Quartz-Enhanced Photoacoustic Detection for Aerosol Optical Characterization

Particulate matter emitted by anthropogenic and natural sources strongly affect the radiative budget of the Earth. Non-absorbing aerosols have a negative radiative forcing effect, acting to cool the planet and thereby masking the warming caused by greenhouse gases. Absorbing aerosols including black carbon, dust and brown carbon can provide positive radiative forcing at the top of the atmosphere depending on their optical properties. Due to its short atmospheric lifetime, black carbon can have a strong regional effect (e.g. in Himalaya and in the Arctic, where surface albedo is high). How much aerosols affect the Earth’s climate however remains highly uncertain. Providing accurate, widespread and unbiased measurements of aerosol optical properties is important for understanding how aerosols will affect the future climate system. However, in depth studies on aerosol optical properties, and in particular absorption, are still lacking. Photoacoustic spectrometry has been recently employed to measure aerosol absorption. The technique is more fundamental and unbiased then traditional filter-based techniques. This type of spectrometry exploits the photoacoustic effect, which is the production of an acoustic wave from the excitation of a particle absorbing a photon. Currently available commercial spectrometers are very useful for laboratory and field experiments, but due to their typical size, they are unpractical for studies employing small payload aircrafts (e.g. unmanned aircrafts) or balloons. A recent development in photoacoustic spectrometry reported by Kosterev et al. in 2002 is the use of a quartz tuning fork for the detection, termed Quartz-Enhanced Photoacoustic Spectrometry (QEPAS). Due to the high resonance frequency (~32 KHz) of the tuning fork, QEPAS has good potential for the miniaturization of a photoacoustic spectrometry system. The quartz tuning fork is piezoelectric, and a signal is generated only when the tines of the tuning fork move in opposite directions. This property of the tuning fork makes QEPAS less sensitive to background noise. The method has been successfully demonstrated on gaseous species, and is less sensitive to background noise than typical acoustic spectrometers. We present preliminary results for the application of QEPAS to the characterization of aerosol optical properties. Here we analyze first laboratory results from our research and propose possible improvements on our current design.