Sam Erik Walker

• Solberg, Sverre; Walker, Sam-Erik; Schneider, Philipp & Guerreiro, Cristina (2021). Quantifying the Impact of the Covid-19 Lockdown Measures on Nitrogen Dioxide Levels throughout Europe. Atmosphere. ISSN 2073-4433. 12. doi: 10.3390/atmos12020131. Full text in Research Archive
• Hamer, Paul David; Walker, Sam-Erik; Sousa Santos, Gabriela; Vogt, Matthias; Vo, Dam Thanh & Lopez-Aparicio, Susana [Show all 9 contributors for this article] (2020). The urban dispersion model EPISODE v10.0 – Part 1: An Eulerian and sub-grid-scale air quality model and its application in Nordic winter conditions. Geoscientific Model Development. ISSN 1991-959X. 13, p. 4323–4353. doi: 10.5194/gmd-13-4323-2020. Full text in Research Archive
• Hamer, Paul David; Walker, Sam-Erik; Sousa Santos, Gabriela; Vogt, Matthias; Vo, Dam Thanh & Lopez-Aparicio, Susana [Show all 8 contributors for this article] (2020). A presentation of the EPISODE urban scale air quality model and its application to Nordic winter conditions. In Moussiopoulos, Nicolas; Sokhi, Ranjeet S.; Tsegas, George; Fragkou, Evangelia; Chourdakis, Eleftherios & Pipilis, Ioannis (Ed.), Proceedings of Abstracts. 12th International Conference on Air Quality Science and Application. Online Conference. The Air quality Conference. ISSN 9781527258297. p. 70–70.
• 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, p. 3357–3399. doi: 10.5194/gmd-12-3357-2019. 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 Samaniego, Francisco J. (Eds.), Proceedings of the 60th World Statistics Congress of the International Statistical Institute, ISI2015. The International Statistical Institute. ISSN 978-90-73592-35-3. p. 3131–3136.
• 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, p. 185–202. doi: 10.1016/j.scitotenv.2015.04.108. Full text in Research Archive
• Karl, Matthias; Castell, Nuria; Simpson, David; Solberg, Sverre; Starrfelt, Jostein & Svendby, Tove Marit [Show all 8 contributors for this article] (2014). Uncertainties in assessing the environmental impact of amine emissions from a CO2 capture plant. Atmospheric Chemistry and Physics (ACP). ISSN 1680-7316. 14(16), p. 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). Time Series Analysis of Significant Wave Height Data for Identification of Trends in the Ocean Wave Climate. In ASME, conference (Eds.), Proceedings of ASME 2013 32nd International Conference on Ocean, Offshore and Arctic Engineering (OMAE 2013). Volume 2A: Structures, Safety and Reliability. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-5532-4. doi: 10.1115/omae2013-10024.
• 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, p. 148–160. doi: 10.1016/j.oceaneng.2012.12.042.
• 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), p. 348–357.
• Denby, Bruce; Dudek, Agnes V; Walker, Sam-Erik; Costa, Ana Margarida; Monteiro, Alexandra & van den Elshout, Sef [Show all 7 contributors for this article] (2011). Towards uncertainty mapping in air-quality modelling and assessment. International Journal of Environment and Pollution. ISSN 0957-4352. 44(1-4), p. 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), p. 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), p. 348–357.
• Berger, Janne; Walker, Sam-Erik; Denby, Bruce; Berkowicz, Ruwim; Løfstrøm, Per & Ketzel, Matthias [Show all 9 contributors for this article] (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), p. 319–334. Full text in Research Archive
• Madsen, Christian; Gehring, Ulrike; Walker, Sam-Erik; Brunekreef, Bert; Stigum, Hein & Næss, Øyvind Erik [Show all 7 contributors for this article] (2010). Ambient air pollution exposure, residential mobility and term birth weight in Oslo, Norway. Environmental Research. ISSN 0013-9351. 110(4), p. 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), p. 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), p. 129–137. doi: 10.1097/EDE.0b013e31815c0827.

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• Weydahl, Torleif; Johnsrud, Mona; Vo, Dam Thanh; Walker, Sam-Erik; Høiskar, Britt Ann Kåstad & Ranheim, Patrick (2020). Revidert tiltaksutredning for lokal luftkvalitet i Stavanger kommune.
• Guerreiro, Cristina; Solberg, Sverre; Walker, Sam-Erik & Schneider, Philipp (2020). Hvordan har luftkvalitet i Europa endret seg under lockdown og hvorfor?
• Schneider, Philipp; Castell, Nuria; Hamer, Paul David; Walker, Sam-Erik & Bartonova, Alena (2020). Networks of air quality sensors and their use for high-resolution mapping of urban air quality.
• 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; Hjort, Nils Lid & Hermansen, Gudmund Horn (2015). Model selection and verification for ensemble based probabilistic forecasting of air pollution in Oslo, Norway .
• Walker, Sam-Erik (2015). Confidence distributions based on M-estimators.
• Madsen, Christian; Gehring, U; Walker, SE; Brunekreef, B; Stigum, Hein & Næss, Øyvind Erik [Show all 7 contributors for this article] (2009). Ambient Air Pollution Exposure, Residential Mobility and Term Birth Weight in Oslo, Nonway. Epidemiology. ISSN 1044-3983. 20(6), p. S151–S151.
• Colette, Augustin; Solberg, Sverre; Aas, Wenche & Walker, Sam-Erik (2021). Understanding Air Quality Trends in Europe. Focus on the relative contribution of changes in emission of activity sectors, natural fraction and meteorological variability. ETC/ATNI. ISSN 978-82-93752-25-7.
• Hamer, Paul David; Walker, Sam-Erik & Schneider, Philipp (2021). Appropriate Assimilation Methods for Air Quality Prediction and Pollutant Emission Inversion. An Urban Data Assimilation Systems Report. NILU. ISSN 978-82-425-3027-1. Full text in Research Archive Show summary

  Rapporten presenterer en oversikt over dataassimilasjonsmetoder som kan anvendes for luftkvalitetsmodellering. Innledningsvis beskrives kort historisk bakgrunn for bruk av dataassimilasjon i numerisk værvarsling, der vi legger vekt på forskjellene mellom anvendelse av assimilasjon i meteorologiske varslingsmodeller og i spredningsmodeller for luftkvalitet. Basert på disse forskjellene beskrives så ønskede egenskaper til assimilasjonsmetoder for luftkvalitetsmodellering. Deretter gis en oversikt over tilgjengelige assimilasjonsmetoder, der vi søker å identifisere de mest aktuelle for vårt bruk som grunnlag for videre anvendelser i prosjektet.

• Ortiz, Alberto González; Guerreiro, Cristina; Soares, Joana; Antognazza, Frederico; Gsella, Artur & Houssiau, Michel [Show all 16 contributors for this article] (2020). Air quality in Europe - 2020 report. European Environment Agency. ISSN 978-92-9480-292-7.
• Solberg, Sverre; Walker, Sam-Erik; Guerreiro, Cristina & Colette, Augustin (2020). Statistical modelling for long-term trends of pollutants - Use of a GAM model for the assessment of measurements of O3, NO2 and PM. ETC/ATNI.
• Walker, Sam-Erik & Schneider, Philipp (2020). A study of the relative expanded uncertainty formula for comparing low-cost sensor and reference measurements. NILU. ISSN 978-82-425-2997-8. Full text in Research Archive Show summary

  In this report, we investigate the relative expanded uncertainty (REU) formula for comparing low-cost sensors (microsensors) and reference measurements. The purpose of the REU formula is to check if microsensor measurements follow the data quality objective (DQO) of the European Air Quality Directive 2008/50/EC to be considered equivalent to a reference instrument. The project aimed to obtain a good understanding of the REU formula for its proper use in current and future projects involving microsensors.

• Weydahl, Torleif; Walker, Sam-Erik; Johnsrud, Mona; Vo, Dam Thanh & Ranheim, Patrick (2019). Air quality assessment for improved local air quality in Tromsø. NILU. ISSN 978-82-425-2996-1. Full text in Research Archive Show summary

  The air quality assessment covers mapping of the air quality in Tromsø through traffic, emission and dispersion calculations of PM10, PM2,5 and NO2 for the present situation (2016) and future scenarios (2023) with and without measures on particulate matter. Based on the calculations and in coordination with Tromsø municipality and the workgroup, a plan for improved local air quality and a management plan for periods with high concentration levels is proposed for political processing.

• Solberg, Sverre; Walker, Sam-Erik & Schneider, Philipp (2019). Trends in measured NO2 and PM. Discounting the effect of meteorology. ETC/ACM.
• Myhre, Cathrine Lund; Svendby, Tove Marit; Hermansen, Ove; Lunder, Chris Rene; Platt, Stephen Matthew & Fiebig, Markus [Show all 11 contributors for this article] (2019). Monitoring of greenhouse gases and aerosols at Svalbard and Birkenes in 2018. Annual report. NILU. ISSN 978-82-425-2985-5. Full text in Research Archive Show summary

  Rapporten presenterer aktiviteter og måleresultater fra klimagassovervåkingen ved Zeppelin-observatoriet på Svalbard for årene 2001-2018 og klimagassmålinger og klimarelevante partikkelmålinger fra Birkenes for 2009-2018.

• Myhre, Cathrine Lund; Svendby, Tove Marit; Hermansen, Ove; Lunder, Chris Rene; Platt, Stephen Matthew & Fiebig, Markus [Show all 11 contributors for this article] (2018). Monitoring of greenhouse gases and aerosols at Svalbard and Birkenes in 2017 - Annual report. NILU. ISSN 978-82-425-2952-7. 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. 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.

• Karl, Matthias & Walker, Sam-Erik (2014). Method and tool selection within the pilot study of air quality in Poland: Data fusion methodologies. NILU. ISSN 978-82-425-2700-4.
• Steinbakk, Gunnhildur Högnadóttir; Thorarinsdottir, Thordis Linda; Lahoz, William A. & Walker, Sam-Erik (2014). Data assimilation and statistical post-processing for numerical air quality predictions. Norsk Regnesentral.
• Clench-Aas, Jocelyne; Aldrin, Magne; Bartonova, A.; Follestad, Turid; Walker, Sam-Erik & Skjønsberg, Ole Henning [Show all 8 contributors for this article] (2000). The short- and long-term effects of air pollution on symptoms of reduced health in a panel of chool-age children. NILU. ISSN 82-425-1131-4.

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