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.

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The formation, properties and impact of secondary organic aerosol: current and emerging issues

The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

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Organic aerosol and global climate modelling: a review

We present a method to describe the relationship between particle dry diameter and cloud condensation nu- clei (CCN) activity using a single hygroscopicity parameter . Values of the hygroscopicity parameter are between 0.5 and 1.4 for highly-CCN-active salts such as sodium chlo- ride, between 0.01 and 0.5 for slightly to very hygroscopic organic species, and 0 for nonhygroscopic components. Ob- servations indicate that atmospheric particulate matter is typ- ically characterized by 0.1<< 0.9. If compositional data are available and if the hygroscopicity parameter of each com- ponent is known, a multicomponent hygroscopicity parame- ter can be computed by weighting component hygroscopic- ity parameters by their volume fractions in the mixture. In the absence of information on chemical composition, exper- imental data for complex, multicomponent particles can be fitted to obtain the hygroscopicity parameter. The hygroscop- icity parameter can thus also be used to conveniently model the CCN activity of atmospheric particles, including those containing insoluble components. We confirm the applica- bility of the hygroscopicity parameter and its mixing rule by applying it to published hygroscopic diameter growth fac- tor and CCN-activation data for single- and multi-component particles containing varying amounts of inorganic, organic and surface active compounds. We suggest that may be fit to CCN data assuming s/a=0.072 J m 2 and present a table of derived for this value and T=298.15 K. The predicted hygroscopicities for mixtures that contain the surfactant ful- vic acid agree within uncertainties with the measured values. It thus appears that this approach is adequate for predict- ing CCN activity of mixed particles containing surface ac- tive materials, but the generality of this assumption requires further verification.

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A single parameter representation of hygroscopic growth and cloud condensation nucleus activity

Aerosols are of central importance for atmospheric chemistry and physics, the biosphere, climate, and public health. The airborne solid and liquid particles in the nanometer to micrometer size range influence the energy balance of the Earth, the hydrological cycle, atmospheric circulation, and the abundance of greenhouse and reactive trace gases. Moreover, they play important roles in the reproduction of biological organisms and can cause or enhance diseases. The primary parameters that determine the environmental and health effects of aerosol particles are their concentration, size, structure, and chemical composition. These parameters, however, are spatially and temporally highly variable. The quantification and identification of biological particles and carbonaceous components of fine particulate matter in the air (organic compounds and black or elemental carbon, respectively) represent demanding analytical challenges. This Review outlines the current state of knowledge, major open questions, and research perspectives on the properties and interactions of atmospheric aerosols and their effects on climate and human health.

Atmospheric aerosols: composition, transformation, climate and health effects.

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Aerosol cloud precipitation interactions. Part 1. The nature and sources of cloud-active aerosols

The organic fraction of atmospheric aerosols contains a multitude of compounds and usually only a small fraction can be identified and quantified. However, a limited number of representative organic compounds can be used to describe the water-soluble organic fraction. In this work, initiated within the EU 5FP project SMOCC, four mixtures containing various amounts of inorganic salts (ammonium sulfate, ammonium nitrate, and sodium chloride) and three model organic compounds (levoglucosan, succinic acid and fulvic acid) were studied. The interaction between water vapor and aerosol particles was studied at different relative humidities: at subsaturation using a hygroscopic tandem differential mobility analyzer (H-TDMA) and at supersaturation using a cloud condensation nuclei spectrometer (CCN spectrometer). Surface tensions as a function of carbon concentrations were measured using a bubble tensiometer. Parameterizations of water activity as a function of molality, based on hygroscopic growth, are given for the pure organic compounds and for the mixtures, indicating van't Hoff factors around 1 for the organics. The Zdanovskii-Stokes-Robinson (ZSR) mixing rule was tested on the hygroscopic growth of the mixtures and it was found to adequately explain the hygroscopic growth for 3 out of 4 mixtures, when the limited solubility of succinic acid is taken into account. One mixture containing sodium chloride was studied and showed a pronounced deviation from the ZSR mixing rule. Critical supersaturations calculated using the parameterizations of water activity and the measured surface tensions were compared with those determined experimentally.

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Hygroscopic growth and critical supersaturations for mixed aerosol particles of inorganic and organic compounds of atmospheric relevance

Aliphatic straight-chain dicarboxylic acids have been identified as common water-soluble organic components of atmospheric aerosols. To model the partitioning of such compounds between gas and particle phase in the atmosphere, information about their vapor pressures is essential. In this work, vapor pressures of C3−C9 dicarboxylic acids are derived from measured evaporation rates of submicron aerosols over the temperature range of 290−314 K using the tandem differential mobility analyzer technique. Vapor pressures obtained from the experimental data were as follows:  log( , Pa) = − 4822 K/T + 12.9, log( , Pa) = − 7196.8 K/T + 19.8, (296 K) = 6.7 × 10-4 Pa, log( , Pa) = −8065.0 K/T + 22.2, log( , Pa) = −7692.8 K/T + 21.8, log( , Pa) = −9629.4 K/T + 26.5, and log( , Pa) = −7968.7 K/T + 21.7. Vapor pressures of C3−C9 dicarboxylic acids are shown to alternate strongly with the parity of the number of carbon atoms. Higher vapor pressures of the odd acids fit the less stable crystal structure, the propensity of...

Even-Odd Alternation of Evaporation Rates and Vapor Pressures of C3-C9 Dicarboxylic Acid Aerosols

Evaporation of methyl- and dimethyl-substituted malonic, succinic, glutaric and adipic acid particles at ambient temperatures

Corrigendum to “Evaporation of methyl- and dimethyl-substituted malonic, succinic, glutaric and adipic acid particles at ambient temperatures”: [Journal of Aerosol Science 35 (2004) 1453–1465]

The properties of atmospheric particles are important to public health, radiative forcing of the atmosphere and to elucidating the chemical reactivity of atmospheric particles. We have constructed a Knudsen cell to study the uptake of organic compounds on soot. This article describes the construction and validation of the instrument, and our results on commercial soot concerning the uptake coefficient of ethanol, acetone, 1-butanol and diethoxymethane. First, a technical description of the instrument is presented. Next, its performance is validated by measuring the uptake of NO2 on hexane soot. Finally, the uptake coefficients of four oxygenated hydrocarbons on commercial soot are presented. The objective is to contribute to the understanding of the formation of particles in motor vehicle exhaust. A Knudsen cell is used to measure the uptake of specific gas-surface systems. A quadrupole mass spectrometer is used to determine the decay rate of a pulse of reagent gas in the reaction chamber. The BET surface area of the commercial soot was 12.6 m2/g. The uptake coefficient (γ) has been determined for ethanol (γ0,BET=7.7+-4.8×10−8), 1-butanol (γ0,BET= 1.4±0.54×10−7), acetone (γ0,BET=1-5±0.15×10−7) and diethoxymethane (γ0,BET=2.6±0.61×10−7). These results are characteristic of the specific soot sample used. The ordering of the uptake coefficients, ethanol < 1-butanol ∼ acetone < diethoxymethane, can be ascribed to a combination of physical (size and mass) and chemical effects. In addition, the initial uptake coefficient for NO2 on fresh hexane soot was determined to be γ0,BET= 1.7±1.1×10−4. In conclusion, we demonstrate that this instrument is able to measure uptake coefficients that are in agreement with accepted literature values. New data is presented concerning four light oxygenated hydrocarbons. A large amount of detailed information concerning individual heterogeneous reactions is necessary in order to model the composition of motor vehicle emissions. We look forward to increasing the size of this database. Results for a series of alcohols and alkanes will be presented in a forthcoming publication.

J. Mønster

Papers

Hygroscopic growth and critical supersaturations for mixed aerosol particles of inorganic and organic compounds of atmospheric relevance

Even-Odd Alternation of Evaporation Rates and Vapor Pressures of C3-C9 Dicarboxylic Acid Aerosols

Evaporation of methyl- and dimethyl-substituted malonic, succinic, glutaric and adipic acid particles at ambient temperatures

Corrigendum to “Evaporation of methyl- and dimethyl-substituted malonic, succinic, glutaric and adipic acid particles at ambient temperatures”: [Journal of Aerosol Science 35 (2004) 1453–1465]

Knudsen cell construction, validation and studies of the uptake of oxygenated fuel additives on soot.