How to measure air quality (especially PM2.5) from distance (scanning like a LiDAR)?
Backscatter (standoff) lidar for fine particulate matter concentrations is very hard, but doable.  There is a company in France that offers it (LEOSPHERE), but their website is down.  I found that through this paper.

There are also people that do this or attempt to do this for roadside emissions sensing.

Both of these systems use ND-YAG lasers that are frequency tripled into the UV.  NG-Yag lasers can be expensive (>$1000 to well over $100k), depending on power, rep rate and pulse size.  They also need sophisticated free-space optical setups for both the laser itself (including the frequency triple stage) and the backscatter return path.  On top of this they need sophisticated data analysis tools in order to get usable numbers out.  These are primarily research tools.