Methods for Real-Time, In Situ Measurement of Aerosol Light Absorption

Abstract

Light extinction by aerosols is due to scattering and absorption. The anthropogenic contribution is generally dominated by light scattering by sulfate particles and light absorption by elemental carbon. While real-time, in situ instrumentation for the measurement of ambient light scattering exists and is widely used (i.e., nephelometers), no such instrumentation is currently in use for the sensitive measurement of ambient light absorption by aerosols. Instrumentation for this purpose has been developed in the past, mostly for the measurement of gaseous light absorption, but it has also been applied to the measurement of aerosol light absorption. This instrumentation is based on measuring the absorbed energy, as opposed to measuring light extinction, which is complicated by the scattering component and is also less sensitive. For aerosols, the absorbed energy heats the gas, leading to its thermal expansion. The two most sensitive techniques to detect this expansion are photoacoustic detection, in which the light source is modulated and the periodic expansion of the gas results in a sound wave at the modulation frequency, which may be detected with a microphone; and optical homodyne interferometry, in which the changed gas density is detected with a Mach-Zehnder type interferometer via the directly related change in refractive index. This article reviews the current state of both photoacoustic and interferometric detection methods. In addition, new ideas are discussed that are currently implemented by our group and should lead to substantial improvements. Size and reliability are being improved by utilizing modern, compact solid state lasers. New designs both for the photoacoustic cell and the interferometer promise to be less susceptible to acoustic background noise. In the case of the photoacoustic cell, the new design also virtually eliminates the previously dominant noise source, coherent window noise. Furthermore, an acoustic amplifier, based on the thermoacoustic effect, is being integrated into the photoacoustic cell to further improve its sensitivity.