PhotoAir is a unique and powerful App that can measure PM2.5 anywhere, anytime simply with a mobile phone taking a photo. The complex features embedded in the photo are extracted using advanced computer vision and artificial intelligence algorithms, based on which PM2.5 concentrations are eventually derived. This App accounts for different illumination attenuations caused by particulate matter and can make measurements during day or night regardless of weather, as long as a photo can be taken with a light source available. It is portable, easy to use, efficient, and suitable for any mobile phone user who has a need for monitoring his/her ambient air environment.