Auflistung nach Autor:in "Ogren, John A."

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  • Vorschaubild
    Publikation
    Aerosol decadal trends – Part 1. In-situ optical measurements at GAW and IMPROVE stations
    (Copernicus, 2013) Collaud Coen, Martine; Andrews, Elisabeth; Asmi, Ari; Baltensperger, Urs; Bukowiecki, Nicolas; Day, Derek; Fiebig, Markus; Fjaeraa, Ann Mari; Flentje, Harald; Hyvärinen, Antti-Pekka; Jefferson, Anne; Jennings, Stephen G.; Kouvarakis, Giorgos; Lihavainen, Heikki; Lund Myhre, Cathrine; Malm, William; Mihalopoulos, Nikolaos; Molenar, John; O'Dowd, Colin; Ogren, John A.; Schichtel, Bret; Sheridan, Patrick; Virkkula, Aki; Weingartner, Ernest; Weller, Rolf; Laj, Paolo [in: Atmospheric Chemistry and Physics]
    Currently many ground-based atmospheric stations include in-situ measurements of aerosol physical and optical properties, resulting in more than 20 long-term (> 10 yr) aerosol measurement sites in the Northern Hemisphere and Antarctica. Most of these sites are located at remote locations and monitor the aerosol particle number concentration, wavelength-dependent light scattering, backscattering, and absorption coefficients. The existence of these multi-year datasets enables the analysis of long-term trends of these aerosol parameters, and of the derived light scattering Ångström exponent and backscatter fraction. Since the aerosol variables are not normally distributed, three different methods (the seasonal Mann-Kendall test associated with the Sen's slope, the generalized least squares fit associated with an autoregressive bootstrap algorithm for confidence intervals, and the least-mean square fit applied to logarithms of the data) were applied to detect the long-term trends and their magnitudes. To allow a comparison among measurement sites, trends on the most recent 10 and 15 yr periods were calculated. No significant trends were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficients (mean slope of −2.0% yr−1) were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. The difference in the timing of emission reduction policy for the Europe and US continents is a likely explanation for the decreasing trends in aerosol optical parameters found for most American sites compared to the lack of trends observed in Europe. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient. The high altitude Pacific island station of Mauna Loa presents positive trends for both scattering and absorption coefficients.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Aerosol decadal trends – Part 2. In-situ aerosol particle number concentrations at GAW and ACTRIS stations
    (Copernicus, 2013) Asmi, Ari; Collaud Coen, Martine; Ogren, John A.; Andrews, Elisabeth; Sheridan, Patrick; Jefferson, Anne; Weingartner, Ernest; Baltensperger, Urs; Bukowiecki, Nicolas ; Lihavainen, Heikki; Kivekäs, Niku; Asmi, Eija; Aalto, Pasi Pekka; Kulmala, Markku; Wiedensohler, Alfred; Birmili, Wolfram; Hamed, Amar; O'Dowd, Colin; Jennings, Stephen G.; Weller, Rolf; Flentje, Harald; Fjaeraa, Ann Mari; Fiebig, Markus; Myhre, Cathrine Lund; Hallar, Anna Gannet; Swietlicki, Erik; Kristensson, Adam; Laj, Paolo [in: Atmospheric Chemistry and Physics]
    We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many northern hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long time series of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Characterization and intercomparison of aerosol absorption photometers. result of two intercomparison workshops
    (Copernicus, 2011) Müller, Thomas; Henzing, Bas; de Leeuw, Gerrit; Wiedensohler, Alfred; Alastuey, Andrés; Angelov, H.; Bizjak, Milan; Collaud Coen, Martine; Engström, J. E.; Gruening, Carsten; Hillamo, Risto; Hoffer, András; Imre, Kornélia; Ivanow, Petko; Jennings, Stephen G.; Sun, Junying; Kalivitis, Nikos; Karlsson, Hanna; Komppula, Mikka; Laj, Paolo; Li, S.-M.; Lunder, Chris; Marinoni, Angela; Martins dos Santos, Sebastiao; Moerman, Marcel; Nowak, Andreas; Ogren, John A.; Petzold, Andreas; Pichon, Jean Marc; Rodriquez, Sergio; Sharma, Sangeeta; Sheridan, Patrick J.; Teinilä, Kimmo; Tuch, Thomas; Viana, Mar; Virkkula, Aki; Weingartner, Ernest; Wilhelm, R.; Wang, Yaqiang [in: Atmospheric Measurement Techniques]
    Absorption photometers for real time application have been available since the 1980s, but the use of filter-based instruments to derive information on aerosol properties (absorption coefficient and black carbon, BC) is still a matter of debate. Several workshops have been conducted to investigate the performance of individual instruments over the intervening years. Two workshops with large sets of aerosol absorption photometers were conducted in 2005 and 2007. The data from these instruments were corrected using existing methods before further analysis. The inter-comparison shows a large variation between the responses to absorbing aerosol particles for different types of instruments. The unit to unit variability between instruments can be up to 30% for Particle Soot Absorption Photometers (PSAPs) and Aethalometers. Multi Angle Absorption Photometers (MAAPs) showed a variability of less than 5%. Reasons for the high variability were identified to be variations in sample flow and spot size. It was observed that different flow rates influence system performance with respect to response to absorption and instrumental noise. Measurements with non absorbing particles showed that the current corrections of a cross sensitivity to particle scattering are not sufficient. Remaining cross sensitivities were found to be a function of the total particle load on the filter. The large variation between the response to absorbing aerosol particles for different types of instruments indicates that current correction functions for absorption photometers are not adequate.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild nicht verfügbar
    Publikation
    Climatology of aerosol radiative properties in the free troposphere
    (Elsevier, 04.12.2011) Andrews, Elisabeth; Ogren, John A.; Bonasoni, Paolo; Marinoni, Angela; Cuevas, Emilio; Rodríguez, Sergio Hugo Sánchez; Sun, Junying; Jaffe, Daniel A.; Fischer, Emily V.; Baltensperger, Urs; Weingartner, Ernest; Collaud Coen, Martine; Sharma, Sangeeta; Macdonald, Annemarie; Leaitch, W. Richard; Lin, Neng Huei; Laj, Paolo; Arsov, Todor; Kalapov, Ivo; Jefferson, Anne; Sheridan, Patrick [in: Atmospheric Research]
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation
    (Copernicus, 26.05.2010) Spracklen, Dominick V.; Carslaw, Kenneth S.; Merikanto, Joonas; Mann, Graham W.; Reddington, Carly L.; Pickering, S.; Ogren, John A.; Andrews, Elisabeth; Baltensperger, Urs; Weingartner, Ernest; Boy, Michael; Kulmala, Markku; Laakso, Lauri; Lihavainen, Heikki; Kivekäs, Niku; Komppula, Mika; Mihalopoulos, Ninolaos; Kouvarakis, Giorgos; Jennings, Stephen G.; O'Dowd, Collin D.; Birmili, Wolfram; Wiedensohler, Alfred; Weller, Rolf; Gras, John; Laj, Paolo; Sellegri, Karine; Bonn, Boris; Krejci, Radovan; Laaksonen, Ari; Hamed, Amar; Minikin, Andreas; Harrison, Roy Michael; Talbot, Robert; Sun, Junying [in: Atmospheric Chemistry and Physics]
    We synthesised observations of total particle number (CN) concentration from 36 sites around the world. We found that annual mean CN concentrations are typically 300–2000 cm−3 in the marine boundary layer and free troposphere (FT) and 1000–10 000 cm−3 in the continental boundary layer (BL). Many sites exhibit pronounced seasonality with summer time concentrations a factor of 2–10 greater than wintertime concentrations. We used these CN observations to evaluate primary and secondary sources of particle number in a global aerosol microphysics model. We found that emissions of primary particles can reasonably reproduce the spatial pattern of observed CN concentration (R2=0.46) but fail to explain the observed seasonal cycle (R2=0.1). The modeled CN concentration in the FT was biased low (normalised mean bias, NMB=−88%) unless a secondary source of particles was included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=−25%). Simulated CN concentrations in the continental BL were also biased low (NMB=−74%) unless the number emission of anthropogenic primary particles was increased or a mechanism that results in particle formation in the BL was included. We ran a number of simulations where we included an empirical BL nucleation mechanism either using the activation-type mechanism (nucleation rate, J, proportional to gas-phase sulfuric acid concentration to the power one) or kinetic-type mechanism (J proportional to sulfuric acid to the power two) with a range of nucleation coefficients. We found that the seasonal CN cycle observed at continental BL sites was better simulated by BL particle formation (R2=0.3) than by increasing the number emission from primary anthropogenic sources (R2=0.18). The nucleation constants that resulted in best overall match between model and observed CN concentrations were consistent with values derived in previous studies from detailed case studies at individual sites. In our model, kinetic and activation-type nucleation parameterizations gave similar agreement with observed monthly mean CN concentrations.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Measurement of relative humidity dependent light scattering of aerosols
    (Copernicus, 21.01.2010) Fierz-Schmidhauser, Rahel; Zieger, Paul; Wehrle, Günther; Jefferson, Anne; Ogren, John A.; Baltensperger, Urs; Weingartner, Ernest [in: Atmospheric Measurement Techniques]
    Relative humidity (RH) influences the water content of aerosol particles and therefore has an important impact on the particles' ability to scatter visible light. The RH dependence of the particle light scattering coefficient (σsp is therefore an important measure for climate forcing calculations. We built a humidification system for a nephelometer which allows the measurement of σsp at a defined RH in the range of 40–90%. This RH conditioner consists of a humidifier followed by a dryer, which enables us to measure the hysteresis behavior of deliquescent aerosol particles. In this paper we present the set-up of a new humidified nephelometer, a detailed characterization with well defined laboratory generated aerosols, and a first application in the field by comparing our instrument to another humidified nephelometer. Monodisperse ammonium sulfate and sodium chloride particles were measured at four different dry particle sizes. Agreement between measurement and prediction based on Mie theory was found for both σsp and f(RH)=σsp(RH)/σsp(dry) within the range of uncertainty. The two humidified nephelometers measuring at a rural site in the Black Forest (Germany) often detected different f(RH), probably caused by the aerosol hysteresis behavior: when the aerosol was metastable, therefore was scattering more light, only one instrument detected the higher f(RH).
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild
    Publikation
    Mobility particle size spectrometers. harmonization of technical standards and data structure to facilitate high quality long-term observations of atmospheric particle number size distributions
    (Copernicus, 29.03.2012) Wiedensohler, Alfred; Birmili, Wolfram; Nowak, Marta; Sonntag, André; Weinhold, Kay; Merkel, Maik; Wehner, Birgit; Tuch, Thomas; Pfeifer, Sascha; Fiebig, Markus; Fjäraa, Ann Mari; Asmi, Eija; Sellegri, Karine; Depuy, R.; Venzac, Hervé; Villani, Paolo; Laj, Paolo; Aalto, Pasi Pekka; Ogren, John A.; Swietlicki, Erik; Williams, Paul I.; Roldin, Pontus; Quincey, Paul; Hüglin, Christoph; Fierz-Schmidhauser, Rahel; Gysel, Martin; Weingartner, Ernest; Riccobono, Francesco; Santos, S.; Gruening, Carsten; Faloon, K.; Beddows, D.; Harrison, Roy; Monahan, C.; Jennings, Stephen G.; O'Dowd, Colin D.; Marinoni, Angela; Horn, H.-G.; Keck, L.; Jiang, Jingkun; Scheckman, Jakob; McMurry, Peter H.; Deng, Zhaoze; Zhao, Chunsheng; Moerman, Marcel; Henzing, Bas; de Leeuw, Gerrit; Löschau, G.; Bastian, S. [in: Atmospheric Measurement Techniques]
    Abstract. Mobility particle size spectrometers often referred to as DMPS (Differential Mobility Particle Sizers) or SMPS (Scanning Mobility Particle Sizers) have found a wide range of applications in atmospheric aerosol research. However, comparability of measurements conducted world-wide is hampered by lack of generally accepted technical standards and guidelines with respect to the instrumental set-up, measurement mode, data evaluation as well as quality control. Technical standards were developed for a minimum requirement of mobility size spectrometry to perform long-term atmospheric aerosol measurements. Technical recommendations include continuous monitoring of flow rates, temperature, pressure, and relative humidity for the sheath and sample air in the differential mobility analyzer. We compared commercial and custom-made inversion routines to calculate the particle number size distributions from the measured electrical mobility distribution. All inversion routines are comparable within few per cent uncertainty for a given set of raw data. Furthermore, this work summarizes the results from several instrument intercomparison workshops conducted within the European infrastructure project EUSAAR (European Supersites for Atmospheric Aerosol Research) and ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure Network) to determine present uncertainties especially of custom-built mobility particle size spectrometers. Under controlled laboratory conditions, the particle number size distributions from 20 to 200 nm determined by mobility particle size spectrometers of different design are within an uncertainty range of around ±10% after correcting internal particle losses, while below and above this size range the discrepancies increased. For particles larger than 200 nm, the uncertainty range increased to 30%, which could not be explained. The network reference mobility spectrometers with identical design agreed within ±4% in the peak particle number concentration when all settings were done carefully. The consistency of these reference instruments to the total particle number concentration was demonstrated to be less than 5%. Additionally, a new data structure for particle number size distributions was introduced to store and disseminate the data at EMEP (European Monitoring and Evaluation Program). This structure contains three levels: raw data, processed data, and final particle size distributions. Importantly, we recommend reporting raw measurements including all relevant instrument parameters as well as a complete documentation on all data transformation and correction steps. These technical and data structure standards aim to enhance the quality of long-term size distribution measurements, their comparability between different networks and sites, and their transparency and traceability back to raw data.
    01A - Beitrag in wissenschaftlicher Zeitschrift