Atmospheric brown carbon (BrC) is an important contributor to the
radiative forcing of climate by organic aerosols. Because of the molecular
diversity of BrC compounds and their dynamic transformations, it is
challenging to predictively understand BrC optical properties. OH
radical and O3 reactions, together with photolysis, lead
to diminished light absorption and lower warming effects of biomass
burning BrC. The effects of night-time aging on the optical properties
of BrC aerosols are less known. To address this knowledge gap, night-time
NO3 radical chemistry with tar aerosols from wood pyrolysis
was investigated in a flow reactor. This study shows that the optical
properties of BrC change because of transformations driven by reactions
with the NO3 radical that form new absorbing species and
lead to significant absorption enhancement over the ultraviolet–visible
(UV-vis) range. The overnight aging increases the mass absorption
coefficients of the BrC by a factor of 1.3–3.2 between 380
nm and 650 nm. Nitrated organic compounds, particularly nitroaromatics,
were identified as the main products that contribute to the enhanced
light absorption in the secondary BrC. Night-time aging of BrC aerosols
represents an important source of secondary BrC and can have a pronounced
effect on atmospheric chemistry and air pollution.
Bitumen is a highly
viscous and chemically complex petroleum-derived
material, which is applied as a binder in road construction. However,
the asphalt undergoes hardening, cracking, and embrittlement not only
due to oxidative short-term aging during the mixing and paving process
but also due to long-term aging during the service time of the pavement.
In this study, chemical changes occurring during short-term aging,
mimicked by a prolonged rotating flask procedure, are investigated
for an artificial bitumen model at the molecular level. The model
bitumen enables the application of two complementary analytical techniques
for obtaining a comprehensive insight into the aging effects: high-resolution
Fourier-transform ion cyclotron mass spectrometry (FT-ICR MS) coupled
to thermogravimetry was applied to investigate the aging effects on
polar to semipolar high-molecular-weight compounds ionized by atmospheric
pressure chemical ionization. Aromatic core structures were analyzed
by alternating collision-induced dissociation. In order to support
structural assignments from FT-ICR MS data in the semivolatile region,
comprehensive two-dimensional gas chromatography mass spectrometry
(GC × GC–HRTOFMS) with electron ionization at 70 eV was
applied for the group-type analysis and the investigation of particular
chemical functionalities. Oxidation processes were revealed to be
the prevalent reactions caused by short-term aging of the hydrocarbons
(CH-class) and sulfur-containing classes. Aromatic species with low
steric hindrance or activated carbon positions as well as high aromatic
core structures are favorably oxidized, forming carbonyl functionalities.
For molecules with one sulfur atom (S1-class), nonaromatic species
such as tetrahydrothiophenes decrease, whereas aromatic S1-compounds
remain constant. Nonaromatic S1O1-species tend to further oxidize,
while higher aromatic species are formed with ongoing aging time.
Moreover, this study highlights the aging behavior of nitrogen-containing
compounds, such as carbazoles. A significant reduction of the N-classes
was observed during aging, indicating thermal-induced condensation
reactions as well as favored oxidation of highly aromatic core structures.
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