Reactions involving the dicarbonyl
species methylglyoxal (MG) have
been suggested as an important pathway for the production of secondary
organic aerosol (SOA) in the atmosphere. Reaction in an aqueous inorganic
salt solution, such as ammonium sulfate (AS), leads to the formation
of light-absorbing brown carbon (BrC) product. We report on an investigation
of the optical properties of BrC aerosol generated from the aqueous-phase
reaction between MG and AS as a function of aging time using calibrated
cavity ring-down spectroscopy (CRDS) at a wavelength of 403 nm. The
retrieved real index of refraction at 403 nm is n = 1.558 ± 0.021, with an imaginary index value of k = 0.002 ± 0.004; these values do not appear to change significantly
with aging time over the course of 22 days and are similar to the
AS aerosol values. The small complex index suggests that BrC aerosol
formed from this pathway may not significantly impact radiative forcing.
Measurements of the aerosol optical properties show significant deviation
from Mie theory simulations for particles with diameters of ≳500
nm, probably as a result of nonspherical particle shape effects. In
addition to the CRDS study, we used UV–vis spectroscopy to
measure the mass absorption coefficient of the solution-phase reaction
products as a function of aging. We also employed atomic force microscopy
(AFM)-based IR spectroscopy to investigate the morphology and chemical
composition of single SOA particles. AFM analysis of the particle
morphology shows that a significant fraction of BrC particles with
diameters of ≳500 nm are nonspherical in shape, consistent
with our observed breakdown in the applicability of Mie theory for
larger particles.
Aerosols generated from aqueous samples of readily obtainable humic material standards are often used as proxies for organic particulates found in the atmosphere in various investigations, such as consideration of radiative forcing effects. Here, we present results for the retrieved complex index of refraction, m = n + ik, at a wavelength of 403 nm for aerosols prepared from six humic material standards using a calibrated cavity ring-down spectrometer: a humic acid sodium salt, Pahokee peat humic and fulvic acids, Elliott soil humic and fulvic acids, and Suwannee river fulvic acid. In addition, we have conducted UV−vis spectrometric studies to measure the mass absorption coefficients, molar absorptivities, and absorption Ångstrom exponents of bulk aqueous solutions of the humic materials. We find clear differences between the humic acid (HA) and fulvic acid (FA) samples with the HA having larger values for the imaginary part of the refractive index, k. The mean value for the HA samples is k = 0.170 while the mean is k = 0.037 for the FA materials. We have examined correlations between the retrieved refractive index and humic material characteristics obtained from spectroscopic and elemental analysis, including aromatic content and the oxygen-to-carbon atomic ratio, where the molar absorption coefficient yields the strongest correlation. Finally, we compare the humic material optical properties to those of authentic and laboratory generated organic carbon samples in order to assess the usefulness of these humic standards as proxies for light absorbing aerosol.
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