Soil HONO emissions at high moisture content are driven by microbial nitrate reduction to nitrite: tackling the HONO puzzle

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

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

Reducing Soil-Emitted Nitrous Acid as a Feasible Strategy for Tackling Ozone Pollution

Severe ozone (O3) pollution has been a major air quality issue and affects environmental sustainability in China. Conventional mitigation strategies focusing on reducing volatile organic compounds and nitrogen oxides (NOx) remain complex and challenging. Here, through field flux measurements and laboratory simulations, we observe substantial nitrous acid (HONO) emissions (FHONO) enhanced by nitrogen fertilizer application at an agricultural site. The observed FHONO significantly improves model performance in predicting atmospheric HONO and leads to regional O3 increases by 37%. We also demonstrate the significant potential of nitrification inhibitors in reducing emissions of reactive nitrogen, including HONO and NOx, by as much as 90%, as well as greenhouse gases like nitrous oxide by up to 60%. Our findings introduce a feasible concept for mitigating O3 pollution: reducing soil HONO emissions. Hence, this study has important implications for policy decisions related to the control of O3 pollution and climate change.

The Impact of Agroecosystems on Nitrous Acid (HONO) Emissions during Spring and Autumn in the North China Plain

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

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.

Atmospheric HONO emissions in China: Unraveling the spatiotemporal patterns and their key influencing factors

Nitrous acid (HONO) can be photolyzed to produce hydroxyl radicals (OH) in the atmosphere. OH plays a critical role in the formation of secondary pollutants like ozone (O3) and secondary organic aerosols (SOA) via various oxidation reactions. Despite the abundance of recent HONO studies, research on national HONO emissions in China remains relatively limited. Therefore, this study employed a "wetting-drying" model and bottom-up approach to develop a high-resolution gridded inventory of HONO emissions for mainland China using multiple data. We used the Monte Carlo method to estimate the uncertainty in HONO emissions. In addition, the primary sources of HONO emissions were identified and their spatiotemporal distribution and main influencing factors were studied. The results indicated that the total HONO emissions in mainland China in 2016 were 0.77 Tg N (R50: 0.28-1.42 Tg N), with soil (0.42 Tg N) and fertilization (0.26 Tg N) as the primary sources, jointly contributing to over 87% of the total. Notably, the North China Plain (NCP) had the highest HONO emission density (3.51 kg N/ha/yr). Seasonal HONO emissions followed the order: summer (0.38 kg N/ha) > spring (0.19 kg N/ha) > autumn (0.17 kg N/ha) > winter (0.06 kg N/ha). Moreover, HONO emissions were strongly correlated with fertilization, cropland, temperature, and precipitation. This study provides vital scientific groundwork for the atmospheric nitrogen cycle and the formation of secondary pollutants.

Metabolomics and metatranscriptomics reveal the influence mechanism of endogenous microbe (Staphylococcus succinus) inoculation on the flavor of fermented chili pepper

This study integrated metabolomic and metatranscriptomic techniques to examine how the endogenous microbe, Staphylococcus succinus, influenced the essential flavor of fermented chili peppers. The mechanisms governing spontaneous fermentation and S. succinus-inoculated fermentation were also elucidated. Esters (e.g., ethyl undecanoate, isoamyl acetate, and methyl salicylate), terpenes (e.g., terpinen-4-ol), and alcohols (e.g., α-terpineol, linalool, and 4-methyl-3-heptanol) were found to be the key aroma-active compounds, aspartic acid (Asp) and glutamic acid (Glu) were identified as primary flavoring free amino acids. Notably, during the early stages of S. succinus-inoculated fermentation, the production of these essential metabolites was abundant, while their gradual increase over time was observed in the case of spontaneous fermentation. Metatranscriptomic analysis revealed that S. succinus inoculation could up-regulate genes related to glycolysis, amino acid metabolism, and aroma compound synthesis. These changes sequentially boosted the production of sweet and umami free amino acids, enhanced organic acid levels, increased unique aroma compound generation, and further improved the flavor and quality of the fermented chili peppers. Therefore, S. succinus inoculation can augment the sensory quality of fermented chili peppers, making this strain a promising candidate for Sichuan pickle fermentation starters.

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

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

Geographical Distribution of Colorectal Cancer in Southwestern Iran Between Years 2011 and 2019

BACKGROUND/AIMS

Colorectal cancer is one of the most common cancers in the world. Various genetic, individual, and environmental factors are associated with this disease. Today, the role of environmental and geographical factors has been considered. Accordingly, the present study was conducted to determine the cumulative incidence and geographical distribution of colorectal cancer in Khuzestan province.

MATERIALS AND METHODS

This study was performed ecologically to determine the cumulative incidence of colorectal cancer and its geographical distribution in Khuzestan province between 2011 and 2019. The required information was extracted from the cancer registration program of the Ministry of Health and after refinement and replication.

RESULTS

The cumulative incidence of colorectal cancer in Khuzestan province between 2011 and 2019 was estimated at 40.18 per 100 000 people. The highest cumulative incidence was related to Ahvaz city (95.87 per 100 000 people) and the lowest was related to DashtAzadegan city (25.35 per 100 000 people). The highest cumulative incidence of colorectal cancer, based on climate conditions, is in dry geographical areas (63.77 per 100 000 people), based on average annual rainfall, is in areas with an average rainfall of 100-200 mm (66.28 per 100 000 people), and based on land use, belonged to areas with agricultural use (74.57 per 100 000 people).

CONCLUSION

The cumulative incidence of colorectal cancer is relatively high in Khuzestan province. The cumulative incidence of the disease was higher in the central regions of the province than in other regions. Also, based on geographical areas, the cumulative incidence of the disease was higher in areas with arid climates, the average rainfall was 100-200 mm, and in areas with agricultural land use.

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.

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

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

Ambient ozone at a rural Central European site and its vertical concentration gradient close to the ground

The representativeness of ambient air quality of an in situ measurement is key in the use and correct interpretation of the measured concentration values. Though the horizontal representativeness aspect is generally not neglected in air pollution studies, a detailed, high-resolution vertical distribution of ambient air pollutant concentrations is rarely addressed. The aim of this study is twofold: (i) to explore the vertical distribution of ground-level ozone (O3) concentrations measured at four heights above the ground-namely at 2, 8, 50, and 230 m-and (ii) to examine in detail the vertical O3 concentration gradient in air columns between 2 and 8, 8 and 50, and 50 and 230 m above the ground. We use the daily mean O3 concentrations measured continuously at the Košetice station, representing the rural Central European background ambient air quality observed during 2015-2021. We use the semiparametric GAM (generalised additive model) approach (with complexity or roughness-penalised splines implementation) to analyse the data with sufficient flexibility. Our models for both O3 concentrations and O3 gradients use (additive) decomposition into annual trend and seasonality (plus an overall intercept). The seasonal and year-to-year patterns of the modelled O3 concentrations look very similar at first glance. Nevertheless, a more detailed look through O3 gradients shows that they differ substantially with respect to their seasonal and long-term dynamics. The vertical O3 concentration gradient in 2-230 m is not uniform but changes substantially with increasing height and shows by far the highest dynamics near the ground between 2 and 8 m, differing in both the seasonal and annual aspects for all the air columns inspected. We speculate that non-linear changes of both seasonal and annual components of vertical O3 gradients are due to atmospheric-terrestrial interactions and to meteorological factors, which we will explore in a future study.

Atmospheric HONO formation during and after the Spring Festival holidays in a coastal city of China

Nitrous acid (HONO) is an important source of hydrogen oxides (HOx), which affects air quality, the atmospheric oxidation capacity, and human health. Here, we present ambient measurements of the HONO concentrations in Zhuhai, a coastal city in Southern China, from February 7 to March 15, 2021. The campaign was classified into two periods during (P1) and after (P2) the Spring Festival holidays. The average HONO mixing ratio during P2 (1.19 ± 0.85 ppbv) was much higher than that during P1 (0.24 ± 0.18 ppbv), likely due to the contribution of homogeneous HONO formation. During nighttime, the heterogeneous conversion rate during P2 (0.0089/hr) was considerably higher than that during P1 (0.0057/hr), suggesting a higher heterogeneous NO₂ conversion potential. However, the heterogeneous NO₂ conversion was the dominant way during P1 with a high percentage of 88%, while comparable ratios of heterogeneous and homogeneous formation were found (54% vs. 46%) during P2, indicating that the homogeneous formation was also important during P2. During daytime, homogeneous reaction was the major known pathway, with a contribution of 16% during P1 and 27% during P2, leaving large unknown HONO sources which reasonably correlated with the photo-enhanced NO₂ conversion. Two case scenarios were additionally explored, showing that there might be a primary emission source during one scenario (February 17-18) and vehicle emissions might be the major unknown HONO source for another scenario (March 3-5). The results suggest that large unknown daytime sources still exist which need more future ambient and laboratory studies.

Large Contribution of Nitrous Acid to Soil-Emitted Reactive Oxidized Nitrogen and Its Effect on Air Quality

Soil emissions have long been recognized as an important source of nitric oxide (NO), which regulates atmospheric oxidative capacity and the production of air pollutants. Recent research has also indicated that nitrous acid (HONO) can be emitted in significant quantities from soil microbial activities. However, only a few studies have quantified emissions of HONO along with NO from a wide range of soil types. In this study, we measured emissions of HONO and NO from soil samples collected from 48 sites across China and found much higher emissions of HONO than of NO, especially for samples from northern China. We performed a meta-analysis of 52 field studies in China, which revealed that long-term fertilization increased the abundance of nitrite-producing genes much more than the abundance of NO-producing genes. This promotion effect was greater in northern China than in southern China. In simulations using a chemistry transport model with laboratory-derived parametrization, we found that HONO emissions had a greater effect than NO emissions on air quality. Moreover, we determined that with projected continuous reductions in anthropogenic emissions, the contribution from soils to maximum 1 h concentrations of hydroxyl radicals and ozone and daily average concentrations of particulate nitrate in the Northeast Plain will increase to 17%, 4.6%, and 14%, respectively. Our findings highlight the need to consider HONO in the assessment of the loss of reactive oxidized nitrogen from soils to the atmosphere and its effect on air quality.

HONO Measurement by Catalytic Conversion to NO on Nafion Surfaces

A selective catalytic converter has been developed to quantify nitrous acid (HONO), a photochemical precursor to NO and OH radicals that drives the formation of ozone and other pollutants in the troposphere. The converter is made from a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) that was found to convert HONO to NO with unity yield under specific conditions. When coupled to a commercially available NO_x (=NO + NO₂) chemiluminescence (CL) analyzer, the system measures HONO with a limit of detection as low as 64 parts-per-trillion (ppt) (1 min average) in addition to NO_x. The converter is selective for HONO when tested against other common gas-phase reactive nitrogen species, although loss of O₃ on Nafion is a potential interference. The sensitivity and selectivity of this method allow for accurate measurement of atmospherically relevant concentrations of HONO. This was demonstrated by good agreement between HONO measurements made with the Nafion-CL method and those made with chemical ionization mass spectrometry in a simulation chamber and in indoor air. The observed reactivity of HONO on Nafion also has significant implications for the accuracy of CL NO_x analyzers that use Nafion to remove water from sampling lines.

Urgency of controlling agricultural nitrogen sources to alleviate summertime air pollution in the North China Plain

Agricultural nitrogen sources (ANS) have played an increasingly important role in the air quality since ANS emission controls are much weaker than those for fossil fuel combustion sources due to the increasing food demand. However, ANS emissions are highly uncertain due to stochastic agricultural management activities and limited field measurements, and impacts of ANS on the air quality remain elusive. In the study, the WRF-Chem model has been used to investigate ANS shares in near surface air pollutant concentrations during a growing season in the North China Plain (NCP), with ANS emissions constrained by satellite retrievals. Soil NOX and agricultural NH3 emissions are about 36% and 92% of their total emissions during the growing season. Sensitivity studies demonstrate that ANS count 16.9 μg m-3 (9.9%) of the mean maximum daily average 8-h ozone concentrations (MDA8 [O3]) and 8.9 μg m-3 (31.7%) of fine particulate matter concentrations ([PM2.5]) on average in the NCP. Additionally, the contributions of ANS to MDA8 [O3] and [PM2.5] increase with the deterioration of air pollution in cities. A 50% emission reduction in ANS decreases MDA8 [O3] ([PM2.5]) from 4.2% to 8.4% (from 19.7% to 31.9%) when the air quality changes from being lightly to heavily polluted in terms of MDA8 [O3] (hourly [PM2.5]). Without fossil fuel combustion emissions, the simulated average MDA8 [O3] and [PM2.5] are 111.7 and 8.2 μg m-3 in cities of the NCP, respectively, exceeding the new standards from the World Health Organization. Our study highlights important contributions of ANS to air quality and the urgency of ANS emission abatement for air pollution alleviation during summertime in the NCP.

Reactive Nitrogen Hotspots Related to Microscale Heterogeneity in Biological Soil Crusts

Biocrusts covering drylands account for major fractions of terrestrial biological nitrogen fixation and release large amounts of gaseous reactive nitrogen (N_r) as nitrous acid (HONO) and nitric oxide (NO). Recent investigations suggested that aerobic and anaerobic microbial nitrogen transformations occur simultaneously upon desiccation of biocrusts, but the spatio-temporal distribution of seemingly contradictory processes remained unclear. Here, we explore small-scale gradients in chemical concentrations related to structural characteristics and organism distribution. X-ray microtomography and fluorescence microscopy revealed mixed pore size structures, where photoautotrophs and cyanobacterial polysaccharides clustered irregularly in the uppermost millimeter. Microsensor measurements showed strong gradients of pH, oxygen, and nitrite, nitrate, and ammonium ion concentrations at micrometer scales in both vertical and lateral directions. Initial oxygen saturation was mostly low (∼30%) at full water holding capacity, suggesting widely anoxic conditions, and increased rapidly upon desiccation. Nitrite concentrations (∼6 to 800 μM) and pH values (∼6.5 to 9.5) were highest around 70% WHC. During further desiccation they decreased, while emissions of HONO and NO increased, reaching maximum values around 20% WHC. Our results illustrate simultaneous, spatially separated aerobic and anaerobic nitrogen transformations, which are critical for N_r emissions, but might be impacted by future global change and land management.

Climate warming, but not Spartina alterniflora invasion, enhances wetland soil HONO and NOx emissions

Climate warming and invasive plant growth (plant invasion) may aggravate air pollution by affecting soil nitrogen (N) cycling and the emissions of reactive N gases, such as nitrous acid (HONO) and nitrogen oxides (NO_x). However, little is known about the response of soil NO_y (HONO + NO_x) emissions and microbial functional genes to the interaction of climate warming and plant invasion. Here, we found that experimental warming (approximately 1.5 °C), but not Spartina alterniflora invasion, increased NO_y emissions (0-140 ng N m^-2 s^-1) of treated wetland soils by 4-10 fold. Warming also decreased soil archaeal and fungal richness and diversity, shifted their community structure (e.g., decreased the archaeal classes Thermoplasmata and Iainarchaeia, and increased the archaeal genus Candidatus Nitrosoarchaeum, and the fungal classes Saccharomycetes and Tritirachiomycetes), and decreased the overall abundance of soil N cycling genes. Structural equation modeling revealed that warming-associated changes in edaphic factors and the microbial N cycling potential are responsible for the observed increase in soil NO_y emissions. Collectively, the results showed that climate warming accelerates soil N cycling by stimulating large soil HONO and NO_x emissions, and influences air quality by contributing to atmospheric reactive N and ozone cycling.

Water-driven microbial nitrogen transformations in biological soil crusts causing atmospheric nitrous acid and nitric oxide emissions

Biological soil crusts (biocrusts) release the reactive nitrogen gases (N_r) nitrous acid (HONO) and nitric oxide (NO) into the atmosphere, but the underlying microbial process controls have not yet been resolved. In this study, we analyzed the activity of microbial consortia relevant in N_r emissions during desiccation using transcriptome and proteome profiling and fluorescence in situ hybridization. We observed that < 30 min after wetting, genes encoding for all relevant nitrogen (N) cycling processes were expressed. The most abundant transcriptionally active N-transforming microorganisms in the investigated biocrusts were affiliated with Rhodobacteraceae, Enterobacteriaceae, and Pseudomonadaceae within the Alpha- and Gammaproteobacteria. Upon desiccation, the nitrite (NO₂^-) content of the biocrusts increased significantly, which was not the case when microbial activity was inhibited. Our results confirm that NO₂^- is the key precursor for biocrust emissions of HONO and NO. This NO₂^- accumulation likely involves two processes related to the transition from oxygen-limited to oxic conditions in the course of desiccation: (i) a differential regulation of the expression of denitrification genes; and (ii) a physiological response of ammonia-oxidizing organisms to changing oxygen conditions. Thus, our findings suggest that the activity of N-cycling microorganisms determines the process rates and overall quantity of N_r emissions.

Measurement of HONO flux using the aerodynamic gradient method over an agricultural field in the Huaihe River Basin, China

To investigate nitrous acid (HONO) levels and potential HONO sources above crop rotation fields. The HONO fluxes were measured by the aerodynamic gradient (AG) method from 14 December 2019 to 2 January 2020 over an agricultural field in the Huaihe River Basin. The ambient HONO levels were measured at two different heights (0.15 and 1.5 m), showing a typical diurnal cycle with low daytime levels and high nighttime levels. The upward HONO fluxes were mostly observed during the day, whereas deposition dominated at night. The diurnal variation of HONO flux followed solar radiation, with a noontime maximum of 0.2 nmol/(m²∙sec). The average upward HONO flux of 0.06 ± 0.17 nmol/(m²∙sec) indicated that the agricultural field was a net source for atmospheric HONO. The higher HONO/NO₂ ratio and NO₂-to-HONO conversion rate close to the surface suggested that nocturnal HONO was formed and released near the ground. The unknown HONO source was derived from the daytime HONO budget analysis, with an average strength of 0.31 ppbV/hr at noontime. The surface HONO flux, which was highly correlated with the photolysis frequency J(NO₂) (R² = 0.925) and the product of J(NO₂) × NO₂ (R² = 0.840), accounted for ∼23% of unknown daytime HONO source. The significant correlation between HONO fluxes and J(NO₂) suggests a light-driven HONO formation mechanism responsible for the surface HONO flux during daytime.

Key Role of Equilibrium HONO Concentration over Soil in Quantifying Soil-Atmosphere HONO Fluxes

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.

Temporal variations of soil NO and NO2 fluxes in two typical subtropical forests receiving contrasting rates of N deposition

Soils have been widely acknowledged as important natural sources of nitric oxide (NO) and meanwhile sinks of nitric dioxide (NO₂). High nitrogen deposition across South China could potentially result in large NO emissions from subtropical forests soils there. In this study, the dynamic chamber method was applied to monitor NO and NO₂ fluxes at two subtropical forest sites in South China, namely "Qianyanzhou" (QYZ) and "Tieshanping" (TSP). Chronically higher N deposition occurred at TSP than that at QYZ. Besides soil water filled pore spaces (WFPS) and temperature, ambient NO concentration could also possibly be important in regulating temporal NO emissions, especially in the winter. For both sites, the optimum soil temperature was above 25 °C, while the optimum WFPS for NO release at QYZ was higher (65-70%) than that at TSP (<23%). Moreover, heavy rainfall could trigger NO emission pulses from moist soils at QYZ, while rainfall-induced NO pulses were only observed after a long drying period at TSP. Distinctly different contents of mineral nitrogen and soil moisture conditions between the two sites might induce the divergent preference of WFPS and responses to rainfall. The cumulative soil emission of NO reached 0.41 ± 0.01 and 0.76 ± 0.01 kg N ha^-1 yr^-1 at QYZ and TSP, contributing to 2.5% and 1.4% of the annual throughfall N input, respectively. At both sites, NO₂ were mainly deposited to soils, accounting for 2% and 21% of soil-emitted NO at QYZ and TSP, respectively. The observed annual NO emissions at these two sites were larger than the median values observed for tropical and temperate forests and unfertilized croplands. Higher N deposition could induce larger NO emission potential, while soil temperature and pH might also be important in regulating regional soil NO emissions as N-loss from subtropical forests.

Nitrous acid production and uptake by Zea mays plants in growth chambers in the presence of nitrogen dioxide

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, NO₂ mixing ratio, light intensity, CO₂ 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 CO₂ 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 CO₂, suggesting a higher mesophyll resistance for CO₂ 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.

Cropland nitrogen dioxide emissions and effects on the ozone pollution in the North China plain

Soil nitrogen dioxide (NO_X = NO₂ + NO) emissions have been measured and estimated to be the second most significant contributor to the NO_X burden following the fossil fuel combustion source globally. NO_X emissions from croplands are subject to being underestimated or overlooked in air pollution simulations of regional atmospheric chemistry models. With constraints of ground and space observations of NO₂, the WRF-Chem model is used to investigate the cropland NO_X emission and its contribution to the near-surface ozone (O₃) pollution in North China Plain (NCP) during a growing season as a case study. Model simulations have revealed that the cropland NO_X emissions are underestimated by around 80% without constraints of satellite measured NO₂ column densities. The biogenic NO_X source is estimated to account for half of the anthropogenic NO_X emissions in the NCP during the growing season. Additionally, the cropland NO_X source contributes around 5.0% of the maximum daily average 8h O₃ concentration and 27.7% of NO₂ concentration in the NCP. Our results suggest the agriculture NO_X emission exerts non-negligible impacts on the summertime air quality and needs to be considered when designing emission abatement strategies.

Nitrous acid emission from soil bacteria and related environmental effect over the North China Plain

Soil bacteria could be one of the important sources for ambient HONO. However, the HONO emission from soil bacteria over North China Plain (NCP) with vast croplands has not yet been evaluated. In this study, high-resolution simulations are created to explore the HONO emission from soil bacteria over NCP and related influences on atmospheric chemistry. Ground measurements of critical air pollutants including O₃, HONO, and PM_2.5 compositions are incorporated to constrain the model simulations. Results show that abundant HONO is emitted from soil bacteria over NCP during summertime and the emission rate varies dramatically for different areas (about 0.2 kg km^-2 d^-1 - 2.0 kg km^-2 d^-1). The HONO emission rate presents clear diurnal cycles with peaks of 1.5 kg km^-2 d^-1 in the afternoon and valleys of 0.4 kg km^-2 d^-1 during the early morning hours. The resulting HONO concentration ranges from 0.2 μg m^-3 to 1.4 μg m^-3, which predominates the total HONO concentration in ambient air, particularly in western NCP. The soil bacteria source can significantly alter the diurnal cycles of ambient HONO and OH concentrations over NCP, but only slightly change O₃ and PM_2.5 concentrations via participating photochemistry and secondary aerosol formations. These results highlight the pressing need for the involvement of HONO emission from soil bacteria in modeling studies regarding atmospheric chemistry, particularly in rural areas.

Contribution of Vehicle Emission and NO2 Surface Conversion to Nitrous Acid (HONO) in Urban Environments: Implications from Tests in a Tunnel

Nitrous acid (HONO) is an important photochemical precursor to hydroxyl radicals particularly in an urban atmosphere, yet its primary emission and secondary production are often poorly constrained. Here, we measured HONO and nitrogen oxides (NO_x) at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China. Multiple linear regression revealed that 73.9% of the inlet-outlet incremental HONO concentration was explained by NO₂ surface conversion, while the rest was directly emitted from vehicles with an average HONO/NO_x ratio of 1.31 ± 0.87%, which was higher than that from previous tunnel studies. The uptake coefficient of NO₂, γ(NO₂), on the tunnel surfaces was calculated to be (7.01 ± 0.02) × 10^-5, much higher than that widely used in models. As tunnel surfaces are typical of urban surfaces in the wall and road materials, the dominance of HONO from surface reactions in the poorly lit urban tunnel demonstrated the importance of NO₂ conversion on urban surfaces, instead of NO₂ conversion on the aerosol surface, for both daytime and night-time HONO even in polluted ambient air. The higher γ(NO₂) on urban surfaces and the elevated HONO/NO_x ratio from this study can help explain the missing HONO sources in urban areas.

Agricultural Fertilization Aggravates Air Pollution by Stimulating Soil Nitrous Acid Emissions at High Soil Moisture

Nitrogen lost from fertilized soil is a potentially large source of atmospheric nitrous acid (HONO), a major precursor of the hydroxyl radical. Yet, the impacts of fertilizer types and other influencing factors on HONO emissions are unknown. As a result, the current state-of-the-art models lack an appropriate parameterization scheme to quantify the HONO impact on air quality after fertilization. Here, we report laboratory measurements of high HONO emissions from soils at a 75-95% water-holding capacity after applying three common fertilizers, which contrasts with previous lower predictions at high soil moisture. Urea use leads to the largest release of HONO compared to the other two commonly used fertilizers (ammonium bicarbonate and ammonium nitrate). The significant promotion effect of fertilization lasted up to 1 week. Implementation of the lab-derived parametrization in a chemistry transport model (CMAQ) significantly improved postfertilization HONO predictions at a rural site in the agriculture-intensive North China Plain and increased the regionally averaged daytime OH, O₃, and daily fine particulate nitrate concentrations by 41, 8, and 47%, respectively. The results of our study underscore the necessity to include this large postfertilization HONO source in modeling air quality and atmospheric chemistry. Fertilizer structure adjustments may reduce HONO emissions and improve the air quality in polluted regions with intense agriculture.

Reactions of nitrite with goethite and surface Fe(II)-goethite complexes

Chemodenitrification-the abiotic (chemical) reduction of nitrite (NO₂^-) by iron (II)-plays an important role in nitrogen cycling due in part to this process serving as a source of nitrous oxide (N₂O). Questions remain about the fate of NO₂^- in the presence of mineral surfaces formed during chemodenitrification, such as iron(III) (hydr) oxides, particularly relative to dissolved iron(II). In this study, stirred-batch kinetic experiments were conducted under anoxic conditions (to mimic iron(III)-reducing conditions) from pH 5.5-8 to investigate NO₂^- reactivity with goethite (FeOOH_(s)) and Fe(II)-treated goethite using wet chemical and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Nitrite removal from solution by goethite was more rapid at pH 5.5 than at pH 7 and 8. Spectral changes upon nitrite adsorption imply an inner-sphere surface interaction (monodentate and bidentate) at pH 5.5 based on ATR-FTIR spectra of the nitrite-goethite interface over time. In iron(II)-amended experiments at pH 5.5 with high aqueous Fe(II) in equilibrium with goethite, nitrous oxide was generated, indicating that nitrite removal involved a combination of sorption and reduction processes. The presence of a surface complex resembling protonated nitrite (HONO) with an IR peak near ~1258 cm^-1 was observed in goethite-only and iron(II)-goethite experiments, with a greater abundance of this species observed in the latter treatment. These results might help explain gaseous losses of nitrogen where nitrite and iron(II)/goethite coexist, with implications for nutrient cycling and release of atmospheric air pollutants.

Linking meta-omics to the kinetics of denitrification intermediates reveals pH-dependent causes of N2O emissions and nitrite accumulation in soil

Soil pH is a key controller of denitrification. We analysed the metagenomics/transcriptomics and phenomics of two soils from a long-term liming experiment, SoilN (pH 6.8) and un-limed SoilA (pH 3.8). SoilA had severely delayed N₂O reduction despite early transcription of nosZ (mainly clade I), encoding N₂O reductase, by diverse denitrifiers. This shows that post-transcriptionally hampered maturation of the NosZ apo-protein at low pH is a generic phenomenon. Identification of transcript reads of several accessory genes in the nos cluster indicated that enzymes for NosZ maturation were present across a range of organisms, eliminating their absence as an explanation for the failure to produce a functional enzyme. nir transcript abundances (for NO₂⁻ reductase) in SoilA suggest that low NO₂⁻ concentrations in acidic soils, often ascribed to abiotic degradation, are primarily due to biological activity. The accumulation of NO₂⁻ in neutral soil was ascribed to high nar expression (nitrate reductase). The -omics results revealed dominance of nirK over nirS in both soils while qPCR showed the opposite, demonstrating that standard primer pairs only capture a fraction of the nirK pool. qnor encoding NO reductase was strongly expressed in SoilA, implying an important role in controlling NO. Production of HONO, for which some studies claim higher, others lower, emissions from NO₂⁻ accumulating soil, was estimated to be ten times higher from SoilA than from SoilN. The study extends our understanding of denitrification-driven gas emissions and the diversity of bacteria involved and demonstrates that gene and transcript quantifications cannot always reliably predict community phenotypes.

Elucidating the effect of HONO on O3 pollution by a case study in southwest China

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 (O₃) 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 O₃ levels and data coverage: the serious O₃ pollution period (Aug 13-18 as P1), the O₃ pollution period (Aug 22-28 as P2) and the clean period (Sep 3-12 as P3), with average O₃ peak values of 96 ppb, 82 ppb and 44 ppb, respectively. There was no significant difference of the levels of O₃ 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 O₃ peak values. Considering the larger contribution of HONO photolysis to HO_X primary production than photolysis of HCHO, O₃ and ozonolysis of Alkenes, sensitivity tests of HONO reduction on O₃ production rate in P1 are conducted by a 0-dimension model. Reduced HONO concentration effectively slows the O₃ 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 O₃ peak values in P1, which explained the correlation (R² = 0.51) between average O₃ 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 NO₂ conversion within P1 was assumed to be the reason for the higher HONO concentration in the following early morning which promoted O₃ peak values.

Heterogeneous photochemical uptake of NO2 on the soil surface as an important ground-level HONO source

Nitrous acid (HONO) production from the heterogeneous photochemical reaction of NO₂ on several Chinese soils was performed in a cylindrical reactor at atmospheric pressure. The NO₂ uptake coefficient (γ) and HONO yield (Y_HONO) on different soils were (0.42-5.16) × 10^-5 and 6.3%-69.6%, respectively. Although the photo-enhanced uptake of NO₂ on different soils was observed, light could either enhance or inhibit the conversion efficiency of NO₂ to HONO, depending on the properties of the soils. Soils with lower pH generally had larger γ and Y_HONO. Soil organics played a key role in HONO formation through the photochemical uptake of NO₂ on soil surfaces. The γ showed a positive correlation with irradiation and temperature, while it exhibited a negative relationship with relative humidity (RH). Y_HONO inversely depended on the soil mass (0.32-3.25 mg cm^-2), and it positively relied on the irradiance and RH (7%-22%). There was a maximum value for Y_HONO at 298 K. Based on the experimental results, HONO source strengths from heterogeneous photochemical reaction of NO₂ on the soil surfaces were estimated to be 0.2-2.7 ppb h^-1 for a mixing layer height of 100 m, which could account for the missing daytime HONO sources in most areas.

N2O and NOy production by the comammox bacterium Nitrospira inopinata in comparison with canonical ammonia oxidizers

Nitrous oxide (N₂O) and NO_y (nitrous acid (HONO) + nitric oxide (NO) + nitrogen dioxide (NO₂)) 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 N₂O and NO_y emissions_. Yet, little is known about HONO and NO₂ 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 N₂O 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 N₂O based on results from NH₂OH inhibition assays and its stimulation during archaeal and bacterial ammonia oxidations; iii) postulate that the lack of hydroxylamine (NH₂OH) and NO transformation enzymatic activities may lead to a buildup of NH₂OH/NO which can abiotically react to N₂O ; iv) collectively confirm restrained N₂O and NO_y emission by comammox bacteria, an unneglectable consortium of microbes in global atmospheric emission of reactive nitrogen gases.

Nitrous acid formation on Zea mays leaves by heterogeneous reaction of nitrogen dioxide in the laboratory

Nitrous acid (HONO) is of considerable interest because it is an important precursor of hydroxyl radicals (OH), a key species in atmospheric chemistry. HONO sources are still not well understood, and air quality models fail to predict OH as well as HONO mixing ratios. As there is little knowledge about the potential contribution of plant surfaces to HONO emission, this laboratory work investigated HONO formation by heterogeneous reaction of NO₂ on Zea mays. Experiments were carried out in a flow tube reactor; HONO, NO₂ and NO were measured online with a Long Path Absorption Photometer (LOPAP) and a NOx analyzer. Tests were performed on leaves under different conditions of relative humidity (5-58%), NO₂ mixing ratio representing suburban to urban areas (10-80 ppbv), spectral irradiance (0-20 W m^-2) and temperature (288-313 K). Additional tests on plant wax extracts from Zea mays leaves showed that this component can contribute to the observed HONO formation. Temperature and NO₂ mixing ratios were the two environmental parameters that showed substantially increased HONO emissions from Zea mays leaves. The highest HONO emission rates on Zea mays leaves were observed at 313 K for 40 ppbv of NO₂ and 40% RH and reached values of (5.6 ± 0.8) × 10⁹ molecules cm^-2 s^-1. Assuming a mixing layer of 300 m, the HONO flux from Zea mays leaves was estimated to be 171 ± 23 pptv h^-1 during summertime, which is comparable to what has been reported for soil surfaces.

Characteristics of HONO and its impact on O3 formation in the Seoul Metropolitan Area during the Korea-US Air Quality Study

Photolysis of nitrous acid (HONO) is recognized as an early-morning source of OH radicals in the urban air. During the Korea-US air quality (KORUS-AQ) campaign, HONO was measured using quantum cascade - tunable infrared laser differential absorption spectrometer (QC-TILDAS) at Olympic Park in Seoul from 17 May, 2016 to 14 June, 2016. The HONO concentration was in the range of 0.07-3.46 ppbv, with an average of 0.93 ppbv. Moreover, it remained high from 00:00-05:00 LST. During this time, the mean concentration was higher during the high-O3 episodes (1.82 ppbv) than the non-episodes (1.20 ppbv). In the morning, the OH radicals that were produced from HONO photolysis were 50% higher (0.95 pptv) during the high-O3 episodes than the non-episodes. Diurnal variations in HOx and O3 concentrations were simulated by the F0AM model, which revealed a difference of ~20 ppbv in the daily maximum O3 concentrations between the high-O3 episodes and non-episodes. Furthermore, the HONO concentration increased with an increase in relative humidity (RH) up to 80%; the highest HONO was associated with the top 10% NO2 in each RH group, confirming that NO2 is one of the main precursors of HONO. At night, the conversion ratio of NO2 to HONO was estimated to be 0.88×10-2 h-1; this ratio was found to increase with an increase in RH. The Aitken mode particles (30-120 nm), which act as catalyst surfaces, exhibited a similar tendency with a conversion ratio that increased along with RH, indicating the coupling of surfaces with HONO conversion. Using an artificial neural network (ANN) model, HONO concentrations were successfully simulated with measured variables (r2 = 0.66 as an average of five models). Among these variables, NOx, aerosol surface area, and RH were found to be the main factors affecting the ambient HONO concentrations. The results reveal that RH facilitates the conversion of NO2 to HONO by constraining the availability of aerosol surfaces. This study demonstrates the coupling of HONO with the HOx-O3 cycle in the Seoul Metropolitan Area (SMA) and provides practical evidence of the heterogeneous formation of HONO by employing the ANN model.

An automated dynamic chamber system for exchange flux measurement of reactive nitrogen oxides (HONO and NOX) in farmland ecosystems of the Huaihe River Basin, China

An automated dynamic chamber system was first developed to simultaneously measure the HONO flux and NOx flux. The new dynamic chamber system was applied to field observation, and the HONO and NO_X exchange flux of farmland in the Huaihe River Basin was obtained for the first time. The performance of the dynamic chamber system was verified in the field. In the field observation, the diurnal variations of the HONO fluxes and NO fluxes before and after a rainfall event exhibited two different trends. Before the rainfall and in the latter stage after the rainfall, the maxima of the HONO fluxes and NO fluxes occurred in the morning, then decreased gradually. However, during the early stage after the rainfall, the HONO fluxes and NO fluxes gradually increased in the morning and reached their maximum values in the afternoon. During the measurement period, the maximum HONO flux was 7.69 ng N m^-2 s^-1 and the maximum NO flux was 34.52 ng N m^-2 s^-1. There was no significant correlation between HONO flux and temperature before the rainfall and in the latter stage after the rainfall period, although the correlation coefficient (R) between HONO flux and temperature reached 0.78 in the early stage after the rainfall period, and the R between NO flux and HONO flux reached more than 0.6 before and after rainfall periods. The HONO flux of fresh soil samples were the same order of magnitude as that of field observations. The field results indicate that soil emissions are an important source of atmospheric HONO during the crop growth stage. Negative NO₂ fluxes were found in most observation periods, and there were significant negative linear correlations between NO₂ fluxes and atmospheric NO₂ concentrations. The R between ambient NO₂ concentration and NO₂ flux was 0.79, and the compensation point of NO₂ was 5 ppbv.

Effect of potential HONO sources on peroxyacetyl nitrate (PAN) formation in eastern China in winter

As an important secondary photochemical pollutant, peroxyacetyl nitrate (PAN) has been studied over decades, yet its simulations usually underestimate the corresponding observations, especially in polluted areas. Recent observations in north China found unusually high concentrations of PAN during wintertime heavy haze events, but the current model still cannot reproduce the observations, and researchers speculated that nitrous acid (HONO) played a key role in PAN formation. For the first time we systematically assessed the impact of potential HONO sources on PAN formation mechanisms in eastern China using the Weather Research and Forecasting/Chemistry (WRF-Chem) model in February of 2017. The results showed that the potential HONO sources significantly improved the PAN simulations, remarkably accelerated the RO_x (sum of hydroxyl, hydroperoxyl, and organic peroxy radicals) cycles, and resulted in 80%-150% enhancements of PAN near the ground in the coastal areas of eastern China and 10%-50% enhancements in the areas around 35-40°N within 3 km during a heavy haze period. The direct precursors of PAN were aldehyde and methylglyoxal, and the primary precursors of PAN were alkenes with C > 3, xylenes, propene and toluene. The above results suggest that the potential HONO sources should be considered in regional and global chemical transport models when conducting PAN studies.

Mechanisms of water regime effects on uptake of cadmium and nitrate by two ecotypes of water spinach (Ipomoea aquatica Forsk.) in contaminated soil

Availability of cadmium (Cd) and nitrate and their transfer to green leafy vegetables is highly dependent on physical, chemical and biochemical conditions of the soil. The phenotypic characteristics, accumulation of hazardous materials and rhizosphere properties of two ecotypes of water spinach in response to water stress were investigated. Flooding significantly enhanced plant growth and decreased Cd and nitrate concentrations in the shoot and root of both ecotypes of water spinach. Flooding extensively changed the physicochemical properties and biological processes in the rhizosphere, including increased pH and activities of urease and acid phosphatase, and decreased availability of Cd and nitrate and activity of nitrate reductase. Furthermore, flooding increased rhizosphere bacteria community diversity (including richness and evenness) and changed their community structure. Denitrifying bacteria (Clostridiales, Azoarcus and Pseudomonas), toxic metal resistant microorganisms (Rhodosporillaceae, Rhizobiales and Geobacter) were enriched in the rhizosphere under flooding conditions, and the plant growth-promoting taxa (Sphingomonadaceae) were preferentially colonized in the high accumulator (HA) rhizosphere region. These results indicated that flooding treatments result in biochemical and microbiological changes in soil, especially in the rhizosphere and reduced the availability of Cd and nitrate to plants, thus decreasing their uptake by water spinach. It is, therefore, possible to promote crop growth and reduce the accumulation of hazardous materials in vegetable crops like water spinach by controlling soil moisture conditions.

Microscale pH variations during drying of soils and desert biocrusts affect HONO and NH3 emissions

Microscale interactions in soil may give rise to highly localised conditions that disproportionally affect soil nitrogen transformations. We report mechanistic modelling of coupled biotic and abiotic processes during drying of soil surfaces and biocrusts. The model links localised microbial activity with pH variations within thin aqueous films that jointly enhance emissions of nitrous acid (HONO) and ammonia (NH₃) during soil drying well above what would be predicted from mean hydration conditions and bulk soil pH. We compared model predictions with case studies in which reactive nitrogen gaseous fluxes from drying biocrusts were measured. Soil and biocrust drying rates affect HONO and NH₃ emission dynamics. Additionally, we predict strong effects of atmospheric NH₃ levels on reactive nitrogen gas losses. Laboratory measurements confirm the onset of microscale pH localisation and highlight the critical role of micro-environments in the resulting biogeochemical fluxes from terrestrial ecosystems.

The mechanisms that determine the composition of nitrogen gas emissions from soil remain unclear. A biocrust mechanistic model was developed to resolve puzzling dynamics of nitrous acid and ammonia emissions from drying soil pointing to previously unknown microscale pH zonation in thinning water films that affect soil biogeochemical fluxes.

Impacts of six potential HONO sources on HOx budgets and SOA formation during a wintertime heavy haze period in the North China Plain

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 HO₂ 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 O₃ 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%).