Effective broadband refractive index retrieval by a white light optical particle counter

Abstract

A new approach to retrieve the effective broadband refractive indices (n_broad,eff) of aerosol particles by a white light aerosol spectrometer (WELAS) optical particle counter (OPC) is presented. Using a tandem differential mobility analyzer (DMA)-OPC system, the n_broad,eff are obtained for both laboratory and field applications. This method was tested in the laboratory using substances with a wide range of optical properties. With the obtained n_broad,eff, WELAS aerosol size distributions can be corrected. Therefore, this method can be used for instrument calibration in both laboratory and field measurements. The retrieved effective broadband refractive indices for the scattering aerosols were: ammonium sulfate ((NH₄)₂SO₄, AS) n_broad,eff = 1.52(±0.01) + i0.0, glutaric acid (HOOC(CH₂)₃COOH, GA) n_broad,eff = 1.45(±0.01) + i0.0, and sodium chloride (NaCl) n_broad,eff = 1.49(±0.02) + i0.0, all within 4% of literature values. A lightly absorbing substance, Suwannee river fulvic acid (SRFA), was also measured, and its retrieved n_broad,eff is like that of a pure scatterer, with a value of n_broad,eff = 1.53(±0.01) + i0.0. The retrieved real part of n_broad,eff is in accordance with literature values and the imaginary part with the behavior of the white light spectrum of the WELAS, which is not sensitive below a wavelength of 380 nm, where SRFA mainly absorbs. For absorbing substances, nigrosine and various mixtures of nigrosine with AS and GA were measured. For nigrosine, n_broad,eff = 1.64(±0.04) + i0.20(±0.03) was retrieved, in very good agreement with values found in the literature. The n_broad,eff retrieved by this method for the mixtures was in accordance with the complex refractive index expected. The n_broad,eff retrieved by this method would be similar to the values obtained using the solar spectrum.