Submicron aerosol chemical composition
At the LRK site, average OA loadings increased from spring (3.2 µg m-3) to summer
(5.3 µg m-3), and then decreased in fall (2.8 µg m-3), which is likely related to BVOC emissions that depend on
leaf surface area, solar radiation and ambient temperature (Fig. 2; Guenther et al., 2006). A different pattern was observed at the urban site
(Fig. 1), where average OA loadings were highest during the fall (8.2 µg m-3) followed by winter (7.2 µg m-3), suggesting
contributions from biomass-burning-related OA and non-biogenic sources. High
concentration of OA in fall is slightly lower than ACSM measurement in fall
2011 (Budisulistiorini et al., 2013), but consistent with HR-ToF-AMS
measurements in November 2012 (Xu et al., 2015a; Table S4), suggesting
the role of meteorology. Average OA contributions to NR-PM1 were higher
in spring and summer at JST and LRK, suggesting that biogenic SOA plays a
significant role during these periods. OA characterization is further
discussed in Sect. 3.2.
Seasonal averages of OA, inorganic species and pH from
JST (solid squares) and LRK (open triangles). Error bars show ±1 standard deviation. Seasons are classified into winter (wtr), spring
(spr), summer (smr) and fall.
![]()
Average sulfate concentrations were highest in summer for LRK (2.1 µg m-3) and fall for JST (∼ 2 µg m-3; Fig. 3).
This suggests that sulfate may contribute to enhanced SOA formation in this
region (Lin et al., 2013a; Xu et al., 2015b; Budisulistiorini et al.,
2015). Changes in sulfate concentration at LRK were mainly affected by
changes in SO2 emissions from electrical-generating units in the region
(Tanner et al., 2015). At JST, sulfate measurements are lower but still
within a standard deviation of those measured by HR-ToF-AMS in May and July
2012 in Atlanta (Xu et al., 2015a). SO2 emissions from coal-fired
power plants nearby Atlanta contributed to spatial variability of sulfate
concentration (Peltier et al., 2007). The average contribution of sulfate
to NR-PM1 loading was quite significant throughout the year, ranging
from 12 to 17 % at JST and from 21 to 31 % at LRK (Table 2). Average
concentrations of ammonium and nitrate were < 1
JST and < 0.5 µg m-3 at LRK. The average ammonium and
nitrate contribution to seasonal average NR-PM1 loadings is small
compared to OA and sulfate (Table 2). Both ammonium and nitrate showed
similar trends at the JST site, where they were highest during colder
seasons (winter and fall), while showing no significant fluctuations during
the duration of the study at LRK. This observation is consistent with
previous studies (Tanner et al., 2004b; Olszyna et al., 2005) reporting
that average contributions of ammonium and nitrate are not significant for
rural PM1. Average non-refractory chloride loadings were low (< 0.1 µg m-3), indicating that it is not a significant contributor
to inorganic aerosol mass in this region. The increasing average
contributions from the sum of sulfate, ammonium and nitrate in winter and
fall at JST suggests the important role of inorganics in NR-PM1, in
accord with observations in other major urban areas (Sun et al., 2011;
Petit et al., 2015).
The lowest seasonal average pH was observed in summer (1.45) for JST (Fig. 3) and in fall (1.53) for LRK (Fig. 3). On the other hand, the highest
seasonal average pH was 2.01 for JST and 1.81 for LRK, which were observed
during winter. Overall, seasonal aerosol pH was 1.5–2.0 at both sites,
indicating that NR-PM1 in the southeastern US is acidic year round.
This is consistent with a recent study by Guo et al. (2015). No direct
correlation (r2 < 0.1) was observed between aerosol pH and OA
at both sites. However, this does not necessarily rule out the potential
role of aerosol acidity in enhancing SOA formation in light of laboratory
studies, demonstrating a significant pH effect (Gao et al., 2004; Surratt
et al., 2007; Lin et al., 2013b). Uncertainty of aerosol acidity estimation
by ISORROPIA-II by omission of organic sulfate as input (Lin et al., 2014)
could lead to underprediction of aerosol acidity and the observed lack of
correlation with OA. Seasonal averages of LWC were highest during summer at
both JST (33.97 mol L-1 of aerosol) and LRK (38.17 mol L-1) sites.
It should be noted that the possible LWC contributions from OA are not
included because the organic hygroscopicity parameter estimated from observed
cloud condensation nuclei activities of OA (Guo et al., 2015) was
not available in this study. Studies have suggested that reactive uptake
decreases with enhanced RH (Nguyen et al., 2014; Gaston et al., 2014);
however, some isoprene-derived SOA tracers were elevated by high RH (Zhang
et al., 2011). Although organic water fraction in total LWC was found to be
significant, Guo et al. (2015) suggested that pH prediction using
ISORROPIA-II based on inorganic ions alone gave a reasonable estimate. The
lack of correlation between OA and pH as well as LWC indicates that pH and
LWC might not be limiting factors in OA production in this region, consistent
with previous studies in Georgia and Alabama (Xu et al., 2015a) and
Tennessee (Budisulistiorini et al., 2015). It should be noted that this
study did not include the contribution of organic water into pH estimation,
which could contribute to the relationship between pH and OA.
Mass spectra of PMF factors resolved from (a) winter,
(b) spring, (c) summer and (d) fall OA measured at JST in 2012.
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OA characterizations
The mass spectra and time series of OA factors resolved from PMF analysis at
JST in 2012 are provided in Figs. 4 and 5, respectively, and at LRK in 2013
are provided in Figs. 6 and 7, respectively. More PMF factors were resolved from JST OA
than from LRK OA, which could be due to a larger number of OA source types
in urban areas. Each factor had a distinctive time trend throughout 2012
(Fig. 5) at JST and 2013 at LRK (Fig. 7). OA measured at JST in 2012 and LRK
in 2013 was composed primarily of low-volatility oxygenated OA (LV-OOA) and
IEPOX-derived OA factor (IEPOX-OA). Concentrations of LV-OOA and IEPOX-OA at
both sites were 1.9 and 1.6 µg m-3 on average, respectively (Fig. 8). Hydrocarbon-like OA (HOA) and semi-volatile oxygenated OA (SV-OOA)
concentrations varied between 1 and 2 µg m-3 at JST and
biomass burning OA (BBOA) was ∼ 1 µg m-3 at both
sites. A biogenically influenced factor (91Fac) was observed only at LRK and
accounted for ∼ 1 µg m-3. Due to a lack of
measurements, the potential role of planetary boundary layer (PBL) height to
diurnal variation of PMF factors was not accounted for in this study.
However, diurnal PBL dynamics or loss processes (e.g. deposition) could
influence diurnal patterns observed here for the PMF factors.
Winter
PMF analysis of winter OA yielded a four-factor solution at JST (Figs. 4a
and 5a) and a two-factor solution at LRK (Figs. 6a and 7a). HOA, BBOA,
SV-OOA and low-volatility oxygenated OA (LV-OOA) factors (Ng et al.,
2011a) were resolved from the JST data set, whereas only the BBOA and LV-OOA
factors were resolved from the LRK data set. Increasing the number of factors
in PMF analysis of LRK data resulted in splitting factors that share
similarities with BBOA factor. Thus, we selected a two-factor solution (p= 2) for LRK in winter.
Annual temporal variation of PMF factors resolved from OA
measured at JST in 2012.
![]()
The temporal variation of the HOA factor correlates well (r2 > 0.7) with black carbon (BC), carbon monoxide (CO) and reactive
nitrogen species (NOy; Table S2). Moreover, its diurnal variation
(Fig. 9) showed a morning peak, consistent with an expected contribution
from vehicular emissions (Zhang et al., 2007).
The BBOA factor concentration increased during the night and decreased
during the day at JST (Fig. 9), which could be related to residential and
non-residential wood burning as well as PBL dynamics. BBOA at the LRK site
also showed a large nighttime peak with a gradual decrease during the day
(Fig. 10). The large peak appears to result from a short period of intense
biomass burning that occurred during 15–18 March 2013. Since a source for this
event could not be identified, we do not report it specifically in this
study. The time series of BBOA showed low to moderate correlation (r2
0.4–0.5 at JST and r2 0.2–0.4 at LRK) with BC, suggesting that it
is likely influenced by some local sources (e.g., fires). BBOA mass spectra
from JST and LRK were highly correlated (r2 ∼ 0.7),
indicating similarity of the sources. Comparison of the BBOA mass spectra
with reference mass spectra showed correlation with other OOA factors
(Tables S2 and S3), a known caveat in resolution of BBOA based on unit mass
resolution (UMR) data such as those from ACSM measurements (Wood et al.,
2010). The similarity of BBOA and OOA factor mass spectra could indicate
aging of the BBOA factor, which was observed to have enhanced signals at
m/z 18, 29 and 44 ions and low signals at m/z 60 and 73 ions (Bougiatioti et
al., 2014). However, the BBOA factor observed at JST and LRK displayed an
enhanced signal at m/z 44 ion but retained signals at m/z 60 and 73 ions,
suggesting that it was not as oxidized as the aged BBOA factor.
Mass spectra of PMF factors resolved from (a) winter,
(b) spring, (c) summer and (d) fall OA measured at LRK in 2013.
![]()
Annual temporal variation of PMF factors resolved from OA
measured at LRK in 2013. OA measurements in the summer included results from
the Southern Oxidant Aerosol Study (SOAS) campaign that have been published in
Budisulistiorini et al. (2015).
![]()
LV-OOA is characterized by a high fraction of total ion intensity at m/z 44
(f44) resulting from high oxygen content (Ng et al., 2011a) and is
the most abundant OA type at both JST and LRK (Table 2). Maxima around
midnight at JST (Fig. 9) and in the mid-afternoon at LRK (Fig. 10) were not
significant, indicating that LV-OOA concentration is relatively constant
throughout the day in this region. LV-OOA has been shown to correlate with
non-volatile secondary species, such as sulfate (Jimenez et al., 2009).
Weak correlation (r2 < 0.2) between LV-OOA and sulfate might be
due to a complex oxidation process, as previously observed in urban ambient
aerosol (Sun et al. 2011a). On the other hand, the mass spectral comparison
of LV-OOA from both sites showed strong correlation (r2 ∼ 1, Fig. S25), possibly suggesting similar sources of LV-OOA at these sites.
SV-OOA, which was observed only at JST, showed an f44 smaller than that
of LV-OOA (Fig. 4a), indicating that the factor is less oxidized and thus
semi-volatile (Ng et al., 2011a). The temporal variation of SV-OOA was
moderately correlated (r2 ∼ 0.4) with nitrate (Table S2)
while the mass spectrum was well correlated with previously resolved 82Fac
and IEPOX-OA factors (82Fac and IEPOX-OA are equivalent and are
characterized by a prominent ion at m/z 82; Robinson et al., 2011;
Budisulistiorini et al., 2013, 2015). Since
isoprene emission is expected to be negligible during winter season, SV-OOA
might not relate to IEPOX-derived SOA. The diurnal profile of SV-OOA showed
an increase in the evening and decrease in the morning, similar to the BBOA
profile and HOA factors. Moreover, it tracked well with the diurnal profile of
NO3-. This suggests a possible influence of nitrate-radical
chemistry on nighttime SOA formation during winter (Xu et al., 2015b;
Rollins et al., 2012).
Seasonal average mass concentrations of PMF factors
resolved from JST (solid squares) and LRK (open triangles). Error bars are
shown as ±1 standard deviation.
![]()
Spring
PMF analysis of spring OA resulted in a three-factor solution (i.e., HOA,
LV-OOA, and IEPOX-OA) for the JST site (Figs. 4b and 5b) and a three-factor
solution (i.e., LV-OOA, 91Fac, and IEPOX-OA) for the LRK site (Figs. 6b and 7b). Increasing the number of factors in PMF analysis of JST resulted in
splitting components, and thus, SV-OOA was not resolved in spring. The lack
of the SV-OOA factor might result from evaporation of semi-volatile species
in warmer periods and/or the inability of the ACSM to pick up on the
variability of a factor with low concentration. Similarly, a splitting
component was observed in PMF analysis of LRK data p= 4. Thus, BBOA and/or
HOA were not resolved from LRK in spring.
The average concentration of HOA in Atlanta was lower in spring (0.7 µg m-3) than in winter
(1.7 µg m-3), which could be influenced
by dilution – from a rise of the PBL –and evaporation of POA
during warmer conditions (Robinson et al., 2007). Although its
concentration decreases, the diurnal pattern of HOA was consistent from
winter to spring (Fig. 9) and correlation with primary species was strong
(r2 ∼ 0.6, Table S2).
Average LV-OOA concentration at JST also was the lowest in spring (1.4 µg m-3), which might be attributed to warming temperatures that elevate
the PBL and enhance atmospheric mixing. Diurnal variation of LRK LV-OOA
(Fig. 10) showed a small diurnal maximum in the afternoon, whereas no
variation was observed for JST LV-OOA (Fig. 9). LRK LV-OOA showed moderate
correlation with sulfate (r2 > 0.4, Table S3), suggesting
influence of sulfate at this site during spring (Tanner et al., 2015).
Although no correlation was found for JST LV-OOA vs. sulfate, comparison
of mass spectra revealed the same strong correlation (r2 ∼ 1, Fig. S25) between JST and LRK LV-OOA factors observed in
winter, suggesting possible similar sources over a regional scale.
Diurnal variations of OA and inorganic species measured
by ACSM (upper panel) and OA factors resolved by PMF analysis (lower panel)
from winter, spring, summer and fall measurements at JST in 2012.
![]()
Diurnal variations of OA and inorganic species measured
by ACSM (upper panel) and OA factors resolved by PMF analysis (lower panel)
from winter, spring, summer and fall measurements at LRK in 2013.
![]()
The IEPOX-OA factor, attributed to IEPOX heterogeneous chemistry
(Budisulistiorini et al., 2013; Lin et al., 2012), was resolved from
data sets at both JST and LRK. It was the second most abundant OA type after
LV-OOA at JST, but the most abundant OA component at LRK (Table 2). The
average IEPOX-OA concentration was slightly higher at LRK than at JST, which
is expected due to abundant emissions of isoprene at the forested site.
Diurnal patterns of IEPOX-OA are different at JST and LRK. At LRK, IEPOX-OA
has insignificant diurnal variability, which is likely influenced by small
variability of sulfate as previously observed at this site (Tanner et al.,
2005). However, a small increase in the afternoon and constant concentration
until the evening suggests that this factor is driven by photooxidation of
isoprene (Budisulistiorini et al., 2013). At JST, the diurnal pattern of
IEPOX-OA followed that of total OA, where it slightly decreased during the
day before it increased again in the evening. This diurnal pattern is
different from previous observations at JST during summer 2011
(Budisulistiorini et al., 2013), but quite similar to isoprene OA from May
2012 reported by Xu et al. (2015a), suggesting influence of year-to-year
changes in meteorology, such as precipitation and solar radiation (Table S1). Nevertheless, the mass spectra of IEPOX-OA at JST and LRK are tightly
correlated (r2 ∼ 1), indicative of similar composition.
91Fac was resolved only at the LRK site and accounted for 0.7–1.2 µg m-3. 91Fac has been attributed to various sources: monoterpenes-derived
SOA (Budisulistiorini et al., 2015; Boyd et al., 2015), biogenic SOA
(Chen et al., 2015) and aged BBOA (Robinson et al., 2011). However, a
recent field study identified ions at m/z 18, 29 and 44 as markers for aged
BBOA but not m/z 91 ion (Bougiatioti et al., 2014). Since BBOA was not
resolved from OA measurements in spring, aging of BBOA seems unlikely to be
the source of 91Fac in this study, although it cannot be conclusively ruled
out. The lack of 91Fac at the JST site suggests that its sources may be
limited to emissions and chemical processes in forested and/or rural areas.
91Fac will be further discussed in Sect. 4.2.
Summer
PMF analysis of summer OA resolved the same factors as spring at both sites:
HOA, LV-OOA and IEPOX-OA factors at JST (Figs. 4c and 5c), and LV-OOA,
91Fac and IEPOX-OA factors at LRK (Figs. 6c and 7c). Average HOA mass
concentration at JST increased in summer to ∼ 1.1 µg m-3 (Fig. 8). Temporal variation of
HOA was well correlated (r2 ∼ 0.6) with BC, CO and NOx (Table S2) and the diurnal
pattern was similar to that of spring (Fig. 9). Similar to spring, SV-OOA
was not resolved in summer, which could be attributed to rapid evaporation
of semi-volatile species under high ambient temperatures (Table S1).
Average LV-OOA concentrations at both sites increased in summer; however,
the proportional contribution decreased as a result of a larger contribution
of IEPOX-OA at JST and 91Fac at LRK (Table 2). The time series of LV-OOA was
weakly correlated with sulfate (r2 ∼ 0.2) at JST, but
more strongly correlated with sulfate at LRK (r2= 0.6–0.7; Tables S2–S3). The diurnal profile of LRK LV-OOA showed a local maximum in mid-afternoon
(Fig. 10) and has a moderate correlation (r2 ∼ 0.4) with
the sulfate (Table S3), suggesting that sulfate plays a role in LV-OOA in
summer at LRK. Comparison of JST and LRK LV-OOA mass spectra revealed a
strong correlation (r2= 0.94), possibly suggesting similar sources
between two sites.
Average concentration of the 91Fac OA at LRK was higher in summer than
spring, which indicates the role of meteorology – an increasing
temperature from ∼ 13 ∘C in spring to
∼ 21 ∘C in summer (Table S1). The relative
contribution of 91Fac to total OA increased at LRK (Table 2) and its diurnal
profile showed a local maximum around noon. A moderate correlation of 91Fac
with nitrate (r2 ∼ 0.5, Table S3) suggests that the
factor is moderately oxidized.
Scatter plots of the m/z 53 (possibly C4H5+),
m/z 75 (possibly C3H7O2+), m/z 100 (possibly
C5H8O2+) and m/z 101 (possibly C5H9O2+)
normalized fragment ions from the IEPOX-OA mass spectra vs. the m/z 82
normalized fragment ion from the same mass spectra over different seasons at
the JST and LRK sites. ACSM measures unit mass resolution (UMR); thus the
proposed formulas are based on previous study using HR-ToF-AMS (Lin et al.,
2012). The asterisk marker is the respective ion fragments of IEPOX-OA mass
spectra resolved from OA measurements during summer 2011 at the JST site
(Budisulistiorini et al., 2013).
![]()
Average concentration of IEPOX-OA at JST and LRK increased during summer. At
LRK, the average concentration of IEPOX-OA reached a maximum in summer, but
its relative contribution to total OA mass was lower due to the increasing
concentration of 91Fac. Concentrations of IEPOX-OA at both sites are
comparable (Fig. 8), suggesting that in summer this factor may become
spatially homogeneous in the southeastern US. Since measurements at JST and
LRK were conducted during different years, meteorological changes might play
a role in site-to-site comparison. At LRK, IEPOX-OA showed a small increase
around noon, while at JST there was a local maximum in the mid-afternoon,
suggesting an influx of IEPOX-OA likely transported from surrounding
forested areas. The time series of IEPOX-OA was moderately correlated with
nitrate (r2 ∼ 0.4) at JST, and at LRK, stronger
correlations (r2 > 0.5) with sulfate and nitrate were
observed, suggesting that this factor is moderately oxidized.
Fall
At JST, PMF analysis of fall OA resulted in a four-factor solution (i.e.,
HOA, BBOA, SV-OOA and LV-OOA), while at LRK a three-factor solution was
resolved (i.e., LV-OOA, 91Fac and IEPOX-OA). Increasing the number of
factors in PMF analysis of JST fall data resulted in factor splitting, and
thus, the IEPOX-OA factor was not resolved from this data set. Similarly, we
could not resolve the BBOA factor from LRK fall data because the analysis
resulted in splitting components.
The concentration of JST HOA increased to a level comparable to that in
winter (Fig. 5), which might be influenced by meteorology – a low
ambient temperature and less solar radiation – in fall and winter.
The correlation of the time series of HOA with BC, CO and NOx
(r2 > 0.7) was similar to spring and summer and slightly stronger
than in winter (Table S2) and the diurnal profile appears similar to that in
winter (Fig. 9). The presence of the HOA factor throughout the year at JST
is expected due to traffic emissions in urban areas (Xu et al., 2015a).
At JST, the BBOA factor was resolved again from OA with average
concentration and fractional contribution to total OA less than observed in
winter. The diurnal profile of BBOA during fall at JST appeared similar to
that in winter, suggesting similar emission sources as well as possible PBL
effect during these two colder seasons. The lack of the BBOA factor at LRK
could be attributed to the inability of the ACSM to capture a factor with
low concentration. In winter, the ACSM could capture the strong signal of
BBOA due to some periods of intense burning which would not be expected in
fall.
SV-OOA was also resolved from JST OA with slightly higher average
concentration and fractional contribution than that observed in winter. The
diurnal profile of fall SV-OOA was similar to that in winter, suggesting
similar sources and the role of the PBL. The return of SV-OOA might be influenced by
decreases in temperature from ∼ 26 ∘C in summer to
∼ 15 ∘C in fall (Table S1), resulting in less
evaporation of semi-volatile species.
LV-OOA was resolved from OA at both JST and LRK. Average concentrations of
LV-OOA remained relatively constant from summer to fall at both the urban
and rural sites (Fig. 8). However, the contribution of LV-OOA to total OA at
LRK increased due to decreasing concentrations of other OA factors (i.e.,
IEPOX-OA and 91Fac; Table 2). JST LV-OOA did not show diurnal variation,
whereas Xu et al. (2015a) observed a small diurnal variation by
HR-ToF-AMS. The mass resolution of the ACSM instrument is not as high as the
HR-ToF-AMS; thus, it might not be able to capture the diurnal variability.
LRK LV-OOA increased in mid-morning and reached a maximum around
mid-afternoon. Temporal variation of LV-OOA was weakly correlated (r2 ∼ 0.2) with inorganics at JST, but moderately correlated
(r2= 0.4–0.5) at LRK. Strong correlation of LV-OOA mass spectra
(r2 ∼ 1, slope = 0.8–1.1, Fig. S25) at JST and LRK
indicates a similar or identical source.
The concentration of 91Fac at LRK dropped significantly in fall. The drop
coincided with a decrease of total OA concentration and ambient temperature
– from around 20 ∘C to around 10 ∘C (Fig. 2).
Temperature has been shown to have a negative effect on SOA formation from
monoterpenes (Emanuelsson et al., 2013), but isoprene SOA is shown to be
dependent on temperature (Worton et al., 2013). Similar to 91Fac, IEPOX-OA
concentration at LRK also decreased in fall, suggesting that their sources
could be similar. The lack of the IEPOX-OA factor at JST is likely due to
reduced isoprene emissions, leading to low SOA formation, consistent with
previous studies (Budisulistiorini et al., 2013; Xu et al., 2015a).
Seasonal changes and contribution of OA sources
HOA was observed throughout the year at JST in 2012 and contributes
significantly to total OA (on average 21 %), while it was not observed at
LRK in 2013. Wider standard deviations in winter and fall suggest more
variability in HOA mass in Atlanta during these seasons.
LV-OOA, which was also observed throughout the year, contributes
30–43 % of the total OA on average. At LRK, LV-OOA was also observed throughout the
year, accounting for a large proportion of total OA in winter, up to 66 %.
Results from JST and LRK sites suggest that LV-OOA is annually and spatially
homogeneous, consistent with previous observations in this region (Xu et
al., 2015a).
BBOA was observed during winter and fall 2012 at JST and accounted for 17 % of total OA on
average. Standard deviations of mass concentrations indicate large variability of BBOA in winter (Fig. 8), which could be
related to increases of biomass burning in urban areas during colder
seasons. BBOA was observed only during winter 2013 at LRK. The average
concentration of BBOA at LRK was consistently ∼ 1 µg m-3, but several episodes of high levels resulted in a large standard
deviation (Fig. 8). The LRK site is located quite far from residential
areas; thus emissions from residential burning activities might not be well
captured by the ACSM during the fall season.
SV-OOA was resolved only at JST and only in the fall and winter, implying
that it might be formed from local aging process or transported from nearby
areas. SV-OOA contributed 27 % of the total OA on average. During spring
and summer, SV-OOA concentrations were probably at or below the ACSM limits
of detection due to higher ambient temperatures that likely promote
evaporation of semi-volatile species.
IEPOX-OA concentrations were elevated from spring to summer of 2012 at JST
and 2013 at LRK in accord with expected enhanced emission and photochemistry
of isoprene. In Atlanta (JST), concentration of IEPOX-OA was
38 % of total OA over spring and summer on average. Mass variability of JST IEPOX-OA
in summer was large, primarily as a result of a sharp peak in early July,
when levels were as high as ∼ 4 µg m-3. At LRK,
IEPOX-OA was observed in spring, summer and fall seasons with average
concentrations of 1.4, 2.1 and 0.8 µg m-3 in spring, summer and fall, respectively, contributing 36 % of total OA mass
on average. The drastic decrease of IEPOX-OA
concentration from summer to fall at LRK (Fig. 7) could be attributed to the
drop of ambient temperature that might affect SOA formation (Worton et
al., 2013). In addition, significant IEPOX-OA drop is consistent with loss of tree
foliage as a major source of isoprene emission, which is supported by a lack of IEPOX-OA during winter.
91Fac factor was observed during spring, summer and fall at LRK in 2013.
Seasonal average concentrations of 91Fac were 0.68
(21 %) in spring, 1.25 (23 %) in summer and 0.25 µg m-3 (9 %) in fall. Further discussion about the possible
source(s) of 91Fac is presented in Sect. 4.2. Decrease of 91Fac factor
from summer to fall (Fig. 7) coincided with decrease total OA and IEPOX-OA
factor, possibly suggesting a similar biogenic source.
Correlations of PMF factors resolved from OA measurements
at LRK, TN, against SOA tracers from monoterpene chemistry and isoprene
ozonolysis quantified during the 2013 SOAS. Some of the monoterpene SOA tracers
have been published in Budisulistiorini et al. (2015).
IEPOX-OA
LV-OOA
91Fac
Ref.
Monoterpene SOA tracers
C10H18O5S
0.28
0.26
0.39
Surratt et al. (2008)
C10H16O7Sa
0.42
0.26
0.37
Surratt et al. (2008)
C10H17NO7S
0.00
0.00
0.01
Surratt et al. (2008)
C9H15NO8Sa
0.12
0.22
0.22
Surratt et al. (2008)
C10H17NO10S
0.11
0.15
0.26
Surratt et al. (2008)
C8H12O4 (Terpenylic acid)a
0.32
0.36
0.41
Claeys et al. (2009)
C9H14O4 (Pinic acid)
0.12
0.21
0.19
C10H16O4 (Hydroxy pinonic acid)
0.15
0.21
0.25
C10H16O3 (Pinonic acid)
0.10
0.17
0.20
C7H10O4 (Terebic acid)
0.21
0.32
0.27
Yasmeen et al. (2010)
C8H12O6 (MBTCA)
0.15
0.27
0.14
Szmigielski et al. (2007)
C10H16O6 (DTAA)
0.35
0.42
0.42
Claeys et al. (2009)
Isoprene ozonolysis tracersb
C4H8O6S
0.46
0.40
0.51
Safi Shalamzari et al. (2013);
Riva et al. (2015)
C5H12O6S
0.39
0.19
0.35
Safi Shalamzari et al. (2013);
Riva et al. (2015)
C5H10O5S
0.19
0.19
0.22
Riva et al. (2015)
C5H10O6S
0.33
0.38
0.41
Riva et al. (2015)
C8H10O4S
0.00
0.07
0.03
Riva et al. (2015)
C6H12O7S
0.24
0.33
0.48
Riva et al. (2015)
C9H14O6S
0.21
0.30
0.38
Riva et al. (2015)
C9H16O7S
0.38
0.50
0.46
Riva et al. (2015)
C10H20O9S
0.36
0.29
0.39
Riva et al. (2015)
a Published in Budisulistiorini et al. (2015). b Only nighttime samples were used in PMF factor correlation
with isoprene ozonolysis tracers.