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 α-Pinene

Abstract

<div><p>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 α-pinene and limonene with ozone and NO_x/sunlight at different HC/NO_x 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 α-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 α-pinene SOA for most parameters measured with the AMS, including H:C, O:C, f₄₃ (<i>m/z</i> 43/organic), f_C4H7⁺, and others. Refractive indices for α-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°C, respectively, with corresponding small changes of f₄₃ 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_rs observed as the experiments progress. However, the possibility that oligomerization reactions play a role cannot be ruled out.</p> </div