SOA formation
In Fig. 3, we plot the time series of RH, ozone and organic aerosol
concentrations during the aging of gas-phase emissions from heated peanut
oil. As described above, the ozone concentration prior to dilution was stable
at approximately 40 ppm. The pulse of RH was caused by disconnection
of the introduction line when changing the Teflon filter. During the initial
10 min of heating, the mass concentration of organics was close to
the detection limit of the instrument, indicating that POA emissions were
thoroughly removed by the Teflon filter. During these periods of experiments
where OH radicals were not present, we found that ozone chemistry had a
negligible influence on SOA formation in this study. Immediately after
oxidation was initiated by turning on the UV lamp, substantial SOA was
formed, and its concentration stabilized after about 20 min. The SOA
concentration subsequently reported is the average for the steady period.
SOA production efficiency and type of fat content
(%) a of different cooking oils.
Slopeb
Saturated
Monounsaturated
Polyunsaturated
Others
Omega-6
Omega-3
(µgmolecules-1s-1)
(%)
(%)
(%)
(%)
Sunflower
3.82×10-15
10
19
64
0
7
Corn
3.31×10-15
12
24
56
1
7
Canola
2.68×10-15
7
59
20
9
5
Olive
2.55×10-15
13
71
8
1
7
Peanut
1.7×10-15
16
44
31
0
9
a The type of fat content of cooking oils was
derived from skillsyouneed.com. b SOA production
efficiency was presented as the slope of the fitted straight line to the SOA
concentration vs. OH exposure.
SOA concentration vs. OH exposure and photochemical age in days (at
[OH] = 1.5×106 moleculescm-3) during the aging of
gas-phase emissions from different heated cooking oils. Error bars represent
the standard deviation (1σ).
Figure 4 shows SOA concentration as a function of OH exposure and
photochemical age in days during the aging of gas-phase emissions from
different heated cooking oils. The OH exposure ranged from 2.7×1010 to 1.7×1011 moleculescm-3s, corresponding
to 0.2–1.3 days of photochemical age, assuming 24 h average ambient OH
concentrations of 1.5×106 moleculescm-3 (Mao et
al., 2009). For all experiments, the SOA concentration almost linearly
increased from 41–107 to 320–565 µgm-3 as OH exposure
increased. This linear increase has also been observed from vehicle exhaust
at a similar range of OH exposures (Tkacik et al., 2014). Typically, VOCs are
oxidized through functionalization reactions to produce less volatile
organics that readily condense to form SOA. Upon further oxidation,
fragmentation reactions and cleavage of carbon bonds can occur and form more
volatile products that reduce SOA levels (Kroll et al., 2009). In this study,
functionalization reactions dominated SOA formation as reflected by the
increase in SOA concentrations shown in Fig. 4.
The slope of the fitted straight line to the SOA data was calculated to
estimate the efficiency of different cooking oils in producing SOA (Table 1).
The efficiency of SOA production, in ascending order, was peanut oil, olive
oil, canola oil, corn oil and sunflower oil. The slope of sunflower oil was
3.82×10-15 µgmolecules-1s-1, more than 2
times that of peanut oil. The different slopes might be related to the
emission rate and composition of VOCs from various cooking oils. Table 1
presents the type of fat content of the different cooking oils. It should be
noted that the organic vapors studied here were not the specific fats present
in the raw oils but the thermal breakdown products of fat lipids.
Unsaturated fat accounts for 75–88 % of the total fat content. A
multivariate linear regression was used to relate the SOA production
efficiency to the fat content of cooking oils. The intercept was set to zero.
The resulting equation was Y=2.62×10-17X1+4.71×10-17X2, where Y is the SOA production efficiency
(µgmolecules-1s-1); X1 and X2 represent the
content of monounsaturated fat (%) and omega-6 fatty acid (%) in
cooking oil, respectively. The SOA production efficiency was strongly
correlated (R2=0.97, p< 0.05) with the content of monounsaturated
fat and omega-6 fatty acids. This indicated that the major SOA precursors
from heated cooking oils were related to the content of monounsaturated fat
and omega-6 fatty acids in cooking oils. Moreover, omega-6 fatty acids
dominated the contribution to SOA production. Omega-6 fatty acids are a
family of poly-unsaturated fatty acids that have in common a final
carbon–carbon double bond in the n-6 position, counting from the methyl end
(Simopoulos, 2002). The peroxyl radical reactions of omega-6 fatty acids
might emit long-chain aldehydes (Gardner, 1989), which have been suggested as
potential SOA precursors (Chacon-Madrid et al., 2010).
Correlation coefficients (R2) between POA and SOA UMR mass
spectra and ambient COA resolved by PMF.
CA Pa
CN P
SR P
PT P
OE P
CA S
CN S
SR S
PT S
OE S
COAb
CA P
1.00
0.99
1.00
0.98
0.97
0.85
0.87
0.91
0.93
0.94
0.96
CN P
0.99
1.00
0.99
0.99
0.99
0.89
0.90
0.94
0.96
0.96
0.95
SR P
1.00
0.99
1.00
0.98
0.97
0.85
0.87
0.91
0.93
0.94
0.96
PT P
0.98
0.99
0.98
1.00
0.98
0.83
0.85
0.90
0.93
0.93
0.96
OE P
0.97
0.99
0.97
0.98
1.00
0.86
0.88
0.93
0.95
0.96
0.94
CA S
0.85
0.89
0.85
0.83
0.86
1.00
0.95
0.98
0.96
0.94
0.74
CN S
0.87
0.90
0.87
0.85
0.88
0.95
1.00
0.95
0.96
0.96
0.77
SR S
0.91
0.94
0.91
0.90
0.93
0.98
0.95
1.00
0.99
0.97
0.83
PT S
0.93
0.96
0.93
0.93
0.95
0.96
0.96
0.99
1.00
0.99
0.87
OE S
0.94
0.96
0.94
0.93
0.96
0.94
0.96
0.97
0.99
1.00
0.88
a CA, CN, SR, PT and OE refer to canola, corn,
sunflower, peanut and olive oil. P and S represent POA and SOA,
respectively. b Lee et al. (2015).
The average SOA PR from gas-phase emissions of the five cooking oils at an OH
exposure of 1.7×1011 moleculescm-3s was calculated
to be 1.35±0.30 µgmin-1. Torkmahalleh et al. (2012)
found that primary PM2.5 emission rates for peanut, canola, corn and
olive oils heated at 197 ∘C ranged from 3.7 to
54 mgmin-1. He et al. (2004) reported a PM2.5 emission rate
for frying in vegetable oils of 2.68±2.18 mgmin-1. The SOA
PR determined in this study was negligible compared with primary PM2.5
emission rates for heated cooking oils and frying in vegetable oils. However,
our results may underestimate SOA production from cooking under real-world
conditions. First, recent studies have demonstrated that the oxidation of
IVOCs and SVOCs evaporated from POA could produce significant SOA (Donahue et
al., 2006; Jimenez et al., 2009). In this study, POA from heated cooking oils
was filtered. SVOCs and IVOCs might not evaporate from the filter given that
they might be at saturation as the aerosol was cooled after the emissions.
Second, emissions of SOA precursors will be enhanced when cooking food
compared with heating cooking oils alone. For instance, long-chain aldehyde
emissions from frying processes can be 10 times those of heated oil (Klein et
al., 2016a). Large amounts of monoterpenes will be emitted when frying
vegetables or cooking with herbs and spices (Klein et al., 2016a, b; Liu et al., 2017). These enhanced
emitted precursors may significantly enhance SOA production. Finally,
laboratory and tunnel studies indicate that SOA production from typical
precursors and vehicle exhaust peaks at OH exposures higher than 5.0×1011 moleculescm-3s (Tkacik et al., 2014; Lambe et
al., 2015). The relatively lower OH exposures in this study compared with
typical conditions in the atmosphere may lead to the underestimation of
cooking SOA.
Mass spectra of POA and SOA
Figure 5 shows high-resolution mass spectra of POA and SOA at an OH exposure
of 2.7×1010 moleculescm-3s from heated canola oil.
Other oils have similar mass spectra, as reflected in the good correlations
shown in Table 2. The mass concentration of POA was approximately
35 µgm-3 for canola oil. The prominent peaks in POA from
canola oil were m/z 41 and 55, followed by m/z 29 and 43. The m/z 41,
43 and 55 were dominated by C3H5+, C3H7+ and
C4H7+ ion series, consistent with the previous observation by Allan
et al. (2010). The m/z 29 was instead dominated by ion CHO+, which
can be used as a tracer for organic compounds with alcohol and carbonyl
functional groups, as a result of thermal decomposition of the oils (Lee et
al., 2012). For the SOA mass spectra, the dominating peaks were m/z 28 and
29, followed by m/z 43 and 44. The m/z 28, 29, 43 and 44 were dominated
by CO+, CHO+, C2H3O+ and CO2+,
respectively. For all cooking oils, the mass fractions of m/z 28 and 44 in
SOA were higher, while the mass fractions of m/z 55 and 57 in SOA were
lower than those of the corresponding POA. The increase in mass fractions of
the oxygen-containing ions in SOA mass spectra indicated the formation of
oxidized organic aerosols.
Mass spectra of POA and SOA at an OH exposure of 2.7×1010 moleculescm-3s from heated canola oil.
Fractions of total organic signal at m/z 43 (f43) vs.
m/z (f44) from SOA data in this work together with the triangle plot
of Ng et al. (2010). SOA data from gasoline (Presto et al., 2014; Liu et
al., 2015) and diesel (Presto et al., 2014) vehicle exhaust measured in smog
chamber studies are shown. Data from this work and the literature are colored
according to OH exposure. Ambient SV-OOA and LV-OOA regions are adapted from
Ng et al. (2010).
The correlation coefficients (R2) between POA and SOA unit mass resolution
(UMR) spectra of heated oil and COA resolved by positive matrix factorization
(PMF) analysis (Lee et al., 2015) were calculated and summarized in Table 2
to evaluate their similarities. The POA mass spectra between different
cooking oils exhibited strong correlations (R2> 0.97) and agreed well
with the ambient COA factor obtained at roadside sites in the commercial and
shopping area of Mongkok in Hong Kong (Lee et al., 2015). The SOA mass
spectra between different cooking oils displayed good correlations
(R2> 0.94), suggesting a high degree of similarity. The mass spectra of
cooking SOA also greatly resemble POA and field-derived COA in ambient air,
with R2 ranging from 0.74 to 0.88. Kaltsonoudis et al. (2016) also
observed that the ambient COA factor in two major Greek cities in spring and
summer strongly resembled the aged SOA from meat charbroiling in a smog
chamber.
Fragments derived from the AMS data have been extensively used to explore the
bulk compositions and properties of ambient organic aerosols (Zhang et
al., 2005; Ng et al., 2010; Heald et al., 2010). Here, we use the approach of
Ng et al. (2010) by plotting the fractions of the total organic signal at
m/z 43 (f43) vs. m/z 44 (f44). The m/z 43 signal is abundant
in C3H7+ and C2H3O+ ions, indicating fresh, less oxidized
organic aerosols. The m/z 44 signal, usually dominated by CO2+ and
formed from the thermal decarboxylation of organic acids, is an indicator of
highly oxygenated organic aerosols (Ng et al., 2010).
In Fig. 6, we plot f43 vs. f44 of cooking SOA and SOA data from
gasoline (Presto et al., 2014; Liu et al., 2015) and diesel (Presto et
al., 2014) vehicle exhaust measured in a smog chamber, together with the
triangle defined by Ng et al. (2010) based on the analysis of ambient AMS
data. The ambient low-volatility oxygenated OA (LV-OOA) and semi-volatile OOA
(SV-OOA) factors fall in the upper and lower regions of the triangle,
respectively. Ng et al. (2010) proposed that aging would converge the
f43 and f44 toward the triangle apex (f43=0.02, f44=0.30). In this study, the f43 and f44 ranged from 0.06 to 0.10 and
from 0.05 to 0.07, respectively; they mainly lie in the lower portion of the
SV-OOA region. As shown in Fig. 6, SOA from gasoline and diesel vehicle
exhaust at a similar range of OH exposures had f44 values of 0.11–0.12.
Compared with vehicle exhaust, SOA formed from gas-phase emissions of heated
cooking oils was less oxidized. The potential SOA precursors from heated
cooking oils might be long-chain aldehydes, which are less volatile than SOA
precursors such as aromatics and long-chain alkanes from vehicle exhaust.
Generally, the presence of additional methylene and aldehyde reduce compound
vapor pressure by factors of 3 and 22, respectively (Pankow and Asher, 2008).
For example, the vapor pressure of n-tridecanal is approximately 14 %
of that of n-tridecane at 25 ∘C, as predicted by the
group-contribution model (Pankow and Asher, 2008). A single polar moiety of
first-generation products from long-chain aldehydes will have low enough
volatility to condense, while more volatile aromatics and long-chain alkanes
require more functionalization to form SOA (Donahue et al., 2012). Therefore,
SOA formed from heated cooking oils was less oxidized. For each cooking oil,
there was little change in f44 and a slight increase in f43 as OH
exposure increased. The increased SOA mass may facilitate the partitioning of
more volatile organics, leading to a slight increase in f43 and little
change in f44. This is consistent with the observation of previous
studies that the f44 of SOA from aromatics and monoterpenes varied
little and that f43 increased slightly for SOA mass loadings higher than
100 µgm-3 (Ng et al., 2010; Kang et al., 2011).
Evolution of (a) oxygen to carbon (O : C)
molar ratios and (b) average carbon oxidation state
(OSc) as a function of OH exposure during the aging of gas-phase
emissions from different heated cooking oils, with error bars indicating
standard error. Data at [OH] = 0 represent POA from cooking oils.
Van Krevelen diagram of POA and SOA from different heated cooking
oils. Error bars represent the standard deviations (1σ). SOA data are
colored by OH exposure. Average carbon oxidation states from Kroll et
al. (2011) and functionalization slopes from Heald et al. (2010) are shown
for reference.
Chemical composition of SOA
The O : C ratio and the estimated average carbon oxidation
state (OSc) (OSc≈2× O : C-H : C) (Kroll et
al., 2011) can be used to evaluate the degree of oxidation of organic
aerosols. Figure 7 shows the evolution of O : C ratios and
OSc of SOA from heated cooking oils as a O : C
ratios and OSc of SOA from heated cooking oils as a function of
OH exposure, together with the POA data. The O : C ratios
and OSc of POA were in the range of 0.14 to 0.23 and -1.61 to
-1.44, respectively, comparable to those of POA from meat charbroiling
(Kaltsonoudis et al., 2016). As shown in Fig. 7, for each cooking oil, the
O : C and OSc of SOA displayed similar trends,
initially decreasing rapidly and then increasing slowly or leveling off (for
canola oil only). In this study, the increased SOA mass loadings led to the
rapid decrease in the oxidation degree when the OH exposure increased from
2.7×1010 to 6.4×1010 moleculescm-3s. As
OH exposure and the resulting OA mass loadings further increase, even less
oxidized and more volatile organics partition into the particle phase and
thus decrease the oxidation degree (Donahue et al., 2006). The difference in
O : C for different cooking oils at the same OH exposure
may be attributed to the differences in gas-phase SOA precursors. In general,
the O : C ratios of SOA formed from gas-phase emissions of
heated cooking oils ranged from 0.24 to 0.46 at OH exposures of 2.7×1010-1.7×1011 moleculescm-3s. The OSc of cooking SOA was -1.51 to -0.81,
falling in the range between ambient hydrocarbon-like organic aerosol (HOA,
OSc = -1.69) and SV-OOA (OSc = -0.57)
corrected by the improved-ambient method (Canagaratna et al., 2015). As
suggested by Canagaratna et al. (2015), the OSc is more robust
than the f43 / f44 relationship for evaluating the oxidation
degree of organic aerosols, as the former has been estimated based on the
full spectra.
In Fig. 8 we plot the H : C and O : C
molar ratios of POA and SOA from heated cooking oils on a Van Krevelen
diagram. The cooking data fell along a line with a slope of approximately 0,
suggesting the chemistry of SOA formation in this study was alcohol/peroxide
formation (Heald et al., 2010; Ng et al., 2011). This slope is different from
ambient OA data of -0.8 determined by the improved-ambient method (Heald et
al., 2010). It is also different from vehicle exhaust data, with slopes
ranging from -0.59 to -0.36 (Presto et al., 2014; Liu et al., 2015).