2011. Dissecting Mott and charge-density wave dynamics in the photoinduced phase of 1T-TaS[sub]2Alberto Simoncig, Matija Stupar, Barbara Ressel, Tanusree Saha, Primož Rebernik Ribič, Giovanni De Ninno, 2021, original scientific article Abstract: The two-dimensional transition-metal dichalcogenide 1T−TaS2 is a complex material standing out for its puzzling low temperature phase marked by signatures amenable to both Mott-insulating and charge-density wave states. Electronic Mott states, coupled to a lattice, respond to coherent optical excitations via a modulation of the lower (valence) Hubbard band. Such dynamics is driven by strong electron-phonon coupling and typically lasts for tens of picoseconds, mimicking coherent structural distortions. Instead, the response occurring at the much faster timescale, mainly dominated by electronic many-body effects, is still a matter of intense research. By performing time- and angle-resolved photoemission spectroscopy, we investigated the photoinduced phase of 1T−TaS2 and found out that its lower Hubbard band promptly reacts to coherent optical excitations by shifting its binding energy towards a slightly larger value. This process lasts for a time comparable to the optical pump pulse length, mirroring a transient change of the onsite Coulomb repulsion energy (U). Such an observation suggests that the correction to the bare value of U, ascribed to the phonon-mediated screening which slightly opposes the Hubbard repulsion, is lost within an interval of a few tens of femtoseconds and can be understood as a fingerprint of electronic states largely decoupled from the lattice. Additionally, these results enforce the hypothesis, envisaged in the current literature, that the transient photoinduced states belong to a sort of crossover phase instead of an equilibrium metallic one.
Keywords: ultrafast phenomena, time resolved photoemission, strongly correlated systems, transition metal dichalcogenide
Published in RUNG: 13.04.2021; Views: 2499; Downloads: 0
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2013. Zofka Kveder : Slavic cultural and feminist icon of the early 20th centuryKatja Mihurko, 2021, independent scientific component part or a chapter in a monograph Keywords: Zofka Kveder, slovenska književnost, Martha Tausk, Zdenka Haskova, Avgust Cesarec, Elvira Dolinar, Zdenka Marković, Julka Chlapec-Đorđević, Minka Govekar, Vladimir Jelovšek, Josef Svetopluk Machar
Published in RUNG: 12.04.2021; Views: 2008; Downloads: 57
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2014. Long-term ozone data analysisVjera Butković, Tomislav Cvitaš, Katja Džepina, Nenad Kezele, Leo Klasinc, 2002, original scientific article Abstract: Various approaches to the analysis of 10-year continuous ozone monitoring from the EUROTRAC-TOR network station Puntijarka near Zagreb are reported. The site has a rural character (45.90degrees N; 15.97degrees E, 980 m a.s.l.) and is representative of the lower troposphere of a wider region. Mean hourly ozone volume fractions measured from 1990-1999, autocorrelation plots for all data and for data for summer periods (May-Sep.), box and whiskers representations of diurnal variations during winter (Nov.-March) and summer periods, mean monthly values and 12-month moving averages, and the Fourier transform of the complete set of 94,248 hourly mean ozone volume fractions are discussed. The data show no increase, or possibly a slight decrease, of the ozone volume fraction toward the end of the decade.
Keywords: long term ozone data, fourier analysis, ozone trend analysis, Puntijarka field station
Published in RUNG: 12.04.2021; Views: 2125; Downloads: 0
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2015. Characterization of ambient aerosols in Mexico City during the MCMA-2003 campaign with aerosol mass spectrometry : results from the CENICA SupersiteDara Salcedo, T. B. Onasch, Katja Džepina, M. R. Canagaratna, Qi Zhang, J. A. Huffman, P. F. DeCarlo, J. Jayne, P. Mortimer, D. Worsnop, 2006, original scientific article Abstract: An Aerodyne Aerosol Mass Spectrometer (AMS) was deployed at the CENICA Supersite, during the Mexico City Metropolitan Area field study (MCMA-2003) from 31 March-4 May 2003 to investigate particle concentrations, sources, and processes. The AMS provides real time information on mass concentration and composition of the non-refractory species in particulate matter less than 1 mu m (NR-PM1) with high time and size-resolution. In order to account for the refractory material in the aerosol, we also present estimates of Black Carbon (BC) using an aethalometer and an estimate of the aerosol soil component obtained from Proton-Induced X-ray Emission Spectrometry (PIXE) analysis of impactor substrates. Comparisons of AMS + BC + soil mass concentration with other collocated particle instruments (a LASAIR Optical Particle Counter, a PM2.5 Tapered Element Oscillating Microbalance (TEOM), and a PM2.5 DustTrak Aerosol Monitor) show that the AMS + BC + soil mass concentration is consistent with the total PM2.5 mass concentration during MCMA-2003 within the combined uncertainties. In Mexico City, the organic fraction of the estimated PM2.5 at CENICA represents, on average, 54.6% (standard deviation sigma=10%) of the mass, with the rest consisting of inorganic compounds ( mainly ammonium nitrate and sulfate/ammonium salts), BC, and soil. Inorganic compounds represent 27.5% of PM2.5 (sigma=10%); BC mass concentration is about 11% (sigma=4%); while soil represents about 6.9% (sigma=4%). Size distributions are presented for the AMS species; they show an accumulation mode that contains mainly oxygenated organic and secondary inorganic compounds. The organic size distributions also contain a small organic particle mode that is likely indicative of fresh traffic emissions; small particle modes exist for the inorganic species as well. Evidence suggests that the organic and inorganic species are not always internally mixed, especially in the small modes. The aerosol seems to be neutralized most of the time; however, there were some periods when there was not enough ammonium to completely neutralize the nitrate, chloride and sulfate present. The diurnal cycle and size distributions of nitrate suggest local photochemical production. On the other hand, sulfate appears to be produced on a regional scale. There are indications of new particle formation and growth events when concentrations of SO2 were high. Although the sources of chloride are not clear, this species seems to condense as ammonium chloride early in the morning and to evaporate as the temperature increases and RH decreases. The total and speciated mass concentrations and diurnal cycles measured during MCMA-2003 are similar to measurements during a previous field campaign at a nearby location.
Keywords: aerosol mass-spectrometer, atmospheric aerosol, atmospheric chemistry, atmospheric physics
Published in RUNG: 12.04.2021; Views: 2217; Downloads: 0
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2016. Sources and transformations of particle-bound polycyclic aromatic hydrocarbons in Mexico CityL. C. Marr, Katja Džepina, Jose L. Jimenez, F. Reisen, H. L. Bethel, Janet Arey, J. S. Gaffney, N. A. Marley, Luisa T. Molina, Mario J. Molina, 2006, original scientific article Abstract: Understanding sources, concentrations, and transformations of polycyclic aromatic hydrocarbons (PAHs) in the atmosphere is important because of their potent mutagenicity and carcinogenicity. The measurement of particle-bound PAHs by three different methods during the Mexico City Metropolitan Area field campaign in April 2003 presents a unique opportunity for characterization of these compounds and intercomparison of the methods. The three methods are ( 1) collection and analysis of bulk samples for time-integrated gas- and particle-phase speciation by gas chromatography/ mass spectrometry; ( 2) aerosol photoionization for fast detection of PAHs on particles' surfaces; and ( 3) aerosol mass spectrometry for fast analysis of size and chemical composition. This research represents the first time aerosol mass spectrometry has been used to measure ambient PAH concentrations and the first time that fast, real-time methods have been used to quantify PAHs alongside traditional filter-based measurements in an extended field campaign. Speciated PAH measurements suggest that motor vehicles and garbage and wood burning are important sources in Mexico City. The diurnal concentration patterns captured by aerosol photoionization and aerosol mass spectrometry are generally consistent. Ambient concentrations of particle-phase PAHs typically peak at similar to 110 ng m(-3) during the morning rush hour and rapidly decay due to changes in source activity patterns and dilution as the boundary layer rises, although surface-bound PAH concentrations decay faster. The more rapid decrease in surface versus bulk PAH concentrations during the late morning suggests that freshly emitted combustion-related particles are quickly coated by secondary aerosol material in Mexico City's atmosphere and may also be transformed by heterogeneous reactions.
Keywords: aerosol mass-spectrometer, aerodynamic diameter measurements, oxygenated organic aerosols, relative rate constants
Published in RUNG: 12.04.2021; Views: 2272; Downloads: 0
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2017. Secondary organic aerosol formation from anthropogenic air pollution : rapid and higher than expectedRainer Volkamer, Jose L. Jimenez, F. M. San Martini, Katja Džepina, Q. Zhang, Dara Salcedo, Luisa T. Molina, D. Worsnop, 2006, original scientific article Abstract: The atmospheric chemistry of volatile organic compounds (VOCs) in urban areas results in the formation of 'photochemical smog', including secondary organic aerosol (SOA). State-of-the-art SOA models parameterize the results of simulation chamber experiments that bracket the conditions found in the polluted urban atmosphere. Here we show that in the real urban atmosphere reactive anthropogenic VOCs (AVOCs) produce much larger amounts of SOA than these models predict, even shortly after sunrise. Contrary to current belief, a significant fraction of the excess SOA is formed from first-generation AVOC oxidation products. Global models deem AVOCs a very minor contributor to SOA compared to biogenic VOCs (BVOCs). If our results are extrapolated to other urban areas, AVOCs could be responsible for additional 3 - 25 Tg yr(-1) SOA production globally, and cause up to - 0.1 W m(-2) additional top-of-the-atmosphere radiative cooling.
Keywords: atmospheric aerosol, atmospheric chemistry, volatile organic compounds, secondary organic aerosols
Published in RUNG: 12.04.2021; Views: 3156; Downloads: 0
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2018. Implementation of a Markov Chain Monte Carlo method to inorganic aerosol modeling of observations from the MCMA-2003 campaign : part IIF. M. San Martini, E. J. Dunlea, R. M. Volkamer, T. B. Onasch, J. Jayne, M. R. Canagaratna, D. Worsnop, C. E. Kolb, J. H. Shorter, Katja Džepina, 2006, original scientific article Abstract: A Markov Chain Monte Carlo model for integrating the observations of inorganic species with a thermodynamic equilibrium model was presented in Part I of this series. Using observations taken at three ground sites, i. e. a residential, industrial and rural site, during the MCMA-2003 campaign in Mexico City, the model is used to analyze the inorganic particle and ammonia data and to predict gas phase concentrations of nitric and hydrochloric acid. In general, the model is able to accurately predict the observed inorganic particle concentrations at all three sites. The agreement between the predicted and observed gas phase ammonia concentration is excellent. The NOz concentration calculated from the NOy, NO and NO2 observations is of limited use in constraining the gas phase nitric acid concentration given the large uncertainties in this measure of nitric acid and additional reactive nitrogen species. Focusing on the acidic period of 9-11 April identified by Salcedo et al. ( 2006), the model accurately predicts the particle phase observations during this period with the exception of the nitrate predictions after 10: 00 a. m. ( Central Daylight Time, CDT) on 9 April, where the model underpredicts the observations by, on average, 20%. This period had a low planetary boundary layer, very high particle concentrations, and higher than expected nitrogen dioxide concentrations. For periods when the particle chloride observations are consistently above the detection limit, the model is able to both accurately predict the particle chloride mass concentrations and provide well-constrained HCl ( g) concentrations. The availability of gas-phase ammonia observations helps constrain the predicted HCl ( g) concentrations. When the particles are aqueous, the most likely concentrations of HCl ( g) are in the sub-ppbv range. The most likely predicted concentration of HCl ( g) was found to reach concentrations of order 10 ppbv if the particles are dry. Finally, the atmospheric relevance of HCl ( g) is discussed in terms of its indicator properties for the possible influence of chlorine-mediated photochemistry in Mexico City.
Keywords: secondary organic aerosols, Mexico City, MCMA-2003 field campaign, thermodynamic equilibrium
Published in RUNG: 11.04.2021; Views: 2236; Downloads: 0
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2019. Technical note : use of a beam width probe in an aerosol mass spectrometer to monitor particle collection efficiency in the fieldDara Salcedo, T. B. Onasch, M. R. Canagaratna, Katja Džepina, J. A. Huffman, J. Jayne, D. Worsnop, C. E. Kolb, S. Weimer, F. Drewnick, 2007, original scientific article Abstract: Two Aerodyne Aerosol Mass Spectrometers (Q-AMS) were deployed in Mexico City, during the Mexico City Metropolitan Area field study (MCMA-2003) from 29 March - 4 May 2003 to investigate particle concentrations, sources, and processes. We report the use of a particle beam width probe (BWP) in the field to quantify potential losses of particles due to beam broadening inside the AMS caused by particle shape (nonsphericity) and particle size. Data from this probe show that no significant mass of particles was lost due to excessive beam broadening; i.e. the shape- and size-related collection efficiency (E-s) of the AMS during this campaign was approximately one. Comparison of the BWP data from MCMA-2003 with other campaigns shows that the same conclusion holds for several other urban, rural and remotes sites. This means that the aerodynamic lens in the AMS is capable of efficiently focusing ambient particles into a well defined beam and onto the AMS vaporizer for particles sampled in a wide variety of environments. All the species measured by the AMS during MCMA-2003 have similar attenuation profiles which suggests that the particles that dominate the mass concentration were internally mixed most of the time. Only for the smaller particles ( especially below 300 nm), organic and inorganic species show different attenuation versus particle size which is likely due to partial external mixing of these components. Changes observed in the focusing of the particle beam in time can be attributed, in part, to changes in particle shape (i.e. due to relative humidity) and size of the particles sampled. However, the relationships between composition, atmospheric conditions, and particle shape and size appear to be very complex and are not yet completely understood.
Keywords: atmospheric aerosol, organic aerosols, aerodynamic lenses, Mexico City
Published in RUNG: 11.04.2021; Views: 2714; Downloads: 0
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2020. Detection of particle-phase polycyclic aromatic hydrocarbons in Mexico City using an aerosol mass spectrometerKatja Džepina, Janet Arey, Linsey C. Marr, D. Worsnop, Dara Salcedo, Q. Zhang, Timothy B. Onasch, Luisa T. Molina, Mario J. Molina, Jose L. Jimenez, 2007, original scientific article Abstract: We report the quantification of ambient particle-bound polycyclic aromatic hydrocarbons (PAHs) for the first time using a real-time aerosol mass spectrometer. These measurements were carried out during the Mexico City Metropolitan Area field study (MCMA-2003) that took place from March 29 to May 4, 2003. This was the first time that two different fast, real-time methods have been used to quantify PAHs alongside traditional filter-based measurements in an extended field campaign. This paper focuses on the technical aspects of PAH detection in ambient air with the Aerodyne AMS equipped with a quadrupole mass analyzer (Q-AMS), on the comparison of PAHs measured by the Q-AMS to those measured
with the other two techniques, and on some features of the ambient results.
PAHs are very resistant to fragmentation after ionization. Based on laboratory experiments with eight PAH standards, we show that their molecular ions, which for most particulate PAHs in ambient particles are larger than 200 amu, are often the largest peak in their Q-AMS spectra. Q-AMS spectra of PAH are similar to those in the NIST database, albeit with more fragmentation. We have developed a subtraction method that allows the removal of the contribution from non-PAH organics to the ion signals of the PAHs in ambient data. We report the mass concentrations of all individual groups of PAHs with molecular weights of 202, 216, 226 + 228, 240 + 242, 250 + 252, 264 + 266, 276 + 278, 288 + 290, 300 + 302, 316 and 326 + 328, as well as their sum as the total PAH mass concentration.
The time series of the Photoelectric Aerosol Sensor (PAS) and Q-AMS PAH measurements during MCMA-2003 are well correlated, with the smallest difference between measured PAH concentrations observed in the mornings when ambient aerosols loadings are dominated by fresh traffic emissions. The Q-AMS PAH measurements are also compared to those from GC–MS analysis of filter samples. Several groups of PAHs show agreement within the uncertainties, while the Q-AMS measurements are larger than the GC–MS ones for several others. In the ambient Q-AMS measurements the presence of ions tentatively attributed to cyclopenta[cd]pyrene and dicyclopentapyrenes causes signals at m/z 226 and 250,
which are significantly stronger than the signals in GC–MS analysis of filter samples. This suggests that very labile, but likely toxic, PAHs were present in the MCMA atmosphere that decayed rapidly due to reaction during filter sampling, and this may explain at least some of the differences between the Q-AMS and GC–MS measurements.
Keywords: AMS, PAH, Mexico City
Published in RUNG: 11.04.2021; Views: 2989; Downloads: 0
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