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ABSTRACT: Measurements of airborne particles with aerodynamic diameter of 10 μm or less (PM10) and meteorological observations are available from 13 stations distributed throughout Switzerland and representing different site types. The effect of all available meteorological variables on PM10 concentrations was estimated using Generalized Additive Models. Data from each season were treated separately. The most important variables affecting PM10 concentrations in winter, autumn and spring were wind gust, the precipitation rate of the previous day, the precipitation rate of the current day and the boundary layer depth. In summer, the most important variables were wind gust, Julian day and afternoon temperature. In addition, temperature was important in winter. A "weekend effect" was identified due to the selection of variable "day of the week" for some stations. Thursday contributes to an increase of 13% whereas Sunday contributes to a reduction of 12% of PM10 concentrations compared to Monday on average over 9 stations for the yearly data. The estimated effects of meteorological variables were removed from the measured PM10 values to obtain the PM10 variability and trends due to other factors and processes, mainly PM10 emissions and formation of secondary PM10 due to trace gas emissions. After applying this process, the PM10 variability was much lower, especially in winter where the ratio of adjusted over measured mean squared error was 0.27 on average over all considered sites. Moreover, PM10 trends in winter were more negative after the adjustment for meteorology and they ranged between −1.25 μg m−3 yr−1 and 0.07 μg m−3 yr−1. The adjusted trends for the other seasons ranged between −1.34 μg m−3 yr−1 and −0.26 μg m−3 yr−1 in spring, −1.40 μg m−3 yr−1 and −0.28 μg m−3 yr−1 in summer and −1.28 μg m−3 yr−1 and −0.11 μg m−3 yr−1 in autumn. The estimated trends of meteorologically adjusted PM10 were in general non-linear. The two urban street sites considered in the study, Bern and Lausanne, experienced the largest reduction in measured and adjusted PM10 concentrations. This indicates a verifiable effect of traffic emission reduction strategies implemented during the past two decades. The average adjusted yearly trends for rural, urban background and urban street stations were −0.37, −0.53 and −1.2 μg m−3 yr−1 respectively. The adjusted yearly trends for all stations range from −0.15 μg m−3 yr−1 to −1.2 μg m−3 yr−1 or −1.2% yr−1 to −3.3% yr−1.
Atmospheric Chemistry and Physics. 01/2011;
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ABSTRACT: A quantum cascade laser based absorption spectrometer (QCLAS) is applied for the first time to perform in situ, continuous and high precision isotope ratio measurements of CO2 in the free troposphere. Time series of the three main CO2 isotopologue mixing ratios (12C16O2, 12C16O2 and 12C18O16O) have simultaneously been measured at one second time resolution over two years (from August 2008 to present) at the High Altitude Research Station Jungfraujoch (3580 m a.s.l., Switzerland). This work focuses on periods in February 2009 only, when sudden and pronounced enhancements in the tropospheric CO2 were observed. These short-term changes were closely correlated with variations in CO mixing ratios measured at the same site, indicating combustion related emissions as potential source. The analytical precision of 0.046‰ (at 50 s integration time) for both δ13C and δ18O and the high temporal resolution allowed the application of the Keeling plot method for source signature identification. The spatial origin of these CO2 emission sources was then determined by backward Lagrangian particle dispersion simulations.
Atmospheric Chemistry and Physics Discussions. 01/2010;
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ABSTRACT: The atmospheric layer closest to the ground is strongly influenced by variable surface fluxes (emissions, surface deposition) and can therefore be very heterogeneous. In order to perform air quality measurements that are representative of a larger domain or a certain degree of pollution, observatories are placed away from population centres or within areas of specific population density. Sites are often categorised based on subjective criteria that are not uniformly applied by the atmospheric community within different administrative domains yielding an inconsistent global air quality picture. A novel approach for the assessment of parameters reflecting site representativeness is presented here, taking emissions, deposition and transport towards 34 sites covering Western and Central Europe into account. These parameters are directly inter-comparable among the sites and can be used to select sites that are, on average, more or less suitable for data assimilation and comparison with satellite and model data. Advection towards these sites was simulated by backward Lagrangian Particle Dispersion Modelling (LPDM) to determine the sites' average catchment areas for the year 2005 and advection times of 12, 24 and 48 h. Only variations caused by emissions and transport during these periods were considered assuming that these dominate the short-term variability of most but especially short lived trace gases. The derived parameters describing representativeness were compared between sites and a novel, uniform and observation-independent categorisation of the sites based on a clustering approach was established. Six groups of European background sites were identified ranging from generally remote to more polluted agglomeration sites. These six categories explained 50 to 80% of the inter-site variability of median mixing ratios and their standard deviation for NO2 and O3, while differences between group means of the longer-lived trace gas CO were insignificant. The derived annual catchment areas strongly depended on the applied LPDM and input wind fields, the catchment settings and the year of analysis. Nevertheless, the parameters describing representativeness showed considerably less variability than the catchment geometry, supporting the applicability of the derived station categorisation.
Atmospheric Chemistry and Physics. 01/2010;
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ABSTRACT: The identification of atmospheric trace species measurements that are representative of well-mixed background air masses is required for monitoring atmospheric composition change at background sites. We present a statistical method based on robust local regression that is well suited for the selection of background measurements and the estimation of associated baseline curves. The bootstrap technique is applied to calculate the uncertainty in the resulting baseline curve. The non-parametric nature of the proposed approach makes it more flexible than other commonly used statistical data filtering methods. Application to carbon monoxide (CO) measured from 1996 to 2009 at the high alpine site Jungfraujoch (Switzerland, 3580 m a.s.l.) demonstrates the feasibility and usefulness of the proposed approach. The determined average annual change for the 1996 to 2009 period as estimated from filtered annual mean CO concentrations is −2.1 ± 1.3 ppb/yr. For comparison, the linear trend of unfiltered CO measurements at Jungfraujoch for this time period is −2.9 ± 1.5 ppb/yr.
Atmospheric Measurement Techniques Discussions. 01/2010;
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ABSTRACT: The particle dispersion model FLEXPART and the trajectory model LAGRANTO are Lagrangian models which are widely used to study synoptic-scale atmospheric air flows such as stratospheric intrusions (SI) and intercontinental transport (ICT). In this study, we focus on SI and ICT events particularly from the North American planetary boundary layer for the Jungfraujoch (JFJ) measurement site, Switzerland, in 2005. Two representative cases of SI and ICT are identified based on measurements recorded at Jungfraujoch and are compared with FLEXPART and LAGRANTO simulations, respectively. Both models well capture the events, showing good temporal agreement between models and measurements. In addition, we investigate the performance of FLEXPART and LAGRANTO on representing SI and ICT events over the entire year 2005 in a statistical way. We found that the air at JFJ is influenced by SI during 19% (FLEXPART) and 18% (LAGRANTO), and by ICT from the North American planetary boundary layer during 13% (FLEXPART) and 12% (LAGRANTO) of the entire year. Through intercomparsion with measurements, our findings suggest that both FLEXPART and LAGRANTO are well capable of representing SI and ICT events if they last for more than 12 h, whereas both have problems on representing short events. It is also shown that although the long-range transported air is characterized by relatively low NOy/CO ratios and elevated CO concentrations, using a combination of NOy/CO and CO as control parameters still encounters difficulty in distinguishing aged air masses by their source regions. Moreover, a sensitivity study indicates that the agreement between models and measurements depends significantly on the threshold values applied to the individual control parameters. Generally, the less strict the thresholds are, the better the agreement between models and measurements. Although the dependence of the agreement on the threshold values is appreciable, it nevertheless confirms the conclusion that both FLEXPART and LAGRANTO are well able to capture SI and ICT events with sustaining time longer than 12 h.
Atmospheric Chemistry and Physics Discussions. 01/2009;
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ABSTRACT: In-situ measurements of 7 volatile hydrocarbons, CxHy, and 3 chlorinated organic compounds, CxHyClz, were performed at Jungfraujoch (Switzerland) during eight years (2000–2007). The analysis of 4-h resolved non-methane volatile organic compounds (NMVOCs) was achieved by using gas-chromatography coupled with mass spectrometry (GC-MS). Variabilities in the NMVOC time series dataset were modeled by factor analysis (positive matrix factorization, PMF). Four factors defined the solution space and could be related to NMVOC sources and atmospheric processes. In order to facilitate factor interpretations the retrieved contributions were compared with independent measurements, such as trace gases (NOx, CO, and CH4) and back trajectories. The most dominant factor (accounting on average for ~42% of the total mixing ratio of the considered NMVOCs) was found to be most active in winter, co-varying with CO and CH4 and could be related to aged combustive emissions as well as natural gas distribution. The other three factors represent both industrial and evaporative sources. Trajectory statistics suggest that the most influential anthropogenic NMVOC sources for Jungfraujoch are located in Eastern Europe, but the Po Valley has been identified as a potential source region for specific industrial sources as well. Aging of the arriving NMVOCs, the derived factors as well as limitations of the methods are discussed. This is the first report of a PMF application on NMVOC data from a background mountain site.
Atmospheric Chemistry and Physics. 01/2009;
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ABSTRACT: In-situ measurements of volatile hydrocarbons, CxHy, and chlorinated organic compounds, CxHyClz, were performed at Jungfraujoch (Switzerland) during eight years (2000–2007). The analysis of 4-hourly resolved non-methane hydrocarbons (NMHCs) was achieved by using gas-chromatography coupled with mass spectrometry (GC-MS). Variabilities in the NMHC series were modeled by factor analysis. Four factors defined the solution space and could be related to hydrocarbon sources. In order to facilitate factor interpretations the retrieved contributions were compared with independent measurements, such as trace gases (NOx, CO, and CH4) and meteorological data (back trajectories). The most dominant factor (accounting on average for 42% of the total mixing ratio of the considered NMHCs) was found to be most active in winter, co-varying with CO and could be related to combustion sources. The other three factors represent both industrial and evaporative sources. Trajectory statistics suggest that the most influential anthropogenic NMHCs sources for Jungfraujoch are located in Eastern Europe, but the Po Valley has been identified as a potential source region for specific industrial sources as well. Aging of the arriving NMHCs, the derived factors as well as limitations of the methods are discussed.
Atmospheric Chemistry and Physics Discussions. 01/2008;
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ABSTRACT: The tropics strongly influence the global atmospheric chemistry budget. However, continuous in-situ observations of trace gases are rare especially in equatorial Africa. The WMO Global Atmosphere Watch (GAW) programme aimed to close this gap with the installation of the Mt. Kenya (MKN) baseline station. Here, the first continuous measurements of carbon monoxide (CO) and ozone (O3) at this site covering the period June 2002 to June 2006 are presented. The representativeness of the site was investigated by means of statistical data analysis, air mass trajectory clustering, interpretation of biomass burning variability and evaluation of O3-CO relationships. Because of its location in eastern equatorial Africa, the site was rarely directly influenced by biomass burning emissions, making it suitable for background observations. Located at 3678 m above sea level the night-time (21:00–04:00 UTC) measurements of CO and O3 were in general representative of the free troposphere, while day-time measurements were influenced by atmospheric boundary layer air. Increased night-time concentrations were observed in 25% of all nights and associated with residual layers of increased CO and water vapour in the free troposphere. Six representative flow regimes towards Mt. Kenya were determined: eastern Africa (21% of the time), Arabian Peninsula and Pakistan (16%), northern Africa free tropospheric (6%), northern Indian Ocean and India (17%), south-eastern Africa (18%) and southern India Ocean (21%) flow regimes. The seasonal alternation of these flow regimes was determined by the monsoon circulation and caused a distinct semi-annual cycle of CO with maxima during February (primary) and August (secondary, annually variable) and with minima in April (primary) and November (secondary, annually variable). O3 showed a weaker annual cycle with a minimum in November and a broad summer maximum. Inter-annual variations were explained with differences in southern African biomass burning and transport towards MKN. Although biomass burning had little direct influence on the measurements at MKN it introduces inter-annual variability in the background concentrations of the southern hemisphere that subsequently reaches Kenya. The measurements at MKN were representative of air masses with little photochemical activity as indicated by weak O3-CO correlations, underlining the baseline character of the site. Inter-comparison of O3 at MKN with sounding data from Nairobi revealed a positive offset of the sounding data, most likely due to additional photochemical production of O3 in the Nairobi city plume. Future extensions of the measurement programme will provide better understanding of the atmospheric chemistry of this globally important region.
Atmospheric Chemistry and Physics. 01/2008;
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ABSTRACT: In this study we present the seasonal variability of ozone production efficiencies ( E<sub>N</sub> ), defined as the net number of ozone molecules produced per molecule of nitrogen oxides (nitrogen oxide (NO) + nitrogen dioxide (NO<sub>2</sub>)=NO<sub>x</sub>) oxidized to NO<sub>z</sub> (total reactive nitrogen (NO<sub>y</sub>)–NO<sub>x</sub>) determined from field measurements of a seven-year period (1998–2004) at the Swiss high-alpine research station Jungfraujoch (JFJ), 3580 m a.s.l. This dataset is a unique long-term data series of nitrogen levels in the free troposphere over Central Europe and hence it offers an excellent opportunity to perform such an analysis and provide further evidence to the photochemical origin of the ozone spring maximum at locations of the northern hemisphere distant from nearby pollution sources. Experimentally derived daily E<sub>N</sub> values have been selected for 571 days out of the 2557 days from 1998 to 2004, from which an average ozone production efficiency of 18.8±1.3 molecules of O<sub>3</sub> produced per molecule of NO<sub>x</sub> oxidized was calculated. This value indicates the great potential and importance of photochemical ozone production in the free troposphere. The monthly means of experimentally derived daily E<sub>N</sub> values show a seasonal variation with lower values from May to August, which can be probably attributed to more efficient vertical transport of polluted air masses from the atmospheric boundary layer up to JFJ. In agreement, theoretically derived monthly E<sub>N</sub> values show similar seasonal variation. The ratio NO<sub>y</sub>/CO, a parameter to assess the aging process that has occurred in an air parcel, was used as a criterion to disaggregate the 571 selected days between undisturbed and disturbed free tropospheric (FT). The monthly means of experimentally derived E<sub>N</sub> values for the undisturbed FT conditions show a distinct seasonal cycle with higher values in the cold season from November to April. The E<sub>N</sub> values for undisturbed FT conditions are particularly higher than the respective monthly E<sub>N</sub> values for disturbed FT conditions from February to October. It should be noted that the monthly E<sub>N</sub> values of March ( E<sub>N</sub> =35.8) and April ( E<sub>N</sub> =34.9) are among the highest values throughout the year for undisturbed FT conditions at JFJ. These results highlight the key and possibly the dominant role for photochemistry in the observed build-up of tropospheric ozone in the winter-spring transition period.
Atmospheric Chemistry and Physics. 01/2007;
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ABSTRACT: An Analysis of Covariance (ANCOVA) was used to derive the influence of the meteorological variability on the daily maximum ozone concentrations at 12 low-elevation sites north of the Alps in Switzerland during the four seasons in the 1992–2002 period. The afternoon temperature and the morning global radiation were the variables that accounted for most of the meteorological variability in summer and spring, while other variables that can be related to vertical mixing and dilution of primary pollutants (afternoon global radiation, wind speed, stability or day of the week) were more significant in winter. In addition, the number of days after a frontal passage was important to account for ozone build-up in summer and ozone destruction in winter. The statistical model proved to be a robust tool for reducing the impact of the meteorological variability on the ozone concentrations. The explained variance of the model, averaged over all stations, ranged from 60.2% in winter to 71.9% in autumn. The year-to-year variability of the seasonal medians of daily ozone maxima was reduced by 85% in winter, 60% in summer, and 50% in autumn and spring after the meteorological adjustment. For most stations, no significantly negative trends (at the 95% confidence level) of the summer medians of daily O<sub>3</sub> or O<sub>x</sub> (O<sub>3</sub>+NO<sub>2</sub>) maxima were found despite the significant reduction in the precursor emissions in Central Europe. However, significant downward trends in the summer 90th percentiles of daily O<sub>x</sub> maxima were observed at 6 sites in the region around Zürich (on average −0.73 ppb yr<sup>-1</sup> for those sites). The lower effect of the titration by NO as a consequence of the reduced emissions could partially explain the significantly positive O<sub>3</sub> trends in the cold seasons (on average 0.69 ppb yr<sup>-1</sup> in winter and 0.58 ppb yr<sup>-1</sup> in autumn). The increase of O<sub>x</sub> found for most stations in autumn (on average 0.23 ppb yr<sup>-1</sup>) and winter (on average 0.39 ppb yr<sup>-1</sup>) could be due to increasing European background ozone levels, in agreement with other studies. The statistical model was also able to explain the very high ozone concentrations in summer 2003, the warmest summer in Switzerland for at least ~150 years. On average, the measured daily ozone maximum was 15 ppb (nearly 29%) higher than in the reference period summer 1992–2002, corresponding to an excess of 5 standard deviations of the summer means of daily ozone maxima in that period.
Atmospheric Chemistry and Physics. 01/2005;
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ABSTRACT: Net vertical air mass export by thermally driven f/lows from the atmospheric boundary layer (ABL) to the free troposphere (FT) above deep Alpine valleys was investigated. The vertical export of pollutants above mountainous terrain is presently poorly represented in global chemistry transport models (GCTMs) and needs to be quantified. Air mass budgets were calculated using aircraft observations obtained in deep Alpine valleys. The results show that on average 3 times the valley air mass is exported vertically per day under fair weather conditions. During daytime the type of valleys investigated in this study can act as an efficient "air pump" that transports pollutants upward. The slope wind system within the valley plays an important role in redistributing pollutants. Nitrogen oxide emissions in mountainous regions are efficiently injected into the FT. This enhances their ozone production efficiency and thus influences tropospheric pollution budgets. Once lifted to the FT above the Alps pollutants are transported horizontally by the synoptic flow and are subject to European pollution export. Forward trajectory studies show that under fair weather conditions two major pathways for air masses above the Alps dominate. Air masses moving north are mixed throughout the whole tropospheric column and further transported eastward towards Asia. Air masses moving south descend within the subtropical high pressure system above the Mediterranean.
Atmospheric Chemistry and Physics Discussions. 01/2003;
