PM2.5 aerosols were collected at Fairbanks (64.51∘ N and
147.51∘ W) in central Alaska during the summer of 2009 and
analyzed for organic tracer compounds using a gas chromatograph–mass
spectrometer. The organic compounds were grouped into 14 classes based
on their functional groups and sources. Concentrations of the total organics
measured ranged from 113 to 1664 ng m-3 (avg 535 ng m-3).
Anhydrosugars (avg 186 ng m-3) and n-alkanoic acids (avg 185 ng m-3) were 2 major classes among the 14 compound classes. The similar
temporal trends and strong positive correlations among anhydrosugars and
n-alkanoic acids demonstrated that biomass burning (BB) is the major source
of organic aerosols (OAs) in central Alaska. The dominance of higher
molecular weight n-alkanoic acids over lower molecular weight homologs and their carbon preference index (5.6–9.8) confirmed that they were mostly
emitted from plant waxes during BB in central Alaska. The mass concentration
ratios of levoglucosan to mannosan denoted that softwood is the main biomass
burned. The rainfall event distinctly enhanced the levels of mannitol and
arabitol due to the growth of fungi and active discharge of fungal spores in
the subarctic region. Molecular compositions of biogenic secondary organic
aerosol (BSOA) tracers inferred that isoprene is a crucial precursor of BSOA
over central Alaska. Our results suggest forest fires and plant emissions to
be the crucial factors controlling the levels and molecular composition of
OAs in central Alaska. We propose that PM2.5 laden with OAs derived in
central Alaska may significantly impact the air quality and climate in
the Arctic via long-range atmospheric transport.
Introduction
Atmospheric aerosols can absorb and scatter solar radiation and alter the
radiative forcing of the atmosphere (Seinfeld and Pandis, 1998; Wilkening et
al., 2000). Fine aerosol particles have a diameter size close to the
wavelengths of visible lights and thus are expected to have a stronger
climatic impact than coarse particles (Kanakidou et al., 2005). They can
also be transported far away from the source regions, and thus their climatic
and environmental effects are delocalized compared to the emission areas.
Aerosol particles that are hydrophilic can act as cloud condensation nuclei
(CCN) and have an indirect climatic effect through modification of cloud
properties (Novakov and Penner, 1993; Novakov and Corrigan, 1996).
Organic aerosols (OAs) that are comprised of thousands of organic compounds
contribute about 20 % to 50 % of the total mass of fine particles in the
continental mid-latitudinal atmosphere (Saxena and Hildemann, 1996), whereas
it is around 90 % in tropical forest areas (Crutzen and Andreae, 1990;
Andreae and Rosenfeld, 2008). They are derived from anthropogenic and
natural sources. They can alter the physical and chemical properties of
atmospheric particles depending on the meteorological conditions. OAs are
highlighted for the past decade because they are related to the changes in
global and regional climate and chemical composition of the atmosphere as
well as public health. Primary organic aerosols (POA) are directly emitted
as particulate forms, whereas secondary organic aerosols (SOA) refer to
particulate organic matters that are transformed to the aerosol phase via
gas-phase oxidation of organic precursors. Emissions of POA particles and
SOA precursors can be released from numerous sources near the ground surface
and subsequently mixed in the boundary layer and to a lesser extent in the
free troposphere. The dry depositional removal of OAs mainly depends on the
sizes of the aerosol particles.
The molecular composition of OAs can be used as a tracer to better understand
the sources and formation pathways. Advances were made during the last
decade to better understand the formation of OAs and their precursors in the
atmosphere. On a global scale, the emission of biogenic volatile organic
compounds (VOCs) is 1 order of magnitude higher than that of anthropogenic
VOCs (Seinfeld and Pandis, 1998). It is notable that biogenic VOCs are
comprised of unsaturated hydrocarbons with double bonds and are more
reactive towards the atmospheric oxidants such as hydroxyl (OH) radicals and
ozone (O3) than anthropogenic VOCs that are largely comprised of
aromatic hydrocarbons. This specific feature of biogenic VOCs further
enhances their significance as a conceivable supplier to the global burden
of OAs in the atmosphere. Laboratory and chamber experiments have also
documented that biogenic VOCs are the potential precursor for SOA formation
in the atmosphere (Kavouras et al., 1998; Jaoui et al., 2007).
Although early Arctic explorers had noticed atmospheric haze (Nordenskiold,
1883), the remote Arctic atmosphere was believed to be extremely clean.
Pilots flying over the North American Arctic in the 1950s observed
widespread haze that could be seen every winter and early spring (Mitchell,
1957). It took until the 1970s for scientists to realize that the haze was
air pollution transported from the middle latitudes (Barrie, 1986). Over the
past 3 decades there has been much research on the climate consequences
of this pollution that is also present in summer. Surface air temperature
has increased more than the global average over the past few decades and is
predicted to warm by about 5 ∘C over a large part of the Arctic
by the end of the 21st century (IPCC, 2001). The Arctic atmosphere
is considered a unique natural laboratory for photochemical reactions and
transformations during the polar sunrise (Kawamura et al., 1996). Arctic
atmosphere is influenced by marine-derived OAs from the Arctic Ocean as well
as continentally derived OAs and their precursors from mid-latitudes in
Eurasia or North America (Stohl et al., 2006; Law and Stohl, 2007).
Previous analyses have reported a substantial contribution of summertime
boreal forest fires to the chemical composition of aerosol over the Arctic
(Iziomon et al., 2006; Kaplan and New, 2006; Stohl et al., 2006). French et
al. (2003) proposed that wildfire contributed a substantial amount of
carbon-based gas from 1950 to 1999 in the atmosphere of the boreal region of
Alaska. Based on the modeling and in situ observations of black carbon (BC)
and soot during the FROSTFIRE campaign, Kim et al. (2005) revealed that BC
and soot particles of 0.4 to 10 µm in radius can be transported to
the Arctic and the whole area of Alaska in a very short time. The results of
Kaplan and New (2006) delivered strong evidence that high-latitude
ecosystems are sensitive to climate change due to the increase in
concentrations of greenhouse gases. Iziomon et al. (2006) examined
summertime aerosols based on column-integrated and surface aerosol
measurements at Borrow in the North Slope of Alaska between 1998 and 2003.
They noticed high loadings of aerosols at least 8 d each summer and
demonstrated that the pollution events with the highest aerosol loadings
were associated with smoke from wildfires in northwestern Canada. Stohl et al. (2006) explored the impact of boreal forest fire emissions on the light-absorbing aerosol levels at the Barrow Arctic station. They proposed that
boreal forest fires could result in elevated concentrations of light-absorbing aerosols throughout the entire Arctic with an impact on the
radiation transmission of the Arctic atmosphere.
The results of Hegg et al. (2009) and Warneke et al. (2009) confirm that biomass burning (BB)
causes a more efficient transport and deposition of BC aerosol in Arctic
snow, causing a strong climate forcing at high latitudes. Based on in situ
measurements in the Arctic and a transport model of carbon monoxide (CO),
Warneke et al. (2010) proposed that BB plumes transported to the Arctic in
spring in 2008 more than doubled the Arctic atmospheric burden in other
seasons. The results of Ward et al. (2012) based on chemical mass balance
modeling revealed that wood smoke was the major source of PM2.5
particles, mainly during the winter months at several locations in Fairbanks.
Ward et al. (2012) and Wang and Hopke (2014) demonstrate that Arctic air
pollution could be so severe that the city of Fairbanks has been labeled
a serious non-attainment area by the United States Environmental Protection
Agency. Biogenic emissions from boreal forest largely increase during the
summertime growing season. The year-round measurements conducted at
Fairbanks by Haque et al. (2016) have shown that SOA derived from biogenic
VOC emissions dominated the organic chemical composition of total suspended
particles during summer in central Alaska. They found higher contributions of
isoprene oxidation products than monoterpene and sesquiterpene oxidation
products to SOA formation in summer due to the higher isoprene emissions and
high levels of oxidants. They estimated isoprene-derived secondary organic
carbon (SOC) approximately 5 times higher than SOA derived from monoterpene
and nearly 2 times higher than sesquiterpene-derived SOA in central Alaska.
Because climate change is generally proceeding fastest at the high latitudes
(Serreze et al., 2000; Hinzman et al., 2005), there is an increasing demand
for better understanding of the chemical compositions and sources of OAs in
the Arctic atmosphere.
We collected PM2.5 samples during the summer of 2009 at Fairbanks
in central Alaska. The samples were analyzed for several organic tracer
compounds to characterize OAs in the North American subarctic region. This
paper discusses the molecular compositions of various organic compound
classes and the factors controlling temporal changes in their concentrations
in central Alaska. We also discuss the sources of organic compounds detected
and the secondary formation processes as well as atmospheric implications
for the burden of OAs in the Arctic and subarctic atmosphere.
MethodologyDescription of the sampling area
Alaska is located in a subarctic zone. Fairbanks is the largest city in
central Alaska. It is situated in the central Tanana Valley connecting the
Chena River near the Tanana River. The location of the sampling site in
Fairbanks (64.51∘ N and 147.51∘ W) and its surroundings
are shown in Fig. 1. The altitude of the sampling location is 136 m above
sea level. The total area of Fairbanks is nearly 85 km2, with a population
of 31 500. The sampling site is located at the downside of Fairbanks, where a
forest is very close to the campus of the University of Alaska Fairbanks.
The highest levels of atmospheric aerosol burden in the United States have
been recorded in Fairbanks (Ward et al., 2012). The National Emission
Inventory database pointed out that forest fires and combustion of fossil
fuels are the two critical sources of air pollution in Fairbanks (Shakya and
Peltier, 2013; Ware et al., 2013).
Atmospheric particle samples of sizes less than 2.5 µm in diameter
(PM2.5) were collected on the rooftop of the International Arctic
Research Center building of the University of Alaska Fairbanks during the
summer season from 5 June to 21 September in 2009 when a forest fire was
active in the region. The collection of samples was performed using a
low-volume air sampler model, URG-2000-39EH (USA), with a flow rate of 16.7 L min-1. PM2.5 particles were retained on a quartz fiber filter of
47 mm in diameter that was pre-combusted at 450 ∘C for 6 h.
The sampler was operated for 3 to several days to get enough aerosol
particles on the filter to detect trace organic species with very low
concentrations. We collected 13 samples (Alaska 01 to 13) and three field blanks
during the campaign. The samples and field blank filters were individually
placed in a pre-heated glass vial with a Teflon-lined screw cap. We stored
the aerosol samples in a dark room at -20∘C to prevent the
samples from microbial degradation and loss of semivolatile organic
compounds.
Analysis of organic tracers
We analyzed the samples for organic compounds using a gas chromatograph–mass
spectrometer (GC–MS) system: a Hewlett-Packard (HP) model 6890 GC coupled to
an HP model 5973 mass-selective detector. A 5.0 cm2 filter area of each
aerosol sample was extracted with a 10 mL dichloromethane (CH2Cl2)
and methanol (CH3OH) mixture (2:1) through ultrasonication (10 min ×3). The solvent extracts were filtered through a Pasteur pipet
packed with pre-combusted (450 ∘C for 6 h) quartz wool to
remove particles and filter debris. The extracts were concentrated by a
rotary evaporator and then dried under a stream of pure nitrogen gas. The
hydroxyl (OH) and carboxyl (COOH) groups of organic compounds in the
extracts were derivatized to trimethylsilyl ethers and esters, respectively,
by the reaction with 50 µL N,O-bis(trimethylsilyl)trifluoroacetamide
including 1 % trimethylsilyl chloride and 10 µL pyridine at 70 ∘C for 3 h (Schauer et al., 1996; Simoneit et al., 2004a).
n-Hexane containing 1.43 ng µL-1 of a C13n-alkane internal
standard (40 µL) was added into the derivatives before injection of
the sample into a GC–MS.
The separation of compounds was performed on a 30 m long DB-5MS fused silica
capillary column (0.25 mm inner diameter and 0.25 µm film thickness).
Helium was used as a carrier gas at a flow rate of 1.0 mL min-1. The GC
oven temperature was programmed from 50 ∘C for 2 min to 120 ∘C at 30 ∘C min-1 and then 300 ∘C at 6 ∘C min-1 with a final isotherm held at 300 ∘C for
16 min. The sample was injected on a splitless mode with an injector
temperature of 280 ∘C. The mass detection was conducted at 70 eV
on an electron ionization mode with a scan range of 50 to 650 Dalton. The
organic compounds were determined by the comparison of the GC retention
times and mass fragmentation patterns of a sample with those of authentic
standards and National Institute of Standards and Technology library data.
The mass spectral data were acquired and processed using HP Chemstation
software. The GC–MS relative response factor of each compound was calculated
using authentic standards or surrogate compounds. The recoveries of
authentic standards or surrogates were above 80 % for target compounds.
The data reported here were not corrected for recoveries. The relative
standard deviation of the measurements based on duplicate analyses was
within 10 %. The field blank filters were analyzed by the procedure
described above. The target compounds were not detected in the blank
filters.
Meteorology and air mass trajectories
Figure 2 shows temporal changes in daily average meteorological parameters
at the campaign site. The daily mean temperature was in a range of 2.0 to 33 ∘C with an average of 13.9 ∘C, whereas the daily
average relative humidity ranged from 19 % to 99 % with a mean of 63 %.
The mean wind speed was 5.2 km h-1 and the total rainfall was 122 mm
during the sampling period. The 5 d air mass backward trajectories at the
height of 500 m above the ground level were computed from the Hybrid Single
Particle Lagrangian Integrated Trajectory model (Draxler and Rolph, 2013).
The air mass backward trajectories arriving over the observation site during
the collection of aerosol samples is presented in Fig. 3.
The daily average variations of meteorological parameters from 5 June to 21 September 2009 at the observation site in central Alaska.
The air mass backward trajectories over the observation site
during the collection of aerosol samples. The color scale shows the height
of the air parcel.
Results and discussionOverview of the molecular composition of organic aerosols
A total of 96 organic compounds were detected in PM2.5 samples
collected at Fairbanks during the sampling period. We grouped them into
14 compound classes as listed in Table 1 together with the mean
concentrations and ranges. Figure 4 shows the chemical compositions of OAs
in individual samples (Alaska 01 to 13). The levels of all the quantified
organic compounds in Alaskan samples ranged from 113 to 1664 ng m-3 (avg 535 ng m-3) with the predominance of anhydrosugars (avg 186 ng m-3) and
n-alkanoic acids (avg 185 ng m-3). Anhydrosugars are produced by
pyrolysis of cellulose and hemicellulose, followed by the subsequent emission
into the atmosphere, and are widely used as specific tracers of BB (Simoneit et al., 1999; Sang et al., 2013). n-Alkanoic acids are derived
directly from the surface of plant leaves and marine phytoplankton as well
as BB and meat cooking (Kawamura and Gagosian, 1987; Rogge et al., 1993;
Fine et al., 2001). Sugar alcohols were detected in ample amounts in three
samples collected during the end of the campaign. We also detected a
substantial amount of isoprene-derived SOA tracers and n-alkanols in Alaskan
samples, while the concentrations of other compound classes are relatively
low.
Chemical compositions of organic compounds in PM2.5 aerosols
from central Alaska. The sample collection periods are 5–12 June (Alaska
01), 12–25 June (Alaska 02), 25 June–4 July (Alaska 03), 4–6 July (Alaska
04), 6–14 July (Alaska 05), 14–23 July (Alaska 06), 23–30 July (Alaska 07),
30 July–4 August (Alaska 08), 4–8 August (Alaska 09), 8–25 August
(Alaska 10), 25–31 August (Alaska 11), 31 August–10 September (Alaska 12)
and 10–21 September (Alaska 13) in 2009.
Concentrations (ng m-3) of organic tracer compound classes
detected in PM2.5 aerosols from central Alaska.
Compound classesMinimumMaximumMeanMedianStandard deviationAnhydrosugars3174918669217Lignin acids0.5153.82.24.2Resin acid0.9196.15.14.7n-Alkanes0.577242123n-Alkanols5.3119462938n-Alkanoic acids9.256218582209Primary sugars0.344131211Sugar alcohols1.02414187.4Phthalate esters0.46.61.70.91.8Aromatic acid0.10.90.30.20.2Polyacids1.2103.32.62.5Isoprene oxidation products2.0142412043Monoterpene oxidation products1.0369.27.08.7Sesquiterpene oxidation products0.13.40.90.31.0Sum of all1131664535251517Anhydrosugars and lignin and resin products: tracers of biomass burning
The combustion of biopolymers including cellulose and hemicellulose as well
as lignin and suberin produces several organic molecules that have been
recognized as important source tracers of OAs. Anhydrosugars and lignin and
resin acids are specific tracers of BB among the pyrolysis products of
biopolymers (Simoneit et al., 1999). The pyrolysis of cellulose and
hemicellulose produces anhydrosugars such as levoglucosan and its isomers
mannosan and galactosan (Simoneit et al., 1999; Engling et al., 2009).
Figure 5a presents molecular distributions of anhydrosugars in Alaskan
PM2.5 samples. Levoglucosan is the dominant anhydrosugar, followed by
mannosan and galactosan in Alaskan samples. Their concentrations ranged from
23 to 463 ng m-3 (avg 125 ng m-3), 4.1 to 180 ng m-3 (avg 36 ng m-3) and 3.5 to 106 ng m-3 (avg 26 ng m-3),
respectively.
Molecular distributions of anhydrosugars and lignin and resin
acids in PM2.5 aerosols collected in central Alaska.
Because 90 % of levoglucosan exists in the atmospheric particles with
an aerodynamic diameter less than 2 µm (Giannoni et al., 2012), it is
reasonable to compare the levoglucosan concentrations of Alaskan PM2.5
samples with those reported in PM10 and TSP during summer or the BB season.
We found that the concentration levels of levoglucosan in central Alaska are
substantially higher than those from the Bering Sea (10 ng m-3) and
Arctic Ocean (5.2 ng m-3) (Hu et al., 2013), Chichijima (0.24 ng m-3) and Okinawa (0.57 ng m-3) islands in the western North
Pacific (Verma et al., 2015; Zhu et al., 2015), northern Japan (7.8 ng m-3) (Agarwal et al., 2010), and Mt. Everest (47.2 ng m-3) (Cong et
al., 2015). They are comparable to those reported at the urban site of Chennai in
India (avg 111 ng m-3) (Fu et al., 2010) but lower than those reported
at the rural site of Lumbini in Nepal (avg 771 ng m-3) (Wan et al., 2017),
the forest site of Rondônia in Brazil (avg 1180 ng m-3) (Graham et al., 2002),
and Chiang Mai in Thailand (avg 1222 ng m-3) (Thepnuan et al., 2019).
The emission strength of BB products and their long-range atmospheric
transport influence the atmospheric levels of anhydrosugars. The backward
trajectories reveal that air masses mostly came from the ocean during the
campaign (Fig. 3). This result shows that anhydrosugars present in the
Alaskan aerosols were mainly associated with the local and regional BB
during the campaign. The higher level of levoglucosan in Fairbanks than
other sites in the Arctic implies a possible effect of BB on the air quality
and climate in the Arctic region. Stocks et al. (2000) and Grell et al. (2011) proposed that the frequency of boreal forest fires recently increased
in summer due to global warming. Figure 6a–c show the temporal trends of
anhydrosugars in the Alaskan aerosols. The levels of anhydrosugars
expressively alter during the campaign period. The lower levoglucosan levels
were found at the beginning of the campaign, whereas they became very high
(241 to 463 ng m-3) on 4–23 July (Fig. 6a). Another peak of
levoglucosan was found on 30 July to 4 August (169 ng m-3). The
concentrations of levoglucosan decreased towards the end of the campaign (23
to 50 ng m-3). Forest fire smokes were seen during 4–23 July and 30 July to 4 August over central Alaska. This observation demonstrates that
levoglucosan levels became high due to the local forest fire in central
Alaska. Mannosan and galactosan presented similar temporal variations to
levoglucosan (Fig. 6b and c). The chemical reaction of anhydrosugars with OH
radicals could also influence their concentrations in the atmosphere.
Although previous studies have reported that levoglucosan can remain stable
in the atmosphere for around 10 d with no substantial degradation (Fraser
and Lakshmanan, 2000; Schkolnik and Rudich, 2006), recent findings (Hoffmann
et al., 2009; Hennigan et al., 2010; Gensch et al., 2018) reported
significant chemical reactivity of levoglucosan and have raised a question
over the stability of levoglucosan in the atmosphere. Hennigan et al. (2010)
carried out a smog chamber experiment and reported the lifetime of
atmospheric levoglucosan to be 0.7 to 2.2 d when exposed to 1×106 molecules of OH cm-3. This lifetime is within the range of 0.5
to 3.4 d predicted by Hoffmann et al. (2009) using the Spectral Aerosol
Cloud Chemistry Interaction Model. Lai et al. (2014) found that the
atmospheric lifetime of levoglucosan ranged from 1.2 to 3.9 d by the
control experiment integrating OH in a flow reactor under different
environmental conditions and different mixing states. Nevertheless, Bai et al. (2013) reported an atmospheric lifetime of levoglucosan to be 26 d
when exposed to an OH level of 2×106 molecules cm-3 that
is much longer than other predictions.
Temporal changes in the concentrations of biomass burning tracers
and other organic compounds in the Alaskan aerosols.
It is notable from the above discussion that the degradation of levoglucosan
is mostly induced by photochemical aging via oxidation by OH radicals during
long-range transport. Therefore, the degradation of levoglucosan could be
insignificant if the receptor site is close to the source region. As
discussed previously, anhydrosugars detected in Alaskan aerosols during the
campaign originated from local and regional BB, and we consider that the
degradation of anhydrosugars may not be important for explaining the low levels
of BB tracers in the samples collected at the beginning and end of the
campaign. The low concentrations of anhydrosugars during the beginning and
end of the campaign might be caused by the decreased emission rate of BB
tracers due to lower BB activities in the source region. Wet deposition may
be another reason to lower the level of anhydrosugars in aerosol samples
collected at the beginning and end of the campaign because we observed
rainfall especially on 5 June to 3 July and 6 August to 17 September in
Fairbanks (Fig. 2). Although the concentrations of both mannosan and
galactosan are much lower than levoglucosan (Fig. 5a), we observed strong
positive correlations (r=0.94–0.97) among these tracers (Table 2). This
result indicates that they might have originated from similar types of
biomass via the burning in central Alaska.
Statistical summary of correlations among the organic
tracers in PM2.5 aerosols from central Alaska.
Linear regressionCorrelationSignificance ofcoefficientcorrelation atP value < 0.05Levoglucosan vs. mannosan0.97SignificantLevoglucosan vs. galactosan0.94SignificantMannosan vs. galactosan0.95SignificantGlucose vs. fructose0.91SignificantGlucose vs. sucrose0.82SignificantFructose vs. sucrose0.94SignificantArabitol vs. mannitol0.95SignificantTrehalose vs. arabitol0.85SignificantTrehalose vs. mannitol0.74SignificantDEPa vs. DBPb0.85SignificantDEPa vs. DiBPc0.87SignificantDEPa vs. DEHPd0.71SignificantDBPb vs. DiBPc0.81SignificantDBPb vs. DEHPd0.88SignificantDiBPc vs. DEHPd0.75SignificantC5-Alkene triols vs. 2-Methyltetrols0.97SignificantGlyceric acid vs. tartaric acid0.84SignificantGlyceric acid vs. citric acid0.67SignificantTartaric acid vs. citric acid0.87SignificantBenzoic acid vs. glyceric acid0.53Not significantBenzoic acid vs. tartaric acid0.39Not significantBenzoic acid vs. citric acid0.17Not significantGlyceric acid vs. isoprene SOA tracer0.78SignificantTartaric acid vs. isoprene SOA tracer0.75SignificantCitric acid vs. isoprene SOA tracer0.67Significant
a Diethyl phthalate. b Dibutyl phthalate. c Diisobutyl phthalate. d Diethylhexyl phthalate.
Levoglucosan (L) is largely produced by thermal decomposition of cellulose,
while mannosan (M) is mainly a pyrolysis product of hemicellulose (Simoneit
et al., 1999). Klemm et al. (2005) found that hardwood contains
almost 55 % to 65 % of cellulose and 20 % to 30 % of hemicellulose.
Accordingly, in a laboratory chamber analysis, Schmidl et al. (2008) found
L / M ratios of nearly 2.5 to 3.9 for softwood burning and around 14 to 15 for
hardwood burning. It is worth using the L / M ratio to identify the relative
contribution of biomass types: hardwood vs. softwood in central Alaska. The
L / M ratios in Alaskan samples ranged from 2.2 to 6.8 (avg 4.6), which are
much lower than the ratios found in smoke samples derived from the burning
of hardwoods but almost similar to the ratios found in samples derived from
the burning of softwoods. The L / M ratios found in Alaskan aerosol samples
are also much lower than the ratios reported for the samples derived from
burning of rice straw (12.3 to 55.0) (Sheesley et al., 2003; Sullivan et
al., 2008; Engling et al., 2009), cereal straw (55.7) (Zhang et al., 2007),
wheat straw (12.7) and corn straw (19.5) (Cheng et al., 2013).
Fine et al. (2001, 2002, 2004) presented the concentrations of BB tracers in
fine particles derived from the burning of several wood species collected
from the United States. Based on their data, we calculated the L / M ratios to
be 3.4 to 6.7 for softwood burning and 10.7 to 83.4 for hardwood burning.
Our values from the Alaskan aerosol (2.2 to 6.8) are well within the range
of L / M ratios for softwood burning from the United States. The ratios in
Alaskan aerosol samples are comparable to those for marine aerosols
collected from the Arctic Ocean (avg 3.5) (Fu et al., 2013), South China
Sea (6.4) and western North Pacific (avg 4.6) (Fu et al., 2011). Our values
are also similar to those found in aerosol samples collected at Montana in
the USA (4.6) (Ward et al., 2006), the Vienna (4.1 to 6.4) and Salzburg (5.4 to
5.7) sites in Austria (Caseiro et al., 2009) and Moitinhos in Portugal (avg 3.5) (Pio et al., 2008), where BB was dominated by burning of softwoods but
was lower than the ratios estimated in aerosol samples collected at Chennai in
India (avg 11.2) (Fu et al., 2010), Karachi in Pakistan (avg 17.5) (Shahid
et al., 2016), Lumbini in Nepal (avg 15.1) (Wan et al., 2017), Morogoro in
Tanzania (9 to 13) (Mkoma et al., 2013), Chiang Mai in Thailand (14.1 to
14.9) (Tsai et al., 2013) and Rondônia in Brazil (avg 14.2) (Claeys et al.,
2010), where hardwoods and crop residues were the major sources of biomass
burning. Our results and the above comparison imply that softwood is most likely
biomass burned in central Alaska during the campaign.
Burning of lignin produces phenolic compounds such as 4-hydroxybenzoic
(4-HBA), vanillic and syringic acids, whereas dehydroabietic acid (DHAA) is a
specific pyrolysis product of resin present in the bark surface and needle
leaves and woody tissues of conifers (Simoneit et al., 1993). We detected
4-HBA and DHAA in the Alaskan aerosols, although their concentrations were
much lower than BB tracers produced from cellulose and hemicellulose burning
(Fig. 5b). The concentrations of 4-HBA and vanillic acid ranged from 0.4 to
6.4 ng m-3 (avg 1.7 ng m-3) and from 0.1 to 8.6 ng m-3 (avg 1.8 ng m-3), respectively, whereas those of syringic acid ranged from 0.02 to
1.1 ng m-3 (avg 0.2 ng m-3). Shakya et al. (2011) and Myers-Pigg
et al. (2016) reported that syringic to vanillic acid ratios for the burning
of woody and non-woody angiosperm range from 0.1 to 2.4, whereas the ratios
of softwood are 0.01 to 0.24. The concentration ratio of syringic to
vanillic acid can therefore be used as a marker to distinguish the type of
vegetation burned. We found that syringic to vanillic acid ratios in
Fairbanks aerosols ranged from 0.02 to 0.5 (avg 0.2), suggesting that
softwood is the more important biomass burned in central Alaska during the
campaign. This conclusion is consistent with the observation on the L / M
ratios as discussed above. The temporal variation of 4-HBA is very similar
to that of anhydrosugars, whereas vanillic and syringic acids presented
rather similar temporal trends to DHAA in Alaskan aerosols (Fig. 6d–g).
Simoneit et al. (1993) proposed that the emission of DHAA is different than
those of lignin and cellulose burning products, and therefore it is a more
specific molecular marker of the burning of conifer trees. The
concentrations of DHAA ranged between 0.9 and 19 ng m-3 (avg 6.1 ng m-3), which are higher than those of lignin pyrolysis products (Fig. 5b). This result suggests that the burning of conifer is a common source of
OAs in central Alaska.
Lipids: tracers of leaf waxes and marine sources
Series of lipid class compounds, including n-alkanes (C21 to C33),
n-alkanols (C8 to C30), and n-alkanoic acids (C12 to C32),
were detected in Alaskan aerosols. n-Alkanoic acids are the major lipid class
compounds in Alaskan aerosols (avg 185 ng m-3), which is several times
higher than those of n-alkanols (avg 46 ng m-3) and n-alkanes (avg 24 ng m-3) (Table 1). Figure 7a–c show the average molecular distributions
of lipid compounds in Alaskan aerosols. The molecular distribution of
n-alkanes is characterized by an odd-carbon-number predominance with maxima
at heptacosane (C27: avg 6.8 ng m-3). Low molecular weight (LMW) n-alkanes
are dominant in particles derived from fossil fuel combustion, whereas those
derived from leaf waxes are enriched with high molecular weight (HMW)
n-alkanes (Rogge et al., 1993; Hays et al., 2005; Wang et al., 2009). A
remarkable feature in the molecular signature of n-alkanes is the presence of
only the HMW species (C21 to C33) in Alaskan aerosols. This
molecular signature in the PM2.5 samples suggests that leaf waxes are
the major source of n-alkanes in central Alaska, with no significant
contribution from fossil fuel combustion. This feature is different from the
result of marine aerosols collected over the Arctic Ocean (Fu et al., 2013),
in which n-alkanes were mostly of fossil fuel origin. We conclude that fossil
fuel combustion is not an important source of OAs over central Alaska during
the summer campaign. This remark is consistent with the fact that the fossil
fuel biomarkers such as hopanes and steranes (Ding et al., 2009; Wang et
al., 2009) were not detected in the Alaskan samples. These results
demonstrate that biogenic n-alkanes emitted from boreal forest fires largely
overwhelmed fossil fuel combustion-derived n-alkanes in central Alaska.
Molecular distributions of lipid compounds in PM2.5 aerosols
collected in central Alaska. See Fig. 5 for the description of the
box-and-whisker diagram.
The carbon preference index (CPI) is a powerful tool to characterize the
anthropogenic vs. biogenic sources of lipid compounds (Simoneit et al.,
1991; Kawamura et al., 2003). The CPI value of n-alkanes in fossil fuel
emission is usually close to 1, while it is more than 5 for leaf waxes
(Peltzer and Gagosian, 1989). The calculated CPI values of n-alkanes in the
Alaskan aerosols are in the range of 5.2 to 9.9, with an average of 6.6.
These values are significantly higher than those reported in urban aerosols
from megacities in China (0.9 to 1.8) (Wang et al., 2006), India (1.2 to
2.3) (Fu et al., 2010) and Japan (1.1 to 2.8) (Kawamura et al., 1994), where
aerosol particles were seriously affected by fossil fuel combustion. The
broader range of CPI values was found in aerosol particle samples collected
over Mt. Tai (1.1 to 8.0) (Fu et al., 2008) and the western North Pacific
(1.8 to 15) (Kawamura et al., 2003), where the input of plant waxes
overwhelms the contribution of fossil fuel combustion. Together with these
assessments our results strongly infer that n-alkanes over the Alaskan
atmosphere mainly originated from plant leaf waxes. The wax covering the
external surface of a plant leaf is composed of a mixture of long-chain
aliphatic compounds. Kolattukudy (1976) found that odd-carbon-number
n-alkanes (C25 to C33) are one of the most abundant compound
classes in the leaf wax. Simoneit et al. (1991) considered the excess of odd
homologs minus the neighboring even homologs to be the abundance of plant-derived n-alkanes in atmospheric samples. The contribution of estimated
plant-derived n-alkanes to the total n-alkanes ranged from 53 % to 70 % (avg 61 %), implying that leaf wax is a major source of n-alkanes in the Alaskan
aerosols.
The average molecular characteristics of n-alkanols and n-alkanoic acids
displayed even-carbon-number predominance (Fig. 7b and c). n-Alkanols
presented maxima at docosanol (C22: avg 9.2 ng m-3), whereas
n-alkanoic acids demonstrated a peak at tetracosanoic acid (C24: avg 63 ng m-3). Microbes and marine phytoplankton are the sources of LMW
n-alkanols and n-alkanoic acids, while their HMW homologs are specifically
derived from higher plant waxes (Kawamura et al., 2003; Wang and Kawamura,
2005). Simoneit (2002) has proposed that BB also emits a large extent of
n-alkanols and n-alkanoic acids into the atmosphere. The average
concentrations of HMWn-alkanols (C21 to C30: 31 ng m-3) and
HMW n-alkanoic acids (C21 to C32: 122 ng m-3) are twice as high as those of LMW n-alkanols (C8 to C20: 15 ng m-3) and LMW
n-alkanoic acids (C12 to C20: 58 ng m-3) in the Alaskan
aerosols. This result shows that locally derived plant waxes might be the
source of HMW n-alkanols and n-alkanoic acids in central Alaska. The CPI
values of n-alkanols and n-alkanoic acids are in the range of 3.0 to 10 (avg 6.2) and 5.6 to 9.8 (avg 7.9), respectively, suggesting a large
contribution of plant waxes to lipid class compounds in central Alaskan
aerosols.
The concentrations of n-alkanes and n-alkanols slightly decreased from
the 5–12 June to late June samples (25 June to 4 July) and then dramatically
increased in the 4–6 July sample (Fig. 6h and i). The concentration peaks of
n-alkanes and n-alkanols were also observed in the sample of 14–23 July, whereas
their concentrations constantly decreased from 30 July to the end of the
campaign. The levels of n-alkanoic acids were low at the beginning of the
campaign and then increased drastically in the 4–6 July sample and remained
high in the two samples collected on 6–23 July (Fig. 6j). Concentrations of
n-alkanoic acids decreased from 30 July to 21 September. Fascinatingly, the
temporal variations of lipid class compounds were similar to those of
anhydrosugars (Fig. 6a–c and h–j). Figure 8a–c show the linear regression
analysis of lipid compounds with levoglucosan. We found strong correlations
(r=0.90–0.96) of lipid compounds with levoglucosan in Alaskan aerosols.
These results suggest that forest fires significantly control the
atmospheric levels of lipids in central Alaska via the evaporative ablation
of leaf waxes of terrestrial plants.
We also detected unsaturated n-alkanoic acids in Alaskan aerosol samples.
Oleic (C18:1) and linoleic (C18:2) acids are major constituents of
the cell membranes in terrestrial plants. They are released into the
atmosphere directly from the leaf surface by wind action (Yokouchi and Ambe,
1986; Noureddini and Kanabur, 1999). Fine et al. (2001) and Hays et al. (2005) proposed that BB also emits significant amounts of C18:1 and
C18:2 into the atmosphere. They are subjected to photochemical oxidation
in the atmosphere. C18:1 and C18:2 are more reactive due to a
double bond than C18:0 in the atmosphere with oxidants such as OH
radicals and O3. The ratio of C18:1+C18:2 to octadecanoic
acid (C18:0) is thus used as an indicator of photochemical processing
of OAs (Kawamura and Gagosian, 1987). The ratios ranged from 0.03 to 0.3,
with an average of 0.2 in Alaskan aerosols. Because average concentrations
of C18:1 (0.9 ng m-3) and C18:2 (0.5 ng m-3) in Alaskan
samples are significantly lower than that of C18:0 (10 ng m-3),
C18:1 and C18:2 may be rapidly degraded in the atmosphere by
photochemical oxidations.
Correlations of organic compounds with the biomass burning tracer
levoglucosan in the Alaskan aerosol samples.
Sugar compounds: tracers of primary biological particles
Nine sugar compounds were detected in Alaskan aerosol samples, with five
primary sugars and four sugar alcohols (Fig. 9). The concentrations of
primary sugars were in the range of 0.3 to 44 ng m-3 (avg 13 ng m-3), whereas those of sugar alcohols ranged from 1.0 to 24 ng m-3
(avg 14 ng m-3). The concentrations of total sugar compounds ranged
from 1.3 to 62 ng m-3 (27 ng m-3), in which sugar alcohols
contributed more to the total sugars (avg 54.2 %) than primary sugars
(avg 45.8 %) in Alaskan aerosols. Primary sugars are abundantly present
in vascular plants. They are produced during the photosynthetic process in
leaves and then accumulated in growing plants (Medeiros et al., 2006).
Figure 9a presents the average molecular distributions of primary sugars in
Alaskan aerosols. Primary sugars are characterized by the predominance of
glucose in Alaskan samples, with a concentration range of 0.1 to 19 ng m-3 (avg 6.8 ng m-3), followed by trehalose (avg 2.6 ng m-3). Although sucrose (avg 1.6 ng m-3) and fructose (avg 1.3 ng m-3) are not abundant (Fig. 9a), glucose showed strong positive
correlations with fructose (r=0.91) and sucrose (r=0.82) (Table 2).
Fructose also presented a strong correlation with sucrose (r=0.94) (Table 2). These correlations indicate their similar source and origin in the
atmosphere of central Alaska.
Molecular distributions of primary sugars and sugar alcohols in
PM2.5 aerosols collected in central Alaska. See Fig. 5 for the
description of the box-and-whisker diagram.
Glucose and fructose are carbohydrates enriched in tree barks as well as
branches and leaves (Medeiros et al., 2006; Li et al., 2016). They are
present in plant nectars and fruits as well as pollen and fern spores (Baker
et al., 1998; Graham et al., 2002). Dust and BB-derived particles have also
been reported as major sources of glucose and fructose in the atmosphere
(Nolte et al., 2001; Rogge et al., 2007). Sucrose is produced in plant
leaves and distributed to several portions of the plant body (Jia et al.,
2010). Sucrose has also been reported in airborne pollen grains produced
from blooming plants (Pacini, 2000), surface soil and associated microbiota
(Simoneit et al., 2004b) and dehydrated plant materials (Ma et al., 2009).
We found that glucose shows moderate correlation (r= 0.48) with
levoglucosan (Fig. 8d). Shafizadeh and Fu (1973) documented that glucose is
a minor product of cellulose pyrolysis. The predominance of glucose among
primary sugars together with a moderate correlation with levoglucosan
suggests that pyrolysis of cellulose and hemicellulose is not the source of
glucose in central Alaska. Pullman and Buchanan (2008) found that
soluble carbohydrates such as glucose are a major component of conifers,
where it can be stored in a large amount as deposited or dissolved free
molecules. The temporal trend of glucose showed a peak in the sample
collected during 14–23 July (Fig. 6k). Interestingly, the same sample showed
a high loading of DHAA that is a unique tracer of the burning of conifer
trees (Fig. 6g). This result suggests that the burning of conifer plants is
the source of glucose in central Alaska.
Trehalose is a well-known constituent of microbes and fungal spores as well
as plant species and suspended soil particles (Graham et al., 2003; Medeiros
et al., 2006). The levels of trehalose stayed constant from 5 June to 14 July,
dramatically decreased on 23 July to 8 August and then increased
towards the end of the campaign when rainfall occurred in central Alaska (Figs. 2 and 6m). This result shows that the major source of trehalose might
be the fungi in the surface soil of central Alaska that were emitted after
the rainfall event. Terrestrial plants and marine phytoplankton as well as
soil dust particles and associated microorganisms release xylose into the
atmosphere (Cowie and Hedges, 1984). Although xylose is a minor primary
sugar in Alaskan aerosols (avg 1.1 ng m-3), its temporal trend is very
similar to that of anhydrosugars (Fig. 6a–c and n). This result together
with a strong positive correlation of xylose with levoglucosan (r=0.92)
implies its BB origin in central Alaska (Fig. 8g). This finding is similar
to that of Sullivan et al. (2011), who documented that atmospheric levels of
xylose in the Midwestern United States were attributed to BB emission.
Sugar alcohols presented the predominance of arabitol (avg 6.6 ng m-3)
and mannitol (avg 6.2 ng m-3) (Fig. 9b). The concentration levels of
erythritol (avg 1.0 ng m-3) and inositol (avg 0.3 ng m-3) are
much lower than those of arabitol and mannitol in Alaskan aerosols. Arabitol
and mannitol concentrations were higher during the beginning and end of the
campaign than those during the middle of the campaign (Fig. 6o and p). We
found that arabitol and mannitol are strongly correlated (r=0.95),
implying their similar source in the Alaskan aerosols (Table 2). The major
sources of arabitol and mannitol are airborne fungal spores (Pashynska et
al., 2002; Bauer et al., 2008). Debris from mature leaves has also been
proposed as a source of arabitol and mannitol in the forest areas (Pashynska
et al., 2002; Zhang et al., 2010). Guasco et al. (2013) and Prather et al. (2013) proposed that bubble bursting of seawater contributes bacteria and
dissolves organic species along with sea salts to aerosol particles. Although
air masses mostly originated from the ocean (Fig. 3), the altitude of most
of the air masses dropped at several places and went on to Fairbanks by
maintaining low height. Therefore, we presume a negligible input of marine
sources to sugar alcohols in Alaskan fine aerosol samples. Arabitol and
mannitol were also detected in aerosol particles derived from BB (Fu et al.,
2012; Yang et al., 2012; Nirmalkar et al., 2015). We found insignificant
correlations of arabitol (r=0.16) and mannitol (r=0.27) with
levoglucosan (Fig. 8h and i). This result suggests that BB is not an
important source of arabitol and mannitol in the Alaskan aerosols.
The source of arabitol and mannitol might be fungi in the surface soil of
Fairbanks whose activities were high during the campaign. Elbert et al. (2007) suggested that the active ejection of fungal spores demands water
from the nearby atmosphere and release through osmotic pressure and surface
tension effects. As shown in Figs. 2 and 6o and p, arabitol and
mannitol concentrations in Alaskan samples are well connected with the
rainfall event. We found that the levels of arabitol and mannitol are high
during and after the rainfall. The rainfall increases the moisture contents
in surface soil, and thus fungal and microbial activities are enhanced in
central Alaska. This study implies that the precipitation stimulates the
release of fungal spores to increase the arabitol and mannitol levels in
Alaskan samples. Gottwald et al. (1997) and Burch and Levetin (2002)
reported that passive discharge of spores is enhanced under windy
conditions. This consideration further implies that fungal spores are
actively ejected into the atmosphere of central Alaska. Our finding is
consistent with the result of Elbert et al. (2007) from the Amazonia rainforest,
where the ambient fungal spores were controlled by the active discharge. It
is noteworthy that primary sugar trehalose presented significant positive
correlations with arabitol (r= 0.85) and mannitol (r=0.74) (Table 2),
showing that trehalose is also produced from surface soil under wet
conditions in central Alaska.
Phthalate esters: tracers of plastic burning
Phthalates are widely used as a plasticizer in synthetic polymers and as a
softener in polyvinylchloride (Thuren and Larsson, 1990; Wang et al., 2006).
They can be emitted into the atmosphere by evaporation from polymers because
they are not chemically bonded (Staples et al., 1997). The compositions of
phthalate esters are the subject of scientific discussion and public concern
due to their potential carcinogenic and endocrine-disrupting properties
(Sidhu et al., 2005; Swan et al., 2005). We detected four phthalate esters
in Alaskan aerosols, including diethyl phthalate (DEP), dibutyl phthalate
(DBP), diisobutyl phthalate (DiBP) and diethylhexyl phthalate (DEHP).
The ambient concentrations of total phthalate esters ranged from 0.4 to 6.6 ng m-3 (avg 1.7 ng m-3), which are slightly higher than those
from the North Sea to the high Arctic (0.4 to 1.0 ng m-3) (Xie et al.,
2007), comparable to or slightly lower than those observed in the North
Pacific (0.72 to 4.48 ng m-3) (Atlas and Giam, 1981), Great Lakes (0.1
to 10 ng m-3) (Eisenreich et al., 1981), and Canadian High Arctic (0.28 to
11 ng m-3) (Fu et al., 2009), but much lower than those reported in
Sweden (0.5 to 127 ng m-3) (Thuren and Larsson, 1990), mountainous
aerosols (9.6 to 985 ng m-3) (Fu et al., 2008) and urban aerosols from
megacities in India and China (62 to 2200 ng m-3) (Wang et al., 2006;
Fu et al., 2010). Figure 10 shows the average molecular distributions of
phthalate esters in Alaskan aerosols. We found DEP (avg 0.8 ng m-3) to be
a dominant phthalate, followed by DBP and DEHP (avg 0.4 ng m-3),
whereas DiBP was less abundant (avg 0.2 ng m-3). The predominance of
DEP among phthalate esters in Alaskan aerosol is different than those found
in marine aerosol from the Arctic Ocean (Xie et al., 2007; Fu et al., 2013)
and urban aerosols from India and China (Wang et al., 2006; Fu et al.,
2010), where DEHP was the dominant species. We found similar temporal
variations with significant positive correlations among detected phthalate
esters (r=0.71–0.88) (Fig. 11a–d and Table 2), suggesting that they have
similar sources in central Alaska.
Molecular distributions of phthalate esters in PM2.5
aerosols collected in central Alaska. See Fig. 5 for the description of
the box-and-whisker diagram.
Tracers of biogenic SOA
Significant progress has been made in the last decade to better understand
SOA formation from BVOCs such as isoprene, monoterpenes and sesquiterpenes
(Carlton et al., 2009; Ding et al., 2014; Jathar et al., 2014; Sarkar et
al., 2017). SOA is a crucial component of the atmosphere that has an impact
on the radiation budget directly by scattering sunlight and indirectly by
acting as CCN (Kanakidou et al., 2005; Carlton et al., 2009). Isoprene has
conjugated double bonds, and thus it is more reactive towards oxidants such
as O3 and NOx, resulting in various intermediates and stable
products via a series of oxidative reactions in the atmosphere. We detected
six organic compounds, including 2-methylglyceric acid (2-MGA), three
C5-alkene triols and two 2-methyltetrols (2-MTLs), as isoprene-SOA
tracers in the Alaskan aerosols. Their total concentrations ranged from 2.0
to 142 ng m-3 (avg 41 ng m-3), which are significantly higher
than those reported over the North Pacific (0.11 to 0.48 ng m-3) (Fu et
al., 2011), Canadian High Arctic (avg 0.30 ng m-3) (Fu et al., 2009),
North Pacific to Arctic (avg 0.62 ng m-3) (Ding et al., 2013), western
North Pacific (0.05 to 7.22 ng m-3) (Zhu et al., 2016), a forest site in
western Germany (avg 20.5 ng m-3) (Kourtchev et al., 2008a), and
Mumbai in India (avg 1.1 ng m-3) (Fu et al., 2016), but lower than
those in Mt. Changbai (22 to 280 ng m-3) (Wang et al., 2008) and Mt.
Fuji (avg 69 ng m-3) (Fu et al., 2014), Research Triangle Park in the USA
(19.9 to 384 ng m-3) (Lewandowski et al., 2007) and several sites in
China (8.65 to 554 ng m-3) (Ding et al., 2014).
Temporal changes in the concentrations of phthalate esters and
other organic compounds in the Alaskan aerosols.
Molecular distributions of biogenic secondary organic aerosol
tracers in PM2.5 aerosols collected in central Alaska. See Fig. 5 for
the description of the box-and-whisker diagram.
Molecular compositions of isoprene-SOA tracers are characterized by the
predominance of C5-alkene triols (avg 20 ng m-3) and 2-MTLs (avg 19 ng m-3) in Alaskan aerosols (Fig. 12). Surratt et al. (2010)
proposed that C5-alkene triols and 2-MTLs are higher-generation
products from the photooxidation of epoxydiols of isoprene under
low-NOx conditions. C5-Alkene triols were strongly corrected with
2-MTLs (r=0.97) in the Alaskan aerosols (Table 2). The abundant
co-presence of C5-alkene triols and 2-MTLs suggests their similar
sources and formation pathways in central Alaska. 2-Methylerythritol (avg 13 ng m-3) is twice as abundant as 2-methylthreitol (avg 5.9 ng m-3), similar to previous studies (Ion et al., 2005; Cahill et
al., 2006). The concentration of 2-MGA is significantly lower (avg 2.2 ng m-3) than C5-alkene triols and 2-MTLs (Fig. 12). Surratt et al. (2006) suggested that 2-MGA is produced by the oxidation of methacrolein and
methacrylic acid and has been detected as an important gas-phase
intermediate in the SOA formation from isoprene under high-NOx
conditions. Temporal variations of isoprene-SOA tracers were very similar to
each other in Alaskan aerosols (Fig. 11e–g). Their concentrations increased
in the sample of 12–25 June to 6–14 July and decreased in the sample of
14–23 July and 23–30 July. They increased significantly in the 30 July to 4 August
sample, quickly decreased in the 4–8 August sample, and then remained
comparable at the end of the campaign.
Molecular distributions of aromatic acids and polyacids in PM2.5
aerosols collected in central Alaska. See Fig. 5 for the description of
the box-and-whisker diagram.
Four organic acids were identified as monoterpene-SOA tracers in Alaskan
aerosols. They include 3-hydroxyglutaric acid (3-HGA), pinonic acid (PNA),
pinic acid (PA) and 3-methyl-1,2,3-butanetricarboxylic acid (3-MBTCA).
Hallquist et al. (2009) suggested that these acids are produced by the
oxidation of pinenes through reactions with OH radicals and O3. Their
total concentrations ranged from 1.0 to 36 ng m-3 (avg 9.2 ng m-3), which are higher than those reported in the North Pacific (0.02
to 0.22 ng m-3) (Fu et al., 2011), Canadian High Arctic (avg 1.6 ng m-3) (Fu et al., 2009), North Pacific to Arctic (avg 0.05 ng m-3)
(Ding et al., 2013), and western North Pacific (0.04 to 10.8 ng m-3) (Zhu
et al., 2016), and comparable to those reported at several sites in China
(3.09 to 33.8 ng m-3) (Ding et al., 2014) but lower than those reported at Mt. Fuji
in Japan (avg 39 ng m-3) (Fu et al., 2014), a forest site in Germany
(avg 25.6 ng m-3) (Kourtchev et al., 2008a) and Finland (11.1 to 217 ng m-3) (Kourtchev et al., 2008b). PA is most abundant (avg 3.4 ng m-3) among monoterpene-SOA tracers, followed by PNA (avg 2.3 ng m-3) (Fig. 12). The dominance of PA over PNA in summertime can be
explained by the much lower vapor pressure of PA than that of PNA. However,
this pattern is different from those found in summertime aerosols at the
summit of Mt. Tai in China (Fu et al., 2008) and other sites in Europe
(Kavouras and Stephanou, 2002) and North America (Cahill et al., 2006), in
which PNA is more abundant than PA.
PA and PNA are the initial photooxidation products of monoterpenes that can
be further photooxidized to 3-MBTCA in the atmosphere (Gómez-González et
al., 2012). The concentration ratio (3-MBTCA to PA+PNA) can therefore be
used to differentiate the fresh and photochemically processed
monoterpene-SOA tracers in the atmosphere. We found the ratios to be 0.1 to
1.4 (avg 0.5), demonstrating that monoterpene-SOA are mostly fresh in
central Alaska. The ratios of 3-HGA to 3-MBTCA are mostly lower when they
are produced from α-pinene as compared to β-pinene.
Lewandowski et al. (2013) documented a greater contribution of α-pinene to monoterpene-SOA tracers based on the 3-HGA / 3-MBTCA ratio of
about 1 in the southeastern United States than those observed in California
(1.8 to 3.8). Ding et al. (2014) also suggested α-pinene as the
major precursor in China based on the low ratios in Hefei (1.16) and
Qianyanzhou (0.75). We found the ratios ranged from 0.5 to 2.0 in Alaskan
aerosols (avg 1.0), indicating that α-pinene mainly contributes to
monoterpene-SOA tracers. 3-HGA and 3-MBTCA displayed somewhat similar
temporal variations, although the patterns are different from PA and PNA
(Fig. 11h–k), which showed very similar variations in Alaskan aerosols.
Sesquiterpenes are BVOCs with high reactivity and relatively low vapor
pressure (Duhl et al., 2008). β-Caryophyllene is the dominant
sesquiterpene. The ozonolysis or photooxidation of β-caryophyllene
produces β-caryophyllinic acid in the atmosphere (Jaoui et al.,
2007). Concentrations of β-caryophyllinic acid in Alaskan aerosols
ranged from 0.1 to 3.4 ng m-3 (avg 0.9 ng m-3), which are higher
than those from the Canadian High Arctic (avg 0.12 ng m-3) and Arctic
Ocean (avg 0.017 ng m-3) (Fu et al., 2009, 2013) but lower
than those reported at several sites in China (0.17 to 17.4 ng m-3)
(Ding et al., 2014) and Research Triangle Park in the USA (5.9 to 25 ng m-3) (Lewandowski et al., 2007). The temporal variation of β-caryophyllinic acid is very different from those of isoprene and
monoterpene-SOA tracers detected in Alaskan aerosols (Fig. 11l). Akagi et al. (2011) reported that biogenic VOCs could also be emitted from biomass
burning. Our result showed a high level of β-caryophyllinic acid in
the samples that were affected by BB in central Alaska. Ciccioli et al. (2014) proposed that sesquiterpenes could be accumulated in leaves and wood
because of low volatility and then abundantly emitted upon heating. The
temporal trend variation of β-caryophyllinic acid is similar to those
of anhydrosugars (Figs. 6a–c and 11l). Interestingly, we found a strong
correlation (r=0.98) of β-caryophyllinic acid with levoglucosan
(Fig. 8j), again indicating that forest fire largely contributes to the
formation of β-caryophyllinic acid in central Alaska.
Aromatic acids and polyacids: tracers of SOA
We detected benzoic acid in the Alaskan aerosol with the concentration range
of 0.1 to 0.9 ng m-3 (avg 0.3 ng m-3). Benzoic acid is produced from several
anthropogenic sources. It is a primary pollutant in the automobile emission
and smokes derived from burning of biomass and biofuels (Rogge et al., 1993;
Kawamura et al., 2002). It is also a secondary product of photochemical
degradation of toluene emitted from anthropogenic sources (Suh et al.,
2003). It can play an important role in enhancing the new particle formation
in the atmosphere (Zhang et al., 2004). The temporal variation of benzoic
acid is similar to anhydrosugars detected in Alaskan samples (Figs. 6a–c and
11m). We also found a strong positive correlation (r=0.95) of benzoic
acid with levoglucosan (Fig. 8k), demonstrating that BB is the source of
benzoic acid in central Alaska.
Contributions ( %) of individual organic compound classes to
organic carbon (OC) in PM2.5 aerosols from central Alaskaa.
Compound classesMinimumMaximumMeanMedianStandarddeviationBiomass burning tracers Anhydrosugars1.328.124.263.642.13Lignin and resin acidsb0.030.510.140.110.13Subtotal1.358.354.403.712.24Lipid compounds n-Alkanes0.058.531.550.982.19n-Alkanols0.4021.33.321.825.47n-Alkanoic acids0.6715.97.486.714.80Subtotal1.1645.812.49.2011.3Primary biological aerosols Primary sugars0.050.850.390.500.26Sugar alcohols0.070.950.460.330.33Subtotal0.171.500.850.740.56Phthalate esters0.021.070.140.050.28Aromatic acid0.010.090.020.010.02Polyacids0.020.250.080.090.06Biogenic SOA tracersIsoprene oxidation products0.073.201.280.831.12Monoterpene oxidation products0.070.750.340.340.20Sesquiterpene oxidation products0.020.040.030.030.01Subtotal0.183.991.661.221.29Dicarboxylic acids and related compounds1.152.971.901.870.58Total detected organic compounds6.3759.221.416.913.8
a All the organic compounds quantified were converted to carbon contents and then divided by OC. See Deshmukh et al. (2018) for OC and dicarboxylic acids and related compounds. b The results of lignin and resin acids were combined due to the very low contribution of resin acid to OC.
Polyacids are also the secondary photooxidation products of atmospheric
organic precursors (Wang et al., 2007; Fu et al., 2012). Concentrations of
total polyacids ranged from 1.2 to 10 ng m-3 (avg 3.3 ng m-3),
among which glyceric acid (avg 1.6 ng m-3) was dominant (Fig. 13).
Significant positive correlations were found among all polyacids (r=0.67–0.87) in Alaskan aerosols (Table 2). These results imply that polyacids
may have similar sources or formation pathways in central Alaska. We found
that polyacids showed no significant correlations with benzoic acid (r=0.17–0.53), which is mostly of BB origin in Alaskan samples, as discussed
above (Table 2). These correlations and different temporal trends of benzoic
acid and polyacids suggest that forest fires are not the main source of
polyacids in the Alaskan samples (Fig. 11m–p). This remark is further
supported by the insignificant correlations of polyacids with levoglucosan
(r=0.29–0.47) (Fig. 8l–n). Claeys et al. (2004) suggested that SOA tracers
such as tartaric acid are produced by the photochemical oxidation of
isoprene. Interestingly, significant positive correlations (r=0.67–0.78)
of polyacids were found with total isoprene-SOA tracers detected in Alaskan
samples (Table 2), suggesting that they may be produced by photooxidation of
isoprene in the Alaskan atmosphere.
Contributions of different compound classes to aerosol organic carbon
The contributions of each compound class to organic carbon (OC) in the
Alaskan aerosols are given in Table 3. BB tracers accounted for 1.35 % to 8.35 % (avg 4.40 %) of OC. The contribution of anhydrosugars to OC was
substantially higher (avg 4.26 %) than that of lignin and resin acids
(avg 0.14 %). This value from Fairbanks is notably higher than those
reported in aerosol samples collected from a round-the-world cruise (avg 0.15 %) (Fu et al., 2011), Gosan, Jeju Island in Korea (avg 0.29 %),
Sapporo (avg 0.44 %) and Chichijima (avg 0.06 %) in Japan (Simoneit
et al., 2004a), and Chennai in India (avg 0.59 %) (Fu et al., 2010). The
lipid compound classes in Fairbanks samples accounted for 1.16 to 45.8 %
(avg 12.4 %) of OC. n-Alkanoic acids contributed on average 7.48 %
(0.67 to 15.9 %), which is much higher than those estimated in samples of
the round-the-world cruise (avg 0.82 %), Sapporo (avg 0.62 %) and
Chichijima (avg 0.78 %) (Simoneit et al., 2004a; Fu et al., 2011). The
tracers of primary biological aerosol particles accounted for on average
0.85 % (0.17 % to 1.50 %), among which comparable contributions of
primary sugars (avg 0.39 %) and sugar alcohols (avg 0.46 %) to OC
were found in Alaskan aerosols. Plastic burning tracers accounted for 0.02 to
1.07 % of OC (avg 0.14 %), which is lower than those from the Sapporo
(avg 1.1 %) and Chichijima samples (avg 1.2 %) (Simoneit et al.,
2004a) and tropical samples from India (avg 4.50 %) (Fu et al., 2010).
Biogenic SOA tracers contributed 0.18 to 3.99 % of OC (avg 1.66 %),
among which the contribution of isoprene-derived SOA tracers was high (avg 1.28 %), followed by monoterpene (avg 0.34 %) and sesquiterpene (avg 0.03 %) SOA tracers. Other SOA tracers with minor contribution to OC
include polyacids (avg 0.08 %) and aromatic acids (avg 0.02 %).
With the consideration of water-soluble dicarboxylic acids and related polar
compounds measured in the same sample sets as reported in Deshmukh et al. (2018), the total organic compounds identified in the Alaskan aerosols
accounted for 6.37 % to 59.2 % with a mean of 21.4 % of OC. This result
indicates that a substantial fraction of OAs studied in the Alaskan site can
be identified at a molecular level.
Conclusions and implications
We identified 96 organic compounds in PM2.5 samples collected at
Fairbanks in central Alaska during the summer campaign in 2009.
Concentrations of total organic compounds ranged from 113 to 1664 ng m-3 (avg 535 ng m-3). The most abundant compound classes in the Alaskan aerosol are
anhydrosugars (avg 186 ng m-3) and n-alkanoic acids (avg 185 ng m-3). The temporal variations of anhydrosugars dramatically changed
during the campaign, showing peaks during BB events. The similar temporal
trends of lipids and strong correlations with levoglucosan demonstrated that
local forest fires likely control the atmospheric levels of OAs in central
Alaska. The concentration ratios of levoglucosan to mannosan (2.2 to 6.8)
and syringic to vanillic acid (0.02 to 0.5) suggest that burning of softwood
is a common source of OAs. The higher levels of HMW n-alkanoic acids and
n-alkanols than their LMW homologs together with high CPI values of
n-alkanes (5.2 to 9.9), n-alkanols (3.0 to 10) and n-alkanoic acids (5.6 to
9.8) further suggest that they were emitted by the thermal ablation of plant
waxes during forest fires in central Alaska. The temporal patterns of
mannitol and arabitol suggested that the rainfall played an important role in
enhancing their levels in central Alaska. The molecular compositions of
phthalate esters showed that diethyl phthalate is a commonly used
plasticizer in central Alaska. The molecular composition of biogenic SOA
tracers with a predominance of isoprene-SOA tracers (avg 41 ng m-3)
suggested that isoprene is a crucial precursor of SOA over central Alaska.
Our results provide valuable information to better understand the
compositions of OAs and their sources and formation pathways in the
subarctic atmosphere.
The Arctic is a critical region on the Earth, with a significant warming and
high sensitivity to climate forcing due to a strong effect on an albedo–sea
ice feedback system. Our results confirmed that forest fires and plant
emissions are important factors controlling the organic chemical composition
of fine aerosol particles in central Alaska. It is worthwhile therefore to note
from the above discussion that Fairbanks exemplifies many of the
problems of pollution in the Arctic regions. The local and regional BB
episodes in the warmer season enhanced the atmospheric levels of OAs in central
Alaska. Because the residence time of fine particles is relatively long in the
atmosphere, we propose that OAs of PM2.5 at Fairbanks can be subjected
to long-range transport to the Arctic causing a significant influence on the
air quality and climate in the Arctic region. Although we studied the
aerosol samples collected in 2009, further research is needed to
characterize the seasonal and interannual trends of OAs using more recent
aerosol samples to better evaluate their current impact in the Arctic
atmosphere.
Data availability
The data set of this paper is given in Table S1 in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-14009-2019-supplement.
Author contributions
KK designed the research. YK collected the aerosol samples. DKD and MMH
analyzed the samples for organic tracer compounds. DKD evaluated the data
and wrote the paper under the supervision of KK. All the authors contributed to
discussing results and commenting on the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge financial support from the Japan Society for the Promotion
of Science (JSPS). This work was partially
conducted under the Japan Agency for Marine-Earth Science and Technology –
International Arctic Research Center (JAMSTEC-IARC) collaboration project
with funding from JAMSTEC. We appreciate the financial support from a JSPS
fellowship to Dhananjay Kumar Deshmukh (PU17907) and the Special Grant Program for
International Students (SGPIS) of Hokkaido University, Sapporo, Japan, for
the financial support to M. Mozammel Haque. We thank Bang-Yong Lee for his support
during the sample collection.
Financial support
This research has been supported by the Japan Society for the Promotion of Science (grant no. 24221001) and National Research Foundation of Korea (grant nos. NRF-2016M1A5A1901769 and KOPRI-PN-19081).
Review statement
This paper was edited by Maria Kanakidou and reviewed by three anonymous referees.
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