Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity. Here we show that binary and ternary colloids and dissolved complexes of As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mossbauer-spectroscopy. Dissolved Fe-OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)(3)). Mossbauer-spectroscopy showed that 95% of the Fe in the Fe-OM fraction were present as ferrihydrite-OM colloids while the remaining 5% were in the dissolved fraction. In As(V) plus Fe-OM systems (containing both dissolved and colloidal Fe-OM), 3.5-8 microg As(V)/mg OC was bound to the Fe-OM complexes/colloids compared to <0.015 microg As(V)/mg OC in As-OM systems (without Fe). Upon filtration of As-Fe-OM complexes/colloids with a 3 kDa filter, approximately 6% As was found in the dissolved fraction and approximately 94% As in colloidal Fe-OM. This suggests that As(V) is associated with Fe-OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe-OM complexes via Fe-bridging. Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe-OM have to be considered when studying As transport.

Abstract. Secondary organic aerosol (SOA) was produced from the aromatic precursors catechol and guaiacol by reaction with ozone in the presence and absence of simulated sunlight and humidity and investigated for its properties as a proxy for HUmic-LIke Substances (HULIS). Beside a small particle size, a relatively low molecular weight and typical optical features in the UV/VIS spectral range, HULIS contain a typical aromatic and/or olefinic chemical structure and highly oxidized functional groups within a high chemical diversity. Various methods were used to characterize the secondary organic aerosols obtained: Fourier transform infrared spectroscopy (FTIR) demonstrated the formation of several carbonyl containing functional groups as well as structural and functional differences between aerosols formed at different environmental conditions. UV/VIS spectroscopy of filter samples showed that the particulate matter absorbs far into the visible range up to more than 500 nm. Ultrahigh resolved mass spectroscopy (ICR-FT/MS) determined O/Cratios between 0.3 and 1 and observed m/z ratios between 200 and 450 to be most abundant. Temperature-programmedpyrolysis mass spectroscopy (TPP-MS) identified carboxylic acids and lactones/esters as major functional groups. Particle sizing using a condensation-nucleus-counter and differentialmobility-particle-sizer (CNC/DMPS) monitored the formation of small particles during the SOA formation process.Correspondence to: H. Grothe (grothe@tuwien.ac.at) Particle imaging, using field-emission-gun scanning electron microscopy (FEG-SEM), showed spherical particles, forming clusters and chains. We conclude that catechol and guaiacol are appropriate precursors for studies of the processing of aromatic SOA with atmospheric HULIS properties on the laboratory scale.

The concurrent presence of high values of organic SOA precursors and reactive halogen species (RHS) at very low ozone concentrations allows the formation of halogen-induced organic aerosol, so-called XOA, in maritime areas where high concentrations of RHS are present, especially at sunrise. The present study combines aerosol smog-chamber and aerosol flow-reactor experiments for the characterization of XOA. XOA formation yields from alpha-pinene at low and high concentrations of chlorine as reactive halogen species (RHS) were determined using a 700 L aerosol smog-chamber with a solar simulator. The chemical transformation of the organic precursor during the aerosol formation process and chemical aging was studied using an aerosol flow-reactor coupled to an FTIR spectrometer. The FTIR dataset was analysed using 2D correlation spectroscopy. Chlorine induced homogeneous XOA formation takes place at even 2.5 ppb of molecular chlorine, which was photolysed by the solar simulator. The chemical pathway of XOA formation is characterized by the addition of chlorine and abstraction of hydrogen atoms, causing simultaneous carbon-chlorine bond formation. During further steps of the formation process, carboxylic acids are formed, which cause a SOA-like appearance of XOA. During the ozone-free formation of secondary organic aerosol with RHS a special kind of particulate matter (XOA) is formed, which is afterwards transformed to SOA by atmospheric aging or degradation pathways.

Environmental context. Inorganic, natural aerosols (sea-salt, mineral dust, glacial flour) and contributions of anthropogenic components (fly ash, dust from steel production and processing, etc.) contain iron that can be dissolved as Fe III in saline media. This study investigates photochemical processes in clouds and aerosols producing gas-phase Cl as a function of salt-and gas-phase composition employing a simulation chamber. Atomic Cl may contribute to the oxidative capacity of the troposphere, and our findings imply local sources.Abstract. Artificial sea-salt aerosol, containing Fe III at various compositions, was investigated in a simulation chamber (made of Teflon) for the influence of pH and of the tropospheric trace gases NO 2 , O 3 and SO 2 on the photochemical activation of chloride. Atomic chlorine (Cl) was detected in the gas phase and quantified by the radical clock technique. Dilute brines with known Fe III content were nebulised until the relative humidity reached 70-90 %. The resulting droplets (most abundant particle diameter: 0.35-0.46 mm, initial surface area: up to 3 Â 10 À2 cm 2 cm À3) were irradiated with simulated sunlight, and the consumption of a test mixture of hydrocarbons was evaluated for Cl, Br and OH. The initial rate of atomic Cl production per aerosol surface increased with Fe III and was ,1. . The observed production of atomic Cl is discussed with respect to pH and speciation of the photolabile aqueous Fe III complexes.

Abstract. Reactive halogen species (RHS), such as X·, X 2 and HOX containing X = chlorine and/or bromine, are released by various sources like photo-activated sea-salt aerosol or from salt pans, and salt lakes. Despite many studies of RHS reactions, the potential of RHS reacting with secondary organic aerosol (SOA) and organic aerosol derived from biomass-burning (BBOA) has been neglected. Such reactions can constitute sources of gaseous organohalogen compounds or halogenated organic matter in the tropospheric boundary layer and can influence physicochemical properties of atmospheric aerosols.Model SOA from α-pinene, catechol, and guaiacol was used to study heterogeneous interactions with RHS. Particles were exposed to molecular chlorine and bromine in an aerosol smog-chamber in the presence of UV/VIS irradiation and to RHS, released from simulated natural halogen sources like salt pans. Subsequently, the aerosol was characterized in detail using a variety of physicochemical and spectroscopic methods. Fundamental features were correlated with heterogeneous halogenation, which results in new functional groups (FTIR spectroscopy), changes UV/VIS absorption, chemical composition (ultrahigh resolution mass spectroscopy (ICR-FT/MS)), or aerosol size distribution. However, the halogen release mechanisms were also found to be affected by the presence of organic aerosol. Those interaction processes, changing chemical and physical properties of the aerosol are likely to influence e.g. the ability of the aerosol to act as cloud condensation nuclei, its potential to adsorb other gases with low-volatility, or its contribution to radiative forcing and ultimately the Earth's radiation balance.