A list of authors and their affiliations appears at the end of the paper New-particle formation is a major contributor to urban smog1,2, but how it occurs in cities is often puzzling3. If the growth rates of urban particles are similar to those found in cleaner environments (1–10 nanometres per hour), then existing understanding suggests that new urban particles should be rapidly scavenged by the high concentration of pre-existing particles. Here we show, through experiments performed under atmospheric conditions in the CLOUD chamber at CERN, that below about +5 degrees Celsius, nitric acid and ammonia vapours can condense onto freshly nucleated particles as small as a few nanometres in diameter. Moreover, when it is cold enough (below −15 degrees Celsius), nitric acid and ammonia can nucleate directly through an acid–base stabilization mechanism to form ammonium nitrate particles. Given that these vapours are often one thousand times more abundant than sulfuric acid, the resulting particle growth rates can be extremely high, reaching well above 100 nanometres per hour. However, these high growth rates require the gas-particle ammonium nitrate system to be out of equilibrium in order to sustain gas-phase supersaturations. In view of the strong temperature dependence that we measure for the gas-phase supersaturations, we expect such transient conditions to occur in inhomogeneous urban settings, especially in wintertime, driven by vertical mixing and by strong local sources such as traffic. Even though rapid growth from nitric acid and ammonia condensation may last for only a few minutes, it is nonetheless fast enough to shepherd freshly nucleated particles through the smallest size range where they are most vulnerable to scavenging loss, thus greatly increasing their survival probability. We also expect nitric acid and ammonia nucleation and rapid growth to be important in the relatively clean and cold upper free troposphere, where ammonia can be convected from the continental boundary layer and nitric acid is abundant from electrical storms4,5.

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Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O2 pressure (8.0×10−7 and 3.7×10−2mbar) at 373K. Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of Cu(100)-c(2×2)-O and Cu(100)-(22×2)R45°-O domains, (2) the growth of well-ordered (22×2)R45°-O islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered Cu2O. We demonstrate that the (22×2)R45°-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu–O islands between two (22×2)R45°-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu2O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.

Abstract. Atmospheric soluble organic aerosol material can become solid or semi-solid. Due to increasing viscosity and decreasing diffusivity, this can impact important processes such as gas uptake and reactivity within aerosols containing such substances. This work explores the dependence of shikimic acid ozonolysis on humidity and thereby viscosity. Shikimic acid, a proxy for oxygenated reactive organic material, reacts with O3 in a Criegee-type reaction. We used an environmental microreactor embedded in a scanning transmission X-ray microscope (STXM) to probe this oxidation process. This technique facilitates in situ measurements with single micron-sized particles and allows to obtain near-edge X-ray absorption fine structure (NEXAFS) spectra with high spatial resolution. Thus, the chemical evolution of the interior of the particles can be followed under reaction conditions. The experiments show that the overall degradation rate of shikimic acid is depending on the relative humidity in a way that is controlled by the decreasing diffusivity of ozone with decreasing humidity. This decreasing diffusivity is most likely linked to the increasing viscosity of the shikimic acid–water mixture. The degradation rate was also depending on particle size, most congruent with a reacto-diffusion limited kinetic case where the reaction progresses only in a shallow layer within the bulk. No gradient in the shikimic acid concentration was observed within the bulk material at any humidity indicating that the diffusivity of shikimic acid is still high enough to allow its equilibration throughout the particles on the timescale of hours at higher humidity and that the thickness of the oxidized layer under dry conditions, where the particles are solid, is beyond the resolution of STXM.

The redox property of ceria is a key factor in the catalytic activity of ceria-based catalysts. The oxidation state of well-defined ceria nanocubes in gas environments was analysed in situ by a novel combination of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) and high-energy XPS at a synchrotron X-ray source. In situ high-energy XPS is a promising new tool to determine the electronic structure of matter under defined conditions. The aim was to quantitatively determine the degree of cerium reduction in a nano-structured ceria-supported platinum catalyst as a function of the gas environment. To obtain a non-destructive depth profile at near-ambient pressure, in situ high-energy XPS analysis was performed by varying the kinetic energy of photoelectrons from 1 to 5 keV, and, thus, the probing depth. In ceria nanocubes doped with platinum, oxygen vacancies formed only in the uppermost layers of ceria in an atmosphere of 1 mbar hydrogen and 403 K. For pristine ceria nanocubes, no change in the cerium oxidation state in various hydrogen or oxygen atmospheres was observed as a function of probing depth. In the absence of platinum, hydrogen does not dissociate and, thus, does not lead to reduction of ceria.

Surface oxidation of Cu(100) in O(2) has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (T(S)=303-423 K) and O(2) pressure (p(O(2) )=3.7 x 10(-2)-213 mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu(2)O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O(2) and the onset of Cu-(2 square root 2 x square root 2)R45 degrees -O (theta(O)=0.5 ML) surface reconstruction, (2) initial formation of Cu(2)O and the appearance of 1.8 A high elongated islands that also adopt the Cu-(2 square root 2 x square root 2)R45 degrees -O structure, and (3) formation of highly corrugated Cu-O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu(2)O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.

Abstract. Atmospheric observations in remote locations offer a possibility of exploring trace gas and particle concentrations in pristine environments. However, data from remote areas are often contaminated by pollution from local sources. Detecting this contamination is thus a central and frequently encountered issue. Consequently, many different methods exist today to identify local contamination in atmospheric composition measurement time series, but no single method has been widely accepted. In this study, we present a new method to identify primary pollution in remote atmospheric datasets, e.g., from ship campaigns or stations with a low background signal compared to the contaminated signal. The pollution detection algorithm (PDA) identifies and flags periods of polluted data in five steps. The first and most important step identifies polluted periods based on the derivative (time derivative) of a concentration over time. If this derivative exceeds a given threshold, data are flagged as polluted. Further pollution identification steps are a simple concentration threshold filter, a neighboring points filter (optional), a median, and a sparse data filter (optional). The PDA only relies on the target dataset itself and is independent of ancillary datasets such as meteorological variables. All parameters of each step are adjustable so that the PDA can be “tuned” to be more or less stringent (e.g., flag more or fewer data points as contaminated). The PDA was developed and tested with a particle number concentration dataset collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic. Using strict settings, we identified 62 % of the data as influenced by local contamination. Using a second independent particle number concentration dataset also collected during MOSAiC, we evaluated the performance of the PDA against the same dataset cleaned by visual inspection. The two methods agreed in 94 % of the cases. Additionally, the PDA was successfully applied to a trace gas dataset (CO2), also collected during MOSAiC, and to another particle number concentration dataset, collected at the high-altitude background station Jungfraujoch, Switzerland. Thus, the PDA proves to be a useful and flexible tool to identify periods affected by local contamination in atmospheric composition datasets without the need for ancillary measurements. It is best applied to data representing primary pollution. The user-friendly and open-access code enables reproducible application to a wide suite of different datasets. It is available at https://doi.org/10.5281/zenodo.5761101 (Beck et al., 2021).

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 TiO 2 (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 TiO 2 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 NO 2 in the dark. Core-level metal cation, O, and N XPS spectra were measured at elevated pressures of O 2 , NO 2 , and H 2 O. 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 NO 2 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.