Wavelength Dependence of Refractive Index of Secondary Organic Aerosols Generated during the Ozonolysis and Photooxidation of α-Pinene

Atmospheric aerosol particles play an important role in climate change by scattering and absorbing solar radiation. To quantify the direct influence of aerosols on the atmospheric radiation balance, a detailed understanding of the complex refractive index (RI) for a variety of aerosols is required. The wavelength dependence of the RI values for secondary organic aerosols (SOAs) generated during the ozonolysis and photooxidation of alpha-pinene, which is a major biogenic volatile organic compound (BVOC), was investigated using a photoacoustic soot spectrometer (PASS) and a cavity ring-down spectrometer (CRDS). The real part of the RI values for SOAs generated from both ozonolysis and photooxidation was found to lie between 1.4 and 1.5 at 405, 532, and 781 nm, and to slightly increase with decreasing wavelength. Additionally, the imaginary part of the RI values for these SOAs was found to be negligible (< 0.003) at all of the wavelengths studied (405, 532, and 781 nm). The obtained complex RI values were also compared with values reported in the literature and the chemical properties of the SOAs, which were measured using a high resolution time-of-flight aerosol mass spectrometer (H-ToF-AMS).