The effect of quartz crystal resonance frequency increase upon light irradiation is studied under various conditions, including vacuum. This effect is most probably of a thermal origin and can be attributed to inhomogeneous heating. The magnitude of this effect depends linearly on the amount of energy absorbed by the quartz crystal electrodes. It allows for additional in situ characterization of nanoparticles (NPs)formed in a flame. This type of characterization is demonstrated by the distinction between different flame-synthesized NPs deposited on the quartz crystal surface via molecular beam sampling.