New particle formation at a peri-urban agricultural site.

New Particle Formation (NPF) is a major source of ultrafine particles that affect both air quality and climate. Despite emissions from agricultural activities having a strong potential to lead to NPF, little is known about NPF within agricultural environments. The aim of the present study was to investigate the occurrence of NPF events at an agricultural site, and any potential relationship between agricultural emissions and NPF events. A field campaign was conducted for 3 months at the FR-Gri-ICOS site (France), at an experimental farm 25 km west of Paris city centre. 16 NPF events have been identified from the analysis of particle number size distributions; 8 during the daytime, and 8 during the night-time. High solar radiation and ozone mixing ratios were observed during the days NPF occurred, suggesting photochemistry plays a key role in daytime NPF. These events were also associated with higher levels of VOCs such as isoprene, methanol, or toluene compared to non-event days. However, ammonia levels were lower during daytime NPF events, contributing to the hypothesis that daytime NPF events were not related to agricultural activities. On the other hand, temperature and ozone were lower during the nights when NPF events were observed, whereas relative humidity was higher. During these nights, higher concentrations of NO2 and ammonia were observed. As a result, agricultural activities, in particular the spreading of fertiliser on surrounding crops, are suspected to contribute to night-time NPF events. Finally, all the identified NPF events were also observed at SIRTA monitoring station 20 km from the FR-Gri ICOS site, showing that both night-time and daytime NPF events were regional processes. We hypothesise that night-time NPF may be related to fertiliser spreading over a regional scale, as opposed to the local activities at the farm. To our knowledge, this is the first time night-time NPF has been observed in the agricultural context.

Reduced microbial diversity induces larger volatile organic compound emissions from soils.

Microorganisms in soil are known to be a source and a sink of volatile organic compounds (VOCs). The role of the microbial VOCs on soil ecosystem regulation has been increasingly demonstrated in the recent years. Nevertheless, little is known about the influence of the microbial soil community structure and diversity on VOC emissions. This novel study analyzed the effect of reduced microbial diversity in soil on VOC emissions. We found that reduced levels of microbial diversity in soil increased VOC emissions from soils, while the number of different VOCs emitted decreased. Furthermore, we found that Proteobacteria, Bacteroidetes and fungi phyla were positively correlated to VOC emissions, and other prokaryotic phyla were either negatively correlated or very slightly positively correlated to VOCs emissions. Our interpretation is that Proteobacteria, Bacteroidetes and fungi were VOC producers while the other prokaryotic phyla were consumers. Finally, we discussed the possible role of VOCs as mediators of microbial interactions in soil.

Characterization of particulate and gaseous pollutants from a French dairy and sheep farm.

Agricultural activities highly contribute to atmospheric pollution, but the diversity and the magnitude of their emissions are still subject to large uncertainties. A field measurement campaign was conducted to characterize gaseous and particulate emissions from an experimental farm in France containing a sheep pen and a dairy stable. During the campaign, more than four hundred volatile organic compounds (VOCs) were characterized using an original combination of online and off-line measurements. Carbon dioxide (CO2) and ammonia (NH3) were the most concentrated compounds inside the buildings, followed by methanol, acetic acid and acetaldehyde. A CO2 mass balance model was used to estimate NH3 and VOC emission rates. To our knowledge, this study constitutes the first evaluation of emission rates for most of the identified VOCs. The measurements show that the dairy stable emitted more VOCs than the sheep pen. Despite strong VOC and NH3 emissions, the chemical composition of particles indicates that gaseous farm emissions do not affect the loading of fine particles inside the farm and is mainly explained by the low residence time inside the buildings. The experimental dataset obtained in this work will help to improve emissions inventories for agricultural activities.

Profiles of volatile organic compound emissions from soils amended with organic waste products.

Volatile Organic Compounds (VOCs) are reactive compounds essential to atmospheric chemistry. They are mainly emitted by living organisms, and mostly by plants. Soil microbes also contribute to emissions of VOCs. However, these emissions have not yet been characterised in terms of quality and quantity. Furthermore, long-term organic matter amendments are known to affect the microbial content of soils, and hence the quantity and quality of VOC emissions. This study investigates which and how much of these VOCs are emitted from soil amended with organic waste products (OWPs). Four OWPs were investigated: municipal solid waste compost (MSW), green waste and sludge co-compost (GWS), bio-waste compost (BIOW) and farmyard manure (FYM). These OWPs have been amended every two years since 1998 until now at a rate of ~4 tC ha-1. A soil receiving no organic inputs was used as a reference (CN). VOCs emissions were measured under laboratory conditions using a Proton Transfer Reaction-Quadrupole ion guide Time of Flight-Mass Spectrometry (PTR-QiToF-MS). A laboratory system was set up made of two Pyrex chambers, one for samples and the second empty, to be used as a blank. Our results showed that total VOC emissions were higher in BIOW than in MSW. Further findings outlined that the most emitted compounds were acetone, butanone and acetaldehyde in all treatments, suggesting a common production mechanism for these compounds, meaning they were not affected by the OWP amendment. We isolated 21 VOCs that had statistically different emissions between the treatments and could therefore be considered as good markers of soil biological functioning. Our results suggest that organic matter and pH jointly influenced total VOC emissions. In conclusion, OWPs in soil affect the type of VOC emissions and the total flux also depends on the pH of the soil and the quantity of organic matter.

Photodegradation of methyl thioglycolate particles as a proxy for organosulphur containing droplets.

Understanding the formation and transformation of sulphur-rich particles is of prime importance since they contribute to the global atmospheric sulphur budget. In this work, we performed a series of experiments on a photoactive organosulphur compound namely, methyl thioglycolate, as a model of an organosulphur species of marine origin. By investigating the photoproducts within levitated droplets, we showed that elemental sulphur (α-S8) and sulphate (SO42-) can be photochemically generated at the gas-liquid interface by heterogeneous interaction with gaseous O2 and H2O. These results demonstrate that the surface of levitated droplets facilitate the oxidation of methyl thioglycolate in the dark, while illumination is necessary to produce the oxidation in bulk experiments.

Fatty Acid Surfactant Photochemistry Results in New Particle Formation.

Organic interfaces that exist at the sea surface microlayer or as surfactant coatings on cloud droplets are highly concentrated and chemically distinct from the underlying bulk or overlying gas phase. Therefore, they may be potentially unique locations for chemical or photochemical reactions. Recently, photochemical production of volatile organic compounds (VOCs) was reported at a nonanoic acid interface however, subsequent secondary organic aerosol (SOA) particle production was incapable of being observed. We investigated SOA particle formation due to photochemical reactions occurring at an air-water interface in presence of model saturated long chain fatty acid and alcohol surfactants, nonanoic acid and nonanol, respectively. Ozonolysis of the gas phase photochemical products in the dark or under continued UV irradiation both resulted in nucleation and growth of SOA particles. Irradiation of nonanol did not yield detectable VOC or SOA production. Organic carbon functionalities of the SOA were probed using X-ray microspectroscopy and compared with other laboratory generated and field collected particles. Carbon-carbon double bonds were identified in the condensed phase which survived ozonolysis during new particle formation and growth. The implications of photochemical processes occurring at organic coated surfaces are discussed in the context of marine SOA particle atmospheric fluxes.

Unravelling New Processes at Interfaces: Photochemical Isoprene Production at the Sea Surface.

Isoprene is an important reactive gas that is produced mainly in terrestrial ecosystems but is also produced in marine ecosystems. In the marine environment, isoprene is produced in the seawater by various biological processes. Here, we show that photosensitized reactions involving the sea-surface microlayer lead to the production of significant amounts of isoprene. It is suggested that H-abstraction processes are initiated by photochemically excited dissolved organic matter which will the degrade fatty acids acting as surfactants. This chemical interfacial processing may represent a significant abiotic source of isoprene in the marine boundary layer.

Photosensitized production of functionalized and unsaturated organic compounds at the air-sea interface.

The sea-surface microlayer (SML) has different physical, chemical and biological properties compared to the subsurface water, with an enrichment of organic matter i.e., dissolved organic matter including UV absorbing humic substances, fatty acids and many others. Here we present experimental evidence that dissolved organic matter, such as humic acids, when exposed to sunlight, can photosensitize the chemical conversion of linear saturated fatty acids at the air-water interface into unsaturated functionalized gas phase products (i.e. saturated and unsaturated aldehydes and acids, alkenes and dienes,...) which are known precursors of secondary organic aerosols. These functionalized molecules have previously been thought to be of biological origin, but here we demonstrate that abiotic interfacial photochemistry has the potential to produce such molecules. As the ocean is widely covered by the SML, this new understanding will impact on our ability to describe atmospheric chemistry in the marine environment.

Photosensitized Production of Atmospherically Reactive Organic Compounds at the Air/Aqueous Interface.

We report on experiments that probe photosensitized chemistry at the air/water interface, a region that does not just connect the two phases but displays its own specific chemistry. Here, we follow reactions of octanol, a proxy for environmentally relevant soluble surfactants, initiated by an attack by triplet-state carbonyl compounds, which are themselves concentrated at the interface by the presence of this surfactant. Gas-phase products are determined using PTR-ToF-MS, and those remaining in the organic layer are determined by ATR-FTIR spectroscopy and HPLC-HRMS. We observe the photosensitized production of carboxylic acids as well as unsaturated and branched-chain oxygenated products, compounds that act as organic aerosol precursors and had been thought to be produced solely by biological activity. A mechanism that is consistent with the observations is detailed here, and the energetics of several key reactions are calculated using quantum chemical methods. The results suggest that the concentrating nature of the interface leads to its being a favorable venue for radical reactions yielding complex and functionalized products that themselves could initiate further secondary chemistry and new particle formation in the atmospheric environment.

Heterogeneous reactivity of chlorine atoms with ammonium sulfate and ammonium nitrate particles.

In this laboratory study, model particles of ammonium sulfate (AS) and ammonium nitrate (AN) were exposed to chlorine atoms and uptake experiments were performed in a coated wall flow tube reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on the halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients vary for (NH(4))(2)SO(4), ranging from γ(Cl)(AS)≈ 1 × 10(-3) for size-selected particles to γ(Cl)(AS)≈ 6 × 10(-2) for thin films prepared by spray. An uptake coefficient of γ(Cl)(AN)≈ 2.5 × 10(-3) of Cl˙ on size-selected NH(4)NO(3) particles was measured. A heterogeneous recombination of Cl atoms to from Cl(2) molecules was observed for the two surfaces. Furthermore, an ageing process was observed for AS particles, this phenomenon leading to the formation of new chlorine species on the solid substrate.

Heterogeneous reactivity of chlorine atoms with sodium chloride and synthetic sea salt particles.

The uptake of chlorine atoms on sodium chloride (NaCl) and synthetic sea salt (SSS) particles was studied using a discharge flow reactor coupled to a molecular beam mass spectrometer. The reactive surfaces were prepared by coating the inner surface of the reactor using two different methods: either by depositing size-selected particles on halocarbon wax or by spray depositing thin films using a constant output atomizer. The observed uptake coefficients of Cl˙ on NaCl particles are γ(NaCl)(Cl) ≈ 2 × 10(-2) for size-selected particles or γ(NaCl)(Cl) ≈ 5 × 10(-2) for thin films and for SSS particles γ(SSS)(Cl) ≈ 4 × 10(-3). Heterogeneous recombination of Cl atoms to Cl(2) molecules was observed for the two solid surfaces. The study was performed over the temperature range 258 to 353 K. The temperature dependence of the uptake was observed and the heat of adsorption of Cl˙ on NaCl particles was estimated at Q(ads) = 63 kJ mol(-1) assuming an Eley-Rideal mechanism. The role of surface adsorbed water has also been shown. The atmospheric implications of these findings are discussed briefly.