| 1. |
- Abdo, A. A., et al.
(författare)
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FERMI LARGE AREA TELESCOPE VIEW OF THE CORE OF THE RADIO GALAXY CENTAURUS A
- 2010
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 719:2, s. 1433-1444
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Tidskriftsartikel (refereegranskat)abstract
- We present gamma-ray observations with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope of the nearby radio galaxy Centaurus A (Cen A). The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of Cen A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum (Gamma = 2.67 +/- 0.10(stat) +/- 0.08(sys) where the photon flux is Phi alpha E-Gamma). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004 to 2008. The fit requires a low Doppler factor, in contrast to BL Lac objects which generally require larger values to fit their broadband SEDs. This indicates that the gamma-ray emission originates from a slower region than that from BL Lac objects, consistent with previous modeling results from Cen A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow. The fit parameters are also consistent with Cen A being able to accelerate ultra-high energy cosmic-rays, as hinted at by results from the Auger observatory.
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| 2. |
- de Laat, A. T. J., et al.
(författare)
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Validation of five years (2003-2007) of SCIAMACHY CO total column measurements using ground-based spectrometer observations
- 2010
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Ingår i: Atmospheric Measurement Techniques. - : Copernicus GmbH. - 1867-1381 .- 1867-8548. ; 3:5, s. 1457-1471
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a validation study of SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) carbon monoxide (CO) total column measurements from the Iterative Maximum Likelihood Method (IMLM) algorithm using ground-based spectrometer observations from twenty surface stations for the five year time period of 2003-2007. Overall we find a good agreement between SCIAMACHY and ground-based observations for both mean values as well as seasonal variations. For high-latitude Northern Hemisphere stations absolute differences between SCIAMACHY and ground-based measurements are close to or fall within the SCIAMACHY CO 2 sigma precision of 0.2 x 10(18) molecules/cm(2) (similar to 10%) indicating that SCIAMACHY can observe CO accurately at high Northern Hemisphere latitudes. For Northern Hemisphere mid-latitude stations the validation is complicated due to the vicinity of emission sources for almost all stations, leading to higher ground-based measurements compared to SCIAMACHY CO within its typical sampling area of 8 degrees x 8 degrees. Comparisons with Northern Hemisphere mountain stations are hampered by elevation effects. After accounting for these effects, the validation provides satisfactory results. At Southern Hemisphere mid-to high latitudes SCIAMACHY is systematically lower than the ground-based measurements for 2003 and 2004, but for 2005 and later years the differences between SCIAMACHY and ground-based measurements fall within the SCIAMACHY precision. The 2003-2004 bias is consistent with previously reported results although its origin remains under investigation. No other systematic spatial or temporal biases could be identified based on the validation presented in this paper. Validation results are robust with regard to the choices of the instrument-noise error filter, sampling area, and time averaging required for the validation of SCIAMACHY CO total column measurements. Finally, our results show that the spatial coverage of the ground-based measurements available for the validation of the 2003-2007 SCIAMACHY CO columns is sub-optimal for validation purposes, and that the recent and ongoing expansion of the ground-based network by carefully selecting new locations may be very beneficial for SCIAMACHY CO and other satellite trace gas measurements validation efforts.
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| 3. |
- Angelbratt, Jon, 1981, et al.
(författare)
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A new method to detect long term trends of methane (CH4) and nitrous oxide (N2O) total columns measured within the NDACC ground-based high resolution solar FTIR network
- 2011
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11:13, s. 6167-6183
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Tidskriftsartikel (refereegranskat)abstract
- Total columns measured with the ground-based solar FTIR technique are highly variable in time due to atmospheric chemistry and dynamics in the atmosphere above the measurement station. In this paper, a multiple regression model with anomalies of air pressure, total columns of hydrogen fluoride (HF) and carbon monoxide (CO) and tropopause height are used to reduce the variability in the methane (CH(4)) and nitrous oxide (N(2)O) total columns to estimate reliable linear trends with as small uncertainties as possible. The method is developed at the Harestua station (60 degrees N, 11 degrees E, 600 ma.s.l.) and used on three other European FTIR stations, i.e. Jungfraujoch (47 degrees N, 8 degrees E, 3600 ma.s.l.), Zugspitze (47 degrees N, 11 degrees E, 3000 ma.s.l.), and Kiruna (68 degrees N, 20 degrees E, 400 ma.s.l.). Linear CH(4) trends between 0.13 +/- 0.01-0.25 +/- 0.02% yr(-1) were estimated for all stations in the 1996-2009 period. A piecewise model with three separate linear trends, connected at change points, was used to estimate the short term fluctuations in the CH(4) total columns. This model shows a growth in 1996-1999 followed by a period of steady state until 2007. From 2007 until 2009 the atmospheric CH(4) amount increases between 0.57 +/- 0.22-1.15 +/- 0.17% yr(-1). Linear N(2)O trends between 0.19 +/- 0.01-0.40 +/- 0.02% yr(-1) were estimated for all stations in the 1996-2007 period, here with the strongest trend at Harestua and Kiruna and the lowest at the Alp stations. From the N(2)O total columns crude tropospheric and stratospheric partial columns were derived, indicating that the observed difference in the N(2)O trends between the FTIR sites is of stratospheric origin. This agrees well with the N(2)O measurements by the SMR instrument onboard the Odin satellite showing the highest trends at Harestua, 0.98 +/- 0.28% yr(-1), and considerably smaller trends at lower latitudes, 0.27 +/- 0.25% yr(-1). The multiple regression model was compared with two other trend methods, the ordinary linear regression and a Bootstrap algorithm. The multiple regression model estimated CH(4) and N(2)O trends that differed up to 31% compared to the other two methods and had uncertainties that were up to 300% lower. Since the multiple regression method were carefully validated this stresses the importance to account for variability in the total columns when estimating trend from solar FTIR data.
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| 4. |
- Angelbratt, Jon, 1981, et al.
(författare)
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Carbon monoxide (CO) and ethane (C2H6) trends from ground-based solar FTIR measurements at six European stations, comparison and sensitivity analysis with the EMEP model
- 2011
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Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 11:17, s. 9253-9269
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Tidskriftsartikel (refereegranskat)abstract
- Trends in the CO and C(2)H(6) partial columns (similar to 0-15 km) have been estimated from four European ground-based solar FTIR (Fourier Transform InfraRed) stations for the 1996-2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to -0.45 +/- 0.16%yr(-1), -1.00 +/- 0.24%yr(-1), -0.62 +/- 0.19%yr(-1) and -0.61 +/- 0.16%yr(-1), respectively. The corresponding trends for C(2)H(6) are -1.51 +/- 0.23%yr(-1), -2.11 +/- 0.30%yr(-1), -1.09 +/- 0.25%yr(-1) and -1.14 +/- 0.18%yr(-1). All trends are presented with their 2-sigma confidence intervals. To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20% decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996-2006 period and a 20% increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH(4) and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP models have been evaluated against the measured CO and C(2)H(6) partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Alesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10-22% deviation for CO and 14-31% deviation for C(2)H(6). Their seasonal amplitude is captured within 6-35% and 9-124% for CO and C(2)H(6), respectively. However, 61-98% of the CO and C(2)H(6) partial columns in the European model are shown to arise from the boundary conditions, making the global-scale model a more suitable alternative when modeling these two species. In the evaluation of the global model the average partial columns for 2006 are shown to be within 1-9% and 37-50% of the measurements for CO and C(2)H(6), respectively. The global model sensitivity for assumptions made in this paper is also analyzed.
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| 5. |
- Buehler, Stefan, et al.
(författare)
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A multi-instrument comparison of integrated water vapour measurements at a high latitude site
- 2012
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 12:22, s. 10925-10943
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Tidskriftsartikel (refereegranskat)abstract
- We compare measurements of integrated water vapour (IWV) over a subarctic site (Kiruna, Northern Sweden) from five different sensors and retrieval methods: Radiosondes, Global Positioning System (GPS), ground-based Fourier-transform infrared (FTIR) spectrometer, ground-based microwave radiometer, and satellite-based microwave radiometer (AMSU-B). Additionally, we compare also to ERA-Interim model reanalysis data. GPS-based IWV data have the highest temporal coverage and resolution and are chosen as reference data set. All datasets agree reasonably well, but the ground-based microwave instrument only if the data are cloud-filtered. We also address two issues that are general for such intercomparison studies, the impact of different lower altitude limits for the IWV integration, and the impact of representativeness error. We develop methods for correcting for the former, and estimating the random error contribution of the latter. A literature survey reveals that reported systematic differences between different techniques are study-dependent and show no overall consistent pattern. Further improving the absolute accuracy of IWV measurements and providing climate-quality time series therefore remain challenging problems.
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