Secondary organic aerosol (SOA) accounts for a significant fraction of ambient tropospheric aerosol and a detailed knowledge of the formation, properties and transformation of SOA is therefore required to evaluate its impact on atmospheric processes, climate and human health. The chemical and physical processes associated with SOA formation are complex and varied, and, despite considerable progress in recent years, a quantitative and predictive understanding of SOA formation does not exist and therefore represents a major research challenge in atmospheric science. This review begins with an update on the current state of knowledge on the global SOA budget and is followed by an overview of the atmospheric degradation mechanisms for SOA precursors, gas-particle partitioning theory and the analytical techniques used to determine the chemical composition of SOA. A survey of recent laboratory, field and modeling studies is also presented. The following topical and emerging issues are highlighted and discussed in detail: molecular characterization of biogenic SOA constituents, condensed phase reactions and oligomerization, the interaction of atmospheric organic components with sulfuric acid, the chemical and photochemical processing of organics in the atmospheric aqueous phase, aerosol formation from real plant emissions, interaction of atmospheric organic components with water, thermodynamics and mixtures in atmospheric models. Finally, the major challenges ahead in laboratory, field and modeling studies of SOA are discussed and recommendations for future research directions are proposed.

SSCI-VIDE+CARE+MBM:NHY:SPR:CGOInternational audienceFilms of organic compounds exposed to the atmosphere are ubiquitously found on surfaces of cloud droplets, aerosol particles, buildings, plants, soils and the ocean. The sea-surface microlayer is one example of organic films that are host to countless biogenic amphiphilic compounds concentrated there with respect to bulk water. Yet, organic materials present in the bulk, such as humic acids and other photosensitizing compounds, can still have a tremendous impact on the surfactants (George, 2015). Here, we present a laboratory investigation on secondary organic aerosol (SOA) formation from the reaction of ozone with volatile organic compounds (VOCs), originating from photo-induced processes in surfactant-containing water or sea-water. The results underscore the environmental importance of photochemical reactions at the air–water interface to produce SOA precursors in significant amounts, leading to significant particle formation and growth.Laboratory experiments were conducted in a custom-built 2 m3 Teflon reaction chamber with a pure or sea-water reservoir containing nonanoic acid, a model surfactant proxy for a surface microlayer, and humic acids when desired. In other experiments nonanol was used as a surfactant for comparison. VOC concentrations were measured over time using a proton transfer reaction–time of flight–mass spectrometer (PTR–ToF–MS) as a function of irradiation (UV and visible light) in a humidified low NOx and low ozone environment. Additionally, photochemical gas- and liquid-phase products generated by irradiation of natural riverine biofilms were investigated. Characterization of organic compounds in water and aerosol particles was performed with ion chromatography and liquid chromatography–high resolution mass spectrometry (LC–HRMS) utilizing a quadrupole-orbitrap detector. Aerosol size distribution and numbers were continuously monitored during all experiments.Production of VOCs occurred while the chamber air and water were irradiated with UV light for the nonanoic acid and pure water system. Gas-phase products included alkenes, aldehydes and dienes, as observed by PTR–ToF–MS in three different ionization modes (H3O+, NO+ and O2+). Aerosol nucleation and growth occurred upon ozone introduction and reaction in the chamber but with lights switched off, further indicating the presence of unsaturated VOCs. Concentrations of aerosol particles were typically on the order of 102–103 cm–3. When nonanoic acid and seawater were present, similar types and concentrations of VOCs were observed. Production of halogenated organic compounds was investigated using O2+ reagent ions. The addition of humic acids typically enhanced photochemical VOC production and aerosol formation, where SOA numbers exceeded 104 cm–3 after ozone injection and particle nucleation. We note that when nonanol was used as a surfactant, VOC and aerosol formation was never observed. This result highlights the potential importance of the carbonyl functionality of the acid group in the photochemical mechanism. The products were not only emitted to the gas phase, but also the bulk water. Using LC–HRMS the same VOCs, observed with the PTR–ToF–MS in the gas phase, were also identified in bulk water samples extracted during and after experiments. In addition, high molecular weight compounds such as highly oxidized organic molecules and molecular recombination products were identified in experiments with the nonanoic acid surfactant.We suggest that light absorption precedes radical chemistry at the organic surfactant interface. Recent experimental findings in our group, supported by quantum chemical calculations, reveal the possibility of exciting nonanoic acid molecules to their triplet state even under irradiation of 300–320 nm wavelengths. Subsequent inter- and intra-molecular reactions have been proposed generating OH and other radicals leading to a suite of products. At an organic interface neighboring organic molecules are likely to react, especially in molecular layers such as those existing in organic or biogenic films. This proximity of organic molecules is unique to an interface, in contrast to the bulk or gas phase and thus, may also result in unique chemical reaction pathways leading to VOC formation. Fatty acid and organic compound coated surfaces are ubiquitous and photon-induced chemistry at organic interfaces may be important, ultimately influencing VOC flux and SOA formation at the Earth’s surface and aloft.ReferencesGeorge, C., Ammann, M., D’Anna, B. Donaldson, D. J., Nizkorodov, S. A. (2015), Chemical Reviews 115 (10), 4218–4258. DOI: 10.1021/cr500648

A workshop was held in the framework of the ACCENT (Atmospheric Composition Change – a European Network) Joint Research Programme on "Aerosols" and the Programme on "Access to Laboratory Data". The aim of the workshop was to hold "Gordon Conference" type discussion covering accommodation and reactive uptake of water vapour and trace pollutant gases on condensed phase atmospheric materials. The scope was to review and define the current state of knowledge of accommodation coefficients for water vapour on water droplet and ice surfaces, and uptake of trace gas species on a variety of different surfaces characteristic of the atmospheric condensed phase particulate matter and cloud droplets. Twenty-six scientists participated in this meeting through presentations, discussions and the development of a consensus review. In this review we present an analysis of the state of knowledge on the thermal and mass accommodation coefficient for water vapour on aqueous droplets and ice and a survey of current state-of the-art of reactive uptake of trace gases on a range of liquid and solid atmospheric droplets and particles. The review recommends consistent definitions of the various parameters that are needed for quantitative representation of the range of gas/condensed surface kinetic processes important for the atmosphere and identifies topics that require additional research.

In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present laboratory study, the effect of light (in the range 300–500 nm) on the uptake kinetics of NO₂ on various surfaces taken as proxies for organic surfaces encountered in the troposphere (as organic aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing aromatic compounds was significantly enhanced when irradiated with light of 300–420 nm, and HONO was formed with high yield when the gas was humidified. Especially organic substrates containing a combination of electron donors, such as phenols, and of compounds yielding excited triplet states, such as aromatic ketones, showed a high reactivity towards NO₂. Based on the results reported a mechanism is suggested, in which photosensitised electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atmosphere at much longer wavelengths compared to the recently proposed nitrate photolysis.