Retrieval of asphalt aggregate exposition to the atmosphere by mivis airborne imagery to support environmental analysis

Mechanic action due to vehicle transit and natural worsening of asphalts may cause changes on surface composition and aggregate reorganization. The major constituent of asphalt composition is composed by the aggregate fraction while the rest usually consists mainly about a 5% of bitumen. Bitumen absorbs main part of solar incident radiation while the aggregate compound cause the increase of reflectance. Recent studies have shown that a significant source of sub-micrometer fine particles are produced by the road–tire interface and aggregate appears to be the major source of heavy metals when exposed to air but also have high capacity for absorbing ions. Wear degree of road surface can therefore affect the contribution associated with road particles emissions/absorption in the environment. When describing adsorption capacities, superficial area is the major factor to be taken into account and it will be more relevant to use the unit per square meter than per gram. Considering that surface area of exposed aggregates is the cause that most influences the production of wear particles and adsorption capacity of metals, the quantification of the aggregate-atmosphere interface can be an important issue to estimate the particle emissions originated from the road-tire interface. For the determination of aggregate exposure to the atmosphere, Digital Imaging Processing (DIP) and spectral signatures can be used. After a pre-processing step for normalization and co- registration of frames, a supervised non-parametric classification (parallelepiped method) was performed to retrieve the Exposed Aggregates Index (EAI). Its calculation allow to quantify the area of exposed aggregates to the atmosphere in each frame and quantifying the superficial bitumen removal. This index is finally compared with spectral data acquired during the field survey, taking into account different bands already used in bibliography to characterize physical and chemical properties of asphalt. Due to recent high resolution satellites and airborne devices, latest researches have started to adopt remote sensing techniques which may provide a synoptic analysis of road condition. Image classification, or simple spectral analysis, can provide wide information for monitoring urban processes. In most of researches the attention is focus on the bitumen removal as a parameter that contribute on asphalt differentiation. The EAI was retrieved by remote sensed images using spectral indices in order to extend the aggregate exposition to the atmosphere to large areas and, consequently, the superficial removal of bitumen. For this study the Multispectral Infrared Visible Imaging Spectrometer (MIVIS), an high spatial and spectral airborne sensor, was adopted to perform the research in the study area inner the Campania District (Italy). EAI maps were performed and then validated using field data. The existing correlation between EAI and spectral indices from field data show its potential to define the aggregate exposition to the atmosphere by the use of remote sensed images. This kind of analysis can effectively support transport pollution studies by quantifying the aggregate-tire/atmosphere able to produce particle emissions or, viceversa, to absorb metal ions.