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Rambouillet Forest is a large forested location, situated about 50km from central Paris. It covers some 200km² and contains mainly a mix of oak and pine trees. Its size and proximity towards Paris makes it an interesting site to study the interaction between polluted air masses and biogenic emissions and the impact on air quality in areas surrounding polluted cities. The ACROSS (Atmospheric ChemistRy Of the Suburban forest) campaign represents a large international collaboration between various French, European and American institutes, which aims to further characterize these interactions. For our contribution, different instruments from ICARE and LPC2E CNRS Orléans performed in-situ measurements in this forest during June and July of 2022, including: VOCs/ BVOCs (GC-MS/FID); oxidants (OH radicals, CIMS); oxidant precursors (NOx, NOy, Ecophysics, CAPS, LOPAP); and highly oxidized products (orbitrap). In general, data coverage for all instrumentation was high throughout the campaign, furthermore this instrumentation was calibrated and characterized under field condition where possible. These observations represent a broad look at the composition of the forested environment, and contain information on organic emissions, their oxidized products, oxidants and oxidant precursors. We therefore anticipate that these measurements will contribute significantly to the understanding of the atmospheric chemistry operating during the ACROSS campaign.

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

A comprehensive field campaign, with measurements of HONO and related parameters, was conducted in summer 2018 at the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). At the summit station, high HONO mixing ratios were observed (mean ± 1σ: 133 ± 106 pptv, maximum: 880 pptv), with a diurnal noontime peak (mean ± 1σ: 133 ± 72 pptv at 12:30 local time). Constraints on the kinetics of aerosol-derived HONO sources (NO2 uptake on the aerosol surface and particulate nitrate photolysis) were performed and discussed, which enables a better understanding of the interaction of HONO and aerosols, especially in the polluted North China Plain. Various evidence of air mass transport from the ground to the summit level was provided. Furthermore, daytime HONO formation from different paths and its role in radical production were quantified and discussed. We found that the homogeneous reaction NO + OH could only explain 8.0 % of the daytime HONO formation, resulting in strong unknown sources (Pun). Campaigned-averaged Pun was about 290 ± 280 pptv h−1, with a maximum of about 1800 pptv h−1. Aerosol-derived HONO formation mechanisms were not the major sources of Pun at the summit station. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO + OH), depending on the chemical kinetic parameters used. Coupled with sensitivity tests on the kinetic parameters used, the NO2 uptake on the aerosol surface and particulate nitrate photolysis contributed 1.5 %–19 % and 0.6 %–9.6 % of the observed Pun, respectively. Based on synchronous measurements at the foot and the summit station, an amount of field evidence was proposed to support the finding that the remaining majority (70 %–98 %) of Pun was dominated by the rapid vertical transport from the ground to the summit level and heterogeneous formation on the mountain surfaces during transport. HONO photolysis at the summit level initialized daytime photochemistry and still represented an essential OH source in the daytime, with a contribution of about one-quarter of O3. We provided evidence that ground-derived HONO played a significant role in the oxidizing capacity of the upper boundary layer through the enhanced vertical air mass exchange driven by mountain winds. The follow-up impacts should be considered in regional chemistry transport models.

Furans are emitted to the atmosphere during biomass burning from the pyrolysis of cellulose. They are one of the major contributing volatile organic compound (VOC) classes to OH and NO3 reactivity in biomass burning plumes. The major removal process of furans from the atmosphere at night is reaction with the nitrate radical, NO3. Here, we report a series of relative rate experiments in the 7300 L indoor simulation chamber at Institut de Combustion Aérothermique Réactivité et Environnement, Centre national de la recherche scientifique (ICARE-CNRS), Orléans, using a number of different reference compounds to determine NO3 reaction rate coefficients for four furans, two furanones, and pyrrole. In the case of the two furanones, this is the first time that NO3 rate coefficients have been reported. The recommended values (cm3 molec.−1 s−1) are as follows: furan, (1.49 ± 0.23) × 10−12; 2-methylfuran, (2.26 ± 0.52) × 10−11; 2,5-dimethylfuran, (1.02 ± 0.31) × 10−10; furfural (furan-2-aldehyde), (9.07 ± 2.3) × 10−14; α-angelicalactone (5-methyl-2(3H)-furanone), (3.01 ± 0.45) × 10−12; γ-crotonolactone (2(5H)-furanone)

In the summer of 2018, a comprehensive field campaign, with measurements on HONO and related parameters, was conducted at the foot (150 m a.s.l.) and the summit of Mt. Tai (1534 m a.s.l.) in the central North China Plain (NCP). With the implementation of a 0-D box model, the HONO budget with six additional sources and its role in radical chemistry at the foot station were explored. We found that the model default source, NO + OH, could only reproduce 13 % of the observed HONO, leading to a strong unknown source strength of up to 3 ppbv h−1. Among the additional sources, the NO2 uptake on the ground surface dominated (∼ 70 %) nighttime HONO formation, and its photo-enhanced reaction dominated (∼ 80 %) daytime HONO formation. Their contributions were sensitive to the mixing layer height (MLH) used for the parameterizations, highlighting the importance of a reasonable MLH for exploring ground-level HONO formation in 0-D models and the necessity of gradient measurements. A ΔHONO/ΔNOx ratio of 0.7 % for direct emissions from vehicle exhaust was inferred, and a new method to quantify its contribution to the observations was proposed and discussed. Aerosol-derived sources, including the NO2 uptake on the aerosol surface and the particulate nitrate photolysis, did not lead to significant HONO formation, with their contributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistry at the foot station. It was also a substantial radical source throughout the daytime, with contributions higher than O3 photolysis to OH initiation. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while modeled NO3 appeared to be significant at night. Peaks of modeled NO3 time series and average diurnal variation reached 22 and 9 pptv, respectively. NO3-induced reactions contribute 18 % of nitrate formation potential (P(HNO3)) and 11 % of the isoprene (C5H8) oxidation throughout the whole day. At night, NO3 chemistry led to 51 % and 44 % of P(HNO3) or the C5H8 oxidation, respectively, implying that NO3 chemistry could significantly affect nighttime secondary organic and inorganic aerosol formation in this high-O3 region. Considering the severe O3 pollution in the NCP and the very limited NO3 measurements, we suggest that besides direct measurements of HOx and primary HOx precursors (O3, HONO, alkenes, etc.), NO3 measurements should be conducted to understand the atmospheric oxidizing capacity and air pollution formation in this and similar regions.

Rate coefficients for the reaction of NO 3 radicals with a series of aromatic aldehydes were measured in a 7300 L simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were k1= 2.6 ± 0.3, k2= 8.7 ± 0.8, k3= 4.9 ± 0.5, k4= 4.9 ± 0.4, k5= 15.1 ± 1.3, k6= 12.8 ± 1.2 and k7= 6.2 ± 0.6, respectively, in the units of 10 −15 cm 3 molec. −1 s −1 at 298 ± 2 K. The rate coefficient k13 for the reaction of the NO 3 radical with deuterated benzaldehyde (benzaldehyde-d 1) was found to be half that of k1 . The end product of the reaction in an excess of NO 2 was measured to be C 6 H 5 C(O)O 2 NO 2 . Theoretical calculations of aldehydic bond energies and reaction pathways indicate that the NO 3 radical reacts primarily with aromatic aldehydes through the abstraction of an aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.

Soil was proposed as an important nitrous acid (HONO) source but has not so far been demonstrated by field flux measurements and its atmospheric impacts are highly uncertain. Through field flux measurements we found strong soil HONO emission flux (FHONO) enhanced by nitrogen fertilizer use for agricultural fields. FHONO showed distinct diurnal variations with noontime peaks up to hundreds of ng-N m-2 s-1. With soil samples where field measurements took place, our laboratory experiments subsequently found that the release of soil nitrification-originated nitrite as HONO was largely accelerated by surface water exchange. This process was parameterized by a modified Arrhenius equation that reproduced field flux observations well. With the consideration of FHONO, the box model could well reproduce the observed ambient HONO and found that FHONO could significantly accelerate ozone production, highlighting the amplified role of nitrogen fertilizer in exacerbating regional ozone pollution. Moreover, we found that nitrification inhibitors could reduce HONO emissions by ~90%, indicating their great potential to mitigate air pollution. Furthermore, we estimated yearly HONO emission of 0.25 Tg-N from global fertilizer use for agriculture, representing a considerable reactive nitrogen source and significantly affecting atmospheric oxidizing capacity and air quality on a global scale.

Soil was proposed as an important nitrous acid (HONO) source but has not so far been demonstrated by field flux measurements and its atmospheric impacts are highly uncertain. Here we conducted field flux measurements and found that soil HONO emission flux (FHONO) could be enhanced by more than two orders of magnitude by the use of nitrogen fertilizers for agricultural fields in the North China Plain (NCP). FHONO showed distinct diurnal variations with noontime peaks up to hundreds of ng-N m-2 s-1. With soil samples where field measurements took place, our laboratory experiments subsequently found that the release of soil nitrification-originated nitrite as HONO was largely accelerated by soil surface water evaporation. This process was parameterized by a modified Arrhenius equation that reproduced field flux observations well. With the consideration of FHONO, the box model could well reproduce the observed ambient HONO and found that FHONO could significantly accelerate ozone (O3) production, highlighting the amplified role of nitrogen fertilizer use in exacerbating O3 pollution in the NCP. Moreover, we estimated yearly HONO emission of 0.25 Tg-N from global fertilizer use for agriculture, representing a considerable reactive nitrogen source and significantly affecting atmospheric oxidizing capacity and air quality on a global scale. Furthermore, we found that nitrification inhibitors could reduce HONO and nitric oxide emissions by ~90%, indicating their great potential to mitigate regional air pollution.

Atmospheric black carbon (BC) concentration over a nearly 5 year period (mid-2017-2021) was continuously monitored over a suburban area of Orléans city (France). Annual mean atmospheric BC concentration were 0.75 ± 0.65, 0.58 ± 0.44, 0.54 ± 0.64, 0.48 ± 0.46 and 0.50 ± 0.72 μg m

This article, the eighth in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers the gas-phase thermal and photochemical reactions of organic species with four, or more, carbon atoms (≥ C4) available on the IUPAC website in 2021, including thermal reactions of closed-shell organic species with HO and NO3 radicals and their photolysis. The present work is a continuation of volume II (Atkinson et al., 2006), with new reactions and updated data sheets for reactions of HO (77 reactions) and NO3 (36 reactions) with ≥ C4 organics, including alkanes, alkenes, dienes, aromatics, oxygenated, organic nitrates and nitro compounds in addition to photochemical processes for nine species. The article consists of a summary table (Table 1), containing the recommended kinetic parameters for the evaluated reactions, and a supplement containing the data sheets, which provide information upon which recommendations are made.

International audience; Summertime HONO concentrations were synchronously measured at two (agricultural and nonagricultural) sites in the North China Plain (NCP). Daytime HONO (1.4 ± 0.6 ppbv) and HONO/NO2 ((12 ± 8)%) over the agricultural field after fertilization were found to be remarkably higher than those before fertilization, implying strong HONO emission from the fertilized fields. Synchronous enhancements of HONO and O3 after fertilization at both sites suggested that the emitted HONO accelerated the local and the regional O3 pollution. HONO budget analysis further revealed that its emission was significantly enhanced after fertilization. Soil HONO emission flux and its uncertainty were estimated and discussed. The estimated emission flux exhibited a distinct diurnal variation with a noontime maximum. The net OH production rate from HONO photolysis greatly exceeded that from O3 photolysis over the agricultural field, and their maximum ratio of 4.7 was obtained after fertilization. We provide field evidence that fertilized fields in the NCP act as a strong HONO source, which accelerates daytime photochemistry, leading to an increase of regional photo-oxidants such as O3. Considering the severe O3 pollution in the summer NCP and that the large area of the agricultural field is regularly treated with high fertilization amount in this region, HONO emission should be taken into account in the regional air quality deterioration.

International audience; Rate coefficients for the reaction of NO3 radicals with 6 unsaturated volatile organic compounds (VOCs) in a 7300 L simulation chamber at ambient temperature and pressure have been determined by the relative rate method. The resulting rate coefficients were determined for isoprene, 2-carene, 3-carene, methyl vinyl ketone (MVK), methacrolein (MACR) and crotonaldehyde (CA), as (6.6 ± 0.8) × 10−13, (1.8 ± 0.6) × 10−11, (8.7 ± 0.5) × 10−12, (1.24 ± 1.04) × 10−16, (3.3 ± 0.9) × 10−15 and (5.7 ± 1.2) × 10−15 cm3/(molecule•sec), respectively. The experiments indicate that NO3 radical reactions with all the studied unsaturated VOCs proceed through addition to the olefinic bond, however, it indicates that the introduction of a carbonyl group into unsaturated VOCs can deactivate the neighboring olefinic bond towards reaction with the NO3 radical, which is to be expected since the presence of these electron-withdrawing substituents will reduce the electron density in the π orbitals of the alkenes, and will therefore reduce the rate coefficient of these electrophilic addition reactions. In addition, we investigated the product formation from the reactions of 2-carene and 3-carene with the NO3 radical. Qualitative identification of an epoxide (C10H16OH+), caronaldehyde (C10H16O2H+) and nitrooxy-ketone (C10H16O4NH+) was achieved using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and a reaction mechanism is proposed.

International audience; Northern China is regularly subjected to intense wintertime "haze events", with high levels of fine particles that threaten millions of inhabitants. While sulfate is a known major component of these fine haze particles, its formation mechanism remains unclear especially under highly polluted conditions, with state-of-the-art air quality models unable to reproduce or predict field observations. These haze conditions are generally characterized by simultaneous high emissions of SO2 and photosensitizing materials. In this study, we find that the excited triplet states of photosensitizers could induce a direct photosensitized oxidation of hydrated SO2 and bisulfite into sulfate S(VI) through energy transfer, electron transfer, or hydrogen atom abstraction. This photosensitized pathway appears to be a new and ubiquitous chemical route for atmospheric sulfate production. Compared to other aqueous-phase sulfate formation pathways with ozone, hydrogen peroxide, nitrogen dioxide, or transition-metal ions, the results also show that this photosensitized oxidation of S(IV) could make an important contribution to aerosol sulfate formation in Asian countries, particularly in China.

Wildfire events are increasing globally due to climate change, with significant adverse impacts on regional air quality and global climate. In the middle of September 2020, a wildfire event occurred in Souesmes (Loir-et-Cher, France), and its plume spread out to 200 km around in the following day as observed by the MODIS satellite. Based on comprehensive field measurements at a suburban atmospheric observation site (~50 km northwest from the wildfire location) in Orléans, young fire plumes were identified. Significant increases in trace gases (CO, CH4, N2O, VOCs, etc.) and particles (including black carbon) were found within the BB plumes. Molar enhancement ratios, defined as EF (X) = ∆X/∆CO (where X represents the target species), of various trace gases and black carbon within young plumes were determined accordingly and compared with previous studies. Changes in the ambient ions (ammonium, sulfate, nitrate, chloride, nitrite, etc. in the particle- and gas-phase) and aerosol properties (e.g., aerosol water content, pH) were also quantified and discussed. Furthermore, along with trajectory model (FLEXPART) simulations, we found that the Global Fire Assimilation System (GFAS) may underestimate emissions (e.g., CO) of this small wildfire while other inventories (GFED, FINN) showed significant overestimation. Estimation of emissions of this fire event was conducted and compared with GFAS emissions. Related atmospheric implications are also presented and discussed.

International audience; Absolute measurements of the rate coefficient of the title reaction are reported from 251 to 373 K using the laser-induced fluorescence–pulsed-laser photolysis technique, together with room-temperature relative rate determinations using a simulation chamber. These are the first reported values of this rate coefficient, which demonstrates a predominantly negative temperature dependence that is more pronounced than with other epoxides, suggesting that pre-reactive complexes contribute to the overall reactivity, and that these complexes are comparatively stable for OH + cyclohexene oxide, which may result from its bicyclic structure. This work provides new insights into the lesser-studied reactivity of the epoxide functionality.

International audience; Nitrous acid (HONO), as a key reservoir of hydroxyl radical (OH), plays a significant role in atmospheric chemistry. To study the sources and atmospheric influence of HONO, continuous observations of HONO and relevant air pollutants were performed from June to July 2019 at an urban site in Beijing. A comparative study on the ambient levels, diurnal variation, the sources in daytime and nighttime, and the formation mechanisms were investigated for a haze pollution period (Period I) and an ozone pollution period (Period II). The average hourly HONO concentrations during Period I, Period II, and the whole observation period, were 0.58 ± 0.23, 0.54 ± 0.19 and 0.44 ± 0.24 ppb, respectively. The emission from vehicle exhaust was an important source of nocturnal HONO formation. During the nighttime, compared with the homogeneous reaction of NO with OH, the heterogeneous conversion from NO2 was the dominant pathway for HONO formation. The heterogeneous conversion frequency was 0.0075 h−1 during Period I, higher than that during Period II (0.0028 h−1), suggesting a higher conversion potential to HONO formation during the haze episode. Based on the analysis of HONO budget, it was found that the daytime unknown source Punknown during Period II was higher than that during Period I. Correlation analysis implied that the photo-enhanced NO2 conversion on the aerosol surface might be a potential source for daytime HONO. Without HONO constraint, the Observation-Based Model (OBM) would largely underestimate the averaged daytime atmospheric oxidative capacity (24%), OH production rate (57%), and net O3 production rate (20%). The study results further demonstrated the necessity for clarifying the formation mechanism of HONO to improve the understanding of the influence of HONO to atmospheric chemical processes.

International audience; Severe volatile organic compound (VOC) pollution has become an urgent problem during the heating season in the North China Plain (NCP), as exposure to hazardous VOCs can lead to chronic or acute diseases. A campaign with online VOC measurements was conducted at a rural site in Wangdu, NCP during the 2018 heating season to characterize the compositions and associated sources of VOCs and to assess their potential health risks. The total concentration of VOCs with 94 identified species was 77.21 ± 54.39 ppb. Seven source factors were identified by non-negative matrix factorization, including coal combustion (36.1%), LPG usage (21.1%), solvent usage (13.9%), biomass burning and secondary formation (14.2%), background (7.0%), industrial emissions (4.5%), and vehicle emissions (3.3%). The point estimate approach and Monte Carlo simulation were used to estimate the carcinogenic and non-carcinogenic risks of harzadous VOCs. The results showed that the cumulative health risk of VOCs was above the safety level. Acrolein, 1.2-dichloroethane, 1,2-dichloropropane, chloroform, 1,3-butadiene, and benzene were identified as the key hazardous VOCs in Wangdu. Benzene had the highest average carcinogenic risk. Solvent usage and secondary formation were the dominant sources of adverse health effects. During the Spring Festival, most sources were sharply reduced; and VOC concentration declined by 49%. However, coal and biomass consumptions remained relatively large, probably due to heating demand. This study provides important references for the control strategies of VOCs during the heating season in heavily polluted rural areas in the NCP.

International audience

International audience; This work reports new kinetic and mechanistic information on the atmospheric chemistry of ketones. Both absolute and relative rate methods were used to determine the rate constants for OH reactions with 2-methyl-3-pentanone (2M3P), 3-methyl-2-pentanone (3M2P) and 4-methyl-2-pentanone (4M2P), three widely used compounds in the industry. This work constitutes the first temperature dependence study of the reactions of OH with 2M3P and 3M2P. The following rate constants values are recommended at 298 K (in 10−12 cm3 molecule−1 s−1): kOH+2M3P = 3.49 ± 0.5; kOH+3M2P = 6.02 ± 0.14 and kOH+4M2P = 11.02 ± 0.42. The following Arrhenius expressions (in units of cm3 molecule−1 s−1) adequately describe the measured rate constants for OH reactions with 2M3P and 3M2P in the temperature range 263–373 K: k2M3P = (2.33 ± 0.06) × 10−12 exp((127.4 ± 18.6)/T) and k3M2P = (1.05 ± 0.14) × 10−12 exp((537 ± 41)/T). Products studies from the reactions of OH with the investigated ketones were conducted in a 7.3 m3 simulation chamber using PTR-ToF-MS, UHPLC-MS and GC–MS. A series of short chain carbonyl compounds including formaldehyde, acetone, acetaldehyde, 2-butanone and 2-methypropanal were observed as products. Combining the yields of carbonyls measured with those estimated from the SAR method, we propose various mechanistic degradation schemes of the investigated ketones initiated by reaction with OH radicals.

International audience; The reactions of two selected ethers, methyl n-butyl ether (MnBE) and methyl s-butyl ether (MsBE), with three atmospheric oxidants (OH/NO3/Cl) have been investigated at 298 +/- 2 K using the relative rate method and gas chromatography-mass spectrometry. The reaction trends of these short chain aliphatic ethers with the different oxidants have been discussed. From the obtained results, the reaction with OH is the main tropospheric loss process for MnBE and MsBE, with lifetimes of 19.2 h and 26.8 h, respectively. These short lifetimes indicate that the chemistry of these aliphatic ethers is important in areas close to their emission sources.

Atmospheric particle is one of the major air pollutants, and believed to be important for air quality, radiative forcing and climate. Measurements of aerosol optical properties, size distribution and PM10 concentration were conducted at Orleans, central France during spring (7 March to 25 April) and autumn (25 October to 5 December) 2013. The average values of aerosol scattering coefficient (b(sca)), absorption coefficient (b(abs)), single scattering albedo (SSA) at 532 nm and PM10 concentration are 54.9 +/- 58.2 Mm(-1), 10.6 +/- 10.9 Mm(-1), 0.81 +/- 0.10 and 30.6 +/- 21.6 mu g/m(3) for the spring campaign, and 35.4 +/- 36.7 Mm(-1), 3.9 +/- 4.4 Mm(-1), 0.83 +/- 0.13 and 17.4 +/- 11.8 mu g/m(3) for the autumn campaign, respectively. During the whole observation, the air parcel transported from Atlantic Ocean plays a role in cleaning up the ambient air in Orleans, while the air mass coining from the Eastern Europe induces the pollution events in Orleans. In this study, a simple approach, which based on the diurnal variation of PM10 concentration, Boundary layer depth (BLD) and the human activity factor derived from anthropogenic emission rate, was introduced to estimate the contribution of secondary aerosol to ambient aerosols. Our results show that secondary particles formation trigged by photochemical reactions and oxidations can contribute maximum of 64% and 32% for PM10 mass concentration during the spring and autumn time, respectively. These results highlight that photochemical reactions can enhance the atmospheric oxidation capacity and may faster the secondary particle formation and then play an important role in air quality. (C) 2018 Elsevier B.V. All rights reserved.

In the summer of 2018, a comprehensive field campaign, with measurements on HONO and related parameters, was conducted at the foot (150 m a.s.l.) and the summit of Mt. Tai (1534 m a.s.l.) in the central North China Plain (NCP). With the implementation of a 0-D box model, the HONO budget with six additional sources and its role in radical chemistry at the foot station were explored. We found that the model default source, NO + OH, could only reproduce the observed HONO by 13 %, leading to a strong unknown source strength up to 3 ppbv h−1. Among the additional sources, the NO2 uptake on the ground surface dominated (~70 %) night-time HONO formation, and its photo-enhanced reaction dominated (~80 %) daytime HONO formation. Their contributions were sensitive to the mixing layer height (MLH) used for the parameterizations, highlighting the importance of a reasonable MLH for exploring ground-level HONO formation in 0-D models and the necessity of gradient measurements. A HONO / NOx ratio of 0.7 % for the direct emission was inferred and a new method to quantify its contribution to the observations was proposed and discussed. Aerosol-derived sources, including the NO2 uptake on the aerosol surface and the particulate nitrate photolysis, did not lead to significant HONO formation, with their contributions lower than NO + OH. HONO photolysis in the early morning initialized the daytime photochemistry at both the foot and the summit stations and also was a substantial radical source throughout the daytime, with contributions higher than or about one-quarter of O3 photolysis to OH initiation at the foot and the summit stations, respectively. Moreover, we found that OH dominated the atmospheric oxidizing capacity in the daytime, while NO3 appeared to be significant at night. Peaks of NO3 time series and diurnal variation reached 22 and 9 pptv, respectively. NO3 induced reactions contribute 18 % of nitrate formation potential (P(HNO3)) and 11 % of the isoprene (C5H8) oxidation throughout the whole day. At night, NO3 chemistry led to 51 % or 44 % of P(HNO3) or the C5H8 oxidation, respectively. NO3 chemistry may significantly affect night-time secondary organic and inorganic aerosol formation in this high-O3 region, implying that NO3 chemistry could significantly affect night-time secondary organic and inorganic aerosol formation in this high-O3 region. Considering the severe O3 pollution in the NCP and the very limited NO3 measurements, we suggest that besides direct measurements of HOx and primary HOx precursors (O3, HONO, alkenes, etc.), NO3 measurements should be conducted to understand the atmospheric oxidizing capacity and air pollution formation in this and similar regions.

As a secondary pollutant of photochemical pollution, peroxyacetyl nitrate (PAN) has attracted a close attention. A four-month campaign was conducted at a rural site in North China Plain (NCP) including the measurement of PAN, O

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl

Rate coefficients for the reaction of NO3 radicals with a series of aromatic aldehydes were measured in a 7300 liter simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were: k1 = 2.6 ± 0.3, k2 = 8.8 ± 0.8, k3 = 4.8 ± 0.5, k4 = 4.9 ± 0.5, k5 = 15.1 ± 1.4, k6 = 12.7 ± 1.2 and k7 = 6.2 ± 0.6, respectively, in the units of 10−15 cm3 molecule−1 s−1 at 298 ± 2 K. The rate coefficient k13 for the reaction of the NO3 radical with deuterated benzaldehyde (benzaldehyde-d1) was found to be half that of k1. The end product of the reaction with an excess of NOx was measured to be C6H5C(O)O2NO2. Theoretical calculations of aldehydic bond energies and reaction pathways indicate that NO3 radical reacts with aromatic aldehydes through the abstraction of aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.

International audience; Understanding the formation mechanisms of secondary air pollution is very important for the formulation of air pollution control countermeasures in China. Thus, a large-scale outdoor atmospheric simulation smog chamber was constructed at Chinese Research Academy of Environmental Sciences (the CRAES Chamber), which was designed for simulating the atmospheric photochemical processes under the conditions close to the real atmospheric environment. The chamber consisted of a 56-m3 fluorinated ethylene propylene (FEP) Teflon film reactor, an electrically-driven stainless steel alloy shield, an auxiliary system, and multiple detection instrumentations. By performing a series of characterization experiments, we obtained basic parameters of the CRAES chamber, such as the mixing ability, the background reactivity, and the wall loss rates of gaseous compounds (propene, NO, NO2, ozone) and aerosols (ammonium sulfate). Oxidation experiments were also performed to study the formation of ozone and secondary organic aerosol (SOA), including α-pinene ozonolysis, propene and 1,3,5-trimethylbenzene photooxidation. Temperature and seed effects on the vapor wall loss and SOA yields were obtained in this work: higher temperature and the presence of seed could reduce the vapor wall loss; SOA yield was found to depend inversely on temperature, and the presence of seed could increase SOA yield. The seed was suggested to be used in the chamber to reduce the interaction between the gas phase and chamber walls. The results above showed that the CRAES chamber was reliable and could meet the demands for investigating tropospheric chemistry.

Experiments at atmospherically relevant conditions were performed in the simulation chamber SAPHIR, investigating the reaction of isoprene with NO3 and its subsequent oxidation. Due to the production of NO3 from the reaction of NO2 with O3 as well as the formation of OH in subsequent reactions, the reactions of isoprene with O3 and OH were estimated to contribute up to 15% of the total isoprene consumption each in these experiments. The ratio of RO2 to HO2 concentrations was varied by changing the reactant concentrations, which modifies the product distribution from bimolecular reactions of the nitrated RO2. The reaction with HO2 or NO3 was found to be the main bimolecular loss process for the RO2 radicals under all conditions examined.Yields of the first-generation isoprene oxygenated nitrates as well as the sum of methyl vinyl ketone (MVK) and methacrolein (MACR) were determined by high resolution proton mass spectrometry using the Vocus PTR-TOF. The experimental time series of these products are compared to model calculations based on the MCM v3.3.1,1 the isoprene mechanism as published by Wennberg et al.2 and the newly developed FZJ-NO3-isoprene mechanism,3 which incorporates theory-based rate coefficients for a wide range of reactions.Among other changes, the FZJ-NO3-isoprene mechanism contains a novel fast oxidation route through the epoxidation of alkoxy radicals, originating from the formation of nitrated peroxy radicals. This inhibits the formation of MVK and MACR from the NO3-initiated oxidation of isoprene to practically zero, which agrees with the observations from chamber experiments. In addition, the FZJ-NO3-isoprene mechanism increases the level of agreement for the main first-generation oxygenated nitrates. 1 M. E. Jenkin, J. C. Young and A. R. Rickard, The MCM v3.3.1 degradation scheme for isoprene, Atmospheric Chem. Phys., 2015, 15, 11433–11459.2 P. O. Wennberg at al., Gas-Phase Reactions of Isoprene and Its Major Oxidation Products, Chem. Rev., 2018, 118, 3337–3390. 3 L. Vereecken et al., Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene, Phys. Chem. Chem. Phys., submitted.

As industries transition towards greener business models, many mass-produced chemicals are being replaced by low-global warming potential (GWP) alternatives. Allyl 1,1,2,2-tetrafluoroethyl ether (CH2=CHCH2OCF2CF2H) represents a potential replacement candidate. This molecule contains an olefinic bond, several abstractable hydrogen atoms, an ether linkage and a non-perfluorinated side-chain. As such it can react with various atmospheric oxidants in a variety of degradation mechanisms, each of which may serve to reduce its atmospheric lifetime and its impact upon the environment. Before widespread usage, it is crucial that these environmental sinks are quantified such that the risk that CH2=CHCH2OCF2CF2H poses to the environment can be thoroughly assessed. We present measurements of gas-phase relative rates with hydroxyl radicals (OH), atomic chlorine (Cl), ozone (O3) and nitrate radical (NO3) carried out in HELIOS simulation chamber at CNRS (Orléans, France) as described by Ren et al.1 and references within. Although previous measurements are available for OH2 and Cl,3 we find some discrepancies in comparison to our new determinations. In the case of O3 and NO3, these represent the first such measurements of which we are aware. Furthermore, we have determined the absolute rate coefficient of CH2=CHCH2OCF2CF2H + OH using a pulsed-laser photolysis–laser-induced fluorescence technique between 273 and 363 K performed at the Physical Chemistry department of UCLM (Ciudad Real, Spain) as described by Blázquez et al.4 and references within, representing the first temperature-dependent kinetic measurements for this molecule with OH radicals. In addition, the infrared absorption cross section is quantified between 400 and 4000 cm-1, in an extended range of wavenumbers with respect to the previously reported ones5. Combining each these observations, we are able to provide an improved estimate for the GWP of this molecule and its likely environmental fate. References:1. Ren, Y.; McGillen, M. R.; Daële, V.; Casas, J.; Mellouki, A. Science Total Environ. 2020, 749, 141406.2. Heathfield, A. E.; Anastasi, C.; Pagsberg, P.; McCulloch, A. Atmos. Environ. 1998, 32, 711–717.3. Papadimitriou, V. C.; Kambanis, K. G.; Lazarou, Y. G.; Papagiannakopoulos, P. J. Phys. Chem. A 2004, 108, 2666–2674.4. Blázquez, S.; Antiñolo, M.; Nielsen, O. J.; Albaladejo, J.; Jiménez, E. Chem. Phys. Lett. 2017, 687, 297–302.5. Heathfield, A. E.; Anastasi, C.; McCulloch, A.; Nicolaisen, F. M. Atmos. Environ. 1998, 32, 2825–2833.

Mountain and high-altitude sites provide representative data for the lower free troposphere and various pathways for aerosol interactions, changing boundary layer heights useful in understanding atmospheric composition. However, few studies exist in African regions despite its diversity in both natural and anthropogenic emissions. For this reason, the ATLAS Mohamed V (AM5) observatory in the Middle Atlas region was established to provide the necessary infrastructure for detailed atmospheric studies in the North African high-altitude region. Here, results of a field study conducted to determine the aerosol chemical composition in this region, understand its variations, and importance in assessing global and regional changes in the atmospheric composition is reported. Particulate matter (PM10) filter samples (200) were collected using a high-volume (500l/min) collector in a 12h sampling interval from August to December 2017. The chemical composition of the samples was analyzed for trace metals, ions, elemental carbon, organic carbon, aliphatic hydrocarbons, and polycyclic aromatic hydrocarbon (PAHs) content. The results show that the high-altitude aerosol composition is influenced by regional and transregional transport of different pollutants. Local sources play an important role during periods when the wind speed is low, especially during autumn. Despite the proximity of the site to the Saharan Desert, its influence on the atmospheric composition was mainly seasonal and accounted for only 14% of the sampling duration. The chemical composition was dominated by inorganic elements, mainly suspended dust (47%) and ionic species (16%), and followed by organic matter (15%), water content (12%), and indeterminate mass (9%). Biogenic organics contributed up to 7% of the organic matter with high contributions from compounds such as Nonacosane, Heptacosane, and 2-Pentadecanone. Four main air masses characterized the inflow to the site, which often leads to different aerosol chemical compositions. Mineral dust influenced was seasonal and ranged between 20 and 70% of the PM mass with peaks observed during the summer and was accompanied by high concentrations of SO42- of up to 1.3 µg/m³. PM10 concentrations during winter were low (< 30 µg/m³), with a dominance of marine air masses (53%) carrying aerosols rich in sea salt and polluted anthropogenic aerosols from the coastal regions (Rabat and Casablanca). During the day-time, mineral dust contribution to PM increased by about 42% due to road dust resuspension. In contrast, during night-time, an increase in the concentrations of PAHs, ketones, and anthropogenic metals such as Pb, Ni, and Cu was found due to variations in the boundary layer height. The results provide first insights into typical North African high-altitude background aerosol chemical composition useful for long-term assessment of climate and regional influence of air pollution in North Africa.

Although the dominant source of H2SO4 in the atmosphere is generally considered to be the reaction of SO2 with OH, it is probable that the rapid reactions of Criegee intermediates (CIs) with SO2 can contribute significantly to the tropospheric H2SO4 budget under certain conditions. CIs are produced from alkene ozonolysis, and the vast quantities of unsaturated biogenic and anthropogenic volatile organic compounds emitted could provide a large and diverse flux of CIs to the atmosphere. There remain several key uncertainties regarding the global importance of CIs towards SO2 oxidation, which are principally related to the ambient concentrations of CIs and the competition between CI reaction with SO2 against the many other bimolecular and unimolecular loss processes. This is especially true of the larger, more complex CIs that are produced from terpene ozonolysis.We present experimental studies of the ozonolysis of tetramethylethylene, α-pinene and limonene, using the HELIOS chamber. HELIOS is a highly instrumented large-scale outdoor atmospheric simulation chamber and consists of a hemispheric 90 m3 Teflon-foil reactor, which is interfaced to a variety of on-line measurements including FTIR, PTR-ToF-MS, FIGAERO-ToF-CIMS, OH/H2SO4-CIMS, Aerolaser HCHO, LOPAP and SMPS, together with several GC-MS/FID and LC-MS instruments and a suite of monitors (NO, NO2, O3). Equipped with this range of instrumentation we are able to conduct alkene ozonolysis under near-ambient conditions, whilst we also have a high coverage of key reactive species in the systems of interest.From our results, we are able to provide new information regarding kinetic and mechanistic behaviour of several atmospherically important CIs and their reactive intermediates, providing new constraints on the role of CIs on the tropospheric H2SO4 budget.Keywords: ozonolysis, Criegee Intermediate, sulfur dioxide, sulfuric acid, kinetics

The term open-air factor (OAF) was coined following microbiological research in the 1960s and 1970s which established that rural air had powerful germicidal properties and attributed this to Criegee intermediates formed in the reaction of ozone with alkenes. We have re-evaluated those early experiments applying the current state of knowledge of ozone–alkene reactions. Contrary to previous speculation, neither Criegee intermediates nor the HO radicals formed in their decomposition are directly responsible for the germicidal activity attributed to the OAF. We identify other potential candidates, which are formed in ozone–alkene reactions and have known (and likely) germicidal properties, but the compounds responsible for the OAF remain a mystery. There has been very little research into the OAF since the 1970s, and this effect seems to have been largely forgotten. In this opinion piece we remind the community of the germicidal open-air factor. Given the current global pandemic spread by an airborne pathogen, understanding the natural germicidal effects of ambient air, solving the mystery of the open-air factor and determining how this effect can be used to improve human welfare should be a high priority for the atmospheric science community.

This article, the seventh in the series, presents kinetic and photochemical data sheets evaluated by the IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation. It covers an extension of the gas-phase and photochemical reactions related to Criegee intermediates previously published in Atmospheric Chemistry and Physics (ACP) in 2006 and implemented on the IUPAC website up to 2020. The article consists of an introduction, description of laboratory measurements, a discussion of rate coefficients for reactions of O3 with alkenes producing Criegee intermediates, rate coefficients of unimolecular and bimolecular reactions and photochemical data for reactions of Criegee intermediates, and an overview of the atmospheric chemistry of Criegee intermediates. Summary tables of the recommended kinetic and mechanistic parameters for the evaluated reactions are provided. Data sheets summarizing information upon which the recommendations are based are given in two files, provided as a Supplement to this article.

To characterize the role of dew water for the ground surface HONO distribution, nitrous acid (HONO) measurements with a Monitor for AeRosols and Gases in ambient Air (MARGA) and a LOng Path Absorption Photometer (LOPAP) instrument were performed at the Leibniz Institute for Tropospheric Research (TROPOS) research site in Melpitz, Germany, from 19 to 29 April 2018. The dew water was also collected and analyzed from 8 to 14 May 2019 using a glass sampler. The high time resolution of HONO measurements showed characteristic diurnal variations that revealed that (i) vehicle emissions are a minor source of HONO at Melpitz station; (ii) the heterogeneous conversion of NO2 to HONO on the ground surface dominates HONO production at night; (iii) there is significant nighttime loss of HONO with a sink strength of 0.16±0.12 ppbv h−1; and (iv) dew water with mean NO2- of 7.91±2.14 µg m−2 could serve as a temporary HONO source in the morning when the dew droplets evaporate. The nocturnal observations of HONO and NO2 allowed the direct evaluation of the ground uptake coefficients for these species at night: γNO2→HONO=2.4×10-7 to 3.5×10-6, γHONO,ground=1.7×10-5 to 2.8×10-4. A chemical model demonstrated that HONO deposition to the ground surface at night was 90 %–100 % of the calculated unknown HONO source in the morning. These results suggest that dew water on the ground surface was controlling the temporal HONO distribution rather than straightforward NO2–HONO conversion. This can strongly enhance the OH reactivity throughout the morning time or in other planted areas that provide a large amount of ground surface based on the OH production rate calculation.

International audience; Airborne bacteria play key roles in terrestrial and marine ecosystems and human health, yet our understanding of bacterial communities and their response to the environmental variables lags significantly behind that of other components of PM2.5 Here, atmospheric fine particles obtained from urban and suburb Shanghai were analyzed by using the qPCR and Illumina Miseq sequencing. The bacteria with an average concentration of 2.12 × 103 cells/m3, were dominated by Sphingomonas, Curvibacter, Acinetobacter, Bradyrhizobium, Methylobacterium, Halomonas, Aliihoeflea, and Phyllobacterium, which were related to the nitrogen, carbon, sulfur cycling and human health risk. Our results provide a global survey of bacterial community across urban, suburb, and high-altitude sites. In Shanghai (China), urban PM2.5 harbour more diverse and dynamic bacterial populations than that in the suburb. The structural equation model explained about 27%, 41%, and 20%–78% of the variance found in bacteria diversity, concentration, and discrepant genera among urban and suburb sites. This work furthered the knowledge of diverse bacteria in a coastal Megacity in the Yangtze river delta and emphasized the potential impact of environmental variables on bacterial community structure.