Air quality monitoring plays an increasingly important role in providing accurate air pollution data for assessing the impacts of air pollution on public health. Development of proper sensor networks, by deploying the right air pollution sensors at the right place, in order to meet the needs of different groups in the city and provide the much needed public services, deserves careful attention, especially when smart city development is being considered. However, air quality monitoring can be a costly measure. To tackle such a challenge, air pollution sensor placement can be carefully designed to achieve certain optimal citizen-centric objectives in the absence of field information, which can be formulated as an optimal sensor placement problem. In this paper, we propose three citizen-centric objectives for the optimal sensor placement problem, which does not require the prior deployment of pollution sensors for obtaining any field information. By citizen-centric, we mean that sensor placement puts the citizens' welfare at the center of attention and be able to fulfill the following objectives: 1) better assessing the vulnerable people's exposure to air pollution; 2) maximizing overall satisfaction of obtaining public information on existing air quality; and 3) better monitoring traffic emissions. We formulate the optimization problem for each scenario and propose an effective method to solve the problem accordingly. Last but not least, we conduct a case study in the city of Cambridge to evaluate the feasibility and effectiveness of our proposed methods. Our case study has shown that in order to optimize our citizen-centric objectives, there is a need to re-orient the current sensor placement strategies in the city of Cambridge, U.K.