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

Formation and growth rates of ultrafine atmospheric particles: a review of observations

 Many atmospheric chemicals occur in the gas phase as well as in liquid cloud droplets and aerosol particles Therefore, it is necessary to understand the distribution between the phases According to Henry's law, the equilibrium ratio between the abundances in the gas phase and in the aqueous phase is constant for a dilute solution Henry's law constants of trace gases of potential importance in environmental chemistry have been collected and converted into a uniform format The compilation contains 17 350 values of Henry's law constants for 4632 species, collected from 689 references It is also available at http://wwwhenrys-laworg 

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Compilation of Henry's law constants (version 4.0) for water as solvent

Stratospheric ozone depletion through cat- alytic chemistry involving man-made chlorofluorocar- bons is an area of focus in the study of geophysics and one of the global environmental issues of the twentieth century. This review presents a brief history of the sci- ence of ozone depletion and describes a conceptual framework to explain the key processes involved, with a focus on chemistry. Observations that may be considered as evidence (fingerprints) of ozone depletion due to chlorofluorocarbons are explored, and the related gas phase and surface chemistry is described. Observations of ozone and of chlorine-related trace gases near 40 km provide evidence that gas phase chemistry has indeed currently depleted about 10% of the stratospheric ozone there as predicted, and the vertical and horizontal struc- tures of this depletion are fingerprints for that process. More striking changes are observed each austral spring in Antarctica, where about half of the total ozone col- umn is depleted each September, forming the Antarctic ozone hole. Measurements of large amounts of ClO, a key ozone destruction catalyst, are among the finger- prints showing that human releases of chlorofluorocar- bons are the primary cause of this change. Enhanced ozone depletion in the Antarctic and Arctic regions is linked to heterogeneous chlorine chemistry that oc- curs on the surfaces of polar stratospheric clouds at cold temperatures. Observations also show that some of the same heterogeneous chemistry occurs on the surfaces of particles present at midlatitudes as well, and the abundances of these particles are enhanced following explosive volcanic eruptions. The partition- ing of chlorine between active forms that destroy ozone and inert reservoirs that sequester it is a central part of the framework for our understanding of the 40-km ozone decline, the Antarctic ozone hole, the recent Arctic ozone losses in particularly cold years, and the observation of record midlatitude ozone de- pletion after the major eruption of Mount Pinatubo in the early 1990s. As human use of chlorofluorocarbons continues to decrease, these changes throughout the ozone layer are expected to gradually reverse during the twenty-first century.

https://iri.columbia.edu/~goddard/EESC_W4400/CC/solomon_1999.pdf

Stratospheric ozone depletion: A review of concepts and history

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The dissociation constant of ammonium nitrate and its dependence on temperature, relative humidity and particle size

The reaction probability, γ, of N2O5 (to form HNO3) with monodisperse NH3/H2SO4/H2O aerosols has been measured in a flow tube reactor at atmospheric pressure. Experiments were performed at temperatures of 274 and 293 K and at relative humidities of 1–76%. It appears that the presence of a liquid phase is necessary for reaction. With NH4HSO4 aerosol, γ values are nonzero, even below the deliquescence point of the pure salt, and show a gradual increase with increasing relative humidity up to constant values of 0.05 and 0.09 for 293 and 274 K, respectively. Experiments with aerosols of varied NH3/H2SO4 ratios suggest that evaporation of NH3 from the NH4HSO4 aerosols creates excess H2SO4 on the surface. This can take up water vapor to provide the reactant for N2O5 below the deliquescence point of NH4HSO4. The present observations are consistent with the reaction of N2O5 with moist aerosols being a major removal mechanism for odd nitrogen and a major source of HNO3 in the nighttime troposphere. The same reaction on H2SO4 aerosols may be of significance in the stratosphere.

Reaction probability of N2O5 on aqueous aerosols

Negative activation energies and curved Arrhenius plots. 1. Theory of reactions over potential wells

A possible source of gas phase bromine in the Arctic winter and early spring is the sea-salt aerosol. In this paper, chemical mechanisms for the release of photochemically active bromine from sea-salt are examined. The first of these is oxidation of bromide to elemental bromine by peroxymonosulfuric acid (Caro's acid) produced by the free radical chain oxidation of S(IV). The chain reaction could be initiated in the dark by NO2 or, following polar sunrise, by the reaction of ozone with superoxide. Although the yield of Caro's acid at 298 K is small, the yield may increase at the low temperatures encountered in the Arctic. This could result in the conversion of a large fraction of the initial sea-salt alkalinity to Caro's acid. Caro's acid is known to oxidize bromide to Br2. Since this mechanism requires low temperatures and high SO: concentrations, it is only effective during the winter and early spring and should not oxidize significant amounts of halides in the global marine boundary layer. A second possible mechanism is the free radical oxidation of aqueous bromide to bromine by OH and HO:. This may be effective at moderate pH and may contribute to Br cycling on a global scale. The evaluation of both mechanisms is highly uncertain because of incomplete physical-chemical data. In addition to these primary release mechanisms, there are possible autocatalytic cycles for the release of bromine from sea-salt. These involve the gas phase production of either BrO or HOBr and the return of these species to the sea-salt particles where they initiate additional oxidation of bromide. The efficiency of these cycles should depend critically on the relative amounts of HOBr and HBr produced by the gas phase chemistry. These mechanisms do not appear to be effective as sources of photochemically active C1.

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Mechanisms for the release of halogens from sea-salt particles by free radical reactions

The rate of reaction of gas phase HO2 radicals with a monodisperse, submicrometer aerosol was measured in a flow tube reactor at atmospheric pressure. At the relative humidity of the experiments (75%), the aerosol consisted of concentrated solution droplets of either LiNO3 or NH4HSO4. When the aerosol contained a sufficient amount of Cu(II) ions, reaction of HO2 with the aerosol was observed. The mass accommodation coefficient for HO2 on aqueous particles was determined to be greater than 0.2. This implies that in clouds HO2 mass transport will be limited by gas-phase diffusion and HO2 will be in equilibrium at the gas-liquid interface. Reactions of HO2 with Cu(I) and Cu(II) ions in submicrometer aerosols may have a significant role in converting atmospheric odd hydrogen radicals into H2O2.

/pdf/mass-accommodation-coefficient-for-ho2-radicals-on-aqueous-1pz1jamhij.pdf

Michael Mozurkewich

Papers

The dissociation constant of ammonium nitrate and its dependence on temperature, relative humidity and particle size

Reaction probability of N2O5 on aqueous aerosols

Negative activation energies and curved Arrhenius plots. 1. Theory of reactions over potential wells

Mechanisms for the release of halogens from sea-salt particles by free radical reactions

Mass accommodation coefficient for HO2 radicals on aqueous particles