2 research outputs found
Excitation–Emission Spectra and Fluorescence Quantum Yields for Fresh and Aged Biogenic Secondary Organic Aerosols
Certain biogenic secondary organic
aerosols (SOA) become absorbent
and fluorescent when exposed to reduced nitrogen compounds such as
ammonia, amines, and their salts. Fluorescent SOA may potentially
be mistaken for biological particles by detection methods relying
on fluorescence. This work quantifies the spectral distribution and
effective quantum yields of fluorescence of water-soluble SOA generated
from two monoterpenes, limonene and α-pinene, and two different
oxidants, ozone (O<sub>3</sub>) and hydroxyl radical (OH). The SOA
was generated in a smog chamber, collected on substrates, and aged
by exposure to ∼100 ppb ammonia in air saturated with water
vapor. Absorption and excitation–emission matrix (EEM) spectra
of aqueous extracts of aged and control SOA samples were measured,
and the effective absorption coefficients and fluorescence quantum
yields (∼0.005 for 349 nm excitation) were determined from
the data. The strongest fluorescence for the limonene-derived SOA
was observed for λ<sub>excitation</sub> = 420 ± 50 nm and
λ<sub>emission</sub> = 475 ± 38 nm. The window of the strongest
fluorescence shifted to λ<sub>excitation</sub> = 320 ±
25 nm and λ<sub>emission</sub> = 425 ± 38 nm for the α-pinene-derived
SOA. Both regions overlap with the EEM spectra of some of the fluorophores
found in primary biological aerosols. Despite the low quantum yield,
the aged SOA particles may have sufficient fluorescence intensities
to interfere with the fluorescence detection of common bioaerosols
Molecular Selectivity of Brown Carbon Chromophores
Complementary
methods of high-resolution mass spectrometry and microspectroscopy
were utilized for molecular analysis of secondary organic aerosol
(SOA) generated from ozonolysis of two structural monoterpene isomers: d-limonene SOA (LSOA) and α-pinene SOA (PSOA). The LSOA
compounds readily formed adducts with Na<sup>+</sup> under electrospray
ionization conditions, with only a small fraction of compounds detected
in the protonated form. In contrast, a significant fraction of PSOA
compounds appeared in the protonated form because of their increased
molecular rigidity. Laboratory simulated aging of LSOA and PSOA, through
conversion of carbonyls into imines mediated by NH<sub>3</sub> vapors
in humid air, resulted in selective browning of the LSOA sample, while
the PSOA sample remained white. Comparative analysis of the reaction
products in the aged LSOA and PSOA samples provided insights into
chemistry relevant to formation of brown carbon chromophores. A significant
fraction of carbonyl-imine conversion products with identical molecular
formulas was detected in both samples. This reflects the high level
of similarity in the molecular composition of these two closely related
SOA materials. Several highly conjugated products were detected exclusively
in the brown LSOA sample and were identified as potential chromophores
responsible for the observed color change. The majority of the unique
products in the aged LSOA sample with the highest number of double
bonds contain two nitrogen atoms. We conclude that chromophores characteristic
of the carbonyl-imine chemistry in LSOA are highly conjugated oligomers
of secondary imines (Schiff bases) present at relatively low concentrations.
Formation of this type of conjugated compounds in PSOA is hindered
by the structural rigidity of the α-pinene oxidation products.
Our results suggest that the overall light-absorbing properties of
SOA may be determined by trace amounts of strong brown carbon chromophores