2 research outputs found
Molecular Chemistry of Atmospheric Brown Carbon Inferred from a Nationwide Biomass Burning Event
Lag Ba’Omer, a nationwide
bonfire festival in Israel, was
chosen as a case study to investigate the influence of a major biomass
burning event on the light absorption properties of atmospheric brown
carbon (BrC). The chemical composition and optical properties of BrC
chromophores were investigated using a high performance liquid chromatography
(HPLC) platform coupled to photo diode array (PDA) and high resolution
mass spectrometry (HRMS) detectors. Substantial increase of BrC light
absorption coefficient was observed during the night-long biomass
burning event. Most chromophores observed during the event were attributed
to nitroaromatic compounds (NAC), comprising 28 elemental formulas
of at least 63 structural isomers. The NAC, in combination, accounted
for 50–80% of the total visible light absorption (>400 nm)
by solvent extractable BrC. The results highlight that NAC, in particular
nitrophenols, are important light absorption contributors of biomass
burning organic aerosol (BBOA), suggesting that night time chemistry
of •NO<sub>3</sub> and N<sub>2</sub>O<sub>5</sub> with particles
may play a significant role in atmospheric transformations of BrC.
Nitrophenols and related compounds were especially important chromophores
of BBOA. The absorption spectra of the BrC chromophores are influenced
by the extraction solvent and solution pH, implying that the aerosol
acidity is an important factor controlling the light absorption properties
of BrC
Evolution of the Complex Refractive Index of Secondary Organic Aerosols during Atmospheric Aging
The wavelength-dependence of the
complex refractive indices (RI)
in the visible spectral range of secondary organic aerosols (SOA)
are rarely studied, and the evolution of the RI with atmospheric aging
is largely unknown. In this study, we applied a novel white light-broadband
cavity enhanced spectroscopy to measure the changes in the RI (400–650
nm) of β-pinene and <i>p</i>-xylene SOA produced and
aged in an oxidation flow reactor, simulating daytime aging under
NO<sub><i>x</i></sub>-free conditions. It was found that
these SOA are not absorbing in the visible range, and that the real
part of the RI, <i>n</i>, shows a slight spectral dependence
in the visible range. With increased OH exposure, <i>n</i> first increased and then decreased, possibly due to an increase
in aerosol density and chemical mean polarizability for SOA produced
at low OH exposures, and a decrease in chemical mean polarizability
for SOA produced at high OH exposures, respectively. A simple radiative
forcing calculation suggests that atmospheric aging can introduce
more than 40% uncertainty due to the changes in the RI for aged SOA