## 1 Introduction  
Smart cities have become a central element when addressing several problems in urban environments, such as mobility, public security, energy efficiency and emergency response [1; 2]. Actually, the growth of population living in urban environments has brought many challenges for the governments, demanding new policies and strategies to improve the urban quality of life. Smart cities initiatives have then been developed to cope with the increasing concern about efficient resource management, exploiting evolving communication and sensing technologies [3; 4]. Currently, Smart Cities are based on the concept of \"system of systems\", with different embedded and intelligent devices cooperating to provide valuable information to support a better life in modern cities, operating over Internet-based or application-specific networks. This complex scenario allows the execution of innovative applications, which will strongly support the transformations of the way of living in cities in this century [5]. A sensor is an electronic device endowed with sensing components to measure simple ambient conditions, related to the environment surrounding the sensor. Scalar information such as temperature, humidity, pressure and luminosity can be sensed and processed to reveal some properties about objects,people or events of interest, which can then be exploited by intelligent systems when taking some actions [6]. Moreover, multimedia data can also be sensed when low-power cameras and microphones are embedded in sensor units. The interconnection of sensors in the form of Wireless Sensor Networks (WSN) allows the creation of distributed infrastructures for monitoring in Smart Cities and retrieving information about the behavior of people, the occurrence of events of interest and general conditions about the environment for an uncountable number of applications. In a Smart City scenario, sensors may be neatly placed to cover well-defined areas, such as roads, public squares and metro stations. In such cases, although energy consumption may not be a concern, since sensors may be supplied by the city's electrical system, the multi-system dynamic nature of Smart City applications may bring specific challenges that have demanded innovative solutions. Among those challenges, hundreds or thousands of sensors may be continuously transmitting information, which may require considerable bandwidth, especially when streaming multimedia content. Furthermore, real-time delivery may also be required for fast response to external systems requests, which may also demand different levels of reliability guarantees for transmitted data [7; 8]. Thus, optimizing transmissions of sensed data in Smart City scenarios is of paramount importance and it is the core motivation of this work. Modern cities will experience plenty of events during a day, continuously and in an unexpected way, which may have different relevance on the people's routine and their safety. We can expect that such events may be central in Smart Cities systems. In a superficial analysis, most cities will be concerned with at least traffic conditions, accidents and public security related events, but many more events may also be relevant (e.g., protests, festivals, rainfall level). In such way, wireless sensor networks can exploit the event-centric nature of modern cities to optimize their operation and achieve higher performance. Actually, one way to perform optimizations is assigning priorities to sensors nodes, assuring better resources to more important information [9]. Nevertheless, the identification and classification of events of interests is not straightforward and different approaches may be adopted. In the context of wireless sensor networks, efficient event detection in multi-systems Smart Cities is still a challenging task that has demanded innovative solutions. Social networks sites, or simply Social Networks or Social Media, have become very popular in recent years due to the increasing proliferation and affordability of Internet enabled devices [10].