Organic aerosols (OA) play a significant role in influencing both climate and human health. However, in source–receptor modelling, a large fraction of OA is typically attributed to highly aged, atmospherically processed species collectively referred to as oxygenated organic aerosol (OOA). Nevertheless, the formation pathways and evolution of OOA as well as their impacts on aerosol optical properties, remain poorly understood. To address this knowledge gap, an experiment was conducted in a suburban site in the Paris region to study the evolution of OOA and their optical properties. Our results show that in regionally transported air masses with mixed biogenic and anthropogenic emissions, the formation of OOA through photochemical processes explains most of the increase in submicron particle mass. Meteorological conditions played a critical role: under dry and strong solar radiation conditions, enhanced formation of more-oxidized OOA (MO-OOA) was observed. BrC absorption increased concurrently, with short-wavelength absorption rising by ∼ 35 % over relatively ∼ 24 h of photochemical aging. Conversely, under humid, low-radiation conditions, the OA composition shifted toward less-oxidized OOA (LO-OOA). Suppressed photochemistry limited MO-OOA production, resulting in a lower overall OA oxidation state. These findings highlight the role of photochemistry in shaping both the chemical evolution and resultant optical properties of OA, underscoring the need to consider meteorological dynamics when evaluating aerosol–climate interactions in suburban forest environments.

A test bench has been developed allowing to simulate air flow in a 20 L cylindrical stainless-steel chamber under conditions of the stratosphere and the troposphere: pressure from about 500 to 30 hPa, air temperature from 293 to 223 K and air flow velocity of about 5 m/s. Humidity of the air flow is controlled in the range of frost temperature from 253 to 193 K with accuracy better than 0.3 K for a frost temperature of 198 K. Specifically designed to test a newly developed frost point hygrometer, this facility may as well be used for testing instruments with suitable dimensions especially those operating with sounding balloons.

The source of nitrous acid (HONO) and its importance in island or marine environments are poorly understood. Herein, based on comprehensive field measurements at a hilltop on Corsica Island, we find an inverse diel variation of HONO with higher concentrations during daytime. Night-time HONO budget analysis indicates significant HONO formation during air mass transport along the hillside. In the daytime, although photosensitized NO2 uptake on the ground and NO + OH make considerable contributions (26% and 5%, respectively), a large part of HONO formation (67%, 320 pptv h(-1)) still cannot be explained with state-of-the-art parametrization. Nevertheless, photosensitized heterogeneous NO2 reactions are likely to account for the missing source, due to underestimation of the source by typical parametrizations at low NO2 levels. Furthermore, we demonstrate a significant role of HONO formation as a OH primary source at this island site, with a OH production rate exceeding one-fourth of that of O-3 photolysis. Our findings underscore a potential role of heterogeneous surface reactions in the oxidizing capacity of the island environments.

Alkyl nitrates (ANs) and peroxycarboxylic nitric anhydrides (PANs) are important reservoirs of reactive nitrogen that contribute significantly to the rate of formation and growth of secondary organic aerosols and support the transport of reactive nitrogen from polluted areas to remote areas. It is therefore critical to understand their sources and sinks in different environments. In this study we use measurements of OH, O3, NO3 reactivity, volatile organic compounds, ∑ANs and ∑PANs during the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) campaign to investigate different production and loss processes of ANs and PANs in a temperate forest. During the daytime OH-initiated processes were the dominant source of ANs (69 %–72 %), followed by NO3 (18 %–20 %) and O3 (8 %–12 %). During the nighttime the contribution from OH decreased to 43 %–53 %, and NO3 increased to 26 %–40 % with that of O3 largely unchanged. Of the measured ∑PANs, 48 %–78 % were modelled to be peroxyacetic nitric anhydride (PAN, commonly known as peroxyacetyl nitrate). Physical loss (e.g. deposition) was an important sink for both ANs and PANs and contributed significantly to the very short lifetimes of 1–4 h for ANs and 0.08–1.5 h for PANs observed during the campaign.

The budget of reactive nitrogen species, which play a central role in atmospheric chemistry (e.g. in photochemical O3 production), is poorly understood in forested regions. In this study, through observations of NO, NO2, NOy, and O3 in the Rambouillet Forest near Paris, France, we have examined nighttime processes controlling NOx in an anthropogenically impacted forest environment. The O3 mixing ratios displayed a strong diel profile at the site that was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O3 diel profile was strongly influenced by relative humidity and temperature inversion. Only when the O3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long) were sustained NO peaks observed above the instrumental detection limit, enabling the derivation of average NO emission rates of ∼1.4 ppbv h−1 from the soil. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O3, enabled an effective lifetime of NO2 of ∼0.5–3 h to be derived. As the loss of NO2 was not compensated for by the formation of gas- or particle-phase reactive nitrogen species, it was presumably either driven by deposition to soil and foliar surfaces or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 d. Our results indicate that the nighttime deposition of NO2 is a major sink of boundary layer NOx in this temperate forest environment.

We present direct measurements of biogenic volatile organic compound (BVOC)-induced nitrate radical (NO3) reactivity (kVOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a 6-week summer period in 2022 as part of the Atmospheric ChemistRy Of the Suburban foreSt (ACROSS) campaign. kVOC was measured in a small (700 m2) clearing mainly at a height of 5.5 m above ground level but also at 40 m (for 5 d and nights). At nighttime, mean values (and 25th–75th percentile ranges) of knightVOC(5.5m) = (0.24-0.06+0.32) s−1 and knightVOC(40m) = (0.016-0.007+0.018) s−1 indicate a significant vertical gradient and low NO3 reactivity above the canopy, whereas knightVOC(5.5 m) showed peak values of up to 2 s−1 close to the ground. The strong vertical gradient in NO3 reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterized by a strong temperature inversion and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and nitric oxide, NO). No strong vertical gradient was observed in the mean daytime NO3 reactivity, with kdayVOC(5.5m) = (0.12 ± 0.04) s−1 for the entire campaign and kdayVOC(40m) = (0.07 ± 0.02) s−1 during the 5 d period. Within the clearing, the fractional contribution of VOCs to the total NO3 loss rate coefficient (ktot, determined by photolysis, reaction with NO and VOCs) was 80 %–90 % during the night and ∼ 50 % during the day. In terms of chemical losses of α-pinene below canopy height in the clearing, we find that at nighttime hydroxyl radicals (OH) and ozone (O3) dominate, with NO3 contributing “only” 17 %, which decreases further to 8.5 % during the day. Based on measured OH, measured O3, and calculated NO3 concentrations, the chemical lifetime of BVOCs at noon is about 1 h and is likely to be longer than timescales of transport out of the canopy (typically of the order of minutes), thus significantly reducing the importance of daytime in-canopy processing. Clearly, in forested regions where sufficient nitric oxide and nitrogen dioxide (NOx) is available, the role of NO3 and OH as initiators of BVOC oxidation is not strictly limited to nighttime and daytime, respectively, as often implied in e.g. atmospheric chemistry textbooks.

In this contribution we report on a subset of ground-based measurements performed by LPC2E and ICARE at a forest site of Rambouillet during June-July of 2022 in the frame of the project ACROSS (Atmospheric ChemistRy Of the Suburban foreSt). In particular, we present the following time-concentration profiles:- OH and sum of peroxy radicals (CIMS , LPC2E);- gaseous H2SO4 (CIMS, LPC2E);- large number of volatile organic compounds (VOCs) and their oxidation products, including N-containing oxygenated organic molecules (OOMs) (Orbitrap High Resolution CIMS, LPC2E);- directly emitted VOCs and their oxidation products measured by GC-MS/FID (ICARE)- HONO (LOPAP, ICARE)- NO and NO2 (ICARE)Based on a first analysis of this reduced data-set some preliminary conclusions are made about an influence of VOCs emissions and NOx variability on the composition of OOMs oxidation products, HONO formation and oxidation capacity in the forest canopy.

The ACROSS campaign (Atmospheric chemistry of the suburban forest) is a collaborative effort to better understand how the mixing between urban and biogenic air masses impacts atmospheric composition and reactivity. A main objective is to provide a detailed description of physico-chemical processes involved in atmospheric transformations of biogenic Volatile Organic Compounds (VOCs), and the myriad of intermediate species formed as a consequence, which have yet to be characterized. This multi-platform campaign took place during summer 2022 for a duration of 6 weeks and included a supersite in the Rambouillet forest located on the path of pollution plumes from Paris. An unprecedented suite of analytical instruments was deployed at this site to probe air masses at the ground level and above the forest canopy using a 40-m tower. This presentation will focus on trace gas observations, including VOCs, ROx (OH+HO2+RO2) and selected inorganic species (O3, NOx – NO+NO2), both above and below the forest canopy. A descriptive analysis will be presented to highlight biogenic VOC oxidation regimes taking place under unpolluted (oceanic air masses) and polluted (Paris plumes) conditions, with a focus on primary/secondary species. Chemical compositions observed above and below-canopy will also be contrasted. On the basis of these observations, this presentation will highlight scientific questions related to ROx, VOC and NOx budgets that will be further investigated to fill important gaps in our understanding of biogenic VOC transformations and secondary organic aerosol formation. Acknowledgments. This work is supported by the French national research agency (ANR-20-CE01-0010) & LABEX-CaPPA (ANR-11-LABX-005-01) and the French national program LEFE/CHAT INSU.

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.

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

Reaction of stabilized Criegee intermediates (SCIs) with SO2 was proposed as an additional pathway of gaseous sulfuric acid (H2SO4) formation in the atmosphere, supplementary to the conventional mechanism of H2SO4 production by oxidation of SO2 in reaction with OH radicals. However, because of a large uncertainty in mechanism and rate coefficients for the atmospheric formation and loss reactions of different SCIs, the importance of this additional source is not well established. In this work, we present an estimation of the role of SCIs in H2SO4 formation at a western Mediterranean (Cape Corsica) remote site, where comprehensive field observations including gas-phase H2SO4, OH radicals, SO2, volatile organic compounds (VOCs) and aerosol size distribution measurements were performed in July–August 2013 as a part of the project ChArMEx (Chemistry-Aerosols Mediterranean Experiment). The measurement site was under strong influence of local emissions of biogenic volatile organic compounds, including monoterpenes and isoprene generating SCIs in reactions with ozone, and, hence, presenting an additional source of H2SO4 via SO2 oxidation by the SCIs. Assuming the validity of a steady state between H2SO4 production and its loss by condensation on existing aerosol particles with a unity accommodation coefficient, about 90 % of the H2SO4 formation during the day could be explained by the reaction of SO2 with OH. During the night the oxidation of SO2 by OH radicals was found to contribute only about 10 % to the H2SO4 formation. The accuracy of the derived values for the contribution of OH + SO2 reaction to the H2SO4 formation is limited mostly by a large, at present factor of 2, uncertainty in the OH + SO2 reaction rate coefficient. The contribution of the SO2 oxidation by SCIs to the H2SO4 formation was evaluated using available measurements of unsaturated VOCs and steady-state SCI concentrations estimated by adopting rate coefficients for SCI reactions based on structure–activity relationships (SARs). The estimated concentration of the sum of SCIs was in the range of (1–3) × 103 molec. cm−3. During the day the reaction of SCIs with SO2 was found to account for about 10 % and during the night for about 40 % of the H2SO4 production, closing the H2SO4 budget during the day but leaving unexplained about 50 % of the H2SO4 formation during the night. Despite large uncertainties in used kinetic parameters, these results indicate that the SO2 oxidation by SCIs may represent an important H2SO4 source in VOC-rich environments, especially during nighttime.

Methylglyoxal (MGLY) is an important atmospheric α-dicarbonyl species for which photolysis acts as a significant source of peroxy radicals, contributing to the oxidizing capacity of the atmosphere and, as such, the formation of secondary pollutants such as organic aerosols and ozone. However, despite its importance, only a few techniques exhibit time resolutions and detection limits that are suitable for atmospheric measurements.This study presents the first field measurements of MGLY by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) performed during the ChArMEx SOP2 field campaign. This campaign took place at a Mediterranean site characterized by intense biogenic emissions and low levels of anthropogenic trace gases. Concomitant measurements of MGLY were performed using the 2,4-dinitrophenylhydrazine (DNPH) derivatization technique and high performance liquid chromatography (HPLC) with UV detection. PTR-ToF-MS and DNPH–HPLC measurements were compared to determine whether these techniques can perform reliable measurements of MGLY.Ambient time series revealed levels of MGLY ranging from 28 to 365 pptv, with a clear diurnal cycle due to elevated concentrations of primary biogenic species during the daytime, and its oxidation led to large production rates of MGLY. A scatter plot of the PTR-ToF-MS and DNPH–HPLC measurements indicates a reasonable correlation (R2 = 0.48) but a slope significantly lower than unity (0.58±0.05) and a significant intercept of 88.3±8.0 pptv. A careful investigation of the differences between the two techniques suggests that this disagreement is not due to spectrometric interferences from H3O+(H2O)3 or methyl ethyl ketone (or butanal) detected at m∕z 73.050 and m∕z 73.065, respectively, which are close to the MGLY m∕z of 73.029. The differences are more likely due to uncorrected sampling artifacts such as overestimated collection efficiency or loss of MGLY into the sampling line for the DNPH–HPLC technique or unknown isobaric interfering compounds such as acrylic acid and propanediol for the PTR-ToF-MS.Calculations of MGLY loss rates with respect to OH oxidation and direct photolysis indicate similar contributions for these two loss pathways.

As part of the CHARMEX (Chemistry Aerosol Mediterranean Experiments) project, more than one hundred or- ganic and inorganic gaseous compounds were measured in the summer of 2013 at the Mediterranean remote site of ERSA in Cape-Corsica. During this period, New Particle formation (NPF) events were identified from July 31th to august 2nd when air masses originated from the North-eastern sector (Southern Europe). The results were compared to a non-NPF event from July 21th to July 23rd for which the same wind sectors were identified. They showed that the particles number [10-20 nm] measured by SMPS (Scanning Mobility Particle Sizer) were more correlated with carbon monoxide (CO) during non-NPF events indicating an influence of more polluted and more aged air masses (residence time of CO of ∼60 days). Sulfuric acid (H2SO4) and sulfur dioxide do not show a sig- nificant influence in the formation of nucleation events. On the other hand, biogenic Volatile Organic Compounds (BVOCs) such as isoprene, and mono-terpenes as well as their oxidation products (e.g. MACR+MVK, MTOP) showed good correlation during NPF-events in the range of (r from 0.45 to 0.59) higher than the ones reported during non-NPF events (0.11-0.34) highlighting the importance of these BVOCs on NPF days. The comparison of measured vs calculated reactivity (Zannoni et al, 2016) showed that during NPF-events, the missing part of OH reactivity was higher. It indicates that unmeasured species like sesquiterpenes, organo-nitrates, or oxygenated compounds may play a significant role in such events.