Evaporation–Condensation Effects on Resonant Photoacoustics of Volatile Aerosols

Abstract

In determining the optical properties of the atmosphere, the measurement of light absorption by aerosols is particularly challenging, and yet it is important because of the influence of strongly absorbing black carbon on climate and atmospheric visibility. The photoacoustic method obtains aerosol light absorption in situ, without use of filters, by acoustic measurement of the heat generated from aerosol light absorption, and its transfer to the surrounding air. However, in the general case, volatile aerosols heated by light absorption may also cool by evaporation (mass transfer). In this paper, the limiting case of the photoacoustic response of a volatile aerosol is compared with that of a dry aerosol to further the understanding of the data obtained with photoacoustic instruments. While the theory of photoacoustics of volatile aerosols for low-frequency, nonresonant cells has already been developed, current methods employ high-frequency, acoustically resonant photoacoustic instruments for quantifying atmospheric aerosol light absorption and vehicle exhaust mass concentration associated with black carbon. In this paper, a complete theory of photoacoustics for volatile aerosols is developed that includes additional terms to allow for higher-frequency devices, large particles, and high particle densities. Numerical calculations are used to determine the limits of various approximations.