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.

Atmospheric aerosol has been shown to contain an organic component that includes a significant fraction of small dicarboxylic acids, particularly in the urban environment. As an initial step toward understanding the phase in which particles may exist, a detailed study into the phase transitions of malonic and oxalic acid aerosols has been carried out. Both the aerosol phase transitions (deliquescence and efflorescence) and bulk solution properties (equilibrium water vapor pressure and the solubility and freezing curves of the aqueous solutions) are reported. An aerosol flow tube-FTIR and a static mode chamber-FTIR have been used to identify particulate phase transitions. In the latter the particles can be observed under ice-supersaturated conditions, allowing investigation of behavior at subeutectic temperatures. We report that both malonic and oxalic acid aerosols sustain a substantial level of solute supersaturation before efflorescence occurs, whereas deliquescence occurs at the thermodynamically predicted relative humidity. At room temperature, malonic acid efflorescence is observed at RH = 6% ± 3% and oxalic acid efflorescence occurs at RH ≤ 5%. Malonic acid particles deliquesce between 69% and 91% RH over the temperature range 293−252 K, and for oxalic acid conditions close to 100% RH are required. We report the first observation of the phase transition of oxalic acid between the anhydrous and dihydrate form and discuss our results in the context of recently published data.

Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the author manuscript from TSpace because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page. Deposition of atmospheric constituents -either gas phase or particulate -onto urban 16 impervious surfaces gives rise to a thin "urban grime" film. The area exposed by these 17 impervious surfaces in a typical urban environment is comparable to, or greater than, that 18 of particles present in the urban boundary layer, however it is largely overlooked as a site 19 for heterogeneous reactions. Here we present the results of a field campaign to determine 20 and compare the chemical composition of urban grime and of particles collected 21 simultaneously during the autumn of 2014 at an urban site in central Leipzig, Germany. 22We see dramatically reduced ammonium and nitrate levels in the film as compared to 23 particles suggesting a significant loss of ammonium nitrate, thus enhancing the mobility 24 of these species in the environment. Nitrate levels are 10% lower for films exposed to 25 sunlight compared to those that were shielded from direct sun, indicating a possible 26 mechanism for recycling nitrate anion to reactive nitrogen species. Finally, chloride 27 levels in the film suggest that it could represent an unrecognized source of continental 28 chloride available for ClNO 2 production even in times of low particulate chloride. Such 29 source and recycling processes could prove to be important to local and regional air 30 quality. 31

This work presents the results of two complementary studies of the heterogeneous reaction of gas-phase ozone with solid pyrene films. In the first study, ozone uptake by the pyrene film was determined using a coated-wall flow tube system. In the second, pyrene loss within the film upon exposure to ozone was monitored using a laser-induced fluorescence technique. The dependence of the reactive loss rate on ozone concentration observed in both methods suggests that the reaction proceeds via a Langmuir-Hinshelwood-type surface mechanism. At a mixing ratio of 50 ppb, the steady-state reactive uptake coefficient of ozone by pyrene films increased from 5.0x10(-6) in the dark to 3.7x10(-5) upon exposure to near-UV radiation (300-420 nm). The uptake coefficient increased linearly as a function of UV-A spectral irradiance and decreased markedly with increasing relative humidity. The loss of surface pyrene upon exposure to ozone also displayed a light enhancement: analysis of Langmuir-Hinshelwood plots for the light and dark reactions revealed a small increase in the two-dimensional reaction rate in the presence of light (lambda>or=295 nm). This modest enhancement, however, was less significant than the corresponding enhancement in the loss of gas-phase ozone. In order to explain these observations, we present an integrated mechanism whereby the light-enhanced ozone uptake arises from the reaction of ozone with O2(1Sigmag+) formed via energy transfer from excited-state pyrene and the enhanced pyrene loss occurs via the formation of a charge-transfer complex between excited-state pyrene and adsorbed ozone. The disparity between surface- and gas-phase results underscores the important role that multifaceted strategies can play in elucidating the mechanisms of heterogeneous atmospheric reactions.

The billions of tons of mineral dust released into the atmosphere each year provide an important surface for reaction with gas-phase pollutants. These reactions, which are often enhanced in the presence of light, can change both the gas-phase composition of the atmosphere and the composition and properties of the dust itself. Because dust contains titanium-rich grains, studies of dust photochemistry have largely employed commercial titanium dioxide as a proxy for its photochemically active fraction; to date, however, the validity of this model system has not been empirically determined. Here, for the first time, we directly investigate the photochemistry of the complement of natural titanium-containing minerals most relevant to mineral dust, including anatase, rutile, ilmenite, titanite, and several titanium-bearing species. Using ozone as a model gas-phase pollutant, we show that titaniumcontaining minerals other than titanium dioxide can also photocatalyze trace gas uptake, that samples of the same mineral phase can display very different reactivity, and that prediction of dust photoreactivity based on elemental/mineralogical analysis and/or lightabsorbing properties is challenging. Together, these results show that the photochemistry of atmospheric dust is both richer and more complex than previously considered, and imply that a full understanding of the scope and impact of dust-mediated processes will require the community to engage with this complexity via the study of ambient mineral dust samples from diverse source regions.