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Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
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Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
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Journal Article
Measuring light absorption by freshly emitted organic aerosols: optical artifacts in traditional solvent-extraction-based methods
2019
Overview
Recent studies have shown that organic aerosol (OA) could have a nontrivial
role in atmospheric light absorption at shorter visible wavelengths. Good
estimates of OA light absorption are therefore necessary to better estimate
radiative forcing due to these aerosols in climate models. One of the common
techniques used to measure OA light absorption is the solvent extraction
technique from filter samples which involves the use of a spectrophotometer
to measure bulk absorbance by the solvent-soluble organic fraction of
particulate matter. Measured solvent-phase absorbance is subsequently
converted to particle-phase absorption coefficient using scaling factors.
The conventional view is to apply a correction factor of 2 to absorption
coefficients obtained from solvent-extracted OA based on Mie calculations.
The appropriate scaling factors are a function of biases due to incomplete
extraction of organic carbon (OC) by solvents and size-dependent absorption properties of OA.
The range for these biases along with their potential dependence on burn
conditions is an unexplored area of research. Here, we performed a comprehensive laboratory study involving three solvents
(water, methanol, and acetone) to investigate the bias in absorption
coefficients obtained from solvent-extraction-based photometry techniques as
compared to in situ particle-phase absorption for freshly emitted OA from
biomass burning. We correlated the bias with OC∕TC (total carbon) mass ratio and single
scattering albedo (SSA) and observed that the conventionally used correction
factor of 2 for water and methanol-extracted OA might not be extensible to
all systems, and we suggest caution while using such correction factors to
estimate particle-phase OA absorption coefficients. Furthermore, a linear
correlation between SSA and the OC∕TC ratio was also established. Finally, from
the spectroscopic data, we analyzed the differences in absorption
Ångström exponents (AÅE) obtained from solution- and
particulate-phase measurements. We noted that AÅE from
solvent-phase measurements could deviate significantly from their OA
counterparts.
