Abstract
The Po Valley (Italy) model inter-comparison exercise (POMI) has been carried out in order to explore the changes in air quality in response to changes in emissions. The starting point was the evaluation of the simulated particulate matter and ozone (O3) modelled concentrations against observations for the year 2005 of the six participating chemical transport models. As models were run with the same configuration in terms of spatial resolution, boundary condition, emissions and meteorology, the differences presented in the models’ results are only related to their formulation. As described in the paper, significant efforts have been made to improve the accuracy of the anthropogenic emissions and meteorological input data. Nevertheless, none of the models using the proposed meteorology succeeded to fulfil the quality performance criteria set in the 2008 Air Quality Directive and in the literature for particulate matter, while also for ozone the results are not very satisfying. Although the overall performances look better for O3 than for particulate matter with an aerodynamic diameter smaller than 10 μm (PM10), the models tend to exhibit a similar behaviour and show the largest model variability in locations where concentrations are the highest (urban areas for PM10 and suburbs and hilly areas for O3). While differences are significant in terms of standard deviation and bias, the correlation remains quite similar among models indicating that models generally capture well the main temporal variations, especially the seasonal ones. Possible explanations for this common behaviour and a discussion of the differences among models’ results are presented in this paper.
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Acknowledgments
Part of this work has been funded by the authorities of the Lombardy region. Giuseppe Triacchini and Cinzia Pastorello are thanked for their contribution to the elaboration of the emission inventory, Jean Philippe Putaud for providing the EMEP Ispra data and Panagiota Dilara for her support in the project activities. The regional agencies of the Po Valley are acknowledged for providing the meteorological and air quality data used in this work. The Cyprus Institute received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° 226144. RSE authors thank Chris Emery and Paola Crippa for their contribution in setting up CAMx simulations.
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Appendix: Statistical indicators
Appendix: Statistical indicators
The root mean square error
The bias
The Pearson correlation coefficient (R)
M denotes modelled value, O denotes observed value, N is the number of paired values considered and over bars denote averaged values.
The index of agreement (IOA)
Index of agreement (IOA) determines the extent to which magnitudes of Ō are related to the predicted deviations about Ō. The perfect value of IOA is 1.
The mean fractional bias
Mean fractional bias (MFB) is a useful indicator because it has the advantage of equally weighting positive and negative bias estimates. It has also the advantage of not considering observations as the true value. MFB ranges from −200 to +200 %.
The mean fractional error
Similarly to the MFB, the mean fractional error (MFE) gives equal weight to under- and over-prediction, is not sensitive to a threshold in measured values and does not assume that observations are the truth (i.e. the denominator is the sum of observed and predicted). MFE ranges from 0 to +200 %.
The factor of predictions within a factor of 2 of observations
The relative directive error after (Denby 2010)
where OLV is the closest observed concentration to the limit value concentration (LV) and MLV is the correspondingly ranked modelled concentrations. The RDE has been defined in relation to the AQD (2008) in order to give a mathematical expression of the “model uncertainty” term in the AQD.
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Pernigotti, D., Thunis, P., Cuvelier, C. et al. POMI: a model inter-comparison exercise over the Po Valley. Air Qual Atmos Health 6, 701–715 (2013). https://doi.org/10.1007/s11869-013-0211-1
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DOI: https://doi.org/10.1007/s11869-013-0211-1