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Broadband optical cavity spectrometers are maturing as a technology fortrace-gas detection, but only recently have they been used to retrieve theextinction coefficient of aerosols. Sensitive broadband extinctionmeasurements allow explicit separation of gas and particle phase spectralcontributions, as well as continuous spectral measurements of aerosolextinction in favourable cases. In this work, we report an intercomparisonstudy of the aerosol extinction coefficients measured by three suchinstruments: a broadband cavity ring-down spectrometer (BBCRDS), acavity-enhanced differential optical absorption spectrometer (CE-DOAS), andan incoherent broadband cavity-enhanced absorption spectrometer (IBBCEAS).Experiments were carried out in the SAPHIR atmospheric simulation chamber aspart of the NO3Comp campaign to compare the measurement capabilities ofNO3 and N2O5 instrumentation. Aerosol extinction coefficientsbetween 655 and 690 nm are reported for secondary organic aerosols (SOA)formed by the NO3 oxidation of β-pinene under dry and humidconditions. Despite different measurement approaches and spectral analysisprocedures, the three instruments retrieved aerosol extinction coefficientsthat were in close agreement. The refractive index of SOA formed from theβ-pinene + NO3 reaction was 1.61, and was not measurablyaffected by the chamber humidity or by aging of the aerosol over severalhours. This refractive index is significantly larger than SOA refractiveindices observed in other studies of OH and ozone-initiated terpeneoxidations, and may be caused by the large proportion of organic nitrates inthe particle phase. In an experiment involving ammonium sulfate particles,the aerosol extinction coefficients as measured by IBBCEAS were found to bein reasonable agreement with those calculated using the Mie theory. The resultsof the study demonstrate the potential of broadband cavity spectrometers fordetermining the optical properties of aerosols

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers