I'm collecting some notes from across the site, including the #air-sensors and #particle-sensing pages in one place, and also interested in the latest updates on prices for different particulate air sensors. Thanks!
Here is a technical sheet for a $12.25 particle sensor by Sharp that can be purchased from here. The Sharp product works by detecting IR radiation that is scattered off the particles. The sensor outputs a voltage that is reported to be proportional to the concentration of particulates in the air. I have not seen code for this, but it looks like it would be pretty straight forward to write.
Also note there are drivers for Arduino (see the comments!) + Raspberry Pi are available for the following devices:
Public Lab has one of each of these, and we're interested in loaning them out, but I don't know that we yet have a huge amount of information about how well they meet specific needs, or in-depth information about what kind of uses their data can be put towards. But please ask questions on those pages to help unpack these devices and what they can do for us!