Limitations of the Photoacoustic Technique for Aerosol Absorption Measurement at High Relative Humidity

Laboratory experiments were conducted to assess the suitability of photoacoustic spectroscopy (PAS) for aerosol absorption measurement at high relative humidity (RH). Initial experiments characterized the PAS sensitivity that increased strongly by approximate to 1.25 between dry conditions and 90% RH. Correction procedures were validated by measuring RH-independent absorption for hydrophobic absorbing particles. Absorption measured by PAS for a range of hygroscopic particles, including different morphologies, hygroscopicities, and absorbing entities, showed strong low biases at high RH (down to 0.4 at 90% RH). The bias was due to water evaporation impacting the PAS signal. Cooling samples to lower absolute humidity while maintaining a constant RH did not significantly reduce the bias magnitude within the temperature range 11-25 degrees C. The magnitude and RH dependence of the bias were not reproduced well using a model of PAS response incorporating coupled heat and mass transfer in the transition regime. This was attributed, in part, to uncertainty related to changes in the water mass accommodation coefficient with RH. Given our inability to correct for evaporation-induced bias effects accurately, or reduce their magnitude experimentally, we conclude that PAS is not a technique well suited to the measurement of absorption at high RH. In order to minimize RH-related errors in PAS measurements made under notionally dry conditions, we recommend operation in the RH range 10-30%.