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International audience; 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.

International audience; The heterogeneous degradation of nitrogen dioxide (NO2) on five samples of natural Icelandic volcanic particles has been investigated. Laboratory experiments were carried out under simulated atmospheric conditions using a coated wall flow tube (CWFT). The CWFT reactor was coupled to a blue light nitrogen oxides analyzer (NOx analyzer), and a long path absorption photometer (LOPAP) to monitor in real time the concentrations of NO2, NO and HONO, respectively. Under dark and ambient relative humidity conditions, the steady state uptake coefficients of NO2 varied significantly between the volcanic samples probably due to differences in magma composition and morphological variation related with the density of surface OH groups. The irradiation of the surface with simulated sunlight enhanced the uptake coefficients by a factor of three indicating that photo-induced processes on the surface of the dust occur. Furthermore, the product yields of NO and HONO were determined under both dark and simulated sunlight conditions. The relative humidity was found to influence the distribution of gaseous products, promoting the formation of gaseous HONO. A detailed reaction mechanism is proposed that supports our experimental observations. Regarding the atmospheric implications, our results suggest that the NO2 degradation on volcanic particles and the corresponding formation of HONO is expected to be significant during volcanic dust storms or after a volcanic eruption.

Air pollution is still an environmental problem that urgently needs to be solved in the North China Plain (NCP). With the aim to explore nitrous acid (HONO) formation and its impact on regional air quality in the NCP, the development and systematic assessment of instruments to measure atmospheric HONO concentration and soil HONO emission flux were realized. Based on field measurements, a 3D chemistry-transport model (the Community Multiscale Air Quality Modeling System, CMAQ) and a box (0D) model (Master Chemical Mechanism, MCM) were used to explore the HONO budget and its impact on regional air pollution, e.g., summer ozone (O3) pollution and winter haze pollution. We found that agricultural soil was an important HONO source with a significant impact on regional O3 pollution in the summertime. A new soil HONO emission mechanism related to nitrification was proposed. HONO also plays an important role in atmospheric oxidizing capacity and deteriorating regional air quality in the wintertime., La pollution de l'air est toujours un problème environnemental qui doit être résolu de toute urgence dans la Plaine de Chine du Nord (PCN). Dans le but d'explorer la formation de l’acide nitreux (HONO) et son impact sur la qualité de l'air régional dans la PCN, le développement et l'évaluation systématique d'instruments pour mesurer la concentration de HONO atmosphérique et son flux d'émission du sol ont été réalisés. Sur la base de mesures sur le terrain, un modèle 3D de chimie-transport (le système communautaire de modélisation de la qualité de l'air à plusieurs échelles : CMAQ, de l’EPA) et un modèle de boîte (0D) de mécanisme chimique (Master Chemical Mechanism : MCM) ont été utilisés pour explorer le bilan de HONO et son impact sur la pollution atmosphérique régionale, par exemple la pollution estivale à l'ozone (O3) et la pollution par la brume hivernale. Nous avons constaté que le sol agricole était une source importante de HONO avec un impact significatif sur la pollution régionale à l'ozone en été. Un nouveau mécanisme d'émission de HONO du sol lié à la nitrification a été proposé. HONO joue également un rôle important dans la capacité oxydante de l’atmosphère et la détérioration de la qualité de l'air régional en hiver.

International audience; This work introduces a newly developed reaction mechanism for the oxidation of ammonia in freely propagating and burner-stabilized premixed flames as well as shock-tube, jet-stirred reactor, and plug-flow reactor experiments. The paper mainly focuses on pure ammonia and ammonia–hydrogen fuel blends. The reaction mechanism also considers the formation of nitrogen oxides as well as the reduction of nitrogen oxides depending upon the conditions of the surrounding gas phase. Doping of the fuel blend with NO2 can result in acceleration of H2 autoignition via the reaction NO2 + HO2 ⇋ HONO + O2, followed by the thermal decomposition of HONO, or deceleration of H2 oxidation via NO2 + OH ⇋ NO + HO2. The concentration of HO2 is decisive for the active reaction pathway. The formation of NO in burner-stabilized premixed flames is shown to demonstrate the capability of the mechanism to be integrated into a mechanism for hydrocarbon oxidation.

An instrument implementing Incoherent BroadBand Cavity-Enhanced Absorption Spectroscopy (IBBCEAS) for quantitative measurements of molecular iodine (I2) at ambient pressure and in the presence of aerosols is presented. The instrument is based on a LED emitting in the green spectral region around 500–550 nm, exciting a multitude of rovibronic transitions in the I2 B ← X electronic absorption spectrum. I2 was generated using a permeation device and was mixed with a dilution flow of Ar and another flow containing NaCl aerosols. Retrieval of the mirror reflectivity, necessary for quantitative IBBCEAS measurements, was not pursued in this study. Instead, calibrated I2 flows were used, and a differential optical absorption spectroscopy type of algorithm was implemented for the data analysis. This approach has several benefits, in particular when light extinction due to aerosols is substantial. Estimated detection limits are roughly 0.04 nmol/l without aerosols and 0.4 nmol/l in the presence of aerosols (1010 particles/l with 60–70 nm mean diameter, leading to I 0/I aerosols ≈ 5.4). A drop in measured I2 concentration in relation to the expected mixing ratio was observed when using the highest aerosol concentration, likely due to adsorption of I2 onto aerosols.

The interaction of HONO with UV irradiated TiO2 solid films was studied using a low pressure flow reactor (1–10 Torr of Helium) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to TiO2 was studied in absence of O2 in the reactor as a function of HONO concentration ([HONO]0 = (0.5–5.0) × 1012 molecule cm−3), relative humidity (RH = 0.001–60%) and temperature (T = 275–320 K). The measured initial uptake coefficient (γ0) of HONO on TiO2 surface was independent of the HONO concentration and showed slight inverse dependence on temperature (activation factor = −1390 ± 150 K) and relative humidity: γ0 = 6.9 × 10−4 (RH)−0.3 (at T = 280 K, calculated using BET surface area, 30% uncertainty). NO2 and NO were observed as products of the HONO reaction with UV irradiated TiO2 surface with sum of their yields corresponding to nearly 100% of the nitrogen mass balance. The yields of the NO and NO2 products were found to be (48 ± 7) and (52 ± 8) %, respectively, independent of relative humidity, temperature and concentration of HONO under experimental conditions used. The HONO loss on mineral aerosol during daytime (calculated with uptake data for HONO on TiO2 surface in presence of UV irradiation) appears to be of minor importance compared with HONO photolysis in the atmosphere.

Kinetics and products of the interaction of HONO with solid films of Al 2 O 3 were investigated under dark and UV irradiation conditions using a low pressure flow reactor (1–10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. The reactive uptake of HONO to Al 2 O 3 was studied as a function of HONO concentration ([HONO] 0 = (0.6–3.5) × 10 12 molecule cm −3 ), relative humidity (RH = 1.4 × 10 −4 to 35.4%), temperature ( T = 275–320 K) and UV irradiation intensity ( J NO 2 = 0.002 – 0.012 s − 1 ). The measured reactive uptake coefficient was independent of the HONO concentration and temperature. In contrast, the relative humidity (RH) was found to have a strong impact on the uptake coefficient: γ = 4.8 × 10 −6 (RH) −0.61 and γ = 1.7 × 10 −5 (RH) −0.44 under dark conditions and on irradiated surface ( J NO 2 = 0.012 s − 1 ), respectively ( γ calculated with BET surface area, 30% conservative uncertainty). NO 2 and NO were observed as products of the HONO reaction with Al 2 O 3 surface with yields of 40 ± 6 and 60 ± 9%, respectively, independent of relative humidity, temperature, concentration of HONO and UV irradiation intensity under experimental conditions used. The HONO uptake on mineral aerosol (calculated with uptake data for HONO on Al 2 O 3 surface) appears to be of minor importance compared with other HONO loss processes in the boundary layer of the earth atmosphere.