# A Crowdsensing Platform for Monitoring of Vehicular Emissions: A Smart City Perspective
## 2 Related Works  
Generally speaking, the monitoring of urban pollution from emissions of vehicular pollutants is a complex task. It involves challenges such as capturing vehicle emissions, data transmission through the communication infrastructure of the cities, mass data storage, and the generation and visualization of the processed information. This section presents some works that have addressed control and monitoring of vehicular emissions, as well as applications with virtual sensors, since they contributed to the development of this article. Regarding emissions control, several measures are being considered in the search for solutions that reduce greenhouse gas emissions. In particular, we can highlight the agreement signed by the United Nations (UN) at the Conference of Parties (COP-21), named the Paris Agreement, where the 195 member countries ratified the need to maintain global warming at sustainable indexes for the planet [35; 36]. It is noteworthy that, due to the predominance of motor vehicles in the transport sector, in association with the limited use of emission control technologies, automobiles have become one of the major sources of urban pollution and consequently one of the leading contributors to the greenhouse effect [37; 38]. Concerns about vehicular pollution are not recent, and they can be tracked back to the mid-1960s in the United States. At that time, the California Air Resources Management Committee (CARB) created legislation to reduce pollution by motor vehicles. The first results were the implantation of the Positive Crankcase Ventilation (PCV) system, which, in addition to avoiding the waste of fuel, reduced the emission of hydrocarbons in the air [39]. In the 1970s and 1980s, there were also significant improvements such as the insertion of electronic control in the injection and ignition systems and the exhaust gas re-circulation system. These solutions significantly reduced the emission of nitrogen oxides (NOs). These gases are harmful to human health, being responsible for problems in the respiratory system. The advent of automotive electronic systems occurred during the same period in which CARB and EPA issued resolutions to control emissions. These events led to the creation of a system capable of self-diagnosis and alerting the driver to possible defects in electronic components. This system was named On-Board Diagnostic (OBD) and had its first version launched in 1988, when it became mandatory for all new vehicles that would circulate in the state of California from that year forward [40]. Since 1996, all cars manufactured and sold in the United States are required to have the OBD-II system. The European Union adopted a similar measure in 2003, while Brazil, Russia, and China followed this trend in 2010 [41]. Recently, several papers in the literature have explored the potential of the OBD system to extract data from vehicles and generate useful information that can be used to monitor pollution in smart cities. In [42], the authors conducted a study on the influence of distance traveled and average vehicle speed on fuel consumption. From machine learning techniques, it was possible to predict the fuel consumption for certain stretches to be traveled. The same authors broadened the idea, creating the concept of eco-driving [43]. From the prediction of the fuel consumption and consequently the emission of CO\({}_{2}\) (estimated from linear approximations of the fuel consumption), routes between two points are drawn, considering, besides the distance, the pollutants emitted by the vehicles. In both solutions, data collected via OBD-II are stored locally in the vehicle through a Secure Digital (SD) card and processed offline by an application. Self-diagnostic systems were created with the goal of more effective control of toxic gas emissions and for the detection of electrical, chemical, or mechanical problems in vehicles [40]. Over time, the exploitation of these systems has become more accessible to ordinary users.