Nowadays more and more urban residents are aware of the importance of the air quality to their health, especially who are living in the large cities that are seriously threatened by air pollution. Meanwhile, being limited by the spare sense nodes, the air quality information is very coarse in resolution, which brings urgent demands for high-resolution air quality data acquisition. In this paper, we refer the real-time and fine-gained air quality data in city-scale by employing the crowdsource automobiles as well as their built-in sensors, which significantly improves the sensing system's feasibility and practicability. The main idea of this paper is motivated by that the air component concentration within a vehicle is very similar to that of its nearby environment when the vehicle's windows are open, given the fact that the air will exchange between the inside and outside of the vehicle though the opening window. Therefore, this paper first develops an intelligent algorithm to detect vehicular air exchange state, then extracts the concentration of pollutant in the condition that the concentration trend is convergent after opening the windows, finally, the sensed convergent value is denoted as the equivalent air quality level of the surrounding environment. Based on our Internet of Things cloud platform, real-time air quality data streams from all over the city are collected and analyzed in our data center, and then a fine-gained city level air quality map can be exhibited elaborately. In order to demonstrate the effective- ness of the proposed method, experiments crowdsourcing 500 floating vehicles are conducted in Beijing city for three months to ubiquitously sample the air quality data. Evaluations of the algorithm's performance in comparison with the ground truth indicate the proposed system is practical for collecting air quality data in urban environments.