Broadband Light Extinction of Atmospheric Aerosols With a Dual – Cavity Enhanced Spectrometer

Abstract:

Aerosol extinction is a key input parameter for radiative forcing calculations. Often extinction coefficients are measured using a few single wavelength lasers limiting the number of wavelengths typically measured simultaneously by an instrument. We developed a spectrometer based on a dual cavity enhanced cell that uses a broadband supercontinuum laser source (SC-BDCEES). The SC-BDCEES simultaneously measures the extinction coefficient in the blue-green part of the visible spectrum from 420 to 540 nm. The dual cavity approach allows to correct for background drifts and spectral fluctuations of the laser. The SC-BDCEES extinction measurements favorably compared with Mie theory and nephelometer measurements for size-selected and poly-disperse non-absorbing aerosols such as polystyrene latex spheres, and sodium chloride. We then used the SC-BDCEES to measure the extinction coefficient of black carbon generated with a kerosene lamp. From the broadband extinction coefficient, we calculated a BC Ångström exponent of ~1.6 in agreement with literature values. The SC-BDCEES is particularly useful to provide detailed wavelength dependence of the extinction coefficient of brown carbon and dust aerosols. The instrument sensitivity is also sufficient for field deployment in moderately polluted conditions; for example, to measure the extinction coefficient of biomass burning particles.