Recent years have witnessed fast advancements in near ambient pressure X-ray photoelectron (NAPP) spectroscopy, which is emerging as a powerful tool for the investigation of surfaces in presence of vapors and liquids. In this paper we present a new chamber for the investigation of solid/vapor interfaces relevant to environmental and atmospheric chemistry that fits to the NAPP endstation at the Swiss Light Source. The new chamber allows for performing X-ray photoelectron spectroscopy (XPS) and electron yield near-edge X-ray absorption fine structure spectroscopy (NEXAFS) using soft, tender and hard X-ray in vacuum and in near-ambient pressures up to 20 mbar at environmentally relevant conditions of temperature and relative humidity. In addition, the flow tube design of the chamber enables the dosing of sticky reactive gases with short pressure equilibration time. The accessible photoelectron kinetic energy ranges from 2 to 7000 eV. This range allows the determination of surface and bulk electronic properties of ice and other environmental materials, such as metal oxides and frozen solutions, which are relevant to understanding atmospheric chemistry. The design of this instrument and first results on systems of great interest to the environmental and atmospheric chemistry community are presented. In particular, near-ambient pressure XPS and NEXAFS, coupled to a UV-laser setup, were used to study the adsorption of water on a TiO2 powder sample. The results are in line with previously proposed adsorption models of water on TiO2, and, furthermore, indicate that the concentration of water molecules tends to increase upon UV irradiation. In a second example we illustrate how NEXAFS spectroscopy measurements at the chlorine K-edge can provide new insight on the structures of eutectic and sub-eutectic frozen NaCl solutions at high and low relative humidity, respectively, indicating the formation of solution and solid NaCl phases, respectively. Finally, we demonstrate the assets of this new chamber for the dosing of sticky acidic gases and, in particular, for the investigation of formic acid uptake on ice surfaces.

Heterogeneous reactions of nitrogen oxides on metal oxide surfaces have been suggested to play a significant role in environmental chemistry, physics, and engineering. Many of the metal oxide compounds found among atmospheric mineral dust particles are inherently semiconducting substrates. Due to their low band gap, they are effective photoactive materials in the environmentally relevant ultraviolet (UVA) range of solar radiation. Here, we have studied nitrogen oxide species evolution and photochemistry on TiO2(110) surfaces in the context of atmospheric chemistry by means of near ambient pressure X-ray photoelectron spectroscopy (AP-XPS) coupled with a 375 nm UV-laser module. In the presence of molecular O-2 only, changes in TiO2 surface potential under UV irradiation were observed, attributed to band flattening. Under humid conditions, a significant increase in the BE range attributed to surface hydroxyl groups was observed, which may be the basis for the light-induced superhydrophilicity observed elsewhere with titania-based nanomaterials. The formation of surface nitrite and nitrate was observed after exposure to NO2 in the dark. Core-level metal cation, O, and N XPS spectra were measured at elevated pressures of O-2, NO2, and H2O. By selective UV irradiation of only the XPS measurement spot on the sample, we obtained differential information on the surface chemical state on the UV-irradiated compared to dark reference spots. Upon UV irradiation, increased oxidation of NO2 was observed, while in turn a substantial increase of a reduced nitrate species possibly from electron transfer to nitrate and of a further reduced nitrogen species was observed during exposure to UV-radiation. The effect of surface hydroxylation and the involvement of carbon-containing surface compounds in the formation of nitrogenated organic species are emphasized.

Dinitrogen pentoxide has aroused significant interest in atmospheric chemistry because of its importance in the night time chemistry of nitrogen oxides to influence the tropospheric oxidation capacity. We have used an established method of N-13 production to synthesize N-13 labeled N2O5 for the first time in order to study N2O5 uptake kinetics on aerosol particles. N-13 is produced via the 16 O(p, alpha) N-13 reaction in a gas target attached to the IP2 endstation of the Injector 2 cyclotron at PSI. The (NO)-N-13 produced in the gas target is transported to a laboratory where it is mixed, under dry conditions, with non-labeled NO and O 3 in a gas reactor, giving (NNO5)-N-13. The N2O5 thus produced is fed into an aerosol flow tube together with a humidified aerosol gas flow. The gaseous species present in the resulting gas flow are selectively separated via a narrow parallel plate diffusion denuder system, while aerosol particles can be trapped on a particle filter placed at the end of the denuder system. The activity of the N-13 labeled species trapped on the denuder plates and in the particle filter can be monitored via scintillation counters. A system for the routine online production of N-13 labeled N2O5 has been assembled and used to assess the conformity of the results by kinetic modeling of gas phase N2O5 chemistry, showing good agreement. A few exemplary experiments of uptake of labelled N2O5 to ammonium sulfate and citric acid particles are presented that are in good agreement with results obtained with other methods reported in the literature.

A Knudsen cell flow reactor was coupled to an online gas phase source of the short-lived radioactive tracer 13N to study the adsorption of nitrogen oxides on ice at temperatures relevant for the upper troposphere. This novel approach has several benefits over the conventional coupling of a Knudsen cell with a mass spectrometer. Experiments at lower partial pressures close to atmospheric conditions are possible. The uptake to the substrate is a direct observable of the experiment. Operation of the experiment in continuous or pulse mode allows to retrieve steady state uptake kinetics and more details of adsorption and desorption kinetics.

A new liquid microjet endstation designed for ultraviolet (UPS) and X-ray (XPS) photoelectron, and partial electron yield X-ray absorption (XAS) spectroscopies at the Swiss Light Source is presented. The new endstation, which is based on a Scienta HiPP-2 R4000 electron spectrometer, is the first liquid microjet endstation capable of operating in vacuum and in ambient pressures up to the equilibrium vapor pressure of liquid water at room temperature. In addition, the Scienta HiPP-2 R4000 energy analyzer of this new endstation allows for XPS measurements up to 7000 eV electron kinetic energy that will enable electronic structure measurements of bulk solutions and buried interfaces from liquid microjet samples. The endstation is designed to operate at the soft X-ray SIM beamline and at the tender X-ray Phoenix beamline. The endstation can also be operated using a Scienta 5 K ultraviolet helium lamp for dedicated UPS measurements at the vapor-liquid interface using either He I or He II α lines. The design concept, first results from UPS, soft X-ray XPS, and partial electron yield XAS measurements, and an outlook to the potential of this endstation are presented.

Scanning transmission X-ray microscopy (STXM), a microscopy method which allows imaging with a spatial resolution of 40nm, and X-ray absorption spectroscopy were used to follow in situ the water uptake and release in submicrometer sized particles on a substrate enclosed in a microreactor. Oxygen K-edge near edge X-ray absorption fine structure (NEXAFS) spectra from supported ammonium sulfate particles in their dry salt, saturated solution and supersaturated solution states were obtained for the first time. The variations at the oxygen edge were related to the water content as a function of relative humidity (RH), consistent with mass growth measurements done on larger samples or suspended particle ensembles. Investigations on morphological changes upon water uptake were performed in mixed ammonium sulfate-adipic acid particles using STXM images and NEXAFS spectra taken at the oxygen and the carbon absorption edges, confirming the two phase structure suspected from previous hygroscopicity studies, where adipic acid forms a separate phase of complex morphology partially enclosed by the ammonium sulfate solution at high RH. This example emphasizes the combination of chemical resolution provided via NEXAFS, spatial resolution via STXM and the in situ capability provided by the novel microreactor to obtain information about the microstructure of mixed organic/inorganic particles under close to ambient conditions.

A new in situ cell to study phase transitions and chemical processes on individual aerosol particles in the x-ray transmission microscope at the PolLux beamline of the Swiss light source has been built. The cell is machined from stainless steel and aluminum components and is designed to be used in the standard mount of the microscope without need of complicated rearrangements of the microscope. The cell consists of two parts, a back part which contains connections for the gas supply, heating, cooling devices, and temperature measurement. The second part is a removable clip, which hosts the sample. This clip can be easily exchanged and brought into a sampling unit for aerosol particles. Currently, the cell can be operated at temperatures ranging from -40 to +50 °C. The function of the cell is demonstrated using two systems of submicron size: inorganic sodium bromide aerosols and soot originating from a diesel passenger car. For the sodium bromide we demonstrate how phase transitions can be studied in these systems and that O1s spectra from aqueous sodium bromide solution can be taken from submicron sized particles. For the case of soot, we demonstrate that the uptake of water onto individual soot particles can be studied.