Unveiling the underestimated direct emissions of nitrous acid (HONO)

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

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

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

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

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

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

The reaction of sulfenic acids with OH and HO2 radicals in different environments

Sulfenic acids are involved in major chemical processes occurring in the atmosphere, in food chemistry and in biological systems. In these diverse environments, oxidation reactions caused by reactive oxygen species, especially hydroxyl (OH) and hydroperoxyl (HO2) radicals, are very important, but their mechanisms remain poorly understood. To address this question, in this paper we present high-level theoretical results on selected reactions in gas phase and in aqueous solution. The study shows that the abstraction of the acidic hydrogen by OH or HO2 is the most important process in all cases. It leads to the formation of sulfinyl radicals and H2O or H2O2, respectively, following a proton-coupled electron transfer (pcet) mechanism. The associated rate constants depend on sulfenic acid derivative when the oxidizing species is HO2, but all processes are diffusion controlled in the case of reaction with OH. From structurally simple systems to a cysteine-derived model peptide, this work provides a systematic study that contributes to a comprehensive understanding of the reactivity of sulfenic acids with radicals.

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

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

Exploring Photochemical Conversion of NO2 to HONO on N-Heterocycles: Unique Variation Trend of Gases, Kinetics, and Mechanism

The photochemical conversion of NO2 to nitrous acid (HONO) on N-heterocycles and the corresponding benzene compounds was investigated through a flow tube reactor coupled with the methods of spectroscopy, mass spectrum, and theoretical calculation. The average uptake coefficient of NO2 on organics during 1 h (γa) varied in the range of (1.1-8.0) × 10-6. Light led to an increase in γa by a factor of 2.2-11.5 times. HONO was generated by electron transfer from excited organics to NO2, as shown by the linear correlation of NO2 uptake and HONO formation with electron (e-) production from organics. Especially, the temporal variation of NO2 and HONO concentrations from the NO2 uptake on 9-methyl acridine (MA-N) showed a unique trend, where the NO2 uptake and HONO formation increased with time. The photochemical reaction of NO2 with MA-N generated nitrogen-/oxygen-containing species as well as demethylation species. The coexistence of oxygen-containing species, including acridone and hydroxy-methylacridine, had a synergistic effect on the photochemical conversion of NO2 to HONO. According to experimental results, it was speculated that the heterogeneous reaction between NO2 and organics could contribute to a HONO source strength lower than 660 ppt h-1 on the near-ground surface and lower than 92 ppt h-1 in the aloft air.

Soil Emissions of Reactive Oxidized Nitrogen Reduce the Effectiveness of Anthropogenic Source Control in China

Nitrogen dioxide (NO2) has decreased by ∼33% across over 1200 monitoring sites in China during 2015-2023, following a series of clean air policies. However, most of these sites are located in or near cities, leading to uncertainties in NO2 trends beyond urban regions due to limited observations. Here, we used satellite measurements to examine the differences in NO2 trends between urban and rural China. In urban areas, NO2 columns decreased by 4.0% per annum (a-1) during summer 2011-2023, consistent with bottom-up anthropogenic emission inventory and in situ measurements. In contrast, rural NO2 columns showed a slower than expected reduction (-2.6 to -0.0% a-1) during the same period. Model simulations with updates in the soil reactive oxidized nitrogen (Nr) scheme indicated that increasing soil Nr emissions can be an important factor contributing to the observed slow NO2 decrease in rural areas. This unregulated source increased summertime pollutant levels, partially offsetting the national efforts to mitigate NO2, ozone (O3), and particulate nitrate (NO3-) levels by 20.9%, 15.4%, and 4.7%, respectively, from 2011 to 2020. In the agriculture-intensive North China Plain, the increase in soil Nr emissions offset 46.6% of the NO2 reductions achieved by clean air policies. Our results highlight the increasing significance of soil emissions and the need to control them in future air-quality policies.

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

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

Resolving the overlooked photochemical nitrophenol transformation mechanism induced by nonradical species under visible light

Nitrophenols present on the surface of particulates are ubiquitous in the atmosphere. However, its atmospheric photochemical transformation pathway remains unknown, for which the crucial effect of visible light is largely overlooked, resulting in an incomplete understanding of the effects of nitrophenols in the atmospheric environment. This study delves into the photolysis mechanism of 4-nitrophenol (4NP), one of the most abundant atmospheric nitrophenol compounds, on the surface of photoactive particulates under visible light irradiation. Unexpectedly, the nonradical species (singlet oxygen, 1O2) was identified as a dominant factor in driving the visible photolysis of 4NP. The pathways of HONO and p-benzoquinone (C6H4O2) generation were clarified by acquiring direct evidence of C-N and O-H bond breakage in the nitro (-NO2) and hydroxyl (-OH) groups of 4NP. The further decomposition of HONO results in the generation of NO and hydroxyl radicals, which could directly contribute to atmospheric oxidizing capacity and complicate the PM2.5 composition. Significantly, the behavior of 1O2-induced visible photolysis of 4NP was universal on the surface of common particulates in the atmosphere, such as A1 dust and Fe2O3. This work advances the understanding of the photochemical transformation mechanism of particulate-phase atmospheric nitrophenols, which is indispensable in elucidating the role of nitrophenols in atmospheric chemistry.

Accurately Predicting Spatiotemporal Variations of Near-Surface Nitrous Acid (HONO) Based on a Deep Learning Approach

Gaseous nitrous acid (HONO) is identified as a critical precursor of hydroxyl radicals (OH), influencing atmospheric oxidation capacity and the formation of secondary pollutants. However, large uncertainties persist regarding its formation and elimination mechanisms, impeding accurate simulation of HONO levels using chemical models. In this study, a deep neural network (DNN) model was established based on routine air quality data (O3, NO2, CO, and PM2.5) and meteorological parameters (temperature, relative humidity, solar zenith angle, and season) collected from four typical megacity clusters in China. The model exhibited robust performance on both the train sets [slope = 1.0, r2 = 0.94, root mean squared error (RMSE) = 0.29 ppbv] and two independent test sets (slope = 1.0, r2 = 0.79, and RMSE = 0.39 ppbv), demonstrated excellent capability in reproducing the spatiotemporal variations of HONO, and outperformed an observation-constrained box model incorporated with newly proposed HONO formation mechanisms. Nitrogen dioxide (NO2) was identified as the most impactful features for HONO prediction using the SHapely Additive exPlanation (SHAP) approach, highlighting the importance of NO2 conversion in HONO formation. The DNN model was further employed to predict the future change of HONO levels in different NOx abatement scenarios, which is expected to decrease 27-44% in summer as the result of 30-50% NOx reduction. These results suggest a dual effect brought by abatement of NOx emissions, leading to not only reduction of O3 and nitrate precursors but also decrease in HONO levels and hence primary radical production rates (PROx). In summary, this study demonstrates the feasibility of using deep learning approach to predict HONO concentrations, offering a promising supplement to traditional chemical models. Additionally, stringent NOx abatement would be beneficial for collaborative alleviation of O3 and secondary PM2.5.

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

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

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

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