## 1 Introduction  
Monitoring studies of atmospheric pollution have become more important in the recent years, from local to planetary scale, because of the changes in human activities and their effects on climate and public health. The progressive degradation of nature, because of pollution, has as a consequence that people are demanding a less aggressive way of life for the environment, claiming clean industries, ecological produces, etc. Moreover, citizens demand to their governing class different measures to benefit the environment where they live, with the aim of favoring a better life quality. The main air pollutants related to adverse effects are carbon monoxide (CO), nitrogen oxides (NO\({}_{\mathrm{x}}\), consisting of NO and NO\({}_{2}\)), ozone (O\({}_{3}\)), sulfur dioxide (SO\({}_{2}\)), volatile organic compounds (VOCs, including, for example, benzene and toluene), and particulate matter (usually considering PM\({}_{10}\), i.e., particulates with diameters lower than 10 \(\upmu\)m). CO and NO are generated during the incomplete combustion of carbon-containing fuels, and NO\({}_{2}\) during more complete combustion conditions, mainly from traffic emissions, industrial activities and fuel-fed heating. Some VOCs are released as a result of human activity; for example, from evaporation of liquid fuels, vehicle exhaust, and from solvents in paints, which are known as anthropogenic sources. Moreover, significant amounts of VOCs are also emitted from vegetation, including urban forest and landscapes, agricultural crops, and natural communities. The VOCs emitted by plants are known as biogenic sources. Consequently, the total VOCs volume in the atmosphere is the sum of anthropogenic and biogenic VOCs. O\({}_{3}\) is a secondary pollutant produced by photochemical reactions of primary pollutants, NO\({}_{\mathrm{x}}\) and VOCs, collectively called ozone precursors, enhanced by favorable meteorological conditions (high temperatures and strong solar radiation). The production and changes in the concentration of O\({}_{3}\) are critically dependent on the levels of precursors. The O\({}_{3}\) cycle starts with the emissions of NOx into the atmosphere from a variety of biogenic and anthropogenic sources. The NO is rapidly converted in the atmosphere by the existing O\({}_{3}\) into NO\({}_{2}\) and O\({}_{2}\). In the presence of sunlight, the NO\({}_{2}\) is photo-dissociated back into NO and atomic oxygen (O). Finally, the O recombines with O\({}_{2}\) to form O\({}_{3}\). There are other species in the atmosphere that convert NO to NO\({}_{2}\) without destroying ozone. These are primarily VOCs, also called reactive hydrocarbons. The main atmospheric reactions of this class of compounds are complex and involve hydroxyl radicals and organic radicals. Particulate pollutants have various natural and anthropogenic sources, and mainly originate from mineral dust and fuel combustion in motor vehicles. Since coal and petroleum often contain sulfur compounds, their combustion generates SO\({}_{2}\), which is also a precursor to particulates in the atmosphere and acid rain. The many environmental and health effects of these air pollutants has been described by the World Health Organization in its air quality guideline [1]. It is known that atmospheric pollution is a very complex variable, which is affected by different chemical and physical individual pollutants. Researchers are making efforts to better analyze the pollutants and their relationships, to identify their sources, and to characterize their temporal and spatial patterns. The main objective of these works is to drag out useful results for the design of remediation strategies and reduction of emissions. Different multivariate approaches have been used to study the atmospheric pollution. One of the most popular methodology has been the principal component analysis (PCA), which has been utilized, for instance, to establish seasonal variations, dependence with meteorological conditions, or source apportionment studies [2; 3; 4].