Abstract. 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.

CONTENTS 1. Introduction and Overview of the Field 4260 2. Experimental Methods 4262 2.1. Chambers for Cloud and Aerosol Studies 4262 2.1.1. Cloud Chamber Studies 4262 2.1.2. Aqueous Aerosol Chamber Studies 4263 2.2. Analytical Techniques 4263 2.2.1. Transfer-MS and ESI-MS 4263 2.2.2. High-Resolution Mass Spectrometry (HRMS) 4263 2.2.3. Other MS-Based Studies 4264 2.2.4. NMR 4264 2.2.5. Droplet Evaporation Techniques 4264 2.2.6. Kinetics 4265 3. A Comparison of Aqueous Aerosol, Fog, and Cloud Chemistry 4266 3.1. Overview of Conditions 4266 3.1.1. Occurrence of the Tropospheric Aqueous Phase: RH, ALW, and Clouds on a Global Scale 4266 3.2. Aqueous-Phase Transfer 4267 3.3. pH Effects 4268 3.3.1. Acid−Base Equilibria of Acids and Diacids 4268 3.3.2. Dehydration Reactions of Reaction Intermediates: Alkyl Radical Reformation 4268 3.3.3. Organic Accretion Reactions 4269 3.4. Ionic Strength Effects and Treatment of Nonideal Solutions 4269 3.4.1. Radical Reactions 4269 3.4.2. Nonradical Reactions 4270 3.4.3. Salting-in and Salting-out 4270 3.4.4. Treatment of Nonideality in ALW Chemistry 4270 4. Photochemistry 4271 4.1. Inorganic Bulk Photolysis and Radical Sources 4.1.1. Hydrogen Peroxide Photolysis 4.1.2. Nitrite Photolysis 4.1.3. Photolysis of Chlorine-Containing Species 4.1.4. Peroxomonosulfate Photolysis 4.1.

Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O 3 and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget.autoxidation | ELVOC | monoterpenes | isoprene | new particle formation F ormation and subsequent growth of new aerosol particles is a major source of cloud condensation nuclei (CCN) in the global troposphere (1, 2), and a big contributor to the large reported uncertainty in the radiative forcing by aerosol−cloud interactions (3-7). Multiple field studies have shown that CCN production is tightly connected with the oxidation of biogenic volatile organic compounds (BVOC) emitted by terrestrial ecosystems (8-11). To explain these observations, large-scale model simulations demonstrate a need for a BVOC oxidation mechanism in the atmosphere that produces very low volatility organic vapors with molar formation yields of at least a few percent per reacted precursor compound (12)(13)(14).The existence and formation mechanisms of essentially nonvolatile organic vapors in the atmosphere have puzzled scientists for some time (14-16). Such extremely low volatile organic compounds (ELVOC) (17) were recently detected, both in laboratory studies and in the ambient atmosphere (18), yet typical atmospheric oxidation chemistry schemes do not explain ELVOC produced on a time scale of minutes or hours. Furthermore, current state-of-the-art models using available chemistry schemes have systematically failed to reproduce the observed concentrations and volatility of organic aerosol components (17, 19). A plausible exp...

Abstract. Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine-particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively constant due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

This critical review addresses the atmospheric gas phase and aqueous phase amine chemistry that is relevant to potential emissions from amine-based carbon capture and storage (CCS). The focus is on amine, nitrosamine and nitramine degradation, and nitrosamine and nitramine formation processes. A comparison between the relative importance of the various atmospheric sinks for amines, nitrosamines and nitramines is presented.

Explaining the formation of secondary organic aerosol is an intriguing question in atmospheric sciences because of its importance for Earth's radiation budget and the associated effects on health and ecosystems. A breakthrough was recently achieved in the understanding of secondary organic aerosol formation from ozone reactions of biogenic emissions by the rapid formation of highly oxidized multifunctional organic compounds via autoxidation. However, the important daytime hydroxyl radical reactions have been considered to be less important in this process. Here we report measurements on the reaction of hydroxyl radicals with α- and β-pinene applying improved mass spectrometric methods. Our laboratory results prove that the formation of highly oxidized products from hydroxyl radical reactions proceeds with considerably higher yields than previously reported. Field measurements support these findings. Our results allow for a better description of the diurnal behaviour of the highly oxidized product formation and subsequent secondary organic aerosol formation in the atmosphere.

The most important radicals which need to be considered for the description of chemical conversion processes in tropospheric aqueous systems are the hydroxyl radical (OH), the nitrate radical (NO(3)) and sulphur-containing radicals such as the sulphate radical (SO(4)(-)). For each of the three radicals their generation and their properties are discussed first in the corresponding sections. The main focus herein is to summarize newly published aqueous-phase kinetic data on OH, NO(3) and SO(4)(-) radical reactions relevant for the description of multiphase tropospheric chemistry. The data compilation builds up on earlier datasets published in the literature. Since the last review in 2003 (H. Herrmann, Chem. Rev. 2003, 103, 4691-4716) more than hundred new rate constants are available from literature. In case of larger discrepancies between novel and already published rate constants the available kinetic data for these reactions are discussed and recommendations are provided when possible. As many OH kinetic data are obtained by means of the thiocyanate (SCN(-)) system in competition kinetic measurements of OH radical reactions this system is reviewed in a subchapter of this review. Available rate constants for the reaction sequence following the reaction of OH+SCN(-) are summarized. Newly published data since 2003 have been considered and averaged rate constants are calculated. Applying competition kinetics measurements usually the formation of the radical anion (SCN)(2)(-) is monitored directly by absorption measurements. Within this subchapter available absorption spectra of the (SCN)(2)(-) radical anion from the last five decades are presented. Based on these spectra an averaged (SCN)(2)(-) spectrum was calculated. In the last years different estimation methods for aqueous phase kinetic data of radical reactions have been developed and published. Such methods are often essential to estimate kinetic data which are not accessible from the literature. Approaches for rate constant prediction include empirical correlations as well as structure activity relationships (SAR) either with or without the usage of quantum chemical descriptors. Recently published estimation methods for OH, NO(3) and SO(4)(-) radical reactions in aqueous solution are finally summarized, compared and discussed.

Oceans dominate emissions of dimethyl sulfide (DMS), the major natural sulfur source. DMS is important for the formation of non-sea salt sulfate (nss-SO 4 2− ) aerosols and secondary particulate matter over oceans and thus, significantly influence global climate. The mechanism of DMS oxidation has accordingly been investigated in several different model studies in the past. However, these studies had restricted oxidation mechanisms that mostly underrepresented important aqueous-phase chemical processes. These neglected but highly effective processes strongly impact direct product yields of DMS oxidation, thereby affecting the climatic influence of aerosols. To address these shortfalls, an extensive multiphase DMS chemistry mechanism, the Chemical Aqueous Phase Radical Mechanism DMS Module 1.0, was developed and used in detailed model investigations of multiphase DMS chemistry in the marine boundary layer. The performed model studies confirmed the importance of aqueousphase chemistry for the fate of DMS and its oxidation products. Aqueous-phase processes significantly reduce the yield of sulfur dioxide and increase that of methyl sulfonic acid (MSA), which is needed to close the gap between modeled and measured MSA concentrations. Finally, the simulations imply that multiphase DMS oxidation produces equal amounts of MSA and sulfate, a result that has significant implications for nss-SO 4 2− aerosol formation, cloud condensation nuclei concentration, and cloud albedo over oceans. Our findings show the deficiencies of parameterizations currently used in higher-scale models, which only treat gas-phase chemistry. Overall, this study shows that treatment of DMS chemistry in both gas and aqueous phases is essential to improve the accuracy of model predictions. ) contribute to the formation of new aerosol particles as well as secondary particulate matter and are, thus, important for human health and the Earth's climate (1). Globally, anthropogenic sulfur emissions in the form of sulfur dioxide (SO 2 ) dominate atmospheric production of gaseous H 2 SO 4 and particle-phase sulfate. However, the main natural source of sulfur is the oxidation of dimethyl sulfide (DMS) emitted by oceans (2), which is the most important precursor for non-sea salt sulfate (nss-SO 4 2− ) aerosols over the open ocean (3). Sulfate aerosols strongly influence the climate both by direct negative radiative forcing (4) and as a dominant source of cloud condensation nuclei (CCN) over the open ocean (5). Because oceans cover about 70% of Earth's surface (6) and have generally low albedo, DMS oxidation plays a major role in influencing the natural radiative forcing of sulfate aerosols as well as cloud properties (3).Investigations of the effect of DMS oxidation on natural sulfate aerosol concentrations and cloud and aerosol properties require an accurate, reduced DMS oxidation scheme in chemical transport models (CTMs) and global climate models (GCMs). Current parameterizations use fixed yields of SO 2 and methyl sulfonic acid (MSA) to calculate new nss...