1 research outputs found
Real Refractive Indices and Formation Yields of Secondary Organic Aerosol Generated from Photooxidation of Limonene and α-Pinene: The Effect of the HC/NO<sub><i>x</i></sub> Ratio
The refractive index is an important property affecting aerosol
optical properties, which in turn help determine the aerosol direct
effect and satellite retrieval results. Here, we investigate the real
refractive indices (<i>m</i><sub>r</sub>) of secondary organic
aerosols (SOA) generated from the photooxidation of limonene and α-pinene
with different HC/NO<sub><i>x</i></sub> ratios. Refractive
indices were obtained from polar nephelometer data using parallel
and perpendicular polarized 532 nm light combined with measured size
distributions, and retrievals were performed using a genetic algorithm
and Mie–Lorenz scattering theory. The absolute error associated
with the <i>m</i><sub>r</sub> retrieval is ±0.03, and
reliable retrievals are possible for mass concentrations above 5–20
μg/m<sup>3</sup> depending on particle size. The limonene SOA
data suggest the most important factor controlling the refractive
index is the HC/NO<sub><i>x</i></sub> ratio; the refractive
index is much less sensitive to the aerosol age or mass concentration.
The refractive index ranges from about 1.34 to 1.56 for limonene and
from 1.36 to 1.52 for α-pinene, and generally decreases as the
HC/NO<sub><i>x</i></sub> ratio increases. Especially for
limonene, the particle diameter is also inversely related to the HC/NO<sub><i>x</i></sub> ratio; the final size mode increases from
220 to 330 nm as the HC/NO<sub><i>x</i></sub> ratio decreases
from 33 to 6. In an effort to explore the ability of models from the
literature to explain the observed refractive indices, a recent limonene
oxidation mechanism was combined with SOA partitioning and a structure–property
relationship for estimating refractive indices of condensing species.
The resulting refractive indices fell in a much narrower range (1.475
± 0.02) of <i>m</i><sub>r</sub> than observed experimentally.
We hypothesize the experimentally observed high <i>m</i><sub>r</sub> values are due to oligomerization and the low values
to water uptake, small soluble molecules such as glyoxal and other
factors, each of which is not included in the oxidation mechanism.
Aerosol formation yields were measured over the mass concentration
range from 6 to ∼150 μg/m<sup>3</sup>, over which they
increased steadily, and were higher for high HC/NO<sub><i>x</i></sub> ratio experiments