A year-long near-real-time characterization of non-refractory submicron
aerosol (NR-PM
Characterization of the chemical composition of atmospheric fine aerosol is
important, because of its adverse human health effects (Pope III and
Dockery, 2006) and possible impacts on the Earth's climate system (Forster
et al., 2007). Aerosol with aerodynamic diameters
Over the past decade, online aerosol mass spectrometry (AMS) has been used
to extensively characterize ambient non-refractory (NR)-PM
Worldwide studies have shown that tropospheric PM
Studies in Atlanta, Georgia, have characterized the chemical components of
ambient aerosol collected during different seasons (Lee et al., 2002; Kim
et al., 2003; Butler et al., 2003); however, they were limited by low time
or mass resolution. A recent study reported characterization of ambient
NR-PM
Biogenic hydrocarbons and their oxidation products are major contributors to
ambient fine aerosol in rural areas where anthropogenic sources are low
(Budisulistiorini et al., 2015). In summer 2001, the fraction of non-fossil
carbon was reported to vary from 66 to 80 % of total carbon at Look Rock
(LRK), Great Smoky Mountains National Park (GSMNP), Tennessee (TN), indicating the
likely importance of photochemical oxidation of biogenic VOCs (BVOCs; Tanner et al., 2004a). Sulfate did not show significant diurnal
variability at LRK, TN, suggesting that local meteorological conditions are
less influential in determining concentrations of long-lived species
(Tanner et al., 2005). SOA is a predominant component of PM
Seasonal classification of measurements at JST and LRK is based on direction of angle of the Earth to the sun and the angle of the sunlight as it hits the Earth.
We present a 2-year study comparing near-real-time chemical
characterizations of NR-PM
Annual temporal variations of OA and inorganic species
(
Real-time continuous chemical measurements were conducted during 2012 at a downtown urban site (JST) in Atlanta, GA, and during 2013 at a rural/forested site (LRK) in GSMNP, TN, respectively. Analysis of data obtained from measurements at JST and LRK was classified by season (Table 1), which was able to capture changes in meteorology, in particular ambient temperature, at JST in 2012 and LRK in 2013 as illustrated in Figs. 1 and 2. The period with the coldest temperatures is classified as the winter season, and when the temperature rises, the period is classified as the spring season. Summer season is signified by constant high temperature at the JST and LRK sites. When temperature decreases after summer, this period is categorized as the fall season.
Organic and inorganic species characterizations during the 2013 Southern Oxidant Aerosol Study (SOAS; Budisulistiorini et al., 2015) were included in analysis of the summer season at the LRK site in this study. Detailed descriptions of both sites have been published (Budisulistiorini et al., 2013, 2015). Briefly, the JST site is one of several research sites of the Southeastern Aerosol Research and Characterization (SEARCH) network. The JST site is located in a mixed industrial-residential area about 4.2 km northwest of downtown Atlanta and within approximately 200 m of a bus maintenance yard and several warehouse facilities to the south and southwest (Hansen et al., 2003; Solomon et al., 2003), and within 53 km of a coal-fired power plant (Plant Bowen; Edgerton et al., 2006). The LRK site is located on a ridge top on the northwestern edge of the GSMNP downwind of urban areas, such as Knoxville and Maryville, TN, and small farms with animal grazing areas. Coal-fired power plants Kingston and Bull Run are located within 50–60 km northwest of LRK site (Tennessee Valley Authority, 2015). In summer, up-slope flow carries pollutants emitted in the valley during early morning to the LRK site by mid-morning, and in the evening down-slope flow accompanies a shift of wind direction to the south and east that could isolate the site from fresh primary emissions from the valley and allows aged secondary species to accumulate (Tanner et al., 2005).
Annual temporal variations of OA and inorganic species
(
Ambient NR-PM
Details of PMF analysis of the organic mass fraction have been described
previously (Lanz et al., 2007; Ulbrich et al., 2009; Zhang, 2011). The
PMF2 algorithm (Paatero and Tapper, 1994) was used in robust mode via PMF
Evaluation Tool panel (PET v2.04) using the methods outlined in Ulbrich et
al. (2009) and Zhang et al. (2011). Only the mass range
PMF analysis of year-long data collected from JST and LRK yielded similar
factor solutions as those obtained from seasonal data, but showed additional
factor splitting that made solid identification of unique factors difficult.
Therefore, we present results from PMF analysis performed separately for
winter, spring, summer and fall seasons for the JST and LRK sites. Solutions
were chosen based on the quality of PMF fits as well as interpretability
when compared to reference mass spectra (Ng et al., 2011a; Robinson et
al., 2011) and independent gas- and particle-phase measurements
(Budisulistiorini et al., 2013, 2015). For each
analysis, uncertainty of selected factor solutions was investigated with
different seeds (seed parameter varied from 0 to 100, in steps of 5), FPEAK
parameters, and 100 bootstrapping runs. PMF analysis of each season is
detailed in Figs. S1–S24 in the Supplement and correlations of selected PMF factors with
external tracers and reference mass spectra are provided in Tables S2–S3.
Seasonal averaged mass concentrations of non-refractory
PM
n.a. denotes values not available or resolved from PMF analysis. PMF analysis yielded some residuals of unresolved OA mass that make up the remaining percentage of OA factors.
The thermodynamic model, ISORROPIA-II, in forward mode (Fountoukis and
Nenes, 2007; Nenes et al., 1999), was used to estimate aerosol pH. Inputs
for the model include aerosol-phase sulfate, nitrate, and ammonium as
Seasonally averaged NR-PM
At the LRK site, average OA loadings increased from spring (3.2
Seasonal averages of OA, inorganic species and pH from
JST (solid squares) and LRK (open triangles). Error bars show
Average sulfate concentrations were highest in summer for LRK (2.1
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-PM
Mass spectra of PMF factors resolved from
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
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 (
Annual temporal variation of PMF factors resolved from OA measured at JST in 2012.
The temporal variation of the HOA factor correlates well (
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 (
Mass spectra of PMF factors resolved from
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
SV-OOA, which was observed only at JST, showed an
Seasonal average mass concentrations of PMF factors
resolved from JST (solid squares) and LRK (open triangles). Error bars are
shown as
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
The average concentration of HOA in Atlanta was lower in spring (0.7
Average LV-OOA concentration at JST also was the lowest in spring (1.4
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 (
91Fac was resolved only at the LRK site and accounted for 0.7–1.2
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
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 (
Average concentration of the 91Fac OA at LRK was higher in summer than
spring, which indicates the role of meteorology – an increasing
temperature from
Scatter plots of the
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 (
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 NO
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
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 (
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
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
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
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
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).
The IEPOX-OA component has been observed in chamber experiments and field OA
(Hu et al., 2015). Mass spectra generated by thermal decomposition of
isomeric authentic 3-MeTHF and IEPOX standards directly atomized into the
Aerodyne HR-ToF-AMS show major fragments at
Laboratory and field studies have reported significant signal of an
Seasonal characterization of NR-PM
Characterization of OA using PMF resolved a combined six factors at JST and
LRK sites, with different factors being resolved depending on location,
season and year. HOA and SV-OOA were only resolved at JST that represent
urban OA. BBOA, LV-OOA and IEPOX-OA were resolved from both sites during
different seasons, while 91Fac was only resolved from LRK site during warmer
seasons. HOA contributions to total OA mass were fairly consistent
(
Average IEPOX-OA contributions during warmer seasons were
This study was supported by the Electric Power Research Institute (EPRI). We thank Jerry Brown of Atmospheric Research and Analysis (ARA) as well as Bill Hicks of the Tennessee Valley Authority (TVA) for their assistance in collecting the collocated monitoring data at the JST and LRK sites, respectively. S. H. Budisulistiorini was supported in part by a Fulbright Presidential Fellowship (2010–2013) for attending the University of North Carolina at Chapel Hill, the UNC Graduate School Off-Campus Dissertation Research Fellowship and EPRI. Edited by: E. Browne