Air quality monitoring has attracted a lot of attention from governments, academia and industry, especially for PM2.5 due to its significant impact on our respiratory systems. In this work, we design Mosaic, a low cost urban PM2.5 monitoring system based on mobile sensing. In Mosaic, a small number of air quality monitoring nodes are deployed on city buses to measure air quality. We have deployed the Mosaic system in Hangzhou and Ningbo, China. We conduct research on system design and deployment issues as well as sensor data calibration. These works have been published in IEEE INFOCOM 2016, IEEE INFOCOM 2017, and UbiComp 2018.