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Cyclic volatile methyl Siloxanes (cVMS) are a group of silicon-based organic compounds of anthropogenic origin that are found ubiquitously in the ambient air. They find wide application in many industrial, automotive products, consumer, and personal care products such as paints, solvents, adhesives, cosmetics, polymers, etc. About 90% of the environmentally released cyclic siloxanes diffuse directly into the atmosphere. They are high–production volume chemicals with some having an annual production rate of 45 to 227 thousand tons worldwide. Siloxanes have been observed in both urban and rural areas. Among the different siloxanes emitted into the atmosphere hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), and decamethylcyclopentasiloxane (D5) are of great importance due to their subsequent concentrations. It should be mentioned that siloxanes are not readily biodegradable which means they can persist in the environment for a long duration. The D4 species has already been identified as a potential persistent organic pollutant (PoPs). D4 and D5 are very persistent and bioaccumulative and have evinced or potential hazards to both humans and the environment.Taking advantage of the Vocus proton-transfer-reaction time-of-flight mass spectrometer (VOCUS PTR–TOF–MS) the D3, D4, and D5 species were measured in real-time downtown Paris in the June–July 2022 period. The measurements were performed during the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) field campaign at the urban site located at Université Paris Cité (48.83°N, 2.38°E) sampling at about 25 m above the ground level. The data collected during the summer 2022 showed that the mean concentrations were 0.099±0.009, 0.045±0.009, and 0.131±0.089 ppb for D3, D4, and D5, respectively. A strong diurnal cycle was observed for the different siloxanes. Concentrations for the D5 siloxane peaked in the morning with an averaged maximum of 0.298± 0.119 ppb while the 25th and the 75th quantile are 0.221 and 0.377 ppb, respectively. In the afternoon concentrations show a minimum value, followed by a slight increase during the night. The D5 is the most abundant siloxane species and shows the strongest diurnal cycle compared to the other two identified siloxanes. Observations during the campaign and the potential reaction products and sources of siloxanes will be discussed in this work.KEYWORDS: Siloxanes; Cyclic Siloxanes; D3,D4 and D5; Ambient Air; PoPs; ACROSS; HR- Mass Spectrometry; VOCUS PTR-Tof

It is generally acknowledged that urban vegetation improves the environmental quality of cities around the world as they can provide ecosystem services like mitigation of urban heat island effect, and capture of atmospheric pollutants by the leaf surfaces. However, vegetation is also a source of Biogenic Volatile Organic Compounds (BVOC), that can be targets of oxidants produced from chemical interactions with anthropogenic atmospheric pollutants. Such oxidation phenomena trigger the formation of secondary pollutants like ozone (O3) and secondary organic aerosols (SOA). As part of the sTREEt (Impact of sTress on uRban trEEs and on city air quality) project, the present work aims to (i) characterize the ambient gaseous composition in the center of Paris (Town Hall) impacted by a local biogenic source (an urban garden and street trees) and subjected to significant anthropogenic contributions (mainly traffic), and (ii) estimate the contribution of BVOC (vs Anthropogenic VOC) and ultimately their impact on the ambient air quality (O3, SOA). A field campaign took place in Paris city center, from June 8th to July 6th, 2022, notably including a heat wave period on June 17th and 18th (with temperature up to 37°C). The experimental set-up comprised continuous measurements of VOC (using PTR-ToF-MS), NOx (as traffic tracer), aerosol chemical composition (using ACSM) and meteorological parameters (i.e., temperature, wind, light irradiation). VOCs speciation (including terpenes) was also performed off-line based on cartridges analyses for selected days. Results are presented and compared here with measurements at a suburban site at the SIRTA-ACTRIS facility (20 km SW of Paris). The overall VOC concentrations at the town hall site were higher than at the SIRTA site. Furthermore, isoprene and monoterpenes were mainly of anthropogenic origin, with a diurnal variability and a strong correlation with traffic markers (NOx and aromatic VOC). However, during the heat wave, the concentrations of isoprene and monoterpenes increased and showed a different variability than that of the traffic markers. The concentration of isoprene became very significant with levels around 6 ppb (comparable to the SIRTA site), due to the increase in the biotic component, which may promote the formation of ozone. On the other hand, the moderate increase in monoterpene concentrations (0.1 to 0.7 ppb), due as well to the increase in the biotic component, could contribute significantly to the SOA formation. Thus, assuming a near future increase in the frequency and intensity of heat waves, the contribution of the biogenic source to the ambient air quality may be even more important in urban areas with strong anthropogenic contributions.

Biogenic volatile organic compounds (BVOCs) emitted from a wide range of trees and plants undergo oxidation reactions in the atmosphere to produce secondary organic aerosol (SOA), which has direct and indirect effects on climate, and is accountable for a large proportion of climate uncertainties. The composition and yield of SOA is determined by the precursor BVOCs, which depends on the emission profile of the plant. In Ireland forestry is dominated by Sitka spruce (Picea Sitchensis), the emissions of which are not well characterised. The goal of this study is to identify the BVOC emissions from Sitka spruce, and to assess their SOA formation potential.To characterise the emission profile of Sitka spruce, 4-year old trees were housed in a plant growth chamber under controlled environmental conditions and the emissions monitored on-line with a time-of-flight chemical ionisation mass spectrometer (ToF-CIMS), and off-line with thermal desorption gas chromatography mass spectrometry. The atmospheric reactions of the VOCs emitted by the Sitka spruce were investigated by oxidising them with hydroxyl (OH) radicals in an atmospheric simulation chamber. BVOC oxidation and gas-phase product formation was monitored by ToF-CIMS. A scanning mobility particle sizer (SMPS) was used to track particle formation and growth, and the SOA composition was determined with the use of a filter inlet for gas and aerosols (FIGAERO) fitted to the ToF-CIMS.Over 60 different BVOCs were identified in the Sitka spruce emissions, with oxygenated species accounting for over 50% of them. The most abundant compound identified was piperitone, C10H16O an oxygenated monoterpene. Other compounds prevalent in the emissions included isoprene and monoterpenes, such as myrcene and β-phellandrene. During oxidation experiments the decay of the Sitka spruce emissions was observed with the ToF-CIMS in C6H6+ mode, while the formation of oxidised gas products was observed in I- mode. The most prevalent gas-phase product was C5H6O3. Analysis of the gas-phase oxidation products indicated that the oxidation of multiple BVOCs led to their formation. Particle formation and growth commenced quickly after the OH reaction was initiated. The composition of the SOA showed C6H8O6 as the dominant species, but the majority of the products had formulas in the range #C7 – 15 and #O5 - 8. Analysis of both gas and particle phase chemistry has been performed to determine the SOA formation potential of Sitka spruce BVOC emissions.

The complex refractive index (CRI) is one of the key parameter driving aerosol spectral optical properties and direct radiative effects (DRE). Its value and spectral variation under different conditions, such as anthropogenic− and biogenic−dominated environments and anthropogenic−biogenic mixing situations, remains not fully understood. As a consequence, oversimplified representations of aerosol optical properties are generally used in climate models. Therefore, measurements of aerosol CRI in different environments and their inclusion in models are needed. The field observations from the ACROSS campaign, performed in June-July 2022 in the Ile de France region, are used in this study to deepen the knowledge of aerosol optical properties, aiming to improve the aerosol representation in the CHIMERE model and provide the best constraint for DRE simulations. Measurements obtained both at the Paris city center and the Rambouilllet rural forest sites during ACROSS are considered, in order to explore the CRI variability from anthropogenic−dominated to biogenic−dominated environments, including anthropogenic−biogenic mixing situations. The CRI retrievals at seven different wavelengths, performed by combining the Mie theory with optical and size distribution measurements, are representative of different atmospheric conditions, aerosol loadings as well as type and chemical compositions. In fact, the June-July 2022 period was characterized by highly diversified weather conditions: 1) two strong heatwaves, promoting SOA build-up and favoring the export of the Paris pollution plume towards the forest site; 2) Saharan dust events transported from the upper atmosphere to the ground; 3) biomass burning episode; 4) periods with reduced anthropogenic influence. The CRI retrievals under these different conditions and their link to particulate chemical composition is investigated. Hence, the CRI dataset presented here constitutes a unique dataset from which models can benefit to validate and constrain simulations and DRE estimations, under both urban and biogenic emissions influence. These data, in conjunction with those from the aircraft observations during ACROSS, are used to initialize and perform sensitivity studies on the aerosol DRE, using the CHIMERE−WRF coupled model, the OPTSIM model for the aerosol optical properties and the Rapid Radiative Transfer Model for GCMs (RRTMG).Keywords: Complex refractive index, direct radiative effect, aerosol mixing, urban, forest

The use of animal-based fertilizers in the forms of manure and slurry is widespread throughout Europe. The process of spreading manure and slurry results in large emissions of greenhouse gases as well as inorganic gases and volatile organic compounds (VOCs). The non-methane VOCs emitted from manure and slurry are not well characterized, even though they are expected to result in the formation of important secondary pollutants such as ozone and secondary organic aerosols (SOA). In this study, we conducted an initial small-scale laboratory measurement of VOCs emitted by cattle slurry using a Time of Flight Chemical Ionisation Mass Spectrometer. Dry purified air was passed through a small chamber containing the slurry and into the instrument, which was operated using both C6H6+ and I- as reagent ions in order to detect a large range of oxygenated and hydrocarbon VOCs. Using the I- reagent ion, a number of low molecular weight carboxylic acids were detected, along with some phenolic compounds and hydrogen disulfide. A much larger number of compounds was detected using the C6H6+reagent ion, including the same phenolic compounds and a range of nitrogen-containing species. The timescales of the emitted species showed considerable variation, with some, e.g. hydrogen disulfide, being very intense and short-lived, and others, e.g. phenol, being continuously emitted at a roughly constant rate for several hours. Further work is currently underway to understand the factors controlling the nature of these emissions, their reactivity and SOA formation.