4. Results and discussion4.1. Additivity testThe additivity test has revealed that for NO2, the modellinghypotheses implemented during the construction of AERIS seemreasonable. The main question here was assessing whether a TM onits own is able to reproduce a change in the pollutant concentrationin the same way that this change is produced by the AQMS alongwith other varying sectors. The goal of the analysis is to gain insighton the flexibility of the modelling framework of AERIS.Fig. 4 depicts the behaviour of the different cells of the domainfor different thresholds of the additivity test. It can be seen thatmost cells (4416e98.1%) are below the lowest threshold, coveringmost of the modelling domain while a minority of cells are abovet ¼ 1.0 mg/m3 and t ¼ 1.5 mg/m3 (73 and 11, 1.6% and 0.2% respectively).The spatial representation of this behaviour is relevant todetect the presence of problematic zones (i.e. hotspots) and toassure that the estimates are valid throughout the modelleddomain. Domain-averaged results for the additivity test are presentedin Table 8 for each of the evaluated sectors. As it can be seen,the resulting NO2 mean annual concentrations calculated by bothmodels are similar, with low mean absolute errors (jXi,AERISXi,AQMSj<0.5mg/m3). Additionally, the normalized mean bias (NMB)for the complete domain equalled 4.1% and the normalized meanerror 13.0% (NME), being both values below the NMB15% andNME35% limits for acceptable performance according to Boylanand Russell (2006). It should be noted that the estimates
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