HONO Sources Research Articles

Exploring the HONO source during the COVID-19 pandemic in a megacity in China

HONO is a critical precursor of •OH, but its sources are controversial due to its complex formation mechanism. This study conducted comprehensive observations in Zhengzhou from April 26 to May 11, 2022. Low NOx concentrations were observed during the Covid epidemic period (EP) (10.4 ± 3.0 ppb), compared to the pre-epidemic period (PEP) (12.5 ± 3.8 ppb). The mean HONO concentration during EP (0.53 ± 0.34 ppb) was 0.09 ppb lower than that during PEP (0.62 ± 0.53 ppb). The decrease in HONO concentration during EP came mainly at night due to the reduction in the direct emission (Pemi) (0.03 ppb/hr), the homogeneous reaction between •OH and NO (POH+NO) (0.02 ppb/hr), and the heterogeneous conversion of NO2 on the ground (0.01 ppb/hr). Notably, there was no significant change in daytime HONO concentration. The daytime HONO budget indicated that the primary HONO sources during PEP were the nitrate photolysis (Pnitrate), followed by the POH+NO, Pemi, the photo-enhanced reaction of NO2 on the ground (Pground+hv) and aerosol surface (Paerosol+hv). The primary HONO sources were Pnitrate, POH+NO, Pemi, and Paerosol+hv during EP, respectively. The missing source has a high correlation with solar radiation, there might be other photo-related HONO sources or the contributions of photosensitized reactions were underestimated. In the extremely underestimated cases, HONO production rates from the Pnitrate, Pground+hv, and Paerosol+hv increased by 0.17, 0.10, and 0.10 ppb/hr during PEP, 0.23, 0.13, and 0.16 ppb/hr during EP, and Pnitrate was still the primary source during both PEP and EP.

Measurement report: Exchange fluxes of HONO over agricultural fields in the North China Plain

Abstract. 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. Herein, we conduct FHONO measurements over agricultural fields under fertilized (FHONO-NP, normal fertilization and irrigation) and non-fertilized (FHONO-CK, normal irrigation but no fertilization) treatments. Our results show that nitrogen fertilizer use causes a remarkable increase in FHONO-NP. FHONO-NP exhibits distinct diurnal variations, with an average noontime peak of 152 ng N m−2 s−1. The average FHONO-NP within 3 weeks after fertilization is 97.7±8.6 ng N m−2 s−1, around 2 orders of magnitude higher than before fertilization, revealing the remarkable promotion effect of nitrogen fertilizer on HONO emissions. We also discuss other factors influencing 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 22.3 and 60.8 Gg N yr−1 in the North China Plain (NCP) and mainland 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 to mitigate regional air pollution effectively, future policies should consider reactive nitrogen emissions from agricultural soils.

HONO chemistry at a suburban site during the EXPLORE-YRD campaign in 2018: formation mechanisms and impacts on O3 production

Abstract. HONO is an important precursor for OH radicals that impact secondary-pollutant production. However, there are still large uncertainties about different HONO sources which hinder accurate predictions of HONO concentration and hence atmospheric oxidation capacity. Here HONO was measured during the EXPLORE-YRD campaign (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation and their Effects in the Yangtze River Delta), along with other important parameters, enabling us to comprehensively investigate HONO variation characteristics and evaluate the relative importance of different HONO sources by using a box model. HONO showed significant variations, ranging from several tens of parts per thousand to 4.4 ppb. The average diurnal pattern of HONO / NOx showed a maximum of 0.17 around noon and resembled that of j(O1D), indicating the existence of photo-induced sources. Modeling simulations with only the default HONO source (OH + NO) largely underestimated HONO concentrations, with the modeled-averaged noontime HONO concentration an order of magnitude lower than the observed concentration. The calculated strength of the unknown HONO source (Punknown) showed a nearly symmetrical diurnal profile with a maximum of 2.5 ppb h−1 around noon. The correlation analysis and sensitivity tests showed that the photo-induced NO2 conversion on the ground was able to explain Punknown. Additional HONO sources incorporated into the box model improved the model's performance in simulating HONO concentrations. The revised box model reproduced the nighttime HONO concentration well but still underestimated the daytime HONO concentration. Further sensitivity tests indicated the underestimation of daytime HONO was not due to uncertainties of photo-induced NO2 uptake coefficients on the ground or aerosol surfaces or the enhancement factor of nitrate photolysis but was more likely due to other sources that were not considered in the model. Among the incorporated HONO sources and the default gas-phase source, photo-induced NO2 conversion on the ground dominated the modeled HONO production during the daytime, accounting for 71 % of the total, followed by NO + OH, NO2 hydrolysis on the ground surface, vehicle emissions, photo-induced NO2 conversion on the aerosol surface, nitrate photolysis and NO2 hydrolysis on the aerosol surface. NO2 hydrolysis on the ground surface was the major source of nighttime HONO, contributing 55 % of total HONO production. HONO photolysis contributed 43 % of ROx production during the daytime, followed by O3 photolysis (17 %), HCHO photolysis (14 %), ozonolysis of alkenes (12 %) and carbonyl photolysis (10 %). With observed HONO as a model constraint, the average peak of net ozone production rate increased by 88 % to 12.6 ppb h−1 compared to that without observed HONO as a model constraint, indicating HONO evidently enhanced O3 production and hence aggravated O3 pollution in summer seasons. Our study emphasized the importance of heterogeneous NO2 conversion on the ground surface in HONO production and accurate parameterization of HONO sources in predicting secondary-pollutant production.

Severe photochemical pollution formation associated with strong HONO emissions from dew and guttation evaporation

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 NH3 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 NH3, which have not been considered in the numerous HONO budget studies investigating its large missing daytime source in the NCP. The dissolved HONO and NH3 in leaf surface water droplets reached 1.4 and 23 mg L−1 during the morning on average, resulting in averaged atmospheric HONO and NH3 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.

Nitrous acid budgets in the coastal atmosphere: potential daytime marine sources

Abstract. Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model (OBM) to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO2 on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model reproduced daytime HONO concentrations well during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO2 on aerosol surfaces (> 50 %), whereas particulate nitrate photolysis (34 %) prevailed during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in a “sea case”, with its HONO production rate reaching up to 0.70 ppbv h−1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or a heterogeneous uptake coefficient of NO2 would be required to match observed HONO concentrations. This missing marine HONO source affected the peak O3 production rate and OH radical concentration by 36 % and 28 %, respectively, at the observation site. Given the limited HONO observation data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere's oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments.

Underestimated benefits of NOx control in reducing SNA and O3 based on missing heterogeneous HONO sources

Underestimated benefits of NOx control in reducing SNA and O3 based on missing heterogeneous HONO sources

Urban Grime Photochemistry and Its Interaction with Air Pollutants NO, NO2, and O3: A Source for HONO and NO2 Impacting OH in Urban Areas

Urban Grime Photochemistry and Its Interaction with Air Pollutants NO, NO₂, and O₃: A Source for HONO and NO₂ Impacting OH in Urban Areas

Combining Chamber Experiments and Model Simulations to Evaluate an Indoor HONO Source with Surface Photochemical Properties

Nitrous acid (HONO) is an emerging indoor pollutant that can exert adverse health effects. The chemical production of indoor HONO has been attributed to NO2 heterogeneous reactions, and the source strength has been extensively evaluated via laboratory and model simulation studies. Photolysis of surface nitrate has recently been proposed as an indoor HONO source based on correlation analysis between indoor HONO accumulation and visible light radiation. However, neither experimental validation of the proposed mechanism nor source strength characterization is currently available. In this work, we designed an outdoor photochemical chamber (OPC) to simulate indoor HONO accumulation and established an indoor photochemical model (ICM) to calculate the indoor HONO budget. Indoor HONO accumulation revealed a distinct diel variation with a daytime maximum. Only with this indoor HONO source, the ICM reproduced the indoor HONO budget determined in the OPC. The enhanced reactive cross section of surface nitrate in visible light accounted for the major portion of the HONO source budget (77.2%) and the distinct diel variation. Success with the ICM encouraged us to simulate the HONO budget in real indoor environments. The calculated HONO production rates from surface nitrate photolysis at noon ranged from 1.4 to 4.1 ppbv h-1 under different indoor scenarios. On a daily average, this indoor HONO source contributed 42.4–52.7% to the total chemical sources in the living room but only contributed 4.7% to that in the kitchen, where NO2 heterogeneous reactions dominated.

Springtime HONO budget and its impact on the O3 production in Zibo, Shandong, China

Nitrous acid (HONO) is a vital source of OH radicals, nevertheless, the potential sources of HONO are still uncertain. To get an insight into the HONO sources and its role in atmospheric photochemistry, a comprehensive field campaign was conducted in Zibo City of the North China Plain in April 2021. A high average HONO concentration of 0.91 ppb was observed during spring. Direct emission was an important HONO source in Zibo City, with an average contribution of 32%. A calculated NO2 heterogeneous conversion efficiency of 0.37% h−1 indicated its limited contribution to the HONO formation. In addition, an unknown HONO source with an average production rate of 1.28 ppb h−1 was found through the daytime budget analysis, and the correlation analysis suggested that the photo-enhanced NO2 heterogeneous reaction on the ground surface and the nitrate (NO3−) photolysis might be the daytime HONO missing source. After incorporating the contributions of the two sources, 65% of the HONO unknown source could be explained, while there was still an unknown source with a production rate of 0.45 ppb h−1 that needed to be further explored. Furthermore, a 0-D box model was used to evaluate the role of HONO in O3 formation chemisry. The results showed that the absence of HONO led to an 83% underestimation of the OH production rate and a 45% underestimation of the net Ox production rate. This study strengthened the understanding of the HONO sources and its impact on atmospheric photochemistry in Zibo City of the North China Plain.

Insights Into NOx and HONO Chemistry in the Tropical Marine Boundary Layer at Cape Verde During the MarParCloud Campaign

AbstractChemical 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.

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

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

Unveiling the underestimated direct emissions of nitrous acid (HONO)

Gaseous nitrous acid (HONO) is a critical source of hydroxyl radicals (OH) in the troposphere. While both direct and secondary sources contribute to atmospheric HONO, direct emissions have traditionally been considered minor contributors. In this study, we developed δ15N and δ18O isotopic fingerprints to identify six direct HONO emission sources and conducted a 1-y case study on the isotopic composition of atmospheric HONO at rural and urban sites. Interestingly, we identified that livestock farming is a previously overlooked direct source of HONO and determined its HONO to ammonia (NH3) emission ratio. Additionally, our results revealed that spatial and temporal variations in atmospheric HONO isotopic composition can be partially attributed to direct emissions. Through a detailed HONO budget analysis incorporating agricultural sources, we found that direct HONO emissions accounted for 39~45% of HONO production in rural areas across different seasons. The findings were further confirmed by chemistry transport model simulations, highlighting the significance of direct HONO emissions and their impact on air quality in the North China Plain. These findings provide compelling evidence that direct HONO emissions play a more substantial role in contributing to atmospheric HONO than previously believed. Moreover, the δ15N and δ18O isotopic fingerprints developed in this study may serve as a valuable tool for further research on the atmospheric chemistry of reactive nitrogen gases.

Budget of atmospheric nitrous acid (HONO) during the haze and clean periods in Shanghai: Importance of heterogeneous reactions

Nitrous acid (HONO) plays a significant role in radical cycling and atmospheric oxidative chemistry. While the source and evolution of HONO in the Yangtze River Delta (YRD) region of China after 2018 remains largely unknown, this work monitored HONO and other air pollutants throughout 2019 at an urban site (Pudong, PD) and a suburban site (Qingpu, QP) in Shanghai. Episodes with high HONO mixing ratios but different PM2.5 levels, namely haze and clean episodes, were chosen for HONO budget analysis. Using an observation-based photochemical box model, relative importance of different sources and sinks of HONO were evaluated. Gas-phase reaction of NO with OH was found to be one of the most important daytime HONO formation sources, especially during the QPhaze period (accounting for 40.3 % of daytime HONO formation). In particular, heterogeneous conversion of NO2 on ground and aerosol surface was found to be the dominant source for nocturnal HONO. Photo-enhanced NO2 conversion on ground surface plays an important role in daytime HONO production (19.4 % in PDhaze vs. 27.6 % in PDclean, and 19.8 % in QPhaze vs. 25.9 % in QPclean). In addition, photo-enhanced NO2 conversion at the aerosol surface during haze episodes made more significant contributions to HONO formation compared to the clean periods (20.9 % in PDhaze vs. 17.1 % in PDclean, and 19.7 % in QPhaze vs. 11.2 % in QPclean). The role of multiphase reactions was found to be increasingly important in HONO generation with enhanced relative humidity (RH) during daytime. Significant unknown HONO source was further analyzed and found to be positively related with photolytic as well as multiphase pathways. Overall, our study sheds light on the budget of HONO in one of the biggest megacities in east China, which would help developing future mitigation strategies for urban HONO and atmospheric oxidation capacity.

Direct Formation of Electronic Excited NO2 Contributes to the High Yield of HONO during Photosensitized Renoxification.

Photosensitized renoxification of HNO3 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 NO2. In this study, by using humic acid (HA) as a model environmental photosensitizer, we provide evidence of the direct formation of NO2 in its electronic excited state (NO2*) as a key intermediate during the photosensitizing renoxification of HNO3. Moreover, the high HONO yield originates from the heterogeneous reaction of the primarily formed NO2* 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 (NO2*) from photolysis of adsorbed HNO3 on HA. This work deepens our understanding of the formation of atmospheric HONO and gives insight into the transformation of RNS.

Roles of Amides on the Formation of Atmospheric HONO and the Nucleation of Nitric Acid Hydrates.

Nitrous acid (HONO) is hazardous to the human respiratory system, and the hydrolysis of NO2 is the source of HONO. Hence, the investigation on the removal and transformation of HONO is urgently established. The effects of amide on the mechanism and kinetics of the formation of HONO with acetamide, formamide, methylformamide, urea, and its clusters of the catalyst were studied theoretically. The results show that amide and its small clusters reduce the energy barrier, the substituent improves the catalytic efficiency, and the catalytic effect order is dimer > monohydrate > monomer. Meanwhile, the clusters composed of nitric acid (HNO3), amides, and 1-6 water molecules were investigated in the amide-assisted nitrogen dioxide (NO2) hydrolysis reaction after HONO decomposes by combining the system sampling technique and density functional theory. The study on thermodynamics, intermolecular forces, optics properties of the clusters, as well as the influence of humidity, temperature, atmospheric pressure, and altitude shows that amide molecules promote the clustering and enhance the optical properties. The substituent facilitates the clustering of amide and nitric acid hydrate and lowers the humidity sensitivity of the clusters. The findings will help to control the atmospheric aerosol particle and then reduce the harm of poisonous organic chemicals on human health.

A new insight into the vertical differences in NO2 heterogeneous reaction to produce HONO over inland and marginal seas

Abstract. Ship-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were conducted along the marginal seas of China from 19 April to 16 May 2018 to measure the vertical profiles of aerosol, nitrogen dioxide (NO2), and nitrous acid (HONO). Along the cruise route, we found five hot spots with enhanced tropospheric NO2 vertical column densities (VCDs) in the Yangtze River Delta, Taiwan Strait, Guangzhou–Hong Kong–Macau Greater Bay Area, port of Zhanjiang, and port of Qingdao. Enhanced HONO concentrations could usually be observed under high-level aerosol and NO2 conditions, whereas the reverse was not always the case. To understand the impacts of relative humidity (RH), temperature, and aerosol on the heterogeneous reaction of NO2 to form HONO in different scenarios, the Chinese Academy of Meteorological Sciences (CAMS) and Southern University of Science and Technology (SUST) MAX-DOAS stations were selected as the inland and coastal cases, respectively. The RH turning points in CAMS and SUST cases were both ∼ 65 % (60 %–70 %), whereas two turning peaks (∼ 60 % and ∼ 85 %) of RH were found in the sea cases. As temperature increased, the HONO / NO2 ratio decreased with peak values appearing at ∼ 12.5∘C in CAMS, whereas the HONO / NO2 gradually increased and reached peak values at ∼ 31.5∘C in SUST. In the sea cases, when the temperature exceeded 18.0∘C, the HONO / NO2 ratio rose with increasing temperature and achieved its peak at ∼ 25.0∘C. This indicated that high temperature can contribute to the secondary formation of HONO in the sea atmosphere. In the inland cases, the correlation analysis between HONO and aerosol in the near-surface layer showed that the ground surface is more crucial to the formation of HONO via the heterogeneous reaction of NO2; however, in the coastal and sea cases, the aerosol surface contributed more. Furthermore, we discovered that the conversion rate of NO2 to HONO through heterogeneous reactions in the sea cases is larger than that in the inland cases in higher atmospheric layers (> 600 m). Three typical events were selected to demonstrate three potential contributing factors of HONO production under marine conditions (i.e., transport, NO2 heterogeneous reaction, and unknown HONO source). This study elucidates the sea–land and vertical differences in the forming mechanism of HONO via the NO2 heterogeneous reaction and provides deep insights into tropospheric HONO distribution, transforming process, and environmental effects.

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

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

Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica

Abstract. Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean ±1σ HONO amount fraction was (2.1 ± 1.5) pmol mol−1 and showed a diurnal cycle (range of 1.0–3.2 pmol mol−1) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4×1012 m-2s-1 and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol-1h-1 of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.

Aging of pollution air parcels acts as the dominant source for nocturnal HONO

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 NO2 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/NO2 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.

Atmospheric NOx oxidation as major sources for nitrous acid (HONO)

Nitrous acid (HONO) is the major precursor of hydroxyl (OH) radicals to initiate tropospheric chemistry leading to formation of secondary pollutants. The sources of atmospheric HONO, however, are not fully understood. Here we show two additional HONO sources that stem from atmospheric oxidation of nitrogen oxide (NOx = NO + NO2). Nitric acid (HNO3) formed from photooxidation of NO2 can be converted into HONO with a yield of ~53%, and dark NO oxidation by NO3 radicals in the presence of H2O produces HONO with a yield of 2%. The diurnal variations of HONO levels from field observations in the urban (Beijing) and rural (Wangdu) areas of the North China Plain can be well reproduced by the WRF-Chem model when the two new HONO sources are taken into account. The findings imply that atmospheric NOx oxidation pathways are the major sources for HONO, which can significantly accelerate ozone formation in polluted regions as well.