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• Hamer, Paul David; Walker, Sam-Erik; Sousa Santos, Gabriela; Vogt, Matthias; Vo, Dam Thanh; Lopez-Aparicio, Susana; Ramacher, Martin O. P. & Karl, Matthias (2020). A presentation of the EPISODE urban scale air quality model and its application to Nordic winter conditions, In Nicolas Moussiopoulos; Ranjeet S. Sokhi; George Tsegas; Evangelia Fragkou; Eleftherios Chourdakis & Ioannis Pipilis (ed.),  Proceedings of Abstracts. 12th International Conference on Air Quality Science and Application. Online Conference.  The Air quality Conference.  ISBN 9781527258297.  Abstracts.  s 70 - 70
• Hamer, Paul David; Walker, Sam-Erik; Sousa Santos, Gabriela; Vogt, Matthias; Vo, Dam Thanh; Lopez-Aparicio, Susana; Schneider, Philipp; Ramacher, Martin O. P. & Karl, Matthias (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, s 4323- 4353 . doi: 10.5194/gmd-13-4323-2020 Full text in Research Archive. Show summary

  This paper describes the Eulerian urban dispersion model EPISODE. EPISODE was developed to address a need for an urban air quality model in support of policy, planning, and air quality management in the Nordic, specifically Norwegian, setting. It can be used for the calculation of a variety of airborne pollutant concentrations, but we focus here on the implementation and application of the model for NO2 pollution. EPISODE consists of an Eulerian 3D grid model with embedded sub-grid dispersion models (e.g. a Gaussian plume model) for dispersion of pollution from line (i.e. roads) and point sources (e.g. chimney stacks). It considers the atmospheric processes advection, diffusion, and an NO2 photochemistry represented using the photostationary steady-state approximation for NO2. EPISODE calculates hourly air concentrations representative of the grids and at receptor points. The latter allow EPISODE to estimate concentrations representative of the levels experienced by the population and to estimate their exposure. This methodological framework makes it suitable for simulating NO2 concentrations at fine-scale resolution (<100 m) in Nordic environments. The model can be run in an offline nested mode using output concentrations from a global or regional chemical transport model and forced by meteorology from an external numerical weather prediction model; it also can be driven by meteorological observations. We give a full description of the overall model function and its individual components. We then present a case study for six Norwegian cities whereby we simulate NO2 pollution for the entire year of 2015. The model is evaluated against in situ observations for the entire year and for specific episodes of enhanced pollution during winter. We evaluate the model performance using the FAIRMODE DELTA Tool that utilises traditional statistical metrics, e.g. root mean square error (RMSE), Pearson correlation R, and bias, along with some specialised tests for air quality model evaluation. We find that EPISODE attains the DELTA Tool model quality objective in all of the stations we evaluate against. Further, the other statistical evaluations show adequate model performance but that the model scores greatly improved correlations during winter and autumn compared to the summer. We attribute this to the use of the photostationary steady-state scheme for NO2, which should perform best in the absence of local ozone photochemical production. Oslo does not comply with the NO2 annual limit set in the 2008/50/EC directive (AQD). NO2 pollution episodes with the highest NO2 concentrations, which lead to the occurrence of exceedances of the AQD hourly limit for NO2, occur primarily in the winter and autumn in Oslo, so this strongly supports the use of EPISODE for application to these wintertime events. Overall, we conclude that the model is suitable for an assessment of annual mean NO2 concentrations and also for the study of hourly NO2 concentrations in the Nordic winter and autumn environment. Further, in this work we conclude that it is suitable for a range of policy applications specific to NO2 that include pollution episode analysis, evaluation of seasonal statistics, policy and planning support, and air quality management. Lastly, we identify a series of model developments specifically designed to address the limitations of the current model assumptions. Part 2 of this two-part paper discusses the CityChem extension to EPISODE, which includes a number of implementations such as a more comprehensive photochemical scheme suitable for describing more chemical species and a more diverse range of photochemical environments, as well as a more advanced treatment of the sub-grid dispersion.

• 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.  The American Society of Mechanical Engineers (ASME).  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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• 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.
• 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. Eionet Report - ETC/ATNI. 14/2019.
• Walker, Sam-Erik & Schneider, Philipp (2020). A study of the relative expanded uncertainty formula for comparing low-cost sensor and reference measurements. NILU rapport. 1/2020. 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.

• 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 (2019). Monitoring of greenhouse gases and aerosols at Svalbard and Birkenes in 2018. Annual report.. NILU rapport. 18/2019. 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 2001-2018, and the greenhouse gas monitoring and aerosol observations from Birkenes for 2009-2018.

• Solberg, Sverre; Walker, Sam-Erik & Schneider, Philipp (2019). Trends in measured NO2 and PM. Discounting the effect of meteorology.. Eionet Report - ETC/ACM. 2018/9. Show summary

  This report documents a study on long-term trends in observed atmospheric levels of NO2, PM10 and PM2.5 based on data from the European Environmental Agency (EEA) Airbase v8 (EEA, 2018). The main aim is to evaluate to what extent the observed time series could be simulated as a function of various local meteorological data plus a time-trend by a Generalized Additive Model (GAM). The GAM could be regarded an advanced multiple regression model. If successful, such a model could be used for several purposes; to estimate the long-term trends in NO2 and PM when the effect of the inter-annual variations in meteorology is removed, and secondly, to “explain” the concentration levels in one specific year in terms of meteorological anomalies and long-term trends. The GAM method was based on a methodology developed during a similar project in 2017 looking at the links between surface ozone and meteorology. The input to the study consisted of gridded model meteorological data provided through the EURODELTA Trends project (Colette et al., 2017) for the 1990-2010 period as well as measured data on NO2, PM10 and PM2.5 extracted from Airbase v8. The measurement data was given for urban, suburban and rural stations, respectively. The analysis was split into two time periods, 1990-2000 and 2000-2010 since the number of stations differ substantially for these periods and since there is reason to believe that the trends differ considerably between these two periods. The study was focused on the 4-months winter period (Nov-Feb) since it was important to assure a period of the year with consistent and homogeneous relationships between the input explanatory data (local meteorology) and the levels of NO2 and PM. For NO2, this period will likely cover the season with the highest concentration levels whereas for PM high levels could be expected outside this period due to processes such as secondary formation, transport of Saharan dust and sea spray. When measured by the R2 statistic, the GAM method performed best for NO2 in Belgium, the Netherlands, NW Germany and the UK. Significantly poorer performance was found for Austria and areas in the south. For PM10 there were less clear geographical patterns in the GAM performance. Based on a comparison between the meteorologically adjusted trends and plain linear regression, our results indicate that for the 1990-2000 period meteorology caused an increase in NO2 concentrations that counteracted the effect of reduced emissions. For the period 2000-2010 we find that meteorology lead to reduced NO2 levels in the northwest and a slight increase in the south. The amount of observational data is much less for PM than for NO2. For the 1990-2000 period the number of sites with sufficient length of time series is too small to apply the GAM method on a European scale. For the 2000-2010 period, we find that the general performance of the GAM method is poorer for PM10 than for NO2. With respect to the link between PM10 and temperature, the results indicate a marked geographical pattern with a negative relationship in central Europe and a positive relationship in Spain, southern France and northern Italy. For PM10 during 2000-2010, the vast majority of the estimated trends are found to be negative. The difference between the GAM trend and the plain linear regression, indicates that meteorology lead to increased PM10 levels in the southern and central parts and decreased levels in the north. For PM2.5 it turned out that the amount of data in the entire period 1990-2010 was too small to use the GAM method in a meaningful way on a European scale. Only a few sites had sufficient time series and thus more recent data are required.

• Weydahl, Torleif; Walker, Sam-Erik; Johnsrud, Mona; Vo, Dam Thanh & Ranheim, Patrick (2019). Tiltaksutredning for lokal luftkvalitet i Tromsø. NILU rapport. 26/2019. Full text in Research Archive. Show summary

  Tiltaksutredningen, med handlingsplan og tiltak, skal bidra til å redusere luftforurensningen til et nivå som tilfredsstiller kravene i forurensningsforskriften. Tiltaksutredningen omfatter en kartlegging av luftkvaliteten i Tromsø ved trafikkberegninger og utslipps- og spredningsberegninger for PM10, PM2,5 og NO2 for Dagens situasjon 2016 og Framtidig situasjon 2023 med og uten tiltak mot svevestøv. Basert på resultatene fra beregningene og i samarbeid med oppdragsgiver og arbeidsgruppen, er det foreslått en revidert handlings- og beredskapsplan som skal behandles politisk.

• 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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