Abstract
Abstract. Broadband optical cavity spectrometers are maturing as a technology for trace-gas detection, but only recently have they been used to retrieve the extinction coefficient of aerosols. Sensitive broadband extinction measurements allow explicit separation of gas and particle phase spectral contributions, as well as continuous spectral measurements of aerosol extinction in favourable cases. In this work, we report an intercomparison study of the aerosol extinction coefficients measured by three such instruments: a broadband cavity ring-down spectrometer (BBCRDS), a cavity-enhanced differential optical absorption spectrometer (CE-DOAS), and an incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS). Experiments were carried out in the SAPHIR atmospheric simulation chamber as part of the NO3Comp campaign to compare the measurement capabilities of NO3 and N2O5 instrumentation. Aerosol extinction coefficients between 655 and 690 nm are reported for secondary organic aerosols (SOA) formed by the NO3 oxidation of β-pinene under dry and humid conditions. Despite different measurement approaches and spectral analysis procedures, the three instruments retrieved aerosol extinction coefficients that were in close agreement. The refractive index of SOA formed from the β-pinene + NO3 reaction was 1.61, and was not measurably affected by the chamber humidity or by aging of the aerosol over several hours. This refractive index is significantly larger than SOA refractive indices observed in other studies of OH and ozone-initiated terpene oxidations, and may be caused by the large proportion of organic nitrates in the particle phase. In an experiment involving ammonium sulfate particles, the aerosol extinction coefficients as measured by IBBCEAS were found to be in reasonable agreement with those calculated using the Mie theory. The results of the study demonstrate the potential of broadband cavity spectrometers for determining the optical properties of aerosols.
Highlights
The contribution of suspended atmospheric particles to radiative forcing is both large and poorly constrained (Forster et al, 2007; Menon, 2004)
3 Results We first compare the aerosol extinction coefficient of ammonium sulfate particles, measured by the incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS) setup, against that calculated from the Mie theory
The aerosol extinction coefficients retrieved by all three instruments are compared in two experiments in which secondary organic aerosols (SOA) was formed following the oxidation of β-pinene by NO3
Summary
The contribution of suspended atmospheric particles to radiative forcing is both large and poorly constrained (Forster et al, 2007; Menon, 2004). Current approaches for studying the optical properties of particles are mainly limited in two respects: firstly, significant experimental artefacts and lack of agreement are associated with some aerosol absorption measurements (Kirchstetter et al, 2004; Andreae and Gelencser, 2006). Miles et al (2010) have recently demonstrated a CRDS system for measuring aerosol extinction spectra, typical CRDS and CAPS system are limited to one or two wavelengths; care must be taken to account for gas absorption when quantifying the aerosol extinction. Broadband optical cavity methods do not attain the sensitivity of typical CRDS and other laser-based cavity approaches, but crucially their spectral information allows absorbing gases to be quantified and their absorption contribution to the total sample extinction removed, thereby permitting the underlying aerosol extinction to be quantified. We report for the first time the refractive index of the secondary organic aerosol (SOA) produced by NO3 oxidation of β-pinene
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