Thermodynamic and noise considerations for the detection of microscopic   particles in a gas by Photoacoustic Raman spectroscopy
  We develop a simple thermodynamic model to describe the heat transfer mechanisms and generation of acoustic waves in photoacoustic Raman spectroscopy by small particulate suspensions in a gas. Using Langevin methods to describe the thermal noise we study the signal and noise properties, and from the noise equivalent power we determine the minimum number density of the suspended particles that can be detected. We find that for some relevant cases, as few as 100 particles per cubic meter can be detected. 
Kunal K. Das, Yuri V. Rostovtsev, Kevin Lehmann, and Marlan O. Scully