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Organic aerosol (OA) still remains one of the most difficult components of the atmospheric aerosols to simulate, given the multitude of its precursors, the uncertainty in its formation pathways, and the lack of measurements of its detailed composition. The LANDEX (LANDes Experiment) project, during its intensive field campaign in summer 2017, gives us the opportunity to compare biogenic secondary OA (BSOA) and its precursors and oxidants obtained within and above the Landes forest canopy to simulations performed with CHIMERE, a state-of-the-art regional chemistry transport model. The Landes forest is situated in the southwestern part of France and is one of the largest anthropized forests in Europe (1×106 ha). The majority of the forest is comprised of maritime pine trees, which are strong terpenoid emitters, providing a large potential for BSOA formation. In order to simulate OA buildup in this area, a specific model configuration setup adapted to the local peculiarities was necessary. As the forest is nonhomogeneous, with interstitial agricultural fields, high-resolution 1 km simulations over the forest area were performed. Biogenic volatile organic compound (BVOC) emissions were predicted by MEGAN, but specific land cover information needed to be used and was thus chosen from the comparison of several high-resolution land cover databases. Moreover, the tree species distribution needed to be updated for the specific conditions of the Landes forest. In order to understand the canopy effect in the forest, canopy effects on vertical diffusivity, winds, and radiation were implemented in the model in a simplified way. The refined simulations show a redistribution of BVOCs with a decrease in isoprene and an increase in terpenoid emissions with respect to the standard case, both of which are in line with observations. Corresponding changes to simulated BSOA sources are tracked. Very low nighttime ozone, sometimes near zero, remains overestimated in all simulations. This has implications for the nighttime oxidant budget, including NO3. Despite careful treatment of physical conditions, simulated BSOA is overestimated in the most refined simulation. Simulations are also compared to air quality sites surrounding the Landes forest, reporting a more realistic simulation in these stations in the most refined test case. Finally, the importance of the sea breeze system, which also impacts species concentrations inside the forest, is made evident.

International audience

International audience

Trace gas measurements were performed during the LANDEX (the LANDes EXperiment) Episode 1 field campaign in the summer 2017, in one of the largest European maritime pine forests (> 95% Pinus pinaster) located in southwestern France. Efforts have been focused on obtaining a good speciation of 20 major biogenic volatile organic compounds (BVOCs, including pinenes, carenes, terpinenes, linalool, camphene, etc.). This was made possible by the development of a new and specific chromatographic method. In order to assess the role of BVOCs in the local gas phase chemistry budget, their reactivity with the main atmospheric oxidants (hydroxyl radicals (OH), ozone (O3) and nitrate radicals (NO3)) and the corresponding consumption rates were determined. When considering the OH reactivity with BVOCs, isoprene and linalool accounted for 10-47% of the OH depletion during daytime, and monoterpenes for 50-65%, whereas monoterpenes were the main contributors during the night (70-85%). Sesquiterpenes and monoterpenes were the main contributors to the ozone reactivity, especially β-caryophyllene (30-70%), with a maximum contribution during nighttime. Nighttime nitrate reactivity was predominantly due to monoterpenes (i.e. 90-95%). Five specific groups have been proposed to classify the 19 BVOCs measured in the forest, according to their reactivity with atmospheric oxidants and their concentrations. The total amount of BVOCs consumed under and above the forest canopy was evaluated for 7 BVOCs (i.e. isoprene, α-pinene, β-pinene, myrcene, limonene + cis-ocimene and Δ3-carene). The reactivity of atmospheric oxidants and BVOCs at a local level are discussed in order to highlight the compounds (BVOCs, other VOCs), the atmospheric oxidants and the main associated reactive processes observed under the canopy of a maritime pine forest.

National audience

Atmospheric organic vapors play essential roles in the formation of secondary organic aerosol. Source identification of these vapors is thus fundamental to understand their emission sources and chemical evolution in the atmosphere and their further impact on air quality and climate change. In this study, a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) was deployed in two forested environments, the Landes forest in southern France and the boreal forest in southern Finland, to measure atmospheric organic vapors, including both volatile organic compounds (VOCs) and their oxidation products. For the first time, we performed binned positive matrix factorization (binPMF) analysis on the complex mass spectra acquired with the Vocus PTR-TOF and identified various emission sources as well as oxidation processes in the atmosphere. Based on separate analysis of low- and high-mass ranges, fifteen PMF factors in the Landes forest and nine PMF factors in the Finnish boreal forest were resolved, showing a high similarity between the two sites. Factors representing monoterpenes dominate the biogenic VOCs in both forests, with less contributions from the isoprene factors and sesquiterpene factors. Particularly, various terpene reaction products were separated into individual PMF factors with varying oxidation degrees, such as lightly oxidized compounds from both monoterpene and sesquiterpene oxidations, monoterpene-derived organic nitrates, and monoterpene more oxidized compounds. These factors display similar mass profiles and diurnal variations between the two sites, revealing similar terpene reaction pathways in these forests. With the distinct characteristics of VOCs and oxygenated VOCs measured by the Vocus PTR-TOF, this study identified various primary emission sources and secondary oxidation processes of atmospheric organic vapors in the European pine forests, providing a more comprehensive understanding of gas-phase atmospheric chemistry.

Organic aerosol (OA) still remains one of the most difficult components of the aerosol to simulate, given the multitude of its formation precursors, the uncertainty of its formation pathways and the lack of measurements of its detailed composition. The LANDEX project (The LANDes Experiment), during its intensive field campaign in summer 2017, gives us the opportunity to compare a detailed list of measurements (VOC, NOx, radicals including NO3, aerosol components, …) obtained within and above the Landes forest canopy, to simulations performed with CHIMERE, a regional Chemistry-Transport Model. The Landes forest is situated in the south-western part of France, and is one of the largest anthropized forest in Europe (1 million ha), composed by a majority of maritime pine trees, strong terpene emitters, providing a large potential for biogenic SOA formation.In order to simulate organic aerosol build-up in this area, the set-up of a specific model configuration, adapted to local peculiarities, was necessary. As the forest is inhomogeneous, with interstitial agricultural fields, high-resolution 1 km simulations over the forest area were performed, imbedded into a 5 km resolved French and a 25 km resolved European domains. BVOC emissions were predicted by MEGAN, but specific land cover needed to be used, chosen from the comparison of several high-resolution land-cover databases. Also, the tree species distribution needed updated for the specific conditions of the Landes forest. In order to understand the canopy effect in the forest, sensitivity tests were also performed and the diffusivity between the first two layers were changed. The impact of each of these refinements with respect to the standard model set-up on the concentration changes of biogenic VOCs and organic aerosol was calculated and compared to observations. In addition, the sensitivity of SOA build-up with respect to the organic aerosol scheme (standard scheme within CHIMERE, VBS schemes with updated yields for OA formation from BVOCs, …) was assessed.The ensemble of simulations allowed tracing back the origin of BSOA build-up within and above the Landes forest canopy. Above the canopy, the major simulated pathway of SOA formation is monoterpene oxidation by NO3, while within the canopy, for sufficiently low mixing during nighttime, the NO3 radical is suppressed and only little contributes to SOA build-up. This is in accordance to observations and reactivity considerations which show that within the canopy, ozone attack on sesquiterpenes is the major nighttime SOA source.

International audience; The present work is an extensive laboratory study of organosulfate (OS) formation from the reaction of α-pinene oxidation products or proxies with acidified ammonium sulfate aerosols in three different acidity conditions (NH$_4$)$_2$SO$_4$ 0.06 M; (NH$_4$)$_2$SO$_4$/H$_2$SO$_4$ 0.06 M/0.005 M; (NH$_4$)$_2$SO$_4$/H$_2$SO$_4$ 0.03 M/0.05 M). The kinetics of the reactions of $\alpha$-pinene, $\alpha$-pinene oxide, isopinocampheol, pinanediol, and myrtenal with ammonium sulfate particles were studied using a quasi-static reactor. The reaction of $\alpha$-pinene oxide with the highly acidic ammonium sulfate particles was determined to be 7, 10, 21, and 24 times faster than for isopinocampheol, $\alpha$-pinene, pinanedial, and myrtenal, respectively, for an OS precursor concentration of 1 ppm and after 1 h reaction time. The effective rate coefficients for OS formation from $\alpha$-pinene oxide were determined to be 2 orders of magnitude higher in highly acidic conditions than for the two other acidity conditions. For $\alpha$-pinene oxide reactions with highly acidic ammonium sulfate particles, OS formation was observed to increase linearly with (i) the time of reaction up to 400 min ($r^2$= > 0.95) and (ii) α-pinene oxide gas-phase concentration. However, OS formation from $\alpha$-pinene oxide reactions with slightly acidic or pure ammonium sulfate particles was limited, with a plateau ([OS]max = 0.62 ± 0.03 $\mu$g) reached after around 15–20 min. Organosulfate dimers (m/z 401 and m/z 481) were detected not only with highly acidic particles but also with slightly acidic and pure ammonium sulfate particles, indicating that oligomerization processes do not require strong acidity conditions. Dehydration products of organosulfates ($m/z$ 231 and $m/z$ 383) were observed only under highly acidic conditions, indicating the key role of H$_2$SO$_4$ on the dehydration of organosulfates and the formation of olefins in the atmosphere. Finally, this kinetic study was completed with simulation chamber experiments in which the mass concentration of organosulfates was shown to depend on the available sulfate amount present in the particle phase ($r^2$ = 0.96). In conclusion, this relative comparison between five organosulfate precursors shows that epoxide was the most efficient reactant to form organosulfates via heterogeneous gas–particle reactions and illustrates how gas–particle reactions may play an important role in OS formation and hence in the atmospheric fate of organic carbon. The kinetic data presented in this work provide strong support to organosulfate formation mechanisms proposed in part 1 ( J. Phys. Chem. A 2016, 120, 7909−7923).

International audience; Total individual concentrations (in both gaseous and particulate phases) of 80 polycyclic aromatic compounds (PACs) including 32 nitro-PAHs, 27 oxy-PAHs (polycyclic aromatic hydrocarbons) and 21 parent PAHs have been investigated over a year in the ambient air of Grenoble (France) together with an extended aerosol chemical characterization. The results indicated that their concentrations were strongly affected by primary emissions in cold period, especially from residential heating (i.e. biomass burning). Besides, secondary processes occurred in summer but also in cold period under specific conditions such as during long thermal inversion layer periods and severe PM pollution events. Different secondary processes were involved during both PM pollution events observed in March–April and in December 2013. During the first one, long range transport of air masses, nitrate chemistry and secondary nitro-PAH formation seemed linked. During the second one, the accumulation of primary pollutants over several consecutive days enhanced secondary chemical processes notably highlighted by the dramatic increase of oxy-PAH concentrations. The study of the time trends of ratios of individual nitro- or oxy-PAHs to parent PAHs, in combination with key primary or secondary aerosol species and literature data, allowed the identification of potential molecular markers of PAH oxidation. Finally, 6H-dibenzo[b,d]pyran-6-one, biphenyl-2,2’-dicarboxaldehyde and 3-nitrophenanthrene have been selected to be the best candidates as markers of PAH oxidation processes in ambient air.

A new online gas chromatographic method dedicated to Biogenic Volatile Organic Compounds (BVOC) analysis was developed for the measurement of a 20 BVOC gaseous mixture (isoprene, β-pinene, α-pinene, limonene, trans-β-ocimene, myrcene, sabinene, Δ3-carene, camphene, 1,8 cineole, terpinolene, linalool, α-phellandrene, nopinone, citral, α-terpinene, β-caryophhyllene, p-cymene, γ-terpinene and 2-carene) at a time resolution of 90 minutes. The optimized method includes an online Peltier-cooled thermodesorption system sample trap made of Carbopack B coupled to a gas chromatographic system equipped with a 60 m, 0.25 mm i.d. BPX5 column. Eluent was analysed using a flame ionization detection (FID). Potassium iodide was identified as the best ozone scrubber for the 20 BVOC mixture. In order to obtain an accurate quantification of BVOC concentrations, the development of a reliable standard mixture was also required. Quantification of BVOCs was reported with a detection limit ranging from 4 ppt for α-pinene to 19 ppt for sabinene. The main source of uncertainty was the calibration step, stressing the need of certified gaseous standards for a wider panel of BVOCs. This new method was applied for the first time to measure BVOCs in a pine forest during the LANDEX-episode-1 field campaign (summer 2017). All targeted BVOCs were detected at least once along the campaign. The two major monoterpenes observed were β-pinene and α-pinene, representing on average 60 % of the measured terpenoïds, while isoprene represented only 17 %. Uncertainties determined were always below 13 % for the six major terpenes. Uncertainties may be larger for the other compounds especially for those presenting a mixing ratio close to the detection limit.

Total OH reactivity measurements were conducted during the LANDEX intensive field campaign in a coniferous temperate forest located in the Landes area, south-western France, during July 2017. In order to investigate inter-canopy and intra-canopy variability, measurements were performed inside (6 m) and above the canopy level (12 m), as well as at two different locations within the canopy, using a Comparative Reactivity Method (CRM) and a Laser Photolysis-Laser Induced Fluorescence (LP-LIF) instrument. The two techniques were intercompared at the end of the campaign by performing measurements at the same location. Volatile organic compounds were also monitored at both levels with a proton transfer-time of flight mass spectrometer and online gas Chromatography instruments to evaluate their contribution to total OH reactivity, with monoterpenes being the main reactive species emitted in this Pinus pinaster Aiton dominated forest. Total OH reactivity varied diurnally, following the trend of BVOCs of which emissions and concentrations were dependent on meteorological parameters. Highest levels of total OH reactivity were observed during nights with a low turbulence (u* ≤ 0.2 m/s) leading to lower mixing of emitted species within the canopy and thus an important vertical stratification, characterized by a strong concentration gradient. By comparing the calculated OH reactivity from contributions of individually measured compounds to the measured OH reactivity, a discrepancy was seen at both heights mainly related to ambient temperature during day-time. It highlights that missing OH sinks could be due to temperature-dependent missing primary emissions or secondary products linked to a temperature-enhanced photochemistry. During night-time hours, atmospheric stability and relative humidity played a key role in the missing reactivity. Lower turbulence showed to be favourable for night-time chemistry, inducing a higher missing OH reactivity. Humid surfaces may have represented an additional sink for oxygenated compounds, escaping de facto total OH reactivity, and leading to lower or no missing OH reactivity.

International audience; he present study proposes an advanced methodology to refine the source apportionment of organic aerosol (OA). This methodology is based on the combination of offline and online datasets in a single Positive Matrix Factorization (PMF) analysis using the multilinear engine (ME-2) algorithm and a customized time synchronization procedure. It has been applied to data from measurements conducted in the Paris region (France) during a PM pollution event in March 2015. Measurements included OA ACSM (Aerosol Chemical Speciation Monitor) mass spectra and specific primary and secondary organic molecular markers from PM10 filters on their original time resolution (30 min for ACSM and 4 h for PM10 filters). Comparison with the conventional PMF analysis of the ACSM OA dataset (PMF-ACSM) showed very good agreement for the discrimination between primary and secondary OA fractions with about 75% of the OA mass of secondary origin. Furthermore, the use of the combined datasets allowed the deconvolution of 3 primary OA (POA) factors and 7 secondary OA (SOA) factors. A clear identification of the source/origin of 54% of the total SOA mass could be achieved thanks to specific molecular markers. Specifically, 28% of that fraction was linked to combustion sources (biomass burning and traffic emissions). A clear identification of primary traffic OA was also obtained using the PMF-combined analysis while PMF-ACSM only gave a proxy for this OA source in the form of total hydrocarbon-like OA (HOA) mass concentrations. In addition, the primary biomass burning-related OA source was explained by two OA factors, BBOA and OPOA-like BBOA. This new approach has showed undeniable advantages over the conventional approaches by providing valuable insights into the processes involved in SOA formation and their sources. However, the origins of highly oxidized SOA could not be fully identified due to the lack of specific molecular markers for such aged SOA.

International audience; The influence of the precursor chemical structure on secondary organic aerosol (SOA) formation was investigated through the study of the ozonolysis of two anthropogenic aromatic alkenes: 2-methylstyrene and indene. Experiments were carried out in three different simulation chambers: ICARE 7300L FEP Teflon chamber (ICARE, Orléans, France), EUPHORE FEP Teflon chamber (CEAM, Valencia, Spain), and CESAM evacuable stainless steel chamber (LISA, Créteil, France). For both precursors, SOA yield and growth were studied on a large range of initial concentrations (from ∼60 ppbv to 1.9 ppmv) and the chemical composition of both gaseous and particulate phases was investigated at a molecular level. Gas phase was described using FTIR spectroscopy and online gas chromatography coupled to mass spectrometry, and particulate chemical composition was analyzed (i) online by thermo-desorption coupled to chemical ionization mass spectrometry and (ii) offline by supercritical fluid extraction coupled to gas chromatography and mass spectrometry. The results obtained from a large set of experiments performed in three different chambers and using several complementary analytical techniques were in very good agreement. SOA yield was up to 10 times higher for indene ozonolysis than for 2-methylstyrene ozonolysis at the same reaction advancement. For 2-methylstyrene ozonolysis, formaldehyde and o-tolualdehyde were the two main gaseous phase products while o-toluic acid was the most abundant among six products detected within the particulate phase. For indene ozonolysis, traces of formic and phthalic acids as well as 11 species were detected in the gaseous phase and 11 other products were quantified in the particulate phase, where phthaldialdehyde was the main product. On the basis of the identified products, reaction mechanisms were proposed that highlight specific pathways due to the precursor chemical structure. These mechanisms were finally compared and discussed regarding SOA formation. In the case of 2-methylstyrene ozonolysis, ozone adds mainly on the external and monosubstituted double bond, yielding only one C8- and monofunctionalized Criegee intermediate and hence more volatile products as well as lower SOA mass than indene ozonolysis in similar experimental conditions. In the case of indene, ozone adds mainly on the five-carbon-ring and disubstituted C═C double bond, leading to the formation of two C9- and bifunctionalized Criegee intermediates, which then evolve via different pathways including the hydroperoxide channel and form highly condensable first-generation products.

International audience; During two growing seasons of a maritime pine stand, in 2014 and 2015, ozone (O-3) fluxes have been determined using the eddy covariance (EC) method and compared to the outputs of a big-leaf O-3 deposition model including stomatal, cuticular and soil pathways. The model developed in this study generally allowed to properly reproduce the measured ozone deposition. Ozone fluxes showed a strong reduction during two water stressed periods in September 2014 and July 2015. The model partly explain this fall due to the reduction of stomatal deposition. Despite this stomatal closure, measured O-3 fluxes presented systematically lower negative values than the model outputs, and sometimes even positive values around midday during periods marked by strong water stress. In other words, the difference between observed and modelled O-3 fluxes (hereinafter referred to as the residual O-3 flux) is systematically positive on daytime during these water-stressed periods. This positive residual flux traduced the existence of an O-3 source below the flux measurement level, responsible for positive fluxes that counterbalance deposition fluxes. We developed an O-3 production module based on a terpene emission algorithm and an OH concentration proxy, to try to explain the observed ozone production. As this parametrisation allowed us to reproduce well the daily and inter-daily dynamics of the residual O-3 flux, it confirms that the latter actually resulted from O-3 production processes. This ozone production is here highlighted for the first time using O-3 fluxes measurements by the EC method. The chemical reactions possibly involved in O-3 production processes in this maritime pine forest have been discussed and different mechanisms are proposed, based on peroxy radicals chemistry or stress-induced BVOCs.

Polycyclic aromatic hydrocarbons (PAHs) are recognized as important secondary organic aerosol (SOA) precursors in the urban atmosphere. In this work, the gas-phase ozonolysis of acenaphthylene was investigated in an atmospheric simulation chamber using a proton transfer reaction time-of-flight-mass spectrometer (PTR-TOF-MS) and an aerosol time-of-flight-mass spectrometer (ATOFMS) for on-line characterization of the oxidation products in the gas and particle phases, respectively. SOA samples were also collected on filters and analyzed by ultra performance liquid chromatography/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) and gas chromatography/electron impact ionization-mass spectrometry (GC/EI-MS). The major gas-phase products included a range of oxygenated naphthalene derivatives such as 1,8-naphthalic anhydride, naphthalene 1,8-dicarbaldehyde and naphthaldehyde, as well as a secondary ozonide. Possible reaction mechanisms are proposed for the formation of these products and favoured pathways have been suggested. Many of these products were also found in the particle phase along with a range of oligomeric compounds. The same range of gas and particle phase products was observed in the presence and absence of excess cyclohexane, an OH scavenger, indicating that OH radical production from the ozonolysis of acenaphthylene is negligible. SOA yields in the range 23–37% were determined and indicate that acenaphthylene ozonolysis may contribute to part of the SOA observed in urban areas.

International audience; Atmospheric particles play a major role in both air quality and climate change. Formation of secondary particles in the atmosphere has been observed over many different environments and is believed to provide up to half of the atmospheric cloud condensation nuclei (CCN) at a global scale. However, high uncertainties are still remaining in the description of mechanisms involved in new particle formation (NPF). Especially, more evidences of the implication of biogenic volatile organic compounds (BVOCs) in NPF from field studies are still needed. To investigate this question, two field campaigns have been set up during July 2014 and July 2015, in the French Landes forest (south west of France). Summer 2015 was characterized by a strong hydric stress, whereas summer 2014 was rainy. In 2015, frequent nocturnal NPF was observed, reaching a frequency of occurrence of similar to 55% of the nights, while only one event was observed in 2014. In July 2015, monoterpene mixing ratios (dominated by alpha- and beta-pinene) were higher, mostly due to high ambient temperatures and drought. A focus was made on the 2015 field campaign, where NPF was mostly observed. The mean diurnal variation of the ratio between alpha- and beta-pinene mixing ratios highlighted in-canopy reactivity of monoterpenes with ozone in the early night. This hypothesis was reinforced by the increasing gas phase levels of pinonaldehyde and nopinone, the main first-generation products arising from alpha- and beta-pinene ozonolysis, at night, before NPF started. It strongly suggests that monoterpene oxidation further generated very-low volatility gases involved in NPF. This finding is also supported by the high concentrations of the SOA traditional biogenic tracers, e.g. pinic and pinonic acids, quantified in the particulate phase. The role of BVOCs in NPF is thus highlighted, as well as the importance of nighttime NPF.

International audience; PM10 source apportionment was performed by positive matrix factorization (PMF) using specific primary and secondary organic molecular markers on samples collected over a one year period (2013) at an urban station in Grenoble (France). The results provided a 9-factor optimum solution, including sources rarely apportioned in the literature, such as two types of primary biogenic organic aerosols (fungal spores and plant debris), as well as specific biogenic and anthropogenic secondary organic aerosols (SOA). These sources were identified thanks to the use of key organic markers, namely, polyols, odd number higher alkanes, and several SOA markers related to the oxidation of isoprene, α-pinene, toluene and polycyclic aromatic hydrocarbons (PAHs). Primary and secondary biogenic contributions together accounted for at least 68% of the total organic carbon (OC) in the summer, while anthropogenic primary and secondary sources represented at least 71% of OC during wintertime. A very significant contribution of anthropogenic SOA was estimated in the winter during an intense PM pollution event (PM10 > 50 μg m− 3 for several days; 18% of PM10 and 42% of OC). Specific meteorological conditions with a stagnation of pollutants over 10 days and possibly Fenton-like chemistry and self-amplification cycle of SOA formation could explain such high anthropogenic SOA concentrations during this period. Finally, PMF outputs were also used to investigate the origins of humic-like substances (HuLiS), which represented 16% of OC on an annual average basis. The results indicated that HuLiS were mainly associated with biomass burning (22%), secondary inorganic (22%), mineral dust (15%) and biogenic SOA (14%) factors. This study is probably the first to state that HuLiS are significantly associated with mineral dust.

The present study proposes a novel approach to combine online and offline measurements in statistical source-receptor model such as Positive matrix factorization (PMF). Offline measurements include here POA (primary organic aerosol) and SOA (secondary organic aerosol) molecular markers. Their combination with data obtained using an Aerosol Chemical Speciation Monitor (ACSM) notably allowed for a better deconvolution of SOA fractions - at short time resolution (30min) - of SOA fractions than usually not obtained with online measurements only.; Ces travaux proposent une nouvelle approche permettant de combiner des mesures automatiques et manuelles de la composition chimique des aérosols à l’aide d’un modèle statistique de type “Positive Matrix Factorization” (PMF). Les mesures manuelles incluent des marqueurs moléculaires spécifiques des aérosols organiques primaires et secondaires (AOP et AOS). La combinaison de ces mesures avec les données issues d’un Aerosol Chemical Speciation Monitor (ACSM) a notamment permis d’obtenir une meilleure discrimination des différentes fractions de l’AOS - sur un pas de temps court (30min) – que ce qu’il est générallement réalisé à l’aide des seules mesures automatiques.

Secondary organic aerosol (SOA) is known to account for a major fraction of airborne particulate matter, with significant impacts on air quality and climate at the global scale. Despite the substantial amount of research studies achieved during these last decades, the source apportionment of the SOA fraction remains difficult due to the complexity of the physicochemical processes involved. The selection and use of appropriate approaches are a major challenge for the atmospheric science community. Several methodologies are nowadays available to perform quantitative and/or predictive assessments of the SOA amount and composition. This review summarizes the current knowledge on the most commonly used approaches to evaluate secondary organic carbon (SOC) contents: elemental carbon (EC) tracer method, chemical mass balance (CMB), SOA tracer method, radiocarbon (14C) measurement and positive matrix factorization (PMF). The principles, limitations, challenges and good practices of each of these methodologies are discussed in the present article. Based on a comprehensive—although not exhaustive—review of research papers published during the last decade (2006⁻2016), SOC estimates obtained using these methodologies are also summarized for different regions across the world. Conclusions of some studies which are directly comparing the performances of different methodologies are then specifically discussed. An overall picture of SOC contributions and concentrations obtained worldwide for urban sites under similar conditions (i.e., geographical and seasonal ones) is also proposed here. Finally, further needs to improve SOC apportionment methodologies are also identified and discussed.

International audience; The present study aimed at performing PM10 source apportionment, using positive matrix factorization (PMF), based on filter samples collected every 4 h at a sub-urban station in the Paris region (France) during a PM pollution event in March 2015 (PM10 > 50 μg m−3 for several consecutive days). The PMF model allowed to deconvolve 11 source factors. The use of specific primary and secondary organic molecular markers favoured the determination of common sources such as biomass burning and primary traffic emissions, as well as 2 specific biogenic SOA (marine + isoprene) and 3 anthropogenic SOA (nitro-PAHs + oxy-PAHs + phenolic compounds oxidation) factors. This study is probably the first one to report the use of methylnitrocatechol isomers as well as 1-nitropyrene to apportion secondary OA linked to biomass burning emissions and primary traffic emissions, respectively. Secondary organic carbon (SOC) fractions were found to account for 47% of the total OC. The use of organic molecular markers allowed the identification of 41% of the total SOC composed of anthropogenic SOA (namely, oxy-PAHs, nitro-PAHs and phenolic compounds oxidation, representing 15%, 9%, 11% of the total OC, respectively) and biogenic SOA (marine + isoprene) (6% in total). Results obtained also showed that 35% of the total SOC originated from anthropogenic sources and especially PAH SOA (oxy-PAHs + nitro-PAHs), accounting for 24% of the total SOC, highlighting its significant contribution in urban influenced environments. Anthropogenic SOA related to nitro-PAHs and phenolic compounds exhibited a clear diurnal pattern with high concentrations during the night indicating the prominent role of night-time chemistry but with different chemical processes involved.

International audience; Improving the understanding of processes related to atmospheric particle sources is essential to better assess future climate. Especially, how biogenic volatile organic compounds (BVOCs) are involved in new particle formation (NPF) is still unclear, highlighting the need for field studies in sites that have not yet been explored. Weakly anthropised, mostly composed of maritime pines (known as strong monoterpene emitters), vast and under the influence of sea spray inputs, the Landes forest (located in the southwestern part of France) is a suitable ecosystem to explore these questions. The aim of the present work was to investigate for the first time NPF in the Landes forest, and to identify the conditions for NPF. During a field campaign conducted in July 2015, clear NPF was observed during nighttime, at a high frequency rate (37.5%), whereas only two daytime episodes were observed. Growth rates during NPF events were in the range 9.0-15.7 nm h(-1), and nucleation rates (J(10)) in the range 0.8-8 particles cm(3) s(-1), typically in the range of reported values from rural sites. Nocturnal NPF started at sunset, lagging the reductions of temperature and ozone concentration as well as the increase of relative humidity, atmospheric stability and monoterpene concentration. We established that NPF occurred during more stratified atmosphere episodes, reflecting that NPF is more influenced by local processes at the Landes forest site (Bilos). Concentration of the sum of monoterpenes, here mainly alpha- and beta-pinene, was observed to be maximal during NPF episodes. On the contrary, ozone concentration was lower, which may indicate a larger consumption during nights where NPF episodes occur. Results strongly suggest the contribution of BVOC oxidation to nocturnal NPF, in both nucleation and the growth stages.

The chlorine atom (Cl)-initiated oxidation of three polycyclic aromatic hydrocarbons (PAHs; namely, naphthalene, acenaphthylene, and acenaphthene) was investigated. Experiments were performed in an atmospheric simulation chamber using a proton transfer reaction time-of-flight mass spectrometer (TOF-MS) and an aerosol TOF-MS to characterize the oxidation products in the gas and particle phases, respectively. The major products identified from the reaction of Cl atoms with naphthalene were phthalic anhydride and chloronaphthalene, indicating that H atom abstraction and Cl addition reaction pathways are both important. Acenaphthenone was the principal product arising from reaction of Cl with acenaphthene, while 1,8-naphthalic anhydride, acenaphthenone, acenaphthenequinone, and chloroacenaphthenone were all identified as products of acenaphthylene oxidation, confirming that the cylcopenta-fused ring controls the reactivity of these PAHs toward Cl atoms. Possible reaction mechanisms are proposed for the formation of these products, and favored pathways have been suggested. Large yields of secondary organic aerosol (SOA) were also observed in all experiments, and the major products were found to undergo significant partitioning to the particle-phase. This work suggests that Cl-initiated oxidation could play an important role in SOA formation from PAHs under specific atmospheric conditions where the Cl atom concentration is high, such as the marine boundary layer.

International audience; This work is focused on the gas-phase oxidation of acenaphthylene and acenaphthene by OH radicals and associated secondary organic aerosol (SOA) formation under low and high-NOx conditions. Experiments were carried out in an atmospheric simulation chamber using a proton transfer reaction time-of-flight-mass spectrometer (PTR-TOF-MS) and an aerosol time-of-flight-mass spectrometer (ATOFMS) to chemically characterize the gas- and particle-phase products, respectively. Due to the structures of these two aromatic compounds, the proposed chemical mechanisms exhibit some differences. In the case of acenaphthene, H-atom abstraction from the saturated cyclopenta-fused ring was found to be competitive with the OH-addition to the aromatic rings. During the photooxidation of acenaphthene using nitrous acid (HONO), aromatic ring-opening products such as indanone and indanone carbaldehyde, generated through OH addition to the aromatic ring, were formed in higher yields compared to low-NOx conditions. In the case of acenaphthylene, OH addition to the unsaturated cyclopenta-fused ring was strongly favored. Hence, ring-retaining species such as acenaphthenone and acenaphthenequinone, were identified as the main reaction products in both gas- and particle-phases, especially under high-NOx conditions. Subsequent SOA formation was observed in all experiments and SOA yields were determined under low/high-NOx conditions to be 0.61/0.46 and 0.68/0.55 from the OH-initiated oxidation of acenaphthylene and acenaphthene, respectively.

Organic aerosol (OA) is a major part of atmospheric fine particulate matter (PM), accounting for approximately 20-60% w/w of PM in the continental mid-altitudes. However, ambient OA remains poorly understood due to mixed source origins and processes (anthropogenic and biogenic, primary and secondary). The objective of this study was to refine source apportionment of PM10 OA by Positive Matrix Factorization (PMF) using specific primary and secondary organic molecular markers. PM10 samples were collected over a one year period (2013) at an urban station in Grenoble (France) every third day (24 h-basis sampling) on quartz filters, and an extended chemical characterization was performed (~216 species quantified) including specific primary organic markers i.e. levoglucosan, polyols (arabitol, mannitol), 1-nitropyrene (diesel emission), PAHs, alkanes, hopanes, etc. and secondary markers i.e. nitro- and oxy-PAHs, hydroxyglutaric acid (α-pinene secondary organic aerosol SOA), α-methyl glyceric acid (isoprene SOA), DHOPA (toluene SOA), etc. together with other PM chemical species such as OC/EC, Humic Like Substances (HuLiS), ions/cations (Na+, Mg2+, NH4+, Cl-, SO42-, NO3-), metals (Ba, Cu, Ti, Zn, Sb,...). Results showed that a better source apportionment of PM10 OA fraction was achieved using specific organic markers with additional sources resolved such as biogenic SOA, anthropogenic SOA, primary biogenic (fungi) and plant debris by comparison to more traditional PMF. More than 50% of OC seemed secondary in nature, and a high contribution of anthropogenic SOA was noticed in winter during a specific PM pollution event. Primary and secondary sources of HuLiS were also investigated in this study. Discussion will further underline the details of the chemical and temporal/seasonal profiles of each factor, and their relative contributions.

Understanding the sources and processes responsible of atmospheric PM composition and concentration is required to implement effective PM control strategies. During the last decade, the use of online non-refractory submicron aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) measurements have successfully allowed real time measurements of organic fraction. However, to profoundly understand the sources and formation processes of organic aerosol, a comprehensive source apportionment analysis is still needed. The combination of different datasets from several measurements to refine the apportionment of OA sources, and notably secondary ones, is probably one of the best way to achieve this goal. To the best of our knowledge, this has never been performed extensively (Crippa et al. 2013; Huang et al. 2014; McGuire et al. 2014; Petit et al. 2014; Sun et al. 2012). In the present study, we propose a novel approach of combining online and offline measurements in statistical source-receptor model such as Positive matrix factorization (PMF). An intensive campaign was performed at the SIRTA atmospheric research observatory, representing the suburban background air quality conditions of the Paris area (about 25 km SW from Paris city center). PM10 samples were collected every 4 hours over a period of intensive PM pollution events (PM > 50 μg m-3 over several days) on March 6- 24 2015, concomitantly with online measurements, carried out using ACSM, 7Aethalometer, TEOMFDMS, NOx and O3 analyzers. Regular PMF was first performed on organic matrix obtained from offline measurements. Figure 1 shows the eight different factors obtained using the specific primary (i.e. levoglucosan (biomass burning), methane sulphonic acid (MSA) (marine), 1-nitropyrene (traffic)) and secondary organic molecular markers (i.e. hydroxyglutaric acid (α-pinene), 3-methyl,5- nitrocatechol (biomass burning), α-methyl glyceric acid (isoprene)). PMF was also performed on ACSM OA matrix and has been deconvolved into four factors including HOA, BBOA1, BBOA2 and OOA (Figure 1). These results show that PMF performed on individual dataset is not truly viable to procure complete information about the different atmospheric processes. Lack of high-resolution time to understand rapid atmospheric processes in case of filter measurements, and lack of specific markers to validate oxygenated factors while using online measurements, have emerged out as a critical limitation of regular PMF. Here, the synergic approach proposes to combine traditional off-line PMF factors, such as primary biomass burning, primary biogenic, secondary biogenic, traffic, with OA matrix from ACSM measurements. The unified matrix was again deconvolved with PMF in order to retrieve factors such as HOA, BBOA, OOA, and to explore additional information about OA sources. Discussion will further focus on the factors obtained by combining online and offline measurements in PMF, and their benefits over the conventional approach.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous compounds emitted by all combustion sources. They are of major health concern because of their toxic properties and are therefore, regulated pollutants in ambient air. In the atmosphere, PAH oxidation through homogeneous and heterogeneous reactions may lead to the formation of oxy- and nitro-PAHs. These latter species are also emitted concomitantly with PAHs during incomplete combustion processes. Oxy- and nitro-PAHs are potentially more toxic than their parent PAHs. The identification of the origins of oxy- and nitro-PAHs is challenging, due to the coexistence of their primary and/or secondary sources. These species are also of prime interest because they are, typically part of the secondary organic aerosol (SOA) generated from gaseous PAH oxidation, which is significant in urban environments. Based on ambient air field observations, the objective of this work is to better understand the chemical processes involved in the formation of nitro- and oxy-PAHs, their sources and to identify specific molecules that could further be used as molecular markers of PAH oxidation and SOA formation. Field measurements were performed at both locations in France (Grenoble, 2013 and at the SIRTA station, 2015 (25 km SW from Paris)) over 1 year with samplings, every third day, of the gaseous and particulate (PM10) phases. Intensive observations at SIRTA have been also performed with PM10 samples collected every 4-hour during a period of severe PM pollution event (PM>50 µg m-3 for several days) in March 2015, concomitantly with online measurements (e.g. ACSM, 7λ-Aethalometer). The study of the occurrence of nitro-, -oxy and parent PAHs in the atmosphere, the seasonal and diurnal variations of their concentrations and substance patterns and the assessment of the cancer risk induced by these compounds have been performed. Based on these observations combined with literature knowledge and an extended aerosol chemical characterization, specific molecules of PAH oxidation have been identified. These substances were then used in source-receptor models such as positive matrix factorization (PMF) to apportion the SOA contribution from PAH oxidation on PM10 mass.

In the present study, quasi-static reactor and atmospheric simulation chamber experiments were performed to investigate the formation of α-pinene-derived organosulfates. Organosulfates (R–OSO3H) were examined for the reactions between acidified ammonium sulfate particles exposed to an individual gaseous volatile organic compound, such as α-pinene and oxidized products (α-pinene oxide, isopinocampheol, pinanediol and myrtenal). Molecular structures were elucidated by liquid chromatography interfaced to high-resolution quadrupole time-of-flight mass spectrometry equipped with electrospray ionization (LC/ESI-HR-QTOFMS). New organosulfate products were detected and identified for the first time in the present study. Reaction with α-pinene oxide was found to be a favored pathway for organosulfate formation (C10H18O5S) and to yield organosulfate dimers (C20H34O6S and C20H34O9S2) and trimers (C30H50O10S2) under dry conditions (RH < 1%) and high particle acidity and precursor concentrations (1 ppm). The role of r...

International audience; 21 PAHs, 27 oxy-PAHs and 32 nitro-PAHs were measured every third day over a year in both gaseous (G) and particulate PM10 (P) phases in ambient air of Grenoble (France). Mean total concentrations (G + P) of PAHs and oxy-PAHs were in the same range and about 10 ng m− 3. Nitro-PAHs were 50 to 100 times less concentrated averaging 100 pg m− 3. Polycyclic aromatic compound (PAC) concentrations were 5 to 7 times higher in “cold” period (October to March) than in “warm” period (April to September). Seasonal variations may be explained by higher primary emissions from residential heating, especially biomass burning in “cold” season. Meteorological conditions and influence of the geomorphology around Grenoble, with the formation of thermal inversion layers leading to the stagnation of pollutants, were additional key parameters. Maximum individual PAC concentrations were observed during two PM10 pollution events in December and February–March. Chemical processes and secondary formation of oxy- and nitro-PAH were probably enhanced by the accumulation of the pollutants during these events. PAC gas/particle partitioning depended on compound molecular weight and vapour pressure. Gas/particle partitioning of oxy- and nitro-PAHs were evaluated using a multi-phase poly-parameter linear free energy relationship model. The PAC cancer risk was assessed using toxic equivalency factors available in the literature (19 PAHs, 10 nitro-PAHs and 1 oxy-PAH). Overall, particle-bound PACs contributed about 76% of the cancer risk. While PAHs accounted for most of the total PAC cancer risk, oxy- and nitro-PAHs could account for up to 24%. The risk quantification across substance classes is limited by toxicological data availability.

Knowledge of the relative contribution of primary and secondary organic aerosol fractions is important in formulating effective control measurements for ambient particulate matter (PM), and the main part of Organic Aerosol (OA) in urban and rural atmospheres is found to be of secondary origin. However, its exact contribution still remains uncertain despite intensive efforts undertaken during the last decades to characterize and understand the chemistry leading to Secondary Organic Aerosol (SOA). Indeed, there is still no direct way to separate secondary to primary organic aerosol fractions. Quantification of Secondary Organic Carbon (SOC) is furthermore difficult because of our limited understanding of the molecular composition of SOC and because of the presence of a large, but unknown, number of secondary organic products. This study has been purposely carried out to estimate the contribution of SOC fraction to OA content, by using various semi-direct and indirect methodologies and evaluate associated uncertainties involved in the following outcomes. Measurements were conducted at SIRTA-LSCE, a well arranged atmospheric site in the vicinity of Paris (France), over a period from 6 to 24 March 2015. This was accomplished by utilizing information obtained from intensive filter measurements conducted at every 4 hours. SOC estimations were proposed using Positive Matrix Factorization applied to Aerosol Chemical Speciation Monitor (ACSM) measurements as well as using off-line data analyses, such as EC-, WSOC-, and SOA tracer-based methods. Results might also assert the photochemical origin of SOA, although the relative contribution to the OC and the SOA formation may differ during the chosen period. Large discrepancies were accounted for in the SOC estimations furnished using different techniques. Results imply that cautions should be taken while estimating SOC. Discussion will lead to profound information about the limitations and challenges associated to SOC estimations using different techniques.

The carbonaceous fraction of aerosols is composed of many compounds which constitute the organic carbon (OC) and elemental carbon (EC) components. Within the organic carbon component, Polycyclic Aromatic Hydrocarbons (PAHs) are the prevalent contributor to atmospheric pollution and emitted by incomplete combustion processes. Atmospheric oxidation of PAHs induces the formation of oxidized compounds such as oxygenated-PAHs (oxy-PAHs) and nitrated-PAHs (nitro- PAHs). The study of these PAH derivatives have become a prime concern as most are known to be carcinogenic and potentially more mutagenic than PAHs. These compounds may also instigate the formation of secondary organic aerosol (SOA), which may represent a large source of SOA in urban areas. The main objective of this study was to investigate the primary and secondary processes leading to the formation of oxy-PAHs and nitro-PAHs. This was accomplished by utilizing information obtained from intensive filter measurements along with online monitoring. To the best of our knowledge, this unique approach has not been explored yet. Measurements were conducted at SIRTA-LSCE, which features, the suburban background conditions of Paris, France. Results highlight high concentrations of oxy-PAHs and nitro-PAHs during the period 14-21, March 2015, including specific secondary PAH derivatives as 6H-dibenz[b,d]pyran-6-one, acenaphthenequinone and 2-nitrofluoranthene. This suggests that such species could be generated through secondary processes related to high NO3 - and SO4 2- concentrations that can be explicitly observed considering ACSM results. Besides, the formation of 6H-dibenz[b,d]pyran-6-one seems to be highly influenced by the daytime chemistry while 2-nitrofluoranthene may be generated during nighttime (potentially involving NO3 radical chemistry). Interestingly, fossil-fuel-based Black Carbon (BCff) and 1-nitropyrene (1-NP, a promising marker for diesel emissions) present similar temporal concentration profiles. Discussion will further accentuate diurnal variations, information about the day- and nighttime chemistry, and also usefulness of combining off-line and on-line measurements to ensure a comprehensive understanding of the prevalent atmospheric processes.

A detailed knowledge of the sources, transformation, and fate of organic aerosols (OA) in the atmosphere has become essential because of their major impacts on climate change and air quality. A complete understanding is still hindered by the immense chemical complexity of the organic fraction emitted from both anthropogenic and biogenic sources (primary organic aerosols, POA) or formed into the atmosphere via the photochemical oxidation of (semi-) volatile organic compounds (secondary organic aerosols, SOA). The chemical speciation of OA and the identification of their major sources are then necessary. So far, detailed information on the chemical components is generally only available for 10–30% of OA. In the present study, the source apportionment of OA is presented based on the characterization of the water-soluble organic fraction of ambient air particles, collected onto conventional filters using offline-AMS analysis. An intensive campaign was performed at SIRTA-LSCE, which illustrates the suburban background conditions of Paris (France). Every 4 hour filter measurements were conducted over a period from 6 to 24, March 2015, concomitantly with online measurements, carried out using ACSM, Aethalometer, TEOM-FDMS, NOx and O3 analyzers. Source apportionment was performed on offline- AMS data by multi-linear engine (ME-2), using the newly developed Source Finder (SoFi) package. External markers (i.e. organic species like MSA, levoglucosan, oxalate and SOA molecular compounds) are used to substantiate the presence of primary and secondary factors. SoFi outputs indicate that some factors highlight different contribution over the first and second half of the sampling campaign, advocating the role of different chemical processes during these periods. Data analysis also provides more information about the different factors, their contribution to OA and existing major OA sources in Paris. Finally, results clearly illustrate the benefit of combining different measurements and the use of external markers, in aerosol source apportionment studies.

Primary products of the reactions of gas-phase ozone with anthracene and phenanthrene adsorbed on silica model particles have been investigated. Silica was selected as proxy for mineral atmospheric particles. The particles, coated with anthracene or phenanthrene and placed on a filter, were exposed in a reaction cell to a gaseous ozone flow. Ozone concentration was constant ((6.0±0.6)×1013 molecule cm−3) during the experiments. Anthracene, phenanthrene and their ozonation products were then extracted by focused microwave-assisted extraction or fluid pressurized extraction and analyzed by gas chromatography coupled to mass spectrometry. Anthraquinone and anthrone on the one hand, and 1,1′-biphenyl-2,2′-dicarboxaldehyde on the other hand were identified as the products of anthracene and phenanthrene, respectively and quantified versus time of ozone exposure. This kinetical approach allowed to show that anthraquinone, anthrone and 1,1′-biphenyl-2,2′-dicarboxaldehyde are the primary products of the studied reactions, and to determine their formation yields (respectively, 0.42±0.04, 0.056±0.005 and 1.0±0.4).

Airborne PM pollution has emerged out as a critical issue all across the world. Quantitative and qualitative source analysis is becoming imperative to imply effective emission control strategies to reduce ambient air pollutants. Receptor oriented models, based on the statistical approach, have been developed to analyze various characteristics of the pollutants measured at the receptor site and to estimate their contributions to the source. Among the multivariate statistical receptor models used for PM source apportionment, Positive Matrix Factorization (PMF) has been adopted world wide as one of the most convenient technique. PMF has a non negative constraint and is able to quantify the factor contribution directly without a subsequent use of multiple regression analysis. More than 40% of European source apportionment studies have applied PMF (Belis et al. 2013). Recent advancements have proposed the use of new organic molecular markers in PMF to better investigate the contribution of biogenic and/or secondary organic aerosols. It has been observed that the use of these compounds improves the efficacy of PM source apportionment (Waked et al. 2014). The main objective of this study was to apportion specific PM10 sources, by using a wide variety of such organic molecular markers as PMF input data, for samples collected at an urban station “Les Frenes” of a local air quality network (Air Rhône-Alpes), considered as representative of a densely populated urban area Grenoble (France). PM10 samples were collected every third day (24 h-basis sampling) on quartz filters over a one year period (2013) and extended chemical characterization was performed including the quantification of species such as OC/EC, ions/cations (Na+, Mg2+, NH4+, Cl-, SO42-, NO3-), Polycyclic Aromatic Hydrocarbon (PAH), oxy-PAH, nitro-PAH, polyols (arabitol, mannitol), Methane Sulfonic Acid (MSA), levoglucosan, sulfur-containing PAH (Benzo[b]naphtha[2,1-d]thiophene, BNT), oxalate, higher odd number alkanes (C27, C29, C31), metals (Ba, Cu, Cr, Zn, Sb, Ni, V, Al, Ti, Fe, Mn, Rb, Ca, K). Results showed that the 10-factor profiles have given the best fit in the PMF analysis including biogenic emissions (marine, soil, plant debris), secondary inorganic (nitrate and sulfate factors) and organic aerosols, dust and aged sea salt particles and anthropogenic sources (oil combustion, traffic exhaust, biomass burning, industry…) (Figure 1). The highest percentage contribution to PM is made by secondary inorganic aerosol (~20%). It is interesting to note that Secondary PAH-aerosol factor accounts for ~ 4%. Discussion will further underline the factor contribution on seasonal basis and the stability of the chosen solution.

In the atmosphere, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous compounds. They are mostly emitted into the troposphere by incomplete combustion processes, such as diesel exhaust, residential heating, and wood burning. PAHs may react with oxidants by either homogeneous or heterogeneous reactions to form nitro and/or oxy-PAHs known to be more carcinogenic and mutagenic than parent PAHs (IARC, 2010, 2013; Rosenkranz and Mermelstein, 1985). It has been recently shown that lighter PAHs such as naphthalene and methylnaphthalene are precursors of secondary organic aerosol (SOA) and could represent one of the missing sources of SOA in urban areas (Chan et al., 2009). The purpose of this study is to better understand the SOA formation from gaseous PAH oxidation under different experimental conditions. Photooxidation experiments were performed in the UNC outdoor smog chamber. Naphthalene and 2-methylnaphthalene were selected as the two most abundant PAHs emitted in the gas phase (Reisen and Arey, 2005). Experiments were performed in the presence of nitrogen oxides, varying the type of the seed aerosol (MgSO4 vs (NH4)2SO4), aerosol acidity and relative humidity inside the chamber, as these parameters are known to have an impact on SOA formation. A complete chemical characterization of the aerosol has been performed using three complementary high-resolution mass spectrometry techniques. The composition of the gas phase has been determined during the time course of the experiments using a chemical ionization time of flight mass spectrometer (ToF-CIMS) using iodide reagent ion chemistry and the composition of the condensed phase has been characterized using ultra-performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-QTOFMS) and gas chromatography coupled to a high-resolution quadrupole time-of-flight mass spectrometry (GC-QTOFMS)...

In the present study, formation of aromatic organosulfates (OSs) from the photo-oxidation of polycyclic aromatic hydrocarbons (PAHs) was investigated. Naphthalene (NAP) and 2-methylnaphthalene (2-MeNAP), two of the most abundant gas-phase PAHs and thought to represent "missing" sources of urban SOA, were photochemically oxidized in an outdoor smog chamber facility in the presence of nonacidified and acidified sulfate seed aerosol. Effects of seed aerosol composition, acidity and relative humidity on OS formation were examined. Chemical characterization of SOA extracts by ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry revealed the formation of OSs and sulfonates from photo-oxidation in the presence of sulfate seed aerosol. Many of the organosulfur compounds identified in the smog chamber extracts were also measured in urban fine aerosol collected at Lahore, Pakistan, and Pasadena, USA, demonstrating that PAH photo-oxidation in the presence of sulfate aerosol is a hitherto unrecognized source of anthropogenic secondary organosulfur compounds, and providing new PAH SOA tracers.

Aging processes of secondary organic aerosol (SOA) may be a source of oxygenated organic aerosols; however, the chemical processes involved remain unclear. In this study, we investigate photochemical aging of SOA produced by the gas-phase oxidation of naphthalene by hydroxyl radicals and acenaphthylene by ozone. We monitored the SOA composition using a high-resolution time-of-flight aerosol mass spectrometer. We initiated SOA aging with UV photolysis alone and with OH radicals in the presence or absence of light and at different NOx levels. For naphthalene, the organic composition of the particulate phase seems to be dominated by highly oxidized compounds such as carboxylic acids, and aging data may be consistent with diffusion limitations. For acenaphthylene, the fate of oxidized products and the moderately oxidized aerosol seem to indicate that functionalization reactions might be the main aging process were initiated by the cumulative effect of light and OH radicals.

In this experimental study, rate constants were measured for the reactions of ozone with 13 polycyclic aromatic hydrocarbons (PAHs) adsorbed on different types of particles. Graphite and silica were chosen to model, respectively, carbonaceous and mineral atmospheric particles. The pseudo-first order rate constants were obtained from the fit of the experimental decay of particulate PAH concentrations versus time. Second order rate constants were calculated considering the ozone gaseous concentration. At room temperature, rate constants varied, in the case of graphite particles, between (1.5 ± 0.5) × 10−17 and (1.3 ± 0.7) × 10−16 cm3 molecule−1 s−1 for chrysene and dibenzo[a,l]pyrene, respectively, and, in the case of silica particles, between (1.5 ± 0.3) × 10−17 and (1.4 ± 0.3) × 10−16 cm3 molecule−1 s−1 for fluoranthene and benzo[a]pyrene, respectively. Different granulometric parameters (particle size, pore size) and different PAH concentrations were tested in the case of silica particles. Heterogeneous reactions of ozone with particulate PAHs are shown to be more rapid than those occurring in the gas-phase, and may be competitive with atmospheric photodegradation.

A new one-step method for the analysis of highly polar components of secondary organic aerosols (SOA) has been developed. This method should lead to a better understanding of SOA formation and evolution since it enables the compounds responsible for SOA formation to be identified. Since it is based on supercritical fluid extraction coupled to gas chromatography–mass spectrometry, it minimizes the analysis time and significantly enhances sensitivity, which makes it suitable for trace-level compounds, which are constituents of SOA. One of the key features of this method is the in situ derivatisation step: an online silylation allowing the measurement of highly polar, polyfunctional compounds, which is a prerequisite for the elucidation of chemical mechanisms. This paper presents the development of this analytical method and highlights its ability to address this major atmospheric issue through the analysis of SOA formed from the ozonolysis of a biogenic hydrocarbon (sabinene). Ozonolysis of sabinene was performed in a 6 m3 Teflon chamber. The aerosol components were derivatised in situ. More than thirty products, such as sabinaketone, sabinic acid and other multifunctional compounds including dicarboxylic acids and oxoacids, were measured. Nine of them were identified and quantified. The sensitivity and the linearity (0.91 < R < 0.98) of the method were both good and detection limits ranged from 1.2 to 6.4 ng for the investigated compounds.

Bimolecular rate coefficients for the reactions of the hydroxyl radical, OH, with methanol, ethanol, tetrahydrofuran, dimethylmalonate [CH3OC(O)CH2C(O)OCH3], dimethylsuccinate [CH3OC(O)CH2CH2C(O)OCH3], dimethylcarbonate [CH3OC(O)OCH3] and diethylcarbonate [CH3CH2OC(O)OCH2CH3] in aqueous solutions have been measured using a novel experimental approach. The centrepiece of the new experimental technique reported in this work is a Teflon AF 2400 liquid core waveguide. The physical properties of the Teflon AF 2400 liquid core waveguide allow for the construction of a micro-flowtube reaction photolysis cell with an extremely low volume and a very long optical pathway. Such a reaction system allows for a very sensitive detection of chemical transients in the aqueous phase. The micro-flowtube experiments involved competition kinetics of the OH radical with the organic reactant of interest and an SCN− anion, (kOH+SCN− = 1.29 × 1010 M−1 s−1). The (SCN)−2 anion was detected using UV-visible spectroscopy following a medium pressure mercury lamp photolysis (λ ≥ 366 nm) of H2O/H2O2/reactant/KSCN mixtures. All experiments were carried out at room temperature. Measured rate coefficients for the reaction of the OH radical with methanol, ethanol, tetrahydrofuran, dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate are (units are 108 M−1 s−1): kOH+methanol = 13 ± 4, kOH+ethanol = 19 ± 5, kOH+THF = 38 ± 10, kOH+dimethylmalonate = 2.7 ± 0.9, kOH+dimethylsuccinate = 5.3 ± 2.9, kOH+dimethylcarbonate = 0.51 ± 0.22, kOH+diethylcarbonate = 7.9 ± 3.2. Uncertainties in the above expressions are ±2σ and represent precision only. The reported rate coefficients for the reactions of OH with ethanol, methanol and THF agree very well with the currently recommended values. To date, there is no kinetic data reported in the literature for the OH radical reaction with dimethylmalonate, dimethylsuccinate, dimethylcarbonate and diethylcarbonate. The reaction mechanism is briefly discussed as a function of bond energies.

As part of a project to determine the fate of contaminated milk in the marine environment (Elliott et al ., 2001), the Biochemical Oxygen Demand (BOD) of unprocessed natural milk in sea water was followed in standard BOD bottles and in an open system. The best estimate for the BOD was ¨ 170,000 r mg - l m 1 although measurable effects could still be seen with dilutions up to one in a million. If incubated in the light, simulating summer conditions, the algae produced oxygen and offset some of the loss through bacterial degradation thereby reducing any slump in oxygen. There was also a "nutrient effect" where the presence of milk increased the oxygen production at low concentrations in sea water. In an open system akin to a slick being dispersed, the change in BOD was exponential after an initial activation period. The length of this activation period was greater in more concentrated mixtures of milk and sea water. These results have been utilised in a model to determine the best strategy for disposing of ...