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

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. INTRODUCTION The absorption of visible light by aerosol particles has been identified as a significant component of light extinction in many locations, and hence is an issue of importance in understanding visibility degradation. Because the same input wavelength spectrum is involved, light absorption by aerosols is also an issue in quantifying the important terms on the input side of the earth’s radiation balance. The total extinction of light passing through the atmosphere is usually regarded as the sum of scattering and absorption, both by particles and by gases. For each component of extinction, a simple Beer’s Law expression often applies: I/IO = exp (−ρσl) where IO and I are the incident and emerging light irradiance, and l is the geometric pathlength. The mass concentration of the absorbing or scattering species is given by ρ, and σ is the specific absorption or scattering coefficient, usually expressed in m2/g. It is usually possible to compute a total extinction σ as the sum of the individual values from each scattering or absorbing species. The product ρσ is often labeled as α, an absorption, scattering, or extinction coefficient with dimensions of inverse length. The significance of a given species can be evaluated if the corresponding value of α attributed to it is known and compared to that of other species, or to the total extinction coefficient. Black carbon is the dominant visible light-absorbing particulate species in the troposphere and usually results from anthropogenic combustion sources. It is usually found in the nucleation or accumulation mode H. Moosmüller, W.P. Arnott, and C.F. Rogers Desert Research Institute, University of Nevada System, Reno, Nevada IMPLICATIONS Light absorption by aerosols is an important component of atmospheric light extinction, which influences both radiative energy transfer and visibility in the atmosphere. This paper reviews the development of optical methods capable of both real-time and in situ measurement of aerosol light absorption. Such instrumentation has the potential for both calibration and replacement of conventional filter-based measurements.