Three-wavelength cavity-enhanced albedometer for measuring wavelength-dependent optical properties and single-scattering albedo of aerosols.

Xuezhe Xu,Shuo Wang,Weixiong Zhao,Bo Fang,Dean S Venables,Jiacheng Zhou,Weijun Zhang,Weidong Chen

doi:10.1364/oe.26.033484

Abstract

The spectral dependence of aerosol light absorption (αabs) and single-scattering albedo-[ω, defined as the ratio of the scattering (αscat) and extinction coefficients (αext = αabs + αscat)]-has proven effective in classifying dominant aerosol types. It is also helpful in understanding aerosol sources, transformation, climate and environmental effects, testing aerosol models, and improving the retrieval accuracy of satellite and remote sensing data. Despite the significant progress that has been made with measurement of light absorption and ω, many of the reported instruments either operate at a fixed wavelength or can only measure a single optical parameter. Quantitative multi-parameter wavelength-dependent measurement remains a challenge. In this work, a three-wavelength cavity-enhanced albedometer was developed. The albedometer can measure multiple optical parameters, αext, αscat, αabs, and ω, at λ = 365, 532, and 660 nm, in real time. The instrument's performance was evaluated using four different type laboratory generated aerosols, including polystyrene latex spheres (PSL, non-absorbing); ammonium sulfate (AS, non-absorbing); suwannee river fulvic acid (SRFA, slightly absorbing; a proxy for light absorbing organic aerosol); and nigrosin (strongly absorbing).

Highlights

Aerosol light absorption and single-scattering albedo [SSA, ω, defined as the ratio of the scattering coefficient to the extinction coefficient] are key inputs in direct radiative forcing (DRF) calculations [1]
We describe a three-wavelength CES-albedometer allowing αext, αscat, αabs, and ω to be measured in situ and in real time
Continuous time series measurement of αext, αscat, and αabs at λ = 365, 532, and 660 nm of a particle-free zero air sample are shown in the upper panel of Fig. 3

Summary

Introduction

The instrument integrated photoacoustic and nephelometric spectroscopy, allowing measurements of scattering and absorption coefficients at five wavelengths (centered at 417, 475, 542, 607, and 675 nm) by rotating an optical filter wheel. CRDS [23], BBCES [24,25], and cavity attenuated phase shift spectroscopy (CAPS) [26] have been combined with integrating spheres (IS) for aerosol albedo measurements These instruments allow simultaneous in situ measurements of the aerosol scattering and extinction coefficients (and absorption and ω) in an exact same sample volume. Despite significant progress in developing in situ methods, many of the reported instruments measure only a single optical parameter (absorption coefficient or extinction coefficient) or operate at a fixed wavelength. The performance of the new instrument for measuring reference laboratory generated aerosols is presented and discussed

Experimental section

Results and discussion

Wavelength dependent measurement of laboratory-generated aerosols

Ammonium sulfate

Nigrosin

Conclusion