All rights reserved.)

Quantum chemical computations and transition state theory are employed to systematically research the influence of a single molecule water on the BrO + HONO reaction. Two distinct reactions, namely BrO + trans-HONO and BrO + cis-HONO are explored for the reaction in the absence of water, which is mainly decided by the configuration of HONO. With introduction a single water molecule to the reaction, the rate coefficient of the channel starting from BrO + cis-HONO and BrO + trans-HONO are 2.43 × 10 -19 and 5.22 × 10 -22  cm 3 molecule -1 s -1 , which is larger than the reaction in the absence of water. For further comprehend the impact of water on the BrO + HONO reaction, it is necessary to compute the effective rate coefficient by taking into account the concentration of water. The water-assisted effective rate coefficients for the BrO + HONO reaction are smaller than that the reaction in the absence of water. The reaction of BrO with cis-HONO is feasible both in absence and existence of water., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Urban grime represents an important environmental surface for heterogeneous reactions in urban environment. Here, we assess the physical and chemical properties of urban grime collected during six consecutive months in downtown of Guangzhou, China. There is a significant variation of the uptake coefficients of NO 2 on the urban grime as a function of the relative humidity (RH). In absence of water molecules (0% RH), the light-induced uptake coefficients of NO 2 on urban grime samples collected during six months are very similar in order of ≈10 -6 . At 80% RH, depending on the sampling month the light-induced uptake coefficient of NO 2 can reach one order of magnitude higher values (1.5 × 10 -5 , at 80% RH) compared to those uptakes at 0% RH. In presence of 80% RH, there are strong correlations between the measured NO 2 uptakes and the concentrations of the water soluble carbon, soluble anions, polycyclic aromatic hydrocarbons and n-alkanes depicted in the urban grime. These correlations, demonstrate that surface adsorbed water on urban grime play an important role for the uptakes of NO 2 . The heterogeneous conversion of NO 2 on two-month old urban grime under sunlight irradiation (68 W m -2 , 300 nm < λ < 400 nm) at 60% RH leads to the formation of unprecedented HONO surface flux of 4.7 × 10 10 molecules cm -2 s -1 which is higher than all previously observed HONO fluxes, thereby affecting the oxidation capacity of the urban atmosphere. During the heterogeneous chemistry of NO 2 with urban grime, the unsaturated and N-containing organic compounds are released in the gas phase which can affect the air quality in the urban environment., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Sasho Gligorovski reports financial support was provided by National Natural Science Foundation of China. Sasho Gligorovski reports a relationship with National Natural Science Foundation of China that includes: funding grants., (Copyright © 2022 Elsevier B.V. All rights reserved.)

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.

Despite implementation of strict emission mitigation measures since 2013, heavy haze with high levels of secondary aerosols still frequently engulfs the Guanzhong Basin (GZB), China, during wintertime, remarkably impairing visibility and potentially causing severe health issues. Although the observed low ozone (O 3 ) concentrations do not facilitate the photochemical formation of secondary aerosols, the measured high nitrous acid (HONO) level provides an alternate pathway in the GZB. The impact of heterogeneous HONO sources on the wintertime particulate pollution and atmospheric oxidizing capability (AOC) is evaluated in the GZB. Simulations by the Weather Research and Forecast model coupled with Chemistry (WRF-Chem) reveal that the observed high levels of nitrate and secondary organic aerosols (SOA) are reproduced when both homogeneous and heterogeneous HONO sources are considered. The heterogeneous sources (HET-sources) contribute about 98% of the near-surface HONO concentration in the GZB, increasing the hydroxyl radical (OH) and O 3 concentration by 39.4% and 22.0%, respectively. The average contribution of the HET-sources to SOA, nitrate, ammonium, and sulfate in the GZB is 35.6%, 20.6%, 12.1%, and 6.0% during the particulate pollution episode, respectively, enhancing the mass concentration of fine particulate matters (PM 2.5 ) by around 12.2%. Our results suggest that decreasing HONO level or the AOC becomes an effective pathway to alleviate the wintertime particulate pollution in the GZB., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

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

Nitrous acid (HONO) is an important component of the global nitrogen cycle and can regulate the atmospheric oxidative capacity. Soil is an important source of HONO. [HONO]*, the equilibrium gas-phase concentration over the aqueous solution of nitrous acid in the soil, has been suggested as a key parameter for quantifying soil fluxes of HONO. However, [HONO]* has not yet been well-validated and quantified. Here, we present a method to retrieve [HONO]* by conducting controlled dynamic chamber experiments with soil samples applied with different HONO concentrations at the chamber inlet. We show a bi-directional soil-atmosphere exchange of HONO and confirm the existence of [HONO]* over soil: when [HONO]* is higher than the atmospheric HONO concentration, HONO will be released from soil; otherwise, HONO will be deposited. We demonstrate that [HONO]* is a soil characteristic, which is independent of HONO concentrations in the chamber but varies with different soil water contents. We illustrate the robustness of using [HONO]* for quantifying soil fluxes of HONO, whereas the laboratory-determined chamber HONO fluxes can largely deviate from those in the real world for the same soil sample. This work advances the understanding of the soil-atmosphere exchange of HONO and the evaluation of its impact on the atmospheric oxidizing capacity.

Emission of 2-chlorophenols (2-CPs) can cause serious air pollution and health problems. Here, the reaction kinetics and products of key radicals in 2-CPs photo-oxidation are explored in both gaseous and heterogeneous reactions. Quantum chemical calculations show that •OH-addition pathways are more preferable than H-abstraction pathways in gas phase, while that is opposite in heterogeneous phase. At 298 K, the overall rate coefficients of the title reactions in gas and heterogeneous phases are 3.48 × 10 -13 and 2.37 × 10 -13  cm 3 molecule -1  s -1 with half-lives of 55.3 h and 81.2 h, respectively. The strong H-bonds between linear Si 3 O 2 (OH) 8 and 2-CPs change the energy barriers of initial •OH-addition and H-abstraction reactions, resulting in the competition between heterogeneous reactions and gas phase reactions. The products in heterogeneous reactions are chloroquinone and HONO, which can cause atmospheric acid deposition and eco-toxicity. In gas phase, self-cyclization of alkoxy radical (RO•) leads to formation of •HO 2 and highly‑oxygenated molecules, which cause formation of secondary organic aerosol. It is emphasized that oxidation of 2-CPs by •OH leads to formation of more toxic products for aquatic organisms. Therefore, more attention should be focused on the products originated from •OH-initiated reactions of (2-)CPs in gaseous and heterogeneous reactions., 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 © 2021 Elsevier B.V. All rights reserved.)

Nitrous acid (HONO) photolysis is an important atmospheric reaction that leads to the formation of hydroxyl radicals (OH), the main diurnal atmospheric oxidants. The process of HONO formation remains unclear, and comparisons between field measurements and model results have highlighted the presence of unknown HONO sources. HONO production on plant surfaces was recently suggested to contribute to atmospheric HONO formation, but there is limited information on the quantification of HONO production and uptake by plants. To address this gap in the existing knowledge, the current study investigated HONO exchange on living Zea mays plants. Experiments were conducted in growth chambers under controlled experimental conditions (temperature, relative humidity, NO 2 mixing ratio, light intensity, CO 2 mixing ratio) at temperatures ranging between 283 and 299 K. To investigate the effect of drought on HONO plant-atmosphere exchanges, experiments were carried out on two sets of Zea mays plants exposed to two different water supply conditions during their growth: optimal watering (70% of the field capacity) and water stress (30% of the field capacity). Results indicated that the uptake of HONO by control Zea mays plants increased linearly with ambient temperature, and was correlated with CO 2 assimilation for temperatures ranging from 283 to 299 K. At 299 K, HONO production on the leaves offset this uptake and Zea mays plants were a source of HONO, with a net production rate of 27 ± 7 ppt h -1 . Deposition velocities were higher for HONO than CO 2 , suggesting a higher mesophyll resistance for CO 2 than HONO. As water stress reduced the stomatal opening, it also decreased plant-atmosphere gas exchange. Thus, climate change, which may limit the availability of water, will have an impact on HONO exchange between plants and the atmosphere., 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 © 2021 Elsevier B.V. All rights reserved.)

Nitrous acid (HONO) is a major source of hydroxyl radicals in the troposphere through its photolysis, and can significantly influence ozone (O 3 ) levels, thereby causing considerable crop yield losses. Previous studies have assessed relative crop yield losses by using exposure-response equations with observed or simulated O 3 , however, the contribution of enhanced O 3 due to potential HONO sources to the crop yield losses has never been quantified. In this study, for the first time, we evaluated the crop yield losses caused by potential HONO sources in the North China Plain (NCP), which is one of the major grain-producing areas in China suffering from heavy O 3 pollution, by using the Weather Research and Forecasting/Chemistry (WRF-Chem) model during the wheat and maize growing seasons of 2016. HONO simulations were significantly improved after including six potential HONO sources in the WRF-Chem model. The potential HONO sources produced a daily maximum 8-h O 3 enhancement of 8.1/8.2 ppb during the wheat/maize growing seasons, respectively, and led to ~11.4%/3.3% relative yield losses for wheat/maize, respectively, corresponding to approximately US$3.78/0.66 billion losses, respectively, in NCP in 2016. The above results suggest that potential HONO sources play a significant role in O 3 formation and could induce high crop yield losses globally., Competing Interests: Declaration of competing interest No conflict of interest exists in the submission of this manuscript., (Copyright © 2021. Published by Elsevier B.V.)

We present a novel method, the Gaussian observational model for edge to center heterogeneity (GOMECH), to quantify the horizontal chemical structure of plumes. GOMECH fits observations of short-lived emissions or products against a long-lived tracer (e.g., CO) to provide relative metrics for the plume width ( w i / w CO ) and center ( b oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO ( i / w CO ). To validate GOMECH, we investigate OH and NO 3 oxidation processes in smoke plumes sampled during FIREX-AQ (Fire Influence on Regional to Global Environments and Air Quality, a 2019 wildfire smoke study). An analysis of 430 crosswind transects demonstrates that nitrous acid (HONO), a primary source of OH, is narrower than CO ( w HONO / w CO = 0.73-0.84 ± 0.01) and maleic anhydride (an OH oxidation product) is enhanced on plume edges ( w maleicanhydride / w CO = 1.06-1.12 ± 0.01). By contrast, NO 3 production [P(NO 3 )] occurs mainly at the plume center ( w P(NO 3 ) / w CO = 0.91-1.00 ± 0.01). Phenolic emissions, highly reactive to OH and NO 3 , are narrower than CO ( w phenol / w CO = 0.96 ± 0.03, w catechol / w CO = 0.91 ± 0.01, and w methylcatechol / w CO = 0.84 ± 0.01), suggesting that plume edge phenolic losses are the greatest. Yet, nitrophenolic aerosol, their oxidation product, is the greatest at the plume center ( w nitrophenolicaerosol / w CO = 0.95 ± 0.02). In a large plume case study, GOMECH suggests that nitrocatechol aerosol is most associated with P(NO 3 ). Last, we corroborate GOMECH with a large eddy simulation model which suggests most (55%) of nitrocatechol is produced through NO 3 in our case study.

Gaseous nitrous acid (HONO) has the potential to greatly contribute to the atmospheric oxidation capacity. Increased attention has been paid to in-particle nitrite or nitrous acid, N(III), as one of the HONO sources. However, sources and formation mechanisms of N(III) remain uncertain. Here, we study a much less examined reaction of Fe(II) and nitrate as a source of N(III). The N(III) production was indirectly probed by its multiphase reaction with SO 2 for sulfate production. Particles containing nitrate and Fe(III) were irradiated for generating Fe(II). Sulfate production was enhanced by the presence of UV and organic compounds likely because of the enhanced redox cycle between Fe(II) and Fe(III). Sulfate production rate increases with the concentration of iron-organic complexes in nitrate particles. Similarly, higher concentrations of iron-organic complexes yield higher nitrate decay rates. The estimated production rates of N(III) under simulated conditions in our study vary from 0.1 to 3.0 μg m -3 of air h -1 . These values are comparable to HONO production rates of 0.2-1.6 ppbv h -1 , which fall in the values reported in laboratory and field studies. The present study highlights a synergistic effect of the coexistence of iron-organic complexes and nitrate under irradiation as a source of N(III).

HONO is an important component of reactive nitrogen (N r ) and precursors of OH radical. However, the source and removal of HONO are not clear. Here, measurements of HONO (May 18-31, 2018) were conducted in Xi'an and Xianyang simultaneously for the first time. The relationship between HONO and other N r (such as NO and NO 2 ) in two cities was analyzed. The mixing ratio of HONO in Xi'an was 1.2 ± 0.8 ppbv, and that in Xianyang was 1.2 ± 1.1 ppbv. The nighttime HONO mixing ratio was higher in Xianyang, while the daytime HONO was higher in Xi'an. Compared with the contribution from heterogeneous process of NO 2 , direct emissions and homogeneous processes (NO + OH) were less important for nocturnal HONO formation in these two cities. The relative contribution of heterogeneous process in Xianyang was more important than that in Xi'an. The reaction of NO 2 upon aerosols surface was identified as an important source of HONO for two sites. The conversion of NO 2 on the other surfaces might attend the heterogeneous formation of HONO in Xianyang site. Daytime HONO budget analysis indicated that there was an additional unknown formation process of HONO at two sites. The net OH production rate from HONO (from 08:00 to 17:00) was 1.6 × 10 7 and 1.3 × 10 7 molecule/(cm 3 s) for Xian and Xianyang, 5.2 and 3.5 times higher than from O 3 photolysis. Besides, a dust storm appeared during this observation period, and the impact of local emission and transport processes was separately analyzed. The sources, characteristics, and effects of HONO identified in this study laid a foundation for further research on HONO and air pollution in the Guanzhong area., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Gas-phase nitrous acid (HONO) is a major precursor of hydroxyl radicals that dominate atmospheric oxidizing capacity. Nevertheless, pathways of HONO formation remain to be explored. This study unveiled an important CO 2 -catalysis mechanism of HONO formation, using Born-Oppenheimer molecular dynamics simulations and free-energy samplings. In the mechanism, HCO 3 - formed from CO 2 dimers to produce HONO at water surfaces, and simultaneously, itself reconverts back to CO 2 dimers to produce HONO at water surfaces, and simultaneously, itself reconverts back to CO 2 and HOC(O)ONO. A flow system experiment was performed to confirm the new mechanism, which indicated that HONO concentrations with CO - and HOC(O)ONO. A flow system experiment was performed to confirm the new mechanism, which indicated that HONO concentrations with CO 2 that catalyzes formation of HONO and affects atmospheric oxidizing capacity.2 that catalyzes formation of HONO and affects atmospheric oxidizing capacity.

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., (Copyright © 2020. Published by Elsevier B.V.)

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 TiO 2 as the substrate, where photoenhanced uptake of NO(g) and formation of NO 2 (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., (© 2020 The Authors. Indoor Air published by John Wiley & Sons Ltd.)

Photolysis of nitrous acid (HONO) is one of the major sources for atmospheric hydroxyl radicals (OH), playing significant role in initiating tropospheric photochemical reactions for ozone (O 3 ) production. However, scarce field investigations were conducted to elucidate this effect. In this study, a field campaign was conducted at a suburban site in southwest China. The whole observation was classified into three periods based on O 3 levels and data coverage: the serious O 3 pollution period (Aug 13-18 as P1), the O 3 pollution period (Aug 22-28 as P2) and the clean period (Sep 3-12 as P3), with average O 3 peak values of 96 ppb, 82 ppb and 44 ppb, respectively. There was no significant difference of the levels of O 3 precursors (VOCs and NOx) between P1 and P2, and thus the evident elevation of OH peak values in P1 was suspected to be the most possible explanation for the higher O 3 peak values. Considering the larger contribution of HONO photolysis to HO X primary production than photolysis of HCHO, O 3 and ozonolysis of Alkenes, sensitivity tests of HONO reduction on O 3 production rate in P1 are conducted by a 0-dimension model. Reduced HONO concentration effectively slows the O 3 production in the morning, and such effect correlates with the calculated production rate of OH radicals from HONO photolysis. Higher HONO level supplying for OH radical initiation in the early morning might be the main reason for the higher O 3 peak values in P1, which explained the correlation (R 2  = 0.51) between average O 3 value during daytime (10:00-19:00 LT) and average HONO value during early morning (00:00-05:00 LT). For nighttime accumulation, a suitable range of relative humidity that favored NO 2 conversion within P1 was assumed to be the reason for the higher HONO concentration in the following early morning which promoted O 3 peak values., 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 © 2020 Elsevier B.V. All rights reserved.)

Nitrous oxide (N 2 O) and NO y (nitrous acid (HONO) + nitric oxide (NO) + nitrogen dioxide (NO 2 )) are released as byproducts or obligate intermediates during aerobic ammonia oxidation, and further influence global warming and atmospheric chemistry. The ammonia oxidation process is catalyzed by groups of globally distributed ammonia-oxidizing microorganisms, which are playing a major role in atmospheric N 2 O and NO y emissions . Yet, little is known about HONO and NO 2 production by the recently discovered, widely distributed complete ammonia oxidizers (comammox), able to individually perform the oxidation of ammonia to nitrate via nitrite. Here, we examined the N 2 O and NO y production patterns by comammox bacterium Nitrospira inopinata during aerobic ammonia oxidation, in comparison to its canonical ammonia-converting counterparts, representatives of the ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Our findings, i) show low yield NO y production by the comammox bacterium compared to AOB; ii) highlight the role of the NO reductase in the biological formation of N 2 O based on results from NH 2 OH inhibition assays and its stimulation during archaeal and bacterial ammonia oxidations; iii) postulate that the lack of hydroxylamine (NH 2 OH) and NO transformation enzymatic activities may lead to a buildup of NH 2 OH/NO which can abiotically react to N 2 O ; iv) collectively confirm restrained N 2 O and NO y emission by comammox bacteria, an unneglectable consortium of microbes in global atmospheric emission of reactive nitrogen gases., 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 © 2020 Elsevier Ltd. All rights reserved.)

Nitrogen dioxide (NO 2 ) is an air pollutant discharged from combustion of human activities. Nitrous acid (HONO), measured as NO 2 , is thought to impact respiratory function more than NO 2 . HONO and NO 2 have an equilibrium relationship, and their reaction is affected by climate conditions. This study was conducted to discuss the extent of HONO contained in NO 2 , depending on the level of urbanization. Whether climate conditions that promote HONO production enhanced the level of NO 2 measured was investigated using time series analysis. Climate and outdoor air pollution data measured in April 2009-March 2017 in urban (Tokyo, Osaka, and Aichi) and rural (Yamanashi) areas in Japan were used for the analysis. Air temperature had a trend of negative associations with NO 2 , which might indicate the decomposition of HONO in the equilibrium between HONO and NO 2 . The associations of relative humidity with NO 2 did not have consistent trends by prefecture: humidity only in Yamanashi was positively associated with NO 2 . In high relative humidity conditions, the equilibrium goes towards HONO production, which was observed in Yamanashi, suggesting the proportion of HONO in NO 2 might be low/high in urban/rural areas.

Nitrous acid (HONO) is an important reservoir of the hydroxyl radical (OH) and thus plays a central role in tropospheric chemistry. Exhaust from engines has long been known as a major primary source of HONO, yet most previous studies focused on vehicle emissions on land. In comparison, ship emissions of HONO have been rarely characterized, and their impacts on the tropospheric oxidation chemistry have not been quantified. In this study, we conducted cruise measurements of HONO and related species over the East China Sea. Contrasting air masses from pristine marine background air to highly polluted ship plumes were encountered. The emission ratio of ΔHONO/ΔNO x (0.51 ± 0.18%) was derived from a large number of fresh ship plumes. Using the in-situ measured emission ratio, a global ship emission inventory of HONO was developed based on the international shipping emissions of NO x in the Community Emission Data System inventory. The global shipping voyage emits approximately 63.9 ± 22.2 Gg yr -1 of HONO to the atmosphere. GEOS-Chem modelling with the addition of ship-emitted HONO showed that HONO concentrations could increase up to 40-100% over the navigation areas, leading to about 5-15% increases of primary OH production in the early-morning time. This study elucidates the potentially considerable effects of ship HONO emissions on the marine atmospheric chemistry, and calls for further studies to better characterize the ship emissions of HONO and other reactive species, which should be taken into account by global and regional models., 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 © 2020 Elsevier B.V. All rights reserved.)

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., 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 © 2020. Published by Elsevier B.V.)

HONO measurement was conducted using a wet-chemistry-based method at the Changzhou Environmental Monitoring Center in April 2017. HONO ranged from 0.2-13.9 μg·m -3 with an average of (2.9±2.3) μg·m -3 . O 3 , HCHO, volatile organic compounds, photolysis frequency, and meteorological parameters were simultaneously monitored.·OH concentration was simulated by a Master Chemical Mechanism box model and the daytime maximum·OH concentration ranged from 1.0×10 6 to 14×10 6 molecules per cubic centimeter. The formation rates of·OH by photolysis of HONO, O 3 , HCHO, H 2 O 2 , and alkene ozonolysis were calculated as well. The effects of the five sources on atmospheric oxidation capacity were revealed:O 3 photolysis (46.4%) > HONO photolysis (41.1%) > alkene ozonolysis (10.9%) > HCHO photolysis (1.5%) > H 2 O 2 photolysis (0.1%). HONO photolysis for OH radical production played a major role in the early morning, before with an increase in O 3 concentration, O 3 photolysis began to account for most of the·OH production. After 17:00, due to a significant decrease in the intensity of solar radiation, the alkene ozonolysis started playing a major role in the formation of·OH. The photolysis of formaldehyde and hydrogen peroxide played a negligible role in·OH radical production in this study.

Atmospheric ammonia (NH 3 ) as the most important alkaline gas in the atmosphere has attracted much attention in recent years due to its critical role in haze formation, especially on the North China Plain (NCP). Comprehensive studies are needed for investigating diurnal variations of NH 3 and underlying mechanisms in different seasons and their potential impacts on atmospheric chemistry. In this study, continuous long-term observation (Mar. 2016 to May 2017) of NH 3 at a rural site in the NCP was used to characterize the diurnal variation of NH 3 in different seasons and to unveil its causes and potential impacts on atmospheric chemistry. NH 3 concentrations displayed rapid increases during the morning, reaching very prominent peaks mostly between 8:00 to 11:00 LT. Such frequent (55%) morning peaks were mainly caused by the evaporation of dew and guttation water droplets. Average dew and guttation water volume concentrations of 750 mL m -2 was estimated for spring, which resulted in approximate NH 3 emissions of 800 ng m -2 s - 1 . Such high emission fluxes from dew and guttation water evaporation have never been reported before, suggesting dew and guttation droplets to be significant night-time reservoirs and strong morning sources for NH 3 . In light of recent studies putting forward that NH 3 can promote the heterogeneous formation of HONO and nitrate under high humidity conditions, we investigated the differences in HONO and aerosol chemical composition diurnal variations between days with and without NH 3 morning spikes during November. HONO, nitrate and sulfate concentrations were significantly higher for days with NH 3 morning spikes, with HONO displaying a morning peak near that of NH 3 . These results demonstrate that the prevailing NH 3 morning spikes on the NCP have significant influences on aerosol formation and atmospheric chemistry. NH 3 emission mitigation strategies and regulations are urgently needed., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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%)., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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., (Copyright © 2019. Published by Elsevier B.V.)

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., (Copyright © 2018 Elsevier B.V. All rights reserved.)

We first coupled indoor emissions and biomass burning emissions into the WRF-Chem model besides the other four potential HONO sources (i.e., traffic emissions, soil emissions, and heterogeneous reactions on aerosol and ground surfaces). Eight simulations were performed in the Beijing-Tianjin-Hebei region (BTH) of China in August of 2006. The results indicated that traffic emissions and heterogeneous reactions on ground and aerosol surfaces were the key sources of HONO at night, accounting for ~41%, ~27% and ~20% of the nighttime simulated HONO concentrations, respectively. The two heterogeneous reactions were the main contributors during the day, accounting for ~66% (ground surfaces) and ~19% (aerosol surfaces) of the daytime simulated HONO concentrations. The indoor emission source could be the second largest contributor during nighttime and led to a maximum hourly enhancement of 0.59 and 0.76 ppb at the central urban sites of Beijing and Tianjin, respectively. The six potential HONO sources enhanced the monthly meridional-mean concentrations of O 3 , OH and HO 2 by 5-44%, 5~>150% and 5~>200%, respectively, leading to an enhancement of 1-3 μg m -3 in the monthly averaged concentrations of secondary organic aerosol (SOA), and that of 10-35 μg m -3 in the largest hourly concentrations of SOA within 1000 m above the ground in the BTH. The major precursors of the enhanced SOA were Xylenes, Toluene and BIGALK (lumped alkanes C > 3). The inclusion of the six potential HONO sources in the WRF-Chem model considerably improved the HONO simulations at both urban and suburban sites compared with the corresponding observations. The above results suggested that the six potential HONO sources significantly enhanced the atmospheric oxidation capacity and thus accelerated SOA chemical aging in the BTH of China, leading to large enhancements in the hourly SOA concentrations and aggravating haze events in this region., (Copyright © 2018 Elsevier B.V. All rights reserved.)