RESUMO
The comprehension of atmospheric pollution levels worldwide is a crucial issue for assessing the health consequences from human exposure to polluted air, and for identifying emission reductions that will be effective for minimizing the potential risks. Advanced interconnected sensors are required that could monitor on real-time toxic gaseous concentration. The success of these instruments depends on the reliability of these devices to quantify and disseminate the pollution levels to nearby citizens. Metal-oxide semiconductors are widely used as functional materials for gas sensing because of their chemo-resistive effect when interacting with gases. Mixed oxides are usually considered for the superior performances shown with respect to the single oxides. In this work, tungsten-tin mixed oxides with different Sn molar fraction (0.0018, 0.12, 0.33 and 0.89 named WS-1, WS-2, WS-3 and WS-4) were synthesized by sol-gel co-precipitation. The powders were characterized by ICP-OES analysis, specific surface area measurements, electron microscopy, X-ray diffraction, UV-Vis-NIR and FT-IR spectroscopies. The powders were also deposited through screen-printing technology, obtaining thick film gas sensors, on which measurements of conductance as a function of temperature, surface potential barrier and dynamical responses in the presence of oxidizing or reducing gases were carried out. Based on the studied properties, the mixed oxides can be divided into two groups: the WO3-like samples (WS-1, WS-2, WS-3) and the SnO2-like sample (WS-4). All samples present pure crystalline structures: this is a new result for the WO3-like samples. There is no literature data reporting about the introduction of so high Sn content in a WO3 structure. The combination of spectroscopic and electrical characterizations allowed the definition of an interpretative model that correlates the deepness of defect levels in the band gap of these materials to the values of the surface potential barrier in air and, as a consequence, to the electrical responses to oxidizing and reducing gases.
RESUMO
The first and second AutoOil programmes were conducted since 1992 as a partnership between the European Commission and the automobile and oil industries. These have introduced emission reductions in Europe based on numerical modelling for a target year. They aimed to identify the most cost-effective way to meet desired future air quality over the whole European Union. In their time, these regulatory efforts were considered an important step towards a new approach for establishing European emission limits. With this work, we review the effectiveness of forecasts carried out with numerical modelling and compare these with the actual measurements at the target year, which was the year 2010. Based on these comparisons and new technological innovations these methodologies can incorporate new sectorial assessments for improving the accuracy of the modelling forecasts and for examining the representativeness of emissions reductions, as well as for the simultaneous assessment of population exposure to cocktails of toxic substances under realistic climatological conditions. We also examined at the ten AutoOil domains the geographical generalisation of the forecasts for CO and NO2 at 1065 European urban areas on the basis of their population and the local population density.
