Complex Refractive Index, Single Scattering Albedo, and Mass Absorption Coefficient of Secondary Organic Aerosols Generated from Oxidation of Biogenic and Anthropogenic Precursors
Creators: name: Dingle, Justin H.
name: Zimmerman, Stephen
name: Frie, Alexander L.
name: Min, Justin
name: Heejung Jung
name: Bahreini, Roya
Publisher: name: Taylor & Francis
Year: 2019
Subjects: subject: Biophysics
subject: Biochemistry
subject: Space Science
subject: 29999 Physical Sciences not elsewhere classified
subject: FOS: Physical sciences
subject: FOS: Physical sciences
subject: Molecular Biology
subject: Neuroscience
subject: Physiology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Ecology
subject: Cancer
subject: 110309 Infectious Diseases
subject: FOS: Health sciences
subject: FOS: Health sciences

description: Refractive index and optical properties of biogenic and anthropogenic secondary organic aerosol (SOA) particles were investigated. Aerosol precursors, namely longifolene, α-pinene, 1-methylnaphthalene, phenol, and toluene were oxidized in a Teflon chamber to produce SOA particles under different initial hydrocarbon concentrations and hydroxyl radical sources, reflecting exposures to different levels of nitrogen oxides (NO<sub>x</sub>). The real and imaginary components (<i>n</i> and <i>k,</i> respectively) of the refractive index at 375 nm and 632 nm were determined by Mie theory calculations through an iterative process, using the χ<sup>2</sup> function to evaluate the fitness of the predicted optical parameters with the measured scattering, absorption, and extinction coefficients from a Photoacoustic Extinctiometer and Cavity Attenuated Phase Shift Spectrometer. Single scattering albedo (SSA) and bulk mass absorption coefficient (MAC) at 375 nm were calculated. SSA values of SOA particles from biogenic precursors (longifolene and α-pinene) were ∼0.98-0.99 (∼6.3% uncertainty), reflecting purely scattering aerosols regardless of the NO<sub>x</sub> regime. However, SOA particles from aromatic precursors were more absorbing and displayed NO<sub>x</sub>-dependent SSA values. For 1-methylnaphthalene SOA particles, SSA values of 0.92-0.95 and ∼0.75-0.90 (∼6.1% uncertainty) were observed under intermediate- and high-NO<sub>x</sub> conditions, respectively, reflecting the absorbing effects of SOA particles and NO<sub>x</sub> chemistry for this aromatic system. In mixtures of longifolene and phenol or longifolene and toluene SOA under intermediate- and high-NO<sub>x</sub> conditions, <i>k</i> values of the aromatic-related component of the SOA mixture were higher than that of 1-methylnaphthalene SOA particles. With the increase in OH exposure, <i>k<sub>phenol</sub></i> decreased from 0.10 to 0.02 and 0.22 to 0.05 for intermediate- and high-NO<sub>x</sub> conditions, respectively. A simple relative radiative forcing calculation for urban environments at λ = 375 nm suggests the influence of absorbing SOA particles on relative radiative forcing at this wavelength is most significant for aerosol sizes greater than 0.4 µm.