Isomeric epoxydiols from isoprene photooxidation (IEPOX) have been shown to produce substantial amounts of secondary organic aerosol (SOA) mass and are therefore considered a major isoprene-derived SOA precursor. Heterogeneous reactions of IEPOX on atmospheric aerosols form various aerosol-phase components or “tracers” that contribute to the SOA mass burden. A limited number of the reaction rate constants for these acid-catalyzed aqueous-phase tracer formation reactions have been constrained through bulk laboratory measurements. We have designed a chemical box model with multiple experimental constraints to explicitly simulate gas- and aqueous-phase reactions during chamber experiments of SOA growth from IEPOX uptake onto acidic sulfate aerosol. The model is constrained by measurements of the IEPOX reactive uptake coefficient, IEPOX and aerosol chamber wall losses, chamber-measured aerosol mass and surface area concentrations, aerosol thermodynamic model calculations, and offline filter-based measurements of SOA tracers. By requiring the model output to match the SOA growth and offline filter measurements collected during the chamber experiments, we derive estimates of the tracer formation reaction rate constants that have not yet been measured or estimated for bulk solutions.
The gas-phase photooxidation of isoprene (2-methyl-1,3-butadiene), the
largest biogenic volatile organic compound (VOC) emitted worldwide
(Guenther et al., 2012), yields isomeric
isoprene epoxydiols (IEPOX) (Paulot et al., 2009). Subsequent
acid-catalyzed multiphase chemistry of IEPOX is a significant source of
secondary organic aerosol (SOA) mass (Lin et al., 2012; Surratt et al.,
2010). In recent field studies, up to 50 % of summertime aerosol mass
loadings in the southeastern United States have been attributed to SOA
resulting from IEPOX heterogeneous reactions (Budisulistiorini et al.,
2013, 2015; Lin et al., 2013b). Similar
IEPOX-derived SOA influences are expected in areas with large isoprene
emissions, such as forests primarily composed of broadleaf vegetation. As a
significant SOA precursor, IEPOX has implications regarding potential
climate forcing due to the scattering of incoming radiation and also impacts
human health due to its large contribution to PM
Gas-phase IEPOX can partition to atmospheric aerosol surface area where it
can react with aerosol liquid water and aerosol-phase constituents –
including sulfate, nitrate, and organics – to form a variety of
lower-volatility organic compounds that can remain in the aerosol and
contribute to total aerosol mass. Because their presence establishes IEPOX
as the precursor, the particle-phase products are referred to as IEPOX-SOA
“tracers” (i.e., “molecular markers”). The efficiency of gas-phase IEPOX
removal by aerosol surface area is thought to be largely a function of
aerosol acidity and concentration of nucleophiles that can react with
accommodated IEPOX by acid-catalyzed oxirane ring opening to yield the
tracer compounds (Eddingsaas et al., 2010; Gaston et al., 2014; Nguyen et
al., 2014; Piletic et al., 2013; Riedel et al., 2015; Surratt et al.,
2007b). Products of the reactions have been proposed to include the
2-methyltetrols (2-methylthreitol and 2-methylerythritol) from addition of
water, and the corresponding isomeric sulfate esters (IEPOX-OS) from sulfate
addition (Reactions R1 and R2) (Claeys et al., 2004; Surratt et al.,
2007a).
Products of nitrate addition, while observed less often, are also thought to
be important in certain cases (Darer et al., 2011; Lin et al., 2012).
Additional condensed-phase reactions are thought to form IEPOX-derived
dimeric species (2-methyltetrol dimers, OS dimers), isomeric C
To date, only the formation of IEPOX-derived 2-methyltetrols and/or organosulfates has been investigated through direct bulk kinetic measurements (Cole-Filipiak et al., 2010), the extension of bulk kinetic measurements of surrogate epoxides (Eddingsaas et al., 2010), and computational estimates (Piletic et al., 2013). While the tetrol and IEPOX-OS tracers are responsible for a sizeable fraction of IEPOX-derived SOA (Lin et al., 2013a, b), the remaining tracer formation reactions have yet to be examined, and accurate estimates would benefit SOA modeling efforts (Karambelas et al., 2014; McNeill et al., 2012; Pye et al., 2013). Here we present an approach that combines chamber experiments, offline quantification of SOA tracers from filter samples using authentic standards, and modeling to estimate the formation reaction rate constants of IEPOX-derived SOA tracers whose formation rates are currently unknown. This has been done for a single seed aerosol system, acidified ammonium sulfate at low relative humidity (RH), but the estimated rate coefficients are anticipated to be independent of the seed aerosol used.
Experiments were conducted under dark conditions in an indoor 10 m
Chamber aerosol number distributions, which were subsequently converted to
total aerosol surface area and volume concentrations, were measured by a
scanning electrical mobility system (SEMS v5.0, Brechtel Manufacturing Inc.
– BMI) containing a differential mobility analyzer (DMA, BMI) coupled to a
mixing condensation particle counter (MCPC Model 1710, BMI). Total volume
concentration of seed aerosols was converted to total mass concentration
assuming a density of 1.6 g mL
On completion of IEPOX injection, a filter sample was collected for analysis
of the chamber-generated SOA. Aerosols were collected onto 46.2 mm Teflon
filters (part no.: SF17471, Tisch Scientific) in a stainless-steel filter
holder for 2 h at
As described in previous studies (Lin et al., 2012; Surratt et al.,
2010), IEPOX-derived SOA components were extracted from filters with
high-purity methanol prior to analysis. Analysis was performed on a gas
chromatograph coupled to a mass spectrometer equipped with an electron
ionization source (GC/EI-MS, Hewlett-Packard 5890 Series II GC coupled to a
Hewlett-Packard 5971A MS) and an ultra-performance liquid
chromatograph/high-resolution quadrupole time-of-flight mass spectrometer
equipped with electrospray ionization (UPLC/ESI-HR-QTOFMS, Agilent 6500
Series). 2-Methyltetrols, C
Reaction kinetics of SOA generation were investigated with a
zero-dimensional time-dependent chemical box model incorporating explicit
aqueous-phase tracer formation. The model is initialized with the amount of
A constant IEPOX–aerosol reaction probability (
The coupled differential equations corresponding to the production and/or
loss of IEPOX
This reaction lowers the molar SOA yield (
The complete set of differential equations used to track each of the individual
species in the model is provided in Eqs. (2)–(12).
Aerosol mass loadings from IEPOX-SOA Exp. no. 1 and corresponding
model output. IEPOX injection starts at experiment time
Summary of conditions for each chamber SOA experiment.
Five chamber experiments were performed with the low-RH
(NH
Model output of aqueous-phase IEPOX concentrations during Exp. no. 1 simulation.
Model output of IEPOX-SOA tracers (left panel) and the associated filter-based tracer measurements (right panel) for Exp. no. 1. The “other SOA” is calculated as the difference between the chamber-measured aerosol mass loadings and the sum of the filter-based tracer loadings.
Figure 3 compares the modeled evolution of the SOA tracers in Exp. no. 1 to
offline measurements of the corresponding tracers. Measured tracer mass
loadings for all experiments are provided in Table 2. The tracer
concentrations predicted by the model agree well with the filter
measurements, differing by
Tracer mass loadings for each chamber SOA experiment.
The model also predicts significant titration of total aqueous inorganic
sulfate species ([SO
Model output of predicted titration of total inorganic aerosol
sulfate ([SO
The model-predicted tracer formation rate constants for Reactions (R1)–(R7) are given in Table 3. These are averaged over all experiments, and the
listed errors correspond to 1 standard deviation (1
Model-predicted formation reaction rate constants for IEPOX-SOA tracers.
Epoxide ring-opening reactions by general acids (i.e., bisulfate) have not
been explicitly included in the model. The contribution is expected to be
negligible as the branching ratio between the bisulfate and
H
Aerosol surface area was held constant at initial seed aerosol levels over
the course of a model run, and thus
As a sensitivity test to the choice of 334 g mole
Given the estimates of the tracer formation rate constants, the calculated
Model-predicted IEPOX-SOA tracer distribution and loadings for atmospherically relevant initial conditions.
Figure 5 shows the model output after 6 h processing time, using as inputs
the rate constants from Table 3 and initial atmospheric conditions which
might be representative of a daytime summer urban–rural mixed air mass:
50 % RH,
Keeping in mind that we cannot hope to capture two field studies perfectly
for such a general model case, the model total IEPOX tracer loading
predictions are in relatively close correspondence to recent measurements in
the southeastern United States. Analysis of tracers in ambient PM
Attempts to replicate the chamber experiments at higher RH (50 %) resulted in large positive deviations (1.2–2.3-fold) in total IEPOX tracer mass loadings compared to measured total aerosol mass loadings by the SEMS-MCPC. This result precluded the extension of these kinetic studies to include humid conditions. A possible explanation for the enhancement of filter mass loadings could be subsequent reactions at the Teflon filter surface; however, appropriate controls are required to confirm such effects. The deviation in mass loadings at higher RH indicates that artifacts may be introduced into field and chamber measurements during filter collection even when sampling through a carbon strip denuder.
Low-molecular-weight tracers with significant vapor pressures may be detected as a result of decomposition of SOA products. Such a possibility would dictate caution in adopting the kinetic estimates presented here. The sum of these formation rates would likely represent an upper limit to the formation of such SOA species under the assumption that more than one tracer could potentially be formed from the degradation of these products. However, in the absence of evidence to the contrary, there is general agreement that tracers constitute a large fraction of IEPOX-SOA, and additional investigations are required prior to the proposal that certain SOA tracers represent decomposition products.
In summary, this study is a first approach to placing kinetic constraints on the formation of species that have been quantified in laboratory and field measurements but lack directly measured experimental rate constraints. While bulk solution rate constant estimates are desirable, such measurements pose a challenge when authentic standards are unavailable or when surrogates do not adequately represent the true compounds. Additionally, it is unclear that bulk-phase kinetics can approximate aerosol-phase reactions where non-ideal conditions likely play a role. The flexible approach described here may readily be extended to other SOA production systems known to have atmospheric importance.
This study approximates tracer branching ratios for the currently proposed SOA tracers resulting from IEPOX uptake, a necessary step to predict isoprene-derived SOA production in regional models that guide policy decisions. Additional laboratory studies to identify SOA products and elucidate formation mechanisms are important to ensure that both chamber and field measurements accurately reflect atmospheric processes. Modeling developed on the basis of such experimental systems can then be extended to large-scale models.
This publication was made possible in part by Environmental Protection Agency (EPA) grant no. R835404. Its contents are solely the responsibility of the grantee and do not necessarily represent the official views of the EPA. Further, the EPA does not endorse the purchase of any commercial products or services mentioned in the publication. This work is also funded in part by the National Science Foundation under CHE 1404644 and CHE 1404573 and through a grant from the Texas Commission on Environmental Quality (TCEQ), administered by The University of Texas through the Air Quality Research Program. The contents, findings, opinions, and conclusions are the work of the authors and do not necessarily represent findings, opinions, or conclusions of the TCEQ. The authors also thank Tianqu Cui and Sri Hapsari Budisulistiorini (UNC) and Felipe Lopez-Hilfiker (UW) for helpful discussions.Edited by: M. Shiraiwa