Abstract
The main purpose of this paper is to present a preliminary study on the impact of stratospheric ozone on tropospheric ozone levels under specific atmospheric dynamical conditions. It is well accepted that stratospheric ozone can be the source of part of the tropospheric ozone. Previous studies indicate that the mechanism responsible for this ozone intrusion occurs generally in several steps or just in a single step, usually associated with strong upward motion. In the first part of this paper, the methodology used in order to identify particular short-term episodes, potentially associated to the abovementioned phenomenon, is presented. Several episodes have been studied occurring during 14 years of ozone data collection, recorded by the Portuguese ozone network. Then, an analysis of the dynamical atmospheric conditions associated to previously identified episodes have been developed in order to verify the possibility of stratospheric contribution for the observed ozone level in each episode. Two of these episodes show a relatively good relationship between synoptical patterns related to stratospheric intrusions and backward trajectories. For these cases, the possibility of stratospheric origin should not be discarded before further study is developed.

Abstract
We present new results on the performance of Minimum Error Entropy (MEE) decision trees, which use a novel node split criterion. The results were obtained in a comparive study with popular alternative algorithms, on 42 real world datasets. Carefull validation and statistical methods were used. The evidence gathered from this body of results show that the error performance of MEE trees compares well with alternative algorithms. An important aspect to emphasize is that MEE trees generalize better on average without sacrifing error performance.