Organosulfates (OSs) derived from biogenic volatile organic compounds (BVOCs)
represent an important class of products formed between anthropogenic sulfur pollution and
natural emissions. Both smog-chamber studies and ambient aerosol measurements have shown
these organosulfates may contribute a significant part of secondary organic aerosol (SOA).
However, a lack of authentic OS standards and a systematic study of distribution of OSs
under different polluted conditions in ambient samples presents significant obstacles to our
quantitative understanding of this chemical interaction between human and nature.
Work in this thesis aims to improve the scientific understanding of factors controlling
ambient abundance and composition, and to gain more insights into formation mechanism of
OSs, with a special emphasis on the aerosols of China. Multiple approaches were employed
for quantification of OSs and the main findings are summarized below:
(1) Organic chemistry was employed to synthesize OS and nitroxy OS (NOS) authentic
standards. We demonstrated successfully the synthesis of α-hydroxy OS compounds
derived from nine monoterpenes (e.g., α- & β- pinene, limonene, limonaketone,
sabinene, Δ
3-carene, terpinolene, α-terpinene, and γ-terpinene) and two sesquiterpenes
(e.g., α-humulene and β-caryophllene) through Upjohn dihydroxylation or Sharpless
asymmetric dihydroxylation followed by monosulfation with the sulfur trioxide-pyridine
complex. Two synthetic routes were firstly reported for the synthesis of hydroxy nitrates,
which could be readily sulfated by sulfur trioxide-pyridine complex to provide eight
terpene-derived-NOS isomers from α-pinene, β-pinene, limonene, limonaketone and
β-caryophllene. The first route explored halohydrination of the terpene with
N-bromosuccinimide followed by S
N2 substitution of the bromide with nitrate to
generate the hydroxy nitrate. The second route involved straightforward approach for
epoxide opening by HNO
3 to form hydroxy nitrate.
(2) The in-house available standards allowed us to achieve the accurate quantification of
OSs by liquid chromatography-mass spectrometry (LC-MS) by either monitoring the
respective [M-H]
- molecular ion or through multiple reaction monitoring (MRM) of
mass transitions between the [M-H]
- and typical daughter ions (e.g., m/z 97, 62 and etc.).
Comparisons were made between calibrations by the synthetic authentic standards and
those by quantification surrogate compounds commonly used in previous studies,
revealing camphor sulfonic acid is a better quantification surrogate for α-hydroxy
terpene OSs and octyl sulfate or camphor sulfonic acid are better surrogates for
quantification of NOSs. In ambient samples, we only detected α-hydroxy terpene OS
regioisomers with sulfate group at the less substituted carbon atom, which indicate
terpene diol was the important intermediate for α-hydroxy OS compounds. Furthermore,
the measurement of terpene NOSs from carbon skeleton rearrangement in
filter-collected ambient samples provided the evidence of acid-catalyzed terpene
epoxide opening heterogeneous reaction may help to explain NOSs formation.
(3) An iodide High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer
(HRToF-CIMS) coupled with the Filter Inlet for Gases and AEROsols (FIGAERO) was
applied for the real-time measurements of gas- and particle-phase ambient species
including C
2-3 OSs and their potential precursors. Comparisons between online and
offline instruments on qualifying C
2-3 OSs highlights the importance of using authentic
standards in characterizing instrument performance on online mass spectrometer. The
cluster ion H
3S
2O
8-, considered as a marker for aerosol acidity, was also identified by
HRTof-CIMS throughout the campaign, allowing examination of the role of aerosol
acidity on formation of C
2-3 OSs. The four OSs were strongly correlated with
H
3S
2O
8-
(R>0.74), providing field observational evidence for acid-catalyzed formation of the
small OSs. Mechanistically, the adjacent carbonyl and hydroxyl groups in the precursors
could facilitate formation of 5-member ring intermediates under acidic conditions,
leading to C
2-3 OS formation.
(4) With the help of non-targeted analysis method by a high-performance liquid
chromatogram coupled to ultra-high resolution orbitrap mass spectrometer, we
quantified/semi-quantified around 300 OSs in ambient samples from Beijing, Shanghai,
Guangzhou and Hong Kong. The spatial coverage of three climatic zones from north to
south China provided an unique opportunity to probe controlling factors on OS
formation. The total concentration of detected OSs was up to 545 ng/m
3 in summer and
234 ng/m
3 in winter, which constituted around 1.4% and 0.5% of PM
2.5, respectively.
The contrasts in seasonal variation and spatial variation of four groups of OSs indicate
that a multitude of factors (e.g., BVOC precursor abundance, sulfate pollutant level, and
transport processes) can influence OSs formation in ambient atmosphere.
This thesis work is the first study reporting the synthesis methods and accurate
abundance of a full range of α-hydroxy terpene OS and terpene NOSs. With the help of
authentic standards, this work provides some guidelines for selecting surrogates in LC/MS
analysis for OS compounds when standards are not commercially available. This work has
identified some possible intermediates in the OSs formation mechanism, which provides
valuable leads for future chamber studies. In addition, a field study employing both real-time
measurement of gas- and particle-phase OSs and off-line filter-based OS measurements
provides an example of illustrating the importance of having authentic standards for on-line
instrumental method development. Finally, contrasting OS measurements in samples from
three climatic zones in China reveals the complexity of the web of factors that affect the
interactions between biogenic VOC emissions and anthropogenic pollution.
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