29 research outputs found
Revealing Brown Carbon Chromophores Produced in Reactions of Methylglyoxal with Ammonium Sulfate
Atmospheric
brown carbon (BrC) is an important contributor to light
absorption and climate forcing by aerosols. Reactions between small
water-soluble carbonyls and ammonia or amines have been identified
as one of the potential pathways of BrC formation. However, detailed
chemical characterization of BrC chromophores has been challenging
and their formation mechanisms are still poorly understood. Understanding
BrC formation is impeded by the lack of suitable methods which can
unravel the variability and complexity of BrC mixtures. This study
applies high performance liquid chromatography (HPLC) coupled to photodiode
array (PDA) detector and high resolution mass spectrometry (HRMS)
to investigate optical properties and chemical composition of individual
BrC components produced through reactions of methylglyoxal (MG) and
ammonium sulfate (AS), both of which are abundant in the atmospheric
environment. A direct relationship between optical properties and
chemical composition of 30 major BrC chromophores is established.
Nearly all of these chromophores are nitrogen-containing compounds
that account for >70% of the overall light absorption by the MG+AS
system in the 300–500 nm range. These results suggest that
reduced-nitrogen organic compounds formed in reactions between atmospheric
carbonyls and ammonia/amines are important BrC chromophores. It is
also demonstrated that improved separation of BrC chromophores by
HPLC will significantly advance understanding of BrC chemistry
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High-Resolution Mass Spectrometry and Molecular Characterization of Aqueous Photochemistry Products of Common Types of Secondary Organic Aerosols
This work presents a systematic investigation
of the molecular
level composition and the extent of aqueous photochemical processing
in different types of secondary organic aerosol (SOA) from biogenic
and anthropogenic precursors including α-pinene, β-pinene,
β-myrcene, d-limonene, α-humulene, 1,3,5-trimethylbenzene,
and guaiacol, oxidized by ozone (to simulate a remote atmosphere)
or by OH in the presence of NO<sub><i>x</i></sub> (to simulate
an urban atmosphere). Chamber- and flow-tube-generated SOA samples
were collected, extracted in a methanol/water solution, and photolyzed
for 1 h under identical irradiation conditions. In these experiments,
the irradiation was equivalent to about 3–8 h of exposure to
the sun in its zenith. The molecular level composition of the dissolved
SOA was probed before and after photolysis with direct-infusion electrospray
ionization high-resolution mass spectrometry (ESI-HR-MS). The mass
spectra of unphotolyzed SOA generated by ozone oxidation of monoterpenes
showed qualitatively similar features and contained largely overlapping
subsets of identified compounds. The mass spectra of OH/NO<sub><i>x</i></sub>-generated SOA had more unique visual appearance
and indicated a lower extent of product overlap. Furthermore, the
fraction of nitrogen-containing species (organonitrates and nitroaromatics)
was highly sensitive to the SOA precursor. These observations suggest
that attribution of high-resolution mass spectra in field SOA samples
to specific SOA precursors should be more straightforward under OH/NO<sub><i>x</i></sub> oxidation conditions compared to the ozone-driven
oxidation. Comparison of the SOA constituents before and after photolysis
showed the tendency to reduce the average number of atoms in the SOA
compounds without a significant effect on the overall O/C and H/C
ratios. SOA prepared by OH/NO<sub><i>x</i></sub> photooxidation
of 1,3,5-trimethylbenzene and guaiacol were more resilient to photolysis
despite being the most light-absorbing. The composition of SOA prepared
by ozonolysis of monoterpenes changed more significantly as a result
of the photolysis. The results indicate that aqueous photolysis of
dissolved SOA compounds in cloud/fog water can occur in various types
of SOA, and on atmospherically relevant time scales. However, the
extent of the photolysis-driven change in molecular composition depends
on the specific type of SOA
Chemical Characterization of Crude Petroleum Using Nanospray Desorption Electrospray Ionization Coupled with High-Resolution Mass Spectrometry
Nanospray desorption electrospray ionization (nano-DESI)
combined
with high-resolution mass spectrometry was used for the first time
for the analysis of the polar constituents of liquid petroleum crude
oil samples. The analysis was performed in both positive and negative
ionization modes using three solvents, one of which (acetonitrile/toluene
mixture) is commonly used in petroleomics studies while two other
polar solvents (acetonitrile/water and methanol/water mixtures) are
generally not compatible with petroleum characterization using mass
spectrometry. The results demonstrate that nano-DESI analysis efficiently
ionizes petroleum constituents soluble in a particular solvent. When
acetonitrile/toluene is used as a solvent, nano-DESI generates electrospray-like
spectra. In contrast, strikingly different spectra were obtained using
acetonitrile/water and methanol/water. Comparison with the literature
data indicates that these solvents selectively extract water-soluble
constituents of the crude oil. Water-soluble compounds are predominantly
observed as sodium adducts in nano-DESI spectra indicating that addition
of sodium to the solvent may be a viable approach for efficient ionization
of water-soluble crude oil constituents. Nano-DESI enables rapid screening
of different classes of compounds in crude oil samples based on their
solubility in solvents that are rarely used for petroleum characterization
providing better coverage of the crude oil composition as compared
to electrospray ionization (ESI). It also enables rapid characterization
of water-soluble components of petroleum samples that is difficult
to perform using traditional approaches
Applications of High-Resolution Electrospray Ionization Mass Spectrometry to Measurements of Average Oxygen to Carbon Ratios in Secondary Organic Aerosols
The applicability of high-resolution electrospray ionization
mass
spectrometry (HR ESI-MS) to measurements of the average oxygen to
carbon ratio (O/C) in secondary organic aerosols (SOAs) was investigated.
Solutions with known average O/C containing up to 10 standard compounds
representative of low-molecular-weight SOA constituents were analyzed
and the corresponding electrospray ionization efficiencies were quantified.
The assumption of equal ionization efficiency commonly used in estimating
O/C ratios of SOAs was found to be reasonably accurate. We found that
the accuracy of the measured O/C ratios increases by averaging the
values obtained from both the posive and negative modes. A correlation
was found between the ratio of the ionization efficiencies in the
positive (+) and negative (−) ESI modes and the octanol–water
partition constant and, more importantly, the compound’s O/C.
To demonstrate the utility of this correlation for estimating average
O/C values of unknown mixtures, we analyzed the ESI (+) and ESI (−)
data for SOAs produced by oxidation of limonene and isoprene and compared
them online to O/C measurements using an aerosol mass spectrometer
(AMS). This work demonstrates that the accuracy of the HR ESI-MS method
is comparable to that of the AMS with the added benefit of molecular
identification of the aerosol constituents
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
Hygroscopic Properties of Internally Mixed Particles Composed of NaCl and Water-Soluble Organic Acids
Atmospheric
aging of naturally emitted marine aerosol often leads
to formation of internally mixed particles composed of sea salts and
water-soluble organic compounds of anthropogenic origin. Mixing of
sea salt and organic components has profound effects on the evolving
chemical composition and hygroscopic properties of the resulted particles,
which are poorly understood. Here, we have studied chemical composition
and hygroscopic properties of laboratory generated NaCl particles
mixed with malonic acid (MA) and glutaric acid (GA) at different molar
ratios using micro-FTIR spectroscopy, atomic force microscopy, and
X-ray elemental microanalysis. Hygroscopic properties of internally
mixed NaCl and organic acid particles were distinctly different from
pure components and varied significantly with the type and amount
of organic compound present. Experimental results were in a good agreement
with the AIM modeling calculations of gas/liquid/solid partitioning
in studied systems. X-ray elemental microanalysis of particles showed
that Cl/Na ratio decreased with increasing organic acid component
in the particles with MA yielding lower ratios relative to GA. We
attribute the depletion of chloride to the formation of sodium malonate
and sodium glutarate salts resulted by HCl evaporation from dehydrating
particles
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
Aqueous Photochemistry of Secondary Organic Aerosol of α‑Pinene and α‑Humulene Oxidized with Ozone, Hydroxyl Radical, and Nitrate Radical
Formation
of secondary organic aerosols (SOA) from biogenic volatile
organic compounds (BVOC) occurs via O<sub>3</sub>- and OH-initiated
reactions during the day and reactions with NO<sub>3</sub> during
the night. We explored the effect of these three oxidation conditions
on the molecular composition and aqueous photochemistry of model SOA
prepared from two common BVOC. A common monoterpene, α-pinene,
and sesquiterpene, α-humulene, were used to form SOA in a smog
chamber via BVOC + O<sub>3</sub>, BVOC + NO<sub>3</sub>, and BVOC
+ OH + NO<sub><i>x</i></sub> oxidation. Samples of SOA were
collected on filters, water-soluble compounds from SOA were extracted
in water, and the resulting aqueous solutions were photolyzed to simulate
the photochemical aqueous processing of SOA. The extent of change
in the molecular level composition of SOA over 4 h of photolysis (approximately
equivalent to 64 h of photolysis under ambient conditions) was assessed
with high-resolution electrospray ionization mass spectrometry. The
analysis revealed significant differences in the molecular composition
between SOA formed by the different oxidation pathways. The composition
further evolved during photolysis with the most notable change corresponding
to the nearly complete removal of nitrogen-containing organic compounds.
Hydrolysis of SOA compounds also occurred in parallel with photolysis.
The preferential loss of larger SOA compounds during photolysis and
hydrolysis made the SOA compounds more volatile on average. This study
suggests that aqueous processes may under certain conditions lead
to a reduction in the SOA loading as opposed to an increase in SOA
loading commonly assumed in the literature
Atmospheric Oxidation of Squalene: Molecular Study Using COBRA Modeling and High-Resolution Mass Spectrometry
Squalene
is a major component of skin and plant surface lipids
and is known to be present at high concentrations in indoor dust.
Its high reactivity toward ozone makes it an important ozone sink
and a natural protectant against atmospheric oxidizing agents. While
the volatile products of squalene ozonolysis are known, the condensed-phase
products have not been characterized. We present an analysis of condensed-phase
products resulting from an extensive oxidation of squalene by ozone
probed by electrospray ionization (ESI) high-resolution mass spectrometry
(HR–MS). A complex distribution of nearly 1300 peaks assignable
to molecular formulas is observed in direct infusion positive ion
mode ESI mass spectra. The distribution of peaks in the mass spectra
suggests that there are extensive cross-coupling reactions between
hydroxy-carbonyl products of squalene ozonolysis. To get additional
insights into the mechanism, we apply a Computational Brewing Application
(COBRA) to simulate the oxidation of squalene in the presence of ozone,
and compare predicted results with those observed by the HR–MS
experiments. The system predicts over one billion molecular structures
between 0 and 1450 Da, which correspond to about 27 000 distinct
elemental formulas. Over 83% of the squalene oxidation products inferred
from the mass spectrometry data are matched by the simulation. The
simulation indicates a prevalence of peroxy groups, with hydroxyl
and ether groups being the second-most important O-containing functional
groups formed during squalene oxidation. These highly oxidized products
of squalene ozonolysis may accumulate on indoor dust and surfaces
and contribute to their redox capacity
Molecular Characterization of Brown Carbon in Biomass Burning Aerosol Particles
Emissions from biomass
burning are a significant source of brown
carbon (BrC) in the atmosphere. In this study, we investigate the
molecular composition of freshly emitted biomass burning organic aerosol
(BBOA) samples collected during test burns of sawgrass, peat, ponderosa
pine, and black spruce. We demonstrate that both the BrC absorption
and the chemical composition of light-absorbing compounds depend significantly
on the type of biomass fuels. Common BrC chromophores in the selected
BBOA samples include nitro-aromatics, polycyclic aromatic hydrocarbon
derivatives, and polyphenols spanning a wide range of molecular weights,
structures, and light absorption properties. A number of biofuel-specific
BrC chromophores are observed, indicating that some of them may be
used as source-specific markers of BrC. On average, ∼50% of
the light absorption in the solvent-extractable fraction of BBOA can
be attributed to a limited number of strong BrC chromophores. The
absorption coefficients of BBOA are affected by solar photolysis.
Specifically, under typical atmospheric conditions, the 300 nm absorbance
decays with a half-life of ∼16 h. A “molecular corridor”
analysis of the BBOA volatility distribution suggests that many BrC
compounds in the fresh BBOA have low saturation mass concentration
(<1 μg m<sup>–3</sup>) and will be retained in the
particle phase under atmospherically relevant conditions