Fundamental investigation of photoacoustic signal generation from single aerosol particles at varying relative humidity.

Photoacoustic (PA) spectroscopy enjoys widespread applications across atmospheric sciences. However, experimental biases and limitations originating from environmental conditions and particle size distributions are not fully understood. Here, we combine single-particle photoacoustics with modulated Mie scattering to unravel the fundamental physical processes occurring during PA measurements on aerosols. We perform measurements on optically trapped droplets of varying sizes at different relative humidity. Our recently developed technique - photothermal single-particle spectroscopy (PSPS) - enables fundamental investigations of the interplay between the heat flux and mass flux from single aerosol particles. We find that the PA phase is more sensitive to water uptake by aerosol particles than the PA amplitude. We present results from a model of the PA phase, which sheds further light onto the dependence of the PA phase on the mass flux phenomena. The presented work provides fundamental insights into photoacoustic signal generation of aerosol particles.