HONO Flux Research Articles

Surface exchange of HONO over paddy fields in the Pearl River Delta, China

Soil can release HONO, affecting atmospheric oxidation and tropospheric chemistry processes through the production of hydroxyl radicals (OH) and nitric oxide (NO) via photolysis. However, there is limited field observation of HONO flux, hindering a comprehensive understanding of its emission mechanisms. In this study, a dual open-top chambers system combined with a long path absorption photometer (LOPAP) was employed to measure HONO flux from paddy fields in the Pearl River Delta area (PRD), showing good reproducibility and effectivity to mitigate greenhouse effects. The average HONO flux was 0.77 ± 0.72 ng N m−2 s−1, displaying a diurnal pattern of higher fluxes during the day and lower fluxes at night, similar to previous flux observations. A strong linear correlation between the HONO flux with the product of NO2 and solar radiation (R2 = 0.90) suggests that surface reactions involving NO2 and sunlight may dominate HONO production from the paddy fields, surpassing microbial activity within the soil. Given the diversity of field environments, more field observations are needed for assessing emission rates of HONO and understanding underlying mechanisms.

Annual emissions of N2O, NO, HONO, and NH3 from maize-wheat fields in the North China Plain

Fertilized agricultural soil is a significant source of gaseous nitrogen compounds (GNCs), including N2O, NO, HONO, and NH3. The North China Plain (NCP) is the “hot region” for the release of these GNCs due to intensive fertilization practices. However, existing research has primarily focused on N2O emissions from fertilized farmland in the NCP, lacking comprehensive observational studies on other GNCs. Therefore, a continuous cumulative sampling technique (open-top dynamic chamber system) was utilized in this study to simultaneously measure the exchange fluxes of N2O, NO, HONO, and NH3 over summer maize-winter wheat rotation fields in the NCP. Results showed that GNC emissions from the soil displayed distinct diurnal variations, with higher emissions during the day attributed to elevated soil temperature. However, N2O emissions remained consistent between day and night, potentially influenced not only by soil temperature but also by soil humidity. Annual cumulative emissions and emission factors (EFs) for four GNCs were determined, indicating that N2O, NO, and NH3 emissions during the maize season were 1.38–2.37 times higher than those during the wheat season, with 98 % of HONO emissions occurring in the maize season. Additionally, the study first presented the annual HONO EFs of 0.36 ± 0.03 % in fertilized farmland. Furthermore, a comparison revealed that the fluxes of N2O, NO, NH3, and HONO using the conventional single-point sampling method were 26.4 %, 13.9 %, and 8.10 % lower, and 7.86 % higher compared to the continuous cumulative sampling method recommended in this study. In general, this study provided precise measurements of GNC emissions from farmland, offering essential foundational data for modeling parameters and contributing to the formulation of regional air pollution prevention and control policies.

Effect of fertilizer deep placement and nitrification inhibitor on N2O, NO, HONO, and NH3 emissions from a maize field in the North China Plain

Fertilized soil is an essential source of atmospheric gaseous nitrogen compounds (GNC, including N2O, NO, HONO, and NH3) that play important roles in global warming and regional air pollution. However, there is still a lack of comprehensive research on the exchange fluxes of these gases between soil and the atmosphere, let alone exploration of mitigation measures. Herein, the fluxes of N2O, NO, HONO, and NH3 under different fertilization treatments (surface fertilization (SF), deep fertilization (DF), DF with added nitrification inhibitor (DN), and control (CK, non-fertilization)) were comparatively investigated by open-top dynamic chambers in a maize field in the North China Plain (NCP) in 2022. The results showed that fertilization significantly promoted soil GNC emissions under SF and DF treatments, while there was no significant increase in GNC emissions except for NH3 under the DN treatment after fertilization. In comparison to the SF treatment, the cumulative emissions under the DF treatment exhibited a significant reduction, amounting to 80.8% for N2O, 89.4% for NO, 88.3% for HONO, 84.8% for NH3, and 83.9% for the total emissions of these four gases. These results emphasize the effect of fertilizer deep placement in mitigating soil GNC emissions, and further reductions were observed when using nitrification inhibitors like dicyandiamide (DCD). Compared with the cumulative emissions under the SF treatment, the DN treatment showed reductions exceeding 95% for N2O, NO, and HONO, and 77.1% for NH3 due to the inhibition of ammonia oxidation by DCD, resulting in an overall reduction of 88.8% for the four gases. Therefore, we advocate deep placement of fertilizer with or without nitrification inhibitors during crop cultivation to achieve a comprehensive reduction in GNC emissions and improve regional air quality.

Observations of HONO and its precursors between urban and its surrounding agricultural fields: The vertical transports, sources and contribution to OH

The insufficient study on vertical observations of main atmospheric reactive nitrogen oxides (NO2 and HONO) posed a great challenge to evaluate their intertransport between urban and agricultural areas, and to further learn the atmospheric nitrogen chemistry and the atmospheric oxidation capacity at high altitudes. A stereoscopic measurement campaign (satellite remote sensing, hyperspectral unmanned aerial vehicle (UAV) remote sensing and MAX-DOAS observation) was performed in a typical inland city Hefei and its surrounding agricultural fields from June to October 2022. Average aerosol vertical profiles exhibited a Gaussian shape above 100 m with maximum values of 0.67 km−1 and 0.55 km−1 at 300–400 m layer at Anhui University (AHU) and Changfeng (CF), respectively. The distinct layered structure was mainly attributed to regional transport. Average H2O and NO2 vertical profiles all showed a Gaussian shape and an exponential shape at AHU and CF, respectively. Moreover, the diurnal evolution of H2O profiles performed one peak and bi-peak patterns at AHU and CF, respectively, whereas the diurnal evolution of NO2 at two stations all exhibited bi-peak patterns attributed to vehicle emissions. Average HONO vertical profiles showed an exponential shape and a Gaussian shape at AHU and CF, respectively. Higher HONO (> 0.05 ppb) above 1.0 km at 14:00–16:00 was observed at CF. The transport flux analysis showed that the northern transport flux always larger than southern transport flux for aerosol and H2O. The maximum northern transport fluxes appeared at 300 m and surface for aerosol and H2O, respectively. It indicated that surrounding agricultural fields was an important source of atmospheric H2O of city. The southern transport flux was larger than northern transport flux for NO2, with a maximum net transport flux of 9.20 ppb m s−1 at 100 m. It demonstrated that NO2 transported from urban areas was an important source of NO2 in agricultural fields. For HONO, the southern transport flux was larger than northern transport flux under 100 m, whereas it was opposite above 100 m. It indicated that the HONO distributed at high altitudes at agricultural fields had potential to enhance the atmospheric oxidation capacity of urban area. The net horizontal transport fluxes of HONO of our defined cropland were 5.25 μg m−2 s−1 and -3.65 μg m−2 s−1 during non-fertilization and fertilization periods, respectively. It indicated that the cropland could obviously export HONO to surrounding atmosphere during the fertilization period. Deducing the contribution of direct emission, heterogeneous process was a major source of HONO at urban and agricultural areas. The average surface conversion rate of NO2-to-HONO (CHONO) was 0.01467 h−1, and this value decreased with the increase of height at urban station. While average surface CHONO was 0.0322 h−1 at agricultural fields, which was ~1.2–2.8 times higher than that at urban area. The CHONO at agricultural fields significantly increased with the increase of height. The average CHONO at 1.0 km was ~2.0–3.6 times higher than that at surface. That suggested that the heterogeneous process was the main HONO source at high altitudes at CF, and this process obviously correlated with aerosol and H2O. The higher OH production from HONO (P(OH)HONO) occurred at 0–200 m and 100–400 m with averaged values of 0.31 ppb h−1 and 0.39 ppb h−1 at AHU and CF, respectively. The high P(OH)HONO above 1.0 km at CF from September to October was strongly correlated with high O3 (> 80 ppb). This study emphasized the importance of the stereoscopic of HONO on the analysis of its distribution, evolution, source and atmospheric oxidizing contribution.

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.

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.

Physical and chemical characterization of urban grime: An impact on the NO2 uptake coefficients and N-containing product compounds.

Urban grime represents an important environmental surface for heterogeneous reactions in urban environment. Here, we assess the physical and chemical properties of urban grime collected during six consecutive months in downtown of Guangzhou, China. There is a significant variation of the uptake coefficients of NO2 on the urban grime as a function of the relative humidity (RH). In absence of water molecules (0% RH), the light-induced uptake coefficients of NO2 on urban grime samples collected during six months are very similar in order of ≈10-6. At 80% RH, depending on the sampling month the light-induced uptake coefficient of NO2 can reach one order of magnitude higher values (1.5 × 10-5, at 80% RH) compared to those uptakes at 0% RH. In presence of 80% RH, there are strong correlations between the measured NO2 uptakes and the concentrations of the water soluble carbon, soluble anions, polycyclic aromatic hydrocarbons and n-alkanes depicted in the urban grime. These correlations, demonstrate that surface adsorbed water on urban grime play an important role for the uptakes of NO2. The heterogeneous conversion of NO2 on two-month old urban grime under sunlight irradiation (68 W m-2, 300 nm < λ < 400 nm) at 60% RH leads to the formation of unprecedented HONO surface flux of 4.7 × 1010 molecules cm-2 s-1 which is higher than all previously observed HONO fluxes, thereby affecting the oxidation capacity of the urban atmosphere. During the heterogeneous chemistry of NO2 with urban grime, the unsaturated and N-containing organic compounds are released in the gas phase which can affect the air quality in the urban environment.

A relaxed eddy accumulation (REA) LOPAP system for flux measurements of nitrous acid (HONO)

Abstract. In the present study a relaxed eddy accumulation (REA) system for the quantification of vertical fluxes of nitrous acid (HONO) was developed and tested. The system is based on a three-channel long-path absorption photometer (LOPAP) instrument, for which two channels are used for the updrafts and downdrafts, respectively, and a third one for the correction of chemical interferences. The instrument is coupled to a REA gas inlet, for which an ultrasonic anemometer controls two fast magnetic valves to probe the two channels of the LOPAP instrument depending on the vertical wind direction. A software (PyREA) was developed, which controls the valves and measurement cycles, which regularly alternates between REA, zero and parallel ambient measurements. In addition, the assignment of the updrafts and downdrafts to the physical LOPAP channels is periodically alternated, to correct for differences in the interferences of the different air masses. During the study, only small differences of the interferences were identified for the updrafts and downdrafts excluding significant errors when using only one interference channel. In laboratory experiments, high precision of the two channels and the independence of the dilution-corrected HONO concentrations on the length of the valve switching periods were demonstrated. A field campaign was performed in order to test the new REA-LOPAP system at the TROPOS monitoring station in Melpitz, Germany. HONO fluxes in the range of -4×1013 molecules m−2 s−1 (deposition) to +1.0×1014 molecules m−2 s−1 (emission) were obtained. A typical diurnal variation of the HONO fluxes was observed with low, partly negative fluxes during night-time and higher positive fluxes around noon. After an intensive rain period the positive HONO emissions during daytime were continuously increasing, which was explained by the drying of the uppermost ground surfaces. Similar to other campaigns, the highest correlation of the HONO flux was observed with the product of the NO2 photolysis frequency and the NO2 concentration (J(NO2)⋅[NO2]), which implies a HONO formation by photosensitized conversion of NO2 on organic surfaces, such as humic acids. Other postulated HONO formation mechanisms are also discussed but are tentatively ranked being of minor importance for the present field campaign.

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.

Denitrification is the major nitrous acid production pathway in boreal agricultural soils

Nitrous acid (HONO) photolysis produces hydroxyl radicals—a key atmospheric oxidant. Soils are strong HONO emitters, yet HONO production pathways in soils and their relative contributions are poorly constrained. Here, we conduct 15N tracer experiments and isotope pool dilution assays on two types of agricultural soils in Finland to determine HONO emission fluxes and pathways. We show that microbial processes are more important than abiotic processes for HONO emissions. Microbial nitrate reduction (denitrification) considerably exceeded ammonium oxidation as a source of nitrite—a central nitrogen pool connected with HONO emissions. Denitrification contributed 97% and 62% of total HONO fluxes in low and high organic matter soil, respectively. Microbial ammonium oxidation only produced HONO in high organic matter soil (10%). Our findings indicate that microbial nitrate reduction is an important HONO production pathway in aerobic soils, suggesting that terrestrial ecosystems favouring it could be HONO emission hotspots, thereby influencing atmospheric chemistry.

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.

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 NOX 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.

Photochemical emissions of HONO, NO2 and NO from the soil surface under simulated sunlight

Emissions of HONO, NO 2 and NO from different soils under simulated sunlight were investigated using a flow reactor at ambient pressure. Enhanced emissions of HONO, NO 2 and NO were observed when soils were exposed to the simulated sunlight. NO 3 − and NO 2 − partly contributed to the HONO and NO formations, whereas NH 4 + and NH 2 OH played minor roles in HONO, NO 2 and NO emissions from the soil. The emission flux densities of HONO ( F HONO ), NO 2 ( F N O 2 ) and NO ( F NO ) significantly depended on environmental conditions, including irradiation intensity, relative humidity (RH), temperature and pH. F HONO , F N O 2 and F NO linearly increased with increasing irradiation intensity in the range of 0–320 W/m 2 . Maximum values of F HONO , F N O 2 and F NO appeared at RH = 55%. F HONO and F NO increased while F N O 2 decreased with increasing the temperature in the range of 278–338 K. Maximum F HONO and F N O 2 values appeared at pH = 8 while larger F NO values occurred at pH of 2 and 8. Atmospheric implications about the photochemical emission of HONO from soil contributing to HONO in the atmosphere were also discussed. • Light significantly enhanced HONO, NO 2 and NO emissions from soils. • NO 3 − and NO 2 − partly contributed to the HONO and NO emissions from the soil. • HONO, NO 2 and NO emission flux densities depended on the environmental conditions.

Development and application of a twin open-top chambers method to measure soil HONO emission in the North China Plain

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

A dual dynamic chamber system based on IBBCEAS for measuring fluxes of nitrous acid in agricultural fields in the North China Plain

A dual dynamic chamber system combined with incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS) was first developed to measure nitrous acid (HONO) emission fluxes from agricultural fields in the North China Plain. The concentrations of HONO were measured with two IBBCEAS instruments in the sample and blank chambers, respectively. To ensure the accuracy and reliability of flux measurement, before the dynamic chamber system was deployed in an agricultural field, the correlation and accuracy of the two IBBCEAS instruments were tested, and the mixing uniformity of the gas in the chamber was verified. Within a week after fertilization, the observed HONO flux exhibited a pronounced diurnal variation profile with a maximum of 20.25 ng N m−2 s−1 at noontime and a minimum of −0.86 ng N m−2 s−1 in the early morning, one order of magnitude higher than the flux observed in previously reported literature from fields. During this period, the concentrations of HONO in both of the chambers was higher in the daytime than that nighttime, and the concentration of HONO in blank chamber reached the maximum of 18.88 ppbv at noon on the 5th day after fertilization. HONO and NO2 in the sample chambers showed opposite diurnal profiles. The field observation results suggested that the high HONO flux is mainly from the direct emission of soil rather than from the heterogeneous reaction of NO2. Agricultural fertilization will affect the balance of the HONO budget, and then affect the atmospheric oxidation in the North China Plain.

Nitrous acid formation in a snow-free wintertime polluted rural area

Abstract. Nitrous acid (HONO) photolysis is an important source of hydroxyl radicals (OH) in the lower atmosphere, in particular in winter when other OH sources are less efficient. The nighttime formation of HONO and its photolysis in the early morning have long been recognized as an important contributor to the OH budget in polluted environments. Over the past few decades it has become clear that the formation of HONO during the day is an even larger contributor to the OH budget and additionally provides a pathway to recycle NOx. Despite the recognition of this unidentified HONO daytime source, the precise chemical mechanism remains elusive. A number of mechanisms have been proposed, including gas-phase, aerosol, and ground surface processes, to explain the elevated levels of daytime HONO. To identify the likely HONO formation mechanisms in a wintertime polluted rural environment we present LP-DOAS observations of HONO, NO2, and O3 on three absorption paths that cover altitude intervals from 2 to 31, 45, and 68 m above ground level (a.g.l.) during the UBWOS 2012 experiment in the Uintah Basin, Utah, USA. Daytime HONO mixing ratios in the 2–31 m height interval were, on average, 78 ppt, which is lower than HONO levels measured in most polluted urban environments with similar NO2 mixing ratios of 1–2 ppb. HONO surface fluxes at 19 m a.g.l., calculated using the HONO gradients from the LP-DOAS and measured eddy diffusivity coefficient, show clear upward fluxes. The hourly average vertical HONO flux during sunny days followed solar irradiance, with a maximum of (4.9 ± 0.2) × 1010 molec. cm−2 s−1 at noontime. A photostationary state analysis of the HONO budget shows that the surface flux closes the HONO budget, accounting for 63 ± 32 % of the unidentified HONO daytime source throughout the day and 90 ± 30 % near noontime. This is also supported by 1-D chemistry and transport model calculations that include the measured surface flux, thus clearly identifying chemistry at the ground as the missing daytime HONO source in this environment. Comparison between HONO surface flux, solar radiation, NO2 and HNO3 mixing ratios, and results from 1-D model runs suggest that, under high NOx conditions, HONO formation mechanisms related to solar radiation and NO2 mixing ratios, such as photo-enhanced conversion of NO2 on the ground, are most likely the source of daytime HONO. Under moderate to low NO2 conditions, photolysis of HNO3 on the ground seems to be the main source of HONO.

Diurnal fluxes of HONO above a crop rotation

Abstract. Nitrous acid (HONO) fluxes were measured above an agricultural field site near Paris during different seasons. Above bare soil, different crops were measured using the aerodynamic gradient (AG) method. Two LOPAPs (LOng Path Absorption Photometer) were used to determine the HONO gradients between two heights. During daytime mainly positive HONO fluxes were observed, which showed strong correlation with the product of the NO2 concentration and the long wavelength UV light intensity, expressed by the photolysis frequency J(NO2). These results are consistent with HONO formation by photosensitized heterogeneous conversion of NO2 on soil surfaces as observed in recent laboratory studies. An additional influence of the soil temperature on the HONO flux can be explained by the temperature-dependent HONO adsorption on the soil surface. A parameterization of the HONO flux at this location with NO2 concentration, J(NO2), soil temperature and humidity fits reasonably well all flux observations at this location.

Sources of atmospheric nitrous acid (HONO) in the European High Arctic

The atmospheric concentrations of nitrous acid (HONO) have been measured from 15 to 25 April 2010 at Ny-Alesund (Svalbard). The results were discussed from the stand point of diurnal variations and meteorological effects. No clear diurnal cycle for the HONO concentrations was observed during our measurements. HONO concentration showed strong and significant correlation with both nitrogen dioxide (NO2) and relative humidity (RH), suggesting the importance of heterogeneous processes as HONO source. The strong correlation between HONO and HONO to NO2 ratio suggested the occurrence of heterogeneous conversion from NO2 to HONO, or HONO direct emission processes, or both. We used a pseudo steady state approach, including homogeneous and heterogeneous reactions and the emissive fluxes of HONO from the snow surface. It allowed accounting for all observed HONO concentrations only from small period. Except on them, about 28 % of the measured HONO was explained. The results showed that the heterogeneous conversion of NO2 on humid surface was a major HONO source, especially during the night. However, the results indicated the occurrence of a daytime HONO source whose strength was enough to compete with the HONO photolysis.

The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces.

Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe(2+) present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe(2+)(aq) present in thin films of water coating the surface, whereas in the range of pH 5-8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe(2+) followed by protonation of nitrite via surface Fe-OH2(+) groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.