Dependence of Real Refractive Indices on O:C, H:C and Mass Fragments of Secondary Organic Aerosol Generated from Ozonolysis and Photooxidation of Limonene and alpha-Pinene

Abstract

The refractive index is a fundamental property controlling aerosol optical properties. Secondary organic aerosols have variable refractive indices, presumably reflecting variations in their chemical composition. Here, we investigate the real refractive indices (m(r)) and chemical composition of secondary organic aerosols (SOA) generated from the oxidation of alpha-pinene and limonene with ozone and NOx/sunlight at different HC/NOx ratios. Refractive indices were retrieved from polar nephelometer measurements using parallel and perpendicular polarized 532-nm light. Particle chemical composition was monitored with a high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS). For photochemically generated SOA, the values of refractive indices are consistent with prior results, and ranged from about 1.34 to 1.55 for limonene and from 1.44 to 1.47 for alpha-pinene, generally increasing as the particles grew. While AMS fragments are strongly correlated to the refractive index for each type of SOA, the relationships are in most cases quite different for different SOA types. Consistent with its wide range of refractive index, limonene SOA shows larger variations compared to alpha-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f(43) (m/z 43/organic), f(C4H7) (+), and others. Refractive indices for alpha-pinene ozonolysis SOA also fell in narrow ranges; 1.43-1.45 and 1.46-1.53 for particles generated at 19-22 and 23-29 degrees C, respectively, with corresponding small changes of f(43) and H:C ratio and other parameters. Overall, H:C ratio, m/z 43 and 55 (C2H3O+, C4H7 (+)) were the best correlated with refractive index for all aerosol types investigated. The relationships between m(r) and most fragments support the notion that increasing condensation of less oxygenated semivolatile species (with a possible role for a concomitant decrease in low refractive index water) is responsible for the increasing m(r)s observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.