Characterization of a real-time tracer for isoprene epoxydiols-derived secondary organic aerosol (IEPOX-SOA) from aerosol mass spectrometer measurements
Substantial amounts of secondary organic aerosol (SOA) can be formed from isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly under low-NO conditions. Total IEPOX-SOA, which may include SOA formed from other parallel isoprene oxidation pathways, was quantified by applying po...
Main Authors: | , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2015-10-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | http://www.atmos-chem-phys.net/15/11807/2015/acp-15-11807-2015.pdf |
Summary: | Substantial amounts of secondary organic aerosol (SOA) can be formed from
isoprene epoxydiols (IEPOX), which are oxidation products of isoprene mainly
under low-NO conditions. Total IEPOX-SOA, which may include SOA formed from
other parallel isoprene oxidation pathways, was quantified by applying
positive matrix factorization (PMF) to aerosol mass spectrometer (AMS)
measurements. The IEPOX-SOA fractions of organic aerosol (OA) in multiple field studies across
several continents are summarized here and show consistent patterns with the
concentration of gas-phase IEPOX simulated by the GEOS-Chem chemical
transport model. During the Southern Oxidant and Aerosol Study (SOAS),
78 % of PMF-resolved IEPOX-SOA is accounted by the measured IEPOX-SOA
molecular tracers (2-methyltetrols, C5-Triols, and IEPOX-derived organosulfate
and its dimers), making it the highest level of molecular identification of
an ambient SOA component to our knowledge. An enhanced signal at
C<sub>5</sub>H<sub>6</sub>O<sup>+</sup> (<I>m/z</I> 82) is found in PMF-resolved IEPOX-SOA spectra.
To investigate the suitability of this ion as a tracer for IEPOX-SOA, we
examine <I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub>
(<I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub>=
C<sub>5</sub>H<sub>6</sub>O<sup>+</sup>/OA) across multiple field,
chamber, and source data sets. A background of
~ 1.7 ± 0.1 ‰ (‰ = parts per thousand) is
observed in studies strongly influenced by urban, biomass-burning, and other
anthropogenic primary organic aerosol (POA). Higher background values of
3.1 ± 0.6 ‰ are found in studies strongly influenced by
monoterpene emissions. The average laboratory monoterpene SOA value
(5.5 ± 2.0 ‰) is 4 times lower than the average for IEPOX-SOA
(22 ± 7 ‰), which leaves some room to separate both
contributions to OA. Locations strongly influenced by isoprene emissions
under low-NO levels had higher <I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub>
(~ 6.5 ± 2.2 ‰ on average) than other sites, consistent
with the expected IEPOX-SOA formation in those studies.
<I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub> in IEPOX-SOA is always elevated
(12–40 ‰) but varies substantially between locations, which is
shown to reflect large variations in its detailed molecular composition. The
low <I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub> (< 3 ‰) reported in
non-IEPOX-derived isoprene-SOA from chamber studies indicates that this
tracer ion is specifically enhanced from IEPOX-SOA, and is not a tracer for
all SOA from isoprene. We introduce a graphical diagnostic to study the
presence and aging of IEPOX-SOA as a <I>triangle plot</I> of <I>f</I><sub>CO<sub>2</sub></sub>
vs. <I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub>. Finally, we develop a simplified
method to estimate ambient IEPOX-SOA mass concentrations, which is shown to
perform well compared to the full PMF method. The uncertainty of the tracer
method is up to a factor of ~ 2, if the
<I>f</I><sub>C<sub>5</sub>H<sub>6</sub>O</sub> of the local IEPOX-SOA is not
available. When only unit mass-resolution data are available, as with the
aerosol chemical speciation monitor (ACSM), all methods may perform less well
because of increased interferences from other ions at <I>m/z</I> 82. This study
clarifies the strengths and limitations of the different AMS methods for
detection of IEPOX-SOA and will enable improved characterization of this OA
component. |
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ISSN: | 1680-7316 1680-7324 |