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No. 9: Comparison between measurements and model calculations of nitrogen dioxide (NO2)

Ellermann, T., Ketzel, M. & Solvang Jensen, S. 2012. Sammenligning af NO2-målinger og OSPM-beregninger for 10 gadestrækninger i København. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 26 s. - Teknisk rapport fra DCE - Nationalt Center for Miljø og Energi nr. 9. www.dmu.dk/Pub/TR9.pdf

Summary

This report from DCE - Danish Centre for Environment and Energy at Aarhus University (DCE) presents results from a comparison between measurements and model calculations of nitrogen dioxide (NO2) at ten selected street sections in Copenhagen. Force Technology has carried out the measurements of NO2 during a few weeks period from October 24th to November 28th 2011. DCE has carried out model calculations using the Operational Street Pollution Model (OSPM) in two stages. The model calculations at stage 1 (“blind test”) are carried out without knowledge about the results from the measurements. Model version and input data corresponds to those used for the most recent model calculations presented in the reports from 2011 for the routine monitoring programme and a surveillance project, respectively (Ellermann et al., 2011; Jensen et al., 2011). At stage 2 input data have been updated and revised in order to analyze the differences between measured and model calculated concentrations at stage 1. This report from AU/DCE presents only the results from the model calculations and the comparison between measurements and model results. Further details on the measurements can be found in Oxbøll, (2011).

This project is funded by the Danish Environmental Protection Agency (EPA). The project is carried out in cooperation with EPA, Force technology and Copenhagen Municipality, which all participated in the selection of the street sections. The ten street sections are selected in order to ensure that the street sections represent high, medium and low concentration levels, and the selection include street sections with both one and two sided building facades.

Comparison for the two permanent measurement stations at both H.C. Andersen’s Boulevard (HCAB) and Jagtvej shows good agreement between measurements from Force Technology using passive sampling and measurements from DCE using the EU reference method.

The results from the ”blind test” of OSPM at stage 1 show that the OSPM calculations are in good agreement with the measurements for seven out of nine street sections. The model results show good agreement for all street sections with two sided building facades. Large discrepancies between measurements and model results are found for two street sections (Sydhavnsgade and Fredensgade) both with one sided building facades. However, there are two other street sections with one sided building facades, where the model is in good agreement with the measurements. The discrepancies observed for Sydhavnsgade and Fredensgade are therefore not solely related to poor representation in the model of street sections with one sided building facades.

At stage 2 AU/DCE made a reevaluation of the input data in order to be able to analyze and determine the explanations for the discrepancies between measurements and model results for NO2 at Sydhavnsgade and Fredensgade. Recalculation of the NO2 concentrations using the improved input data lead to the following conclusions:

It is observed that input data in some cases are far from representing the “real” world. There is therefore a need for improving the quality assurance of the input data.

The modified input data on traffic and street configuration lead to insignificant changes in the calculated NO2 concentrations for the main part of the selected street sections. For HCAB (1) and Ågade the improved input data lead to a 10 % higher and 6 % lower concentration, respectively. For Sydhavsgade and Fredensgade the improved input data lead to larger changes in the calculated concentrations. This is the streets where there are the largest difference between measurements and model calculations.

Using the modified input data, the model calculations lead to results that are in agreement with the observations for eight out of nine street sections, where measurements have been carried out. The main difference between model and measurements are at Sydhavnsgade where the model calculation leads to an about 20 % higher concentration than measured. Over all the performance of the model is therefore satisfactory.

The difference between measurements and model result at Fredensgade is a result of incorrect input data concerning street configuration.

For Sydhavnsgade, the difference between measurements and model result is mainly due to incorrect traffic data.

At HCAB there may be larger discrepancies due to the impacts of the construction work and the related changes in traffic on the NO2 concentrations; Impacts on the concentrations that are not described in the model. Hence an additional evaluation of the model performance was carried out for the period 2003 to 2011. This evaluation showed that a scaling of the model input were necessary in order to obtain agreement between model and measurements for this period. Moreover, the model showed lower results than the measurements for 2010 and 2011. This may be explained by the extra contribution to the emissions due to construction work in the area around the measurement station.

Based on the results from the project it is concluded that it is necessary to make minor adjustments in the procedures used to determine the street configuration for the street sections with one sided building facades. Moreover, it would be advisable to repeat the measurements and perform model evaluation for additional street sections with one sided building facades including Nørre Søgade, where the measurements unfortunately were lost in the present campaign. Finally, it would be advisable to improve the input data for traffic in order to reduce the uncertainties related to these data.