Sam Erik Walker

• Karl, Matthias; Walker, Sam-Erik; Solberg, Sverre & Ramacher, Martin O. P. (2019). The Eulerian urban dispersion model EPISODE – Part 2: Extensions to the source dispersion and photochemistry for EPISODE–CityChem v1.2 and its application to the city of Hamburg. Geoscientific Model Development.  ISSN 1991-959X.  12, s 3357- 3399 . doi: 10.5194/gmd-12-3357-2019 Full text in Research Archive. Show summary

  This paper describes the CityChem extension of the Eulerian urban dispersion model EPISODE. The development of the CityChem extension was driven by the need to apply the model in largely populated urban areas with highly complex pollution sources of particulate matter and various gaseous pollutants. The CityChem extension offers a more advanced treatment of the photochemistry in urban areas and entails specific developments within the sub-grid components for a more accurate representation of dispersion in proximity to urban emission sources. Photochemistry on the Eulerian grid is computed using a numerical chemistry solver. Photochemistry in the sub-grid components is solved with a compact reaction scheme, replacing the photo-stationary-state assumption. The simplified street canyon model (SSCM) is used in the line source sub-grid model to calculate pollutant dispersion in street canyons. The WMPP (WORM Meteorological Pre-Processor) is used in the point source sub-grid model to calculate the wind speed at plume height. The EPISODE–CityChem model integrates the CityChem extension in EPISODE, with the capability of simulating the photochemistry and dispersion of multiple reactive pollutants within urban areas. The main focus of the model is the simulation of the complex atmospheric chemistry involved in the photochemical production of ozone in urban areas. The ability of EPISODE–CityChem to reproduce the temporal variation of major regulated pollutants at air quality monitoring stations in Hamburg, Germany, was compared to that of the standard EPISODE model and the TAPM (The Air Pollution Model) air quality model using identical meteorological fields and emissions. EPISODE–CityChem performs better than EPISODE and TAPM for the prediction of hourly NO2 concentrations at the traffic stations, which is attributable to the street canyon model. Observed levels of annual mean ozone at the five urban background stations in Hamburg are captured by the model within ±15 %. A performance analysis with the FAIRMODE DELTA tool for air quality in Hamburg showed that EPISODE–CityChem fulfils the model performance objectives for NO2 (hourly), O3 (daily max. of the 8 h running mean) and PM10 (daily mean) set forth in the Air Quality Directive, qualifying the model for use in policy applications. Envisaged applications of the EPISODE–CityChem model are urban air quality studies, emission control scenarios in relation to traffic restrictions and the source attribution of sector-specific emissions to observed levels of air pollutants at urban monitoring stations.

• Karl, Matthias; Svendby, Tove Marit; Walker, Sam-Erik; Velken, Anna von Streng; Castell Balaguer, Nuria & Solberg, Sverre (2015). Modelling atmospheric oxidation of 2-aminoethanol (MEA) emitted from post-combustion capture using WRF-Chem. Science of the Total Environment.  ISSN 0048-9697.  527-528, s 185- 202 . doi: 10.1016/j.scitotenv.2015.04.108 Full text in Research Archive.
• Walker, Sam-Erik; Hermansen, Gudmund Horn & Hjort, Nils Lid (2015). Model selection and verification for ensemble based probabilistic forecasting of air pollution in Oslo, Norway, In Francisco J. Samaniego (ed.),  Proceedings of the 60th World Statistics Congress of the International Statistical Institute, ISI2015.  The International Statistical Institute.  ISBN 978-90-73592-35-3.  1.  s 3131 - 3136 Show summary

  In this paper, we discuss building time series models for forecasting of air pollution during wintertime conditions in Oslo, Norway, using ensembles of air pollution model data. Since such ensembles becomes increasingly available as part of regular air quality forecast modelling, it is important to build properly calibrated statistical models utilising such data. In particular, we focus on model selection using the Akaike and Bayesian information criteria, and verification of the forecasts using Probability Integral Transform (PIT) histograms and Brier scores. Three time series models are considered, using ensemble mean values as a primary covariate in a linear regression setting explaining observations, and modelling the residual errors as an autoregressive process, using either a constant variance; a timevarying (heteroscedastic) variance only depending on the ensemble variances; or as a combination of both. We show that for the limited, although representative, data analysed, the model incorporating both terms, seems to have an edge according to the model selection criteria and forecast verification tools used. Finally, we briefly discuss the possibility of introducing more focused model selection criteria for these types of models and data.

• Karl, Matthias; Castell, Nuria; Simpson, David; Solberg, Sverre; Starrfelt, Jostein; Svendby, Tove Marit; Walker, Sam-Erik & Wright, Richard Frederic (2014). Uncertainties in assessing the environmental impact of amine emissions from a CO2 capture plant. Atmospheric Chemistry and Physics.  ISSN 1680-7316.  14(16), s 8533- 8557 . doi: 10.5194/acp-14-8533-2014 Full text in Research Archive. Show summary

  In this study, a new model framework that couples the atmospheric chemistry transport model system WRF-EMEP and the multimedia fugacity level III model was used to assess the environmental impact of amine emissions to air from post-combustion carbon dioxide capture. The modelling framework was applied to a typical carbon capture plant artificially placed at Mongstad, west coast of Norway. WRF-EMEP enables a detailed treatment of amine chemistry in addition to atmospheric transport and deposition. Deposition fluxes of WRF-EMEP simulations were used as input to the fugacity model in order to derive concentrations of nitramines and nitrosamine in lake water. Predicted concentrations of nitramines and nitrosamines in ground-level air and drinking water were found to be highly sensitive to the description of amine chemistry, especially of the night time chemistry with the nitrate (NO3) radical. Sensitivity analysis of the fugacity model indicates that catchment characteristics and chemical degradation rates in soil and water are among the important factors controlling the fate of these compounds in lake water. The study shows that realistic emission of commonly used amines result in levels of the sum of nitrosamines and nitramines in ground-level air (0.6–10 pgm−3) and drinking water (0.04–0.25 ngL−1) below the current safety guideline for human health enforced by the Norwegian Environmental Directorate. The modelling framework developed in this study can be used to evaluate possible environmental impacts of emissions of amines from post-combustion capture in other regions of the world.

• Vanem, Erik & Walker, Sam-Erik (2013). Identifying trends in the ocean wave climate by time series analyses of significant wave height data. Ocean Engineering.  ISSN 0029-8018.  61, s 148- 160 . doi: 10.1016/j.oceaneng.2012.12.042
• Vanem, Erik & Walker, Sam-Erik (2013). Time Series Analysis of Significant Wave Height Data for Identification of Trends in the Ocean Wave Climate, In conference ASME (ed.),  Proceedings of ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2013). Volume 2A: Structures, Safety and Reliability.  ASME Press.  ISBN 978-0-7918-5532-4.  Paper No. OMAE2013-10024.
• Walker, Sam-Erik (2012). WORM: A new open road line source model for low wind speed conditions. International Journal of Environment and Pollution.  ISSN 0957-4352.  47(1/2/3/4), s 348- 357
• Denby, Bruce; Dudek, Agnes V; Walker, Sam-Erik; Costa, Ana Margarida; Monteiro, Alexandra; van den Elshout, Sef & Fisher, Bernard (2011). Towards uncertainty mapping in air-quality modelling and assessment. International Journal of Environment and Pollution.  ISSN 0957-4352.  44(1-4), s 14- 23 . doi: 10.1504/IJEP.2011.038398
• Slini, Theodora; Walker, Sam-Erik & Moussiopoulos, Nicolas (2011). Data assimilation within the Air4EU project: the Athens case. International Journal of Environment and Pollution.  ISSN 0957-4352.  44(1-4), s 298- 306 . doi: 10.1504/IJEP.2011.038430
• Walker, Sam-Erik (2011). WORM: A new open road line source model for low wind speed conditions. International Journal of Environment and Pollution.  ISSN 0957-4352.  47(1-4), s 348- 357
• Berger, Janne; Walker, Sam-Erik; Denby, Bruce; Berkowicz, Ruwim; Løfstrøm, Per; Ketzel, Matthias; Härkönen, Jari; Nikmo, Juha & Karppinen, Ari (2010). Evaluation and inter-comparison of open road line source models currently in use in the Nordic countries. Boreal environment research.  ISSN 1239-6095.  15(3), s 319- 334 Full text in Research Archive.
• Madsen, Christian; Gehring, Ulrike; Walker, Sam-Erik; Brunekreef, Bert; Stigum, Hein; Næss, Øyvind Erik & Nafstad, Per (2010). Ambient air pollution exposure, residential mobility and term birth weight in Oslo, Norway. Environmental Research.  ISSN 0013-9351.  110(4), s 363- 371 . doi: 10.1016/j.envres.2010.02.005
• Oftedal, Bente Margaret; Walker, Sam-Erik; Gram, Frederick; McInnes, Harold & Nafstad, Per (2009). Modelling long-term averages of local ambient air pollution in Oslo, Norway: evaluation of nitrogen dioxide, PM10 and PM2.5. International Journal of Environment and Pollution.  ISSN 0957-4352.  36(1-3), s 110- 126
• Oftedal, Bente Margaret; Brunekreef, Bert; Nystad, Wenche; Madsen, Christian; Walker, Sam-Erik & Nafstad, Per (2008). Residential outdoor air pollution and lung function in schoolchildren. Epidemiology.  ISSN 1044-3983.  19(1), s 129- 137 . doi: 10.1097/EDE.0b013e31815c0827

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• Myhre, Cathrine Lund; Svendby, Tove Marit; Hermansen, Ove; Lunder, Chris Rene; Platt, Stephen Matthew; Fiebig, Markus; Fjæraa, Ann Mari; Hansen, Georg Heinrich; Schmidbauer, Norbert; Krognes, Terje & Walker, Sam-Erik (2018). Monitoring of greenhouse gases and aerosols at Svalbard and Birkenes in 2017 - Annual report. NILU rapport. 29/2018. Full text in Research Archive. Show summary

  The report summaries the activities and results of the greenhouse gas monitoring at the Zeppelin Observatory situated on Svalbard in Arctic Norway during the period 2001-2017, and the greenhouse gas monitoring and aerosol observations from Birkenes for 2009-2017.

• Solberg, Sverre; Walker, Sam-Erik; Schneider, Philipp; Guerreiro, Cristina & Colette, Augustin (2018). Discounting the effect of meteorology on trends in surface ozone: Development of statistical tools. ETC/ACM Technical Paper. 2017/15. Show summary

  This report presents the results using a statistical method to single out the influence of interannual meteorological variability on surface ozone. The reason for using such a tool is two-fold: Firstly, to explain the ozone levels in one specific year in terms of weather anomalies and secondly, to estimate the part of long-term ozone trends that is due to the meteorology alone. The method is a so-called GAM (generalized additive model), which could be regarded an advanced multiple regression method relating daily ozone levels to certain meteorological variables. The performance of the method was evaluated by comparing observed ozone data with those predicted by the GAM. This revealed a good to very good agreement in central Europe and Germany in particular. For southern Europe the performance was poorer. The method indicated that meteorology contributed to the downward trend in ozone seen at most sites for both 1990-2000 and 2000-2010.

• Walker, Sam-Erik & Hjort, Nils Lid (2018). Focused model selection and inference using robust estimators..
• Walker, Sam-Erik & Hjort, Nils Lid (2017). Estimation and model selection by data-driven weighted likelihoods..
• Walker, Sam-Erik & Hjort, Nils Lid (2017). Estimation and model selection by data-driven weighted likelihoods..
• Walker, Sam-Erik & Hjort, Nils Lid (2017). Estimation and model selection via maximum weighted likelihoods..
• Walker, Sam-Erik & Hjort, Nils Lid (2017). Focused model selection and inference using robust estimators..
• Walker, Sam-Erik & Hjort, Nils Lid (2016). Model selection and focused inference using robust estimators..
• Walker, Sam-Erik & Hjort, Nils Lid (2016). Model selection and focused inference using robust estimators..
• Walker, Sam-Erik (2015). Confidence distributions based on M-estimators..
• Walker, Sam-Erik; Hjort, Nils Lid & Hermansen, Gudmund Horn (2015). Model selection and verification for ensemble based probabilistic forecasting of air pollution in Oslo, Norway.
• Karl, Matthias & Walker, Sam-Erik (2014). Method and tool selection within the pilot study of air quality in Poland: Data fusion methodologies. NILU OR (NILU Oppdragsrapport). 34/2014.
• Steinbakk, Gunnhildur Högnadóttir; Thorarinsdottir, Thordis Linda; Lahoz, William A. & Walker, Sam-Erik (2014). Data assimilation and statistical post-processing for numerical air quality predictions. NR-notat. SAMBA/49/14.
• Madsen, Christian; Gehring, U; Walker, SE; Brunekreef, B; Stigum, Hein; Næss, Øyvind Erik & Nafstad, Per (2009). Ambient Air Pollution Exposure, Residential Mobility and Term Birth Weight in Oslo, Nonway. Epidemiology.  ISSN 1044-3983.  20(6), s S151- S151
• Clench-Aas, Jocelyne; Aldrin, Magne; Bartonova, A.; Follestad, Turid; Walker, Sam-Erik; Skjønsberg, Ole Henning; Giæver, Petter & Moseng, J. (2000). The short- and long-term effects of air pollution on symptoms of reduced health in a panel of chool-age children. NILU OR (NILU Oppdragsrapport). 65/99.

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