Introduction
The Qinghai–Xizang (Tibet) Plateau (QXP), often called “the third pole”
(Yao et al., 2012), is one of the most remote and isolated regions in the
world. The high altitude of this region has long been recognized as ideal
for studying the long-range transported air pollutants. However,
measurements from this area have been rare, usually due to the harsh natural
conditions and logistic difficulties. These restrictions have become less
problematic in the last decade because of the development of mountain
observatories and improvements in sampling instrumentation (Li et al., 2000;
Cong et al., 2007; Bonasoni et al., 2008; Hegde and Kawamura, 2012; Sang et
al., 2013).
Previous studies in the QXP region have focused on the chemical properties
of aerosols and their source signatures due to the important roles of
aerosols on climate forcing. A few studies conducted at the Himalayas revealed
that aerosols in this region are a complex mixture of inorganic and organic
compounds (Carrico et al., 2003; Rengarajan et al., 2007; Decesari et al.,
2010; Ram et al., 2010). Mineral dust was generally found to be an important
constituent of aerosols in the QXP region because of the presence of large
arid and semi-arid areas in “high Asia”. A relatively large proportion of
carbonaceous aerosols was also observed when the region was influenced by
air masses transported from South Asia, where widespread usage of biofuels
has led to large emissions of biomass burning aerosols (Engling et al.,
2011; Zhao et al., 2013). Indeed, analysis of the chemical compositions of
snow pit samples collected from a glacier in the central Himalayas indicated
that biomass burning particles were significantly enhanced in snow during
the winter–spring periods, due to transport of polluted air masses from
northwest India and Nepal (Xu et al., 2013b).
Although inorganic species (such as mineral dust) are important aerosol
components in the QXP, their chemical process in the atmosphere is
reasonably well characterized due to the small number of inorganic species
and their relatively simple chemistry. There have been increased interests
in the organic constituents of aerosol particles in recent years because of
their high abundances, complex chemical processing, and important roles in
affecting cloud properties (e.g., Kanakidou et al., 2005; Jimenez et al.,
2009). Organic aerosol (OA) is usually dominated by secondary species in
remote regions (Zhang et al., 2007) because of atmospheric aging processes
during long-range transport. For example, the average oxygen-to-carbon (O / C)
atomic ratio, which is an indicator for the oxidation degree of OA, observed
in a remote site in western Canada, was 0.83 (Sun et al., 2009), similar to
the O / C ratios of highly aged, low-volatility (LV) oxygenated organic aerosol (OOA) in the atmosphere determined via positive matrix factorization
(PMF) analysis of the aerosol mass spectrometer (AMS) spectra of OA (Ulbrich
et al., 2009; Zhang et al., 2011). However, the extent of OA oxidation and
the composition of organic species are closely related to the source
characteristics, and the aging processes that are involved, which have never
been carefully evaluated and determined in the QXP.
Most previous studies of aerosol chemistry in the QXP were conducted in the
Himalayan regions because of the key roles of the Himalayas on regional climate
and environment. There have been very few studies on aerosols reported from
the northern QXP (Meng et al., 2013), despite the fact that their
atmospheric behaviors might be significantly different from those found in
the Himalayan regions because of the different climate pattern and aerosol
sources in these two regions. For example, aerosols in the northern QXP
mainly originate from inland China, whereas aerosols in the Himalayas
mainly originate in India. Also, fine particles in the northern QXP tend to
contain large proportions of sulfate (Xu et al., 2014a; Zhang et al.,
2014b), while fine particles in the Himalayas are usually dominated by
carbonaceous material.
The Qilian Shan Station of Glaciology and Ecologic Environment (QSS) is
situated on the northern slope of the western Qilian Shan Mountains, in the
northeastern part of the QXP (Fig. S1 in the Supplement). Due to its high elevation (4180 m a.s.l)
and long distance (200 km) from local pollution sources, the QSS is
well suited for sampling background air masses. In addition, the QSS is
located
near the termini of several glaciers, making this area a unique atmospheric
environment where the photochemistry of snow and glaciers has a relatively
strong effect.
In our previous studies, the seasonal variations of the mass concentration
of water-soluble ions and the number concentration of particles at the QSS
have been characterized (Xu et al., 2013a, 2014a), which
presented two maxima in spring and summer. The first maximum
corresponded to the period during which dust storms predominantly occur in
North China, while the second peak corresponded to the period when the
thermal circulation between areas of high and low elevation is strongest,
where by the prevailing valley wind in the northeast of the QSS blew the
polluted air masses to this region. In the present study, we performed an
intensive field measurement study during the summer of 2012 with the aim of
determining the chemical characteristics and sources of fine particles at the
QSS during summer period. Here, we investigated the PM2.5 (particle matter) chemical
compositions and the properties of the associated organic chemical species,
such as elemental ratios, by applying a suite of instruments including a
high-resolution time-of-flight mass spectrometer (HR-ToF-AMS). The elemental
ratios of organic species are valuable in understanding the oxidation state
of OA and thus the aging processes that had occurred during its long-range
transport. The size distributions of the chemical species were also assessed
to understand the sources and chemical processes of aerosol.
Sample collection and analysis
Aerosol sampling
The samples were collected at the QSS atmospheric chemistry observatory
(39.50∘ N, 96.51∘ E; 4180 m a.s.l.; Fig. S1). The
sampling site and the QSS were described in detail by Xu et al. (2013a).
The summer climate at the QSS is dominated by the East Asian monsoon, which
brings about half of the annual precipitation (360 mm during 2008–2010).
This study was conducted from 11 July to 6 September 2012. PM2.5
samples were collected using a low-volume (16.7 L min-1 flow rate) aerosol
sampler (BGI, USA, model PQ 200), powered by solar cells. The instrument was
regularly calibrated using a TetraCal® calibrator (BGI, USA). All samples were collected on 47 mm
quartz fiber filters (Whatman, Maidstone, England). The flow rate was
measured at 5 min intervals by an internal volume flow meter, and the
recorded flow data were used to calculate the volume of air sampled.
Meteorological parameters including wind speed, wind direction, temperature,
precipitation, and relative humidity were recorded at 30 min intervals at
the meteorological station at the QSS, and the recorded data were used to
calculate the air volume sampled at standard temperature and pressure (STP;
1013 hPa and 273 K). Collection of each sample started in the morning and
continued for 3 days, and a total of 19 aerosol filter samples were obtained.
Three procedure blanks were collected in the field to assess potential
contamination that could have occurred during sampling preparation, transportation, and
storage. The sampling volume ranged from 42 to 44 m3 under STP (i.e., sm3),
with a mean (±1σ) value of 43.2 ± 0.5 sm3. Note that the mass concentrations reported here are all based on
STP (e.g., µg sm-3).
In addition, measurements of the size distribution of chemical species were
made at the QSS from 21 July to 4 September 2012. A 10-stage multi-nozzle
micro-orifice uniform deposit impactor (MOUDI; Model R110, MSP Corp.,
Shoreview, MN) was used to sample particles at a flow rate of 30 L min-1 over
a size range of 0.056–18 µm with nominal cut offsets of 0.056, 0.10,
0.18, 0.32, 0.56, 1.0, 1.8, 2.5, 5.6, 10, and 18 µm. The air pump was
calibrated before each sample was collected and the pump was closely
monitored to identify any changes of the flow rate. The collection
substrates were 47 mm quartz fiber filters which had been heated at 500 ∘C for 8 h prior to sample collection to remove adsorbed organic
material. The sampling time varied between ∼ 20 and
∼ 120 h depending on weather conditions. A total of four sets
of filters were obtained. All the filters were placed in individual
aluminum-lined plastic boxes and stored at -15 ∘C prior to
analysis.
Chemical analysis
The samples were analyzed to characterize their chemical compositions using
a series of instruments, namely ion chromatography (IC) instruments, a total
water-soluble organic carbon / total nitrogen (TOC / TN) analyzer, a HR-ToF-AMS,
and an organic carbon / element carbon (OC / EC) analyzer. For each OC / EC
analysis, a piece of filter measuring at 0.526 cm2 was punched from a
sample filter and analyzed directly using the instrument. The rest of that
filter was extracted by sonication in 15 mL deionized water for 30 min at
∼ 0 ∘C, and the extract was immediately filtered
using a 0.45 µm Acrodisc syringe filter (Pall Life Sciences, Ann
Arbor, MI, USA).
IC analysis
Eight ionic species (Na+, NH4+, K+,
Ca2+, Mg2+, Cl-, NO3-, and SO42-) were
determined using two IC systems (881 Compact IC Pro, Metrohm, Herisau,
Switzerland). One of the IC systems was used to determine cations, and was
equipped with a Metrosep C4 guard/2.0 column and Metrosep C4 250/2.0 column
(Metrohm), which were kept at 30 ∘C during measurements. The other IC
system, equipped with a Metrosep RP2 guard/3.6 column and a Metrosep A
Supp15 250/4.0 column (Metrohm), and kept at 45 ∘C during measurements,
was used to determine anions. The mobile phase in the cation IC system was
1.75 mM nitric acid (made from 70 % nitric acid, Sigma-Aldrich, St Louis,
MO, USA) and 0.75 mM dipicolinic acid (made from ≥ 99.5 % pure
dipicolinic acid, Sigma-Aldrich), and eluted at a flow rate of 0.3 mL min-1.
The eluent of anion IC system was 5 mM sodium carbonate and 0.3 mM sodium hydroxide (made from ≥ 98 % pure sodium hydroxide,
Sigma-Aldrich), and was used at a flow rate of 0.8 mL min-1. The
instruments were calibrated using standard cation and anion solutions
(Dionex, CA, USA). The IC analysis results were evaluated in terms of the
reproducibilities of peak retention times, peak heights, and the linearity
of each calibration curve. More details on the IC analysis methods are given
in Ge et al. (2014).
TOC and TN analysis
An aliquot of each sample was analyzed for TOC
and TN contents using a high-sensitivity TOC/TN analyzer
(TOC-VCPH
with a TNM-1unit, Shimadzu, Kyoto, Japan). The measurements were carried
out using the total carbon (TC) and inorganic carbon method. The TC was
determined by combusting the sample at 720 ∘C in a combustion tube
filled with an oxidation catalyst, which converted all carbon-containing
components into CO2. The CO2 was detected by a non-dispersive
infrared (NDIR) gas analyzer. The inorganic carbon was defined as the carbon
in carbonates and dissolved CO2 in the sample. The carbonates were
transformed into CO2 by treating the sample with 25 % (by weight)
phosphoric acid (H3PO4) in an inorganic carbon reaction vessel.
The CO2 was then volatilized using a sparging procedure and detected by
the NDIR analyzer. The TOC content was calculated by subtracting the
inorganic carbon content from TC content. In the TN analysis the
nitrogen-containing species were decomposed to NO in the combustion tube at
720 ∘C, then the sample was cooled and dehumidified using an
electronic dehumidifier, and the NO was measured using a chemiluminescence
gas analyzer. The total organic nitrogen (TON) was determined by subtracting
the inorganic nitrogen (which included ammonium and nitrate) content,
quantified by IC, from the TN content. The samples were contained in
well-sealed, pre-cleaned glass vials during analysis, and the instruments
automatically withdrew a sample aliquot after piercing the vial seal. The
calibrations were carried out using a potassium hydrogen phthalate standard
for the TC determination, a sodium hydrogen carbonate for the inorganic
carbon determination, and potassium hydrogen phthalate and potassium nitrate
standards for the TN determination.
HR-ToF-AMS analysis
Each filtered sample extract was aerosolized
using argon and dehumidified using a diffusion dryer. The resulting aerosol
particles were sampled into a HR-ToF-AMS instrument (Aerodyne Inc.,
Billerica, MA, USA) through an aerodynamic lens inlet, vaporized at
∼ 600 ∘C, ionized by 70 eV electrons in an electron
impact ionization chamber, and analyzed using the mass spectrometer. The
HR-ToF-AMS analysis procedure for aqueous samples and associated data
processing are described in detail elsewhere (Sun et al., 2010, 2011; Xu et
al., 2013b; Yu et al., 2014). The HR-ToF-AMS was operated in the “V” and
“W” ion optical modes alternatively, spending 2.5 min in each mode. In the
V-mode the HR-ToF-AMS spent 6 s in the mass spectrum (MS) mode and then 4 s
in the particle time-of-flight (PToF) mode, then continuously repeated this
10 s cycle. In the W-mode the instrument used only the MS mode, with 6 s in
each cycle. Between every two samples, purified waters (> 18.2 M cm-1, Millipore, USA) were analyzed in the same way to generate
analytical blanks. Each sample was measured twice to check the
reproducibility of the analysis. Elemental analyses were performed on
high-resolution mass spectra (W-mode, with m/z up to 120), and these were used
to determine the elemental ratios for oxygen to carbon (O / C), hydrogen to
carbon (H / C), nitrogen to carbon (N / C), and organic mass-to-organic carbon (OM / OC)
ratio of the water-soluble organic matter (WSOM) (Aiken et al., 2008). The
elemental contributions of C, O, H, and N reported in this study are
mass based. The signals of H2O+ and CO+ for organic compounds
were not directly measured but scaled to that of CO2+ based on the
evaluation of the H2O+ and CO+ signal in the samples (Fig. S3):
H2O+=0.94× CO2+, CO+=0.46× CO2+.
Potential interferences on the CO2+ signals in the aerosol mass
spectra caused by the presence of carbonate salts were evaluated by
acidifying a sample (QSS1) to pH 4 using sulfuric acid and then analyzing
the sample using the HR-ToF-AMS. Meanwhile, the volatility of the sample was
investigated by a digitally controlled thermodenuder (TD) system. The sample
was aerosolized and passed through a diffusion dryer, switched between the
by-pass (BP) mode and TD mode every 5 min, and finally analyzed using the
HR-ToF-AMS instrument. The TD system was programmed to cycle through 12
temperature steps (30, 50, 70, 100, 150, 200, 180, 130, 110, 88, 66, and
then 40 ∘C). The design and use of the TD system have been
described elsewhere (Fierz et al., 2007). The data were processed in a
similar way as the normal HR-ToF-AMS data analysis, and the remaining
particle mass calculated from the difference between the results of the TD
mode and the BP mode analyses were assumed to indicate the volatility of
individual aerosol species.
OC and EC analysis
The samples were analyzed for OC / EC using a
Thermal/Optical Carbon Analyzer (DRI Model 2001). The procedure used has
been described in detail elsewhere (Cao et al., 2003). Briefly, the system
heated the 0.526 cm2 punched quartz filter aliquot gradually to 120 ∘C
(fraction OC1), 250 ∘C (fraction OC2), 450 ∘C (fraction OC3), and 550 ∘C (fraction OC4) in a
non-oxidizing helium atmosphere, and then to 550 ∘C (fraction
EC1), 700 ∘C (fraction EC2), and 800 ∘C (fraction EC3)
in an oxidizing atmosphere of 2 % oxygen in helium. The carbon evolved at
each temperature was oxidized to CO2, then reduced to methane
(CH4) and quantified using a flame ionization detector. Some of the
organic carbon was pyrolyzed to form black carbon as temperature increased
in the helium atmosphere, resulting in the darkening of the filter. This
darkening was monitored by measuring the decrease in the reflectance of the
sample using light at 633 nm from a He–Ne laser. The original black carbon
and the pyrolized black carbon combusted after being exposed to the
oxygen-containing atmosphere, and the reflectance increased. The amount of
carbon measured after exposure to the oxygen-containing atmosphere until the
reflectance reached its original value was reported as the optically
detected pyrolized carbon (OPC). The eight fractions, OC1, OC2, OC3, OC4,
EC1, EC2, EC3, and OPC, were reported separately. OC was defined as OC1 +
OC2 + OC3 + OC4 + OPC and EC was defined as EC1 + EC2 + EC3 - OPC
in the IMPROVE (Interagency Monitoring of Protected Visual Environments) protocol.
Determination of the WSOM and water-insoluble organic mass concentrations
Time series of (a) meteorological data, (b) wind speed and wind
direction colored by time of day (Beijing Time), (c) mass concentrations of water-soluble ions, organic carbon (OC), elemental carbon (EC), and total organic
nitrogen (TON) in PM2.5, and (d) percent contributions of various
species to total mass.
![]()
The
mass concentrations of WSOM and water-insoluble OM (WIOM), and the average
OM / OC ratio for the organic matter (WSOM + WIOM) in PM2.5 are
estimated using Eqs. (1)–(3)
WSOM=WSOC×OM/OCWSOM ,WIOM=(OC-WSOC)×1.3,OM/OCOM=(WSOM+WIOM)/OC,
where WSOC is the water-soluble organic carbon content in the filter extract
measured by the TOC / TN analyzer, OM / OCWSOM is the OM / OC ratio of the
WSOM (determined from the HR-ToF-AMS measurement), OC is taken from the
filter measurements using the thermo-/optical carbon analyzer, the constant
1.3 in Eq. (2) is the estimated OM / OC for WIOM (Sun et al., 2011), and
OM / OCOM is the average OM / OC ratio of organic matter in PM2.5.
Estimation of the secondary organic aerosol concentrations
The
secondary organic carbon (SOC) content was estimated by determining the
primary organic carbon (POC) content using EC as a tracer, and then
subtracting the POC from the measured total OC. The primary OA (POA)
concentration was estimated based on POC, which was subtracted from the
total OM calculated as the product of the measured OC, and the OM / OCOM
determined in Eq. (3) to determine the secondary OA (SOA) concentration.
The equations for these calculations are shown below:
POC=(OC/EC)pri×EC,SOC=OC-POC,POA=OM/OCPOA×POC,SOA=OC×OM/OCOM-POA.
The calculation of POC was based on the hypothesis that OC and EC correlate
strongly and stay at a constant ratio in primary particles within a
geographical region. Organic aerosol observed at the QSS mostly
originated from lower attitude regions including an urban area east and west
of the QSS based on the typical diurnal pattern of the wind field around the QSS (Xu
et al., 2014a). Based on previous studies, the OM / OC for fresh urban
organics in northern China is between 1.2 and 1.6 (e.g., Xu et al., 2014b;
Zhang et al., 2014a). Since it is expected that organics would be oxidized
gradually during transport, we use the ratio of 1.4 for OM / OC of the
POA (OM / OCPOA) at the QSS.
Results and discussion
Chemical speciation of PM<inline-formula><mml:math display="inline"><mml:msub><mml:mi/><mml:mn>2.5</mml:mn></mml:msub></mml:math></inline-formula>
The meteorological conditions during the measurement period were overall
cold and humid. The air temperature (T) ranged from -5.9 to 14.3 ∘C, with an average of 4.2 ∘C, and the relative
humidity (RH) ranged from 10 to 99 %, with an average of 65 % (Fig. 1a);
3-day air mass back trajectories originating at ∼ 100 m a.g.l. were acquired every 6 h during the sampling period and
showed that 72 and 23 % of the air masses came from west and east of
the sampling site, respectively (Fig. S1). Light precipitation occurred
frequently between 11 July and 19 August (Fig. 1a) because of the
topographic effect, but it was relatively dry from 19 August to 6 September,
probably because of the occurrence of a different synoptic-scale weather
pattern. The 3-day average wind data also showed that the wind speed
from west increased during the late part of the sampling period (Fig. S2).
Wind direction changed diurnally, with moderate mountain wind (from the
southeast at ∼ 2 m s-1) during the night and stronger valley
wind (from the north at ∼ 4 m s-1) during the day (Fig. 1b). The
concentrations of various chemical species, including water-soluble ionic
species (WSIs), OC, EC, and TON, changed significantly according to weather
condition throughout the sampling period. No dust storm event was observed
during the study even though the QSS is close to the desert regions.
(a) The average composition of the species analyzed and (b) the
charge balance between the cations (Na++ NH4++ K+
+ Mg2++ Ca2+) and anions (Cl-+ SO42-+
NO3-).
![]()
The total mass concentrations of the measured species (WSIs + OC + EC
+ TON) throughout the sampling period were in the range of 1.8–8.0 µg sm-3
with the average (±1σ) at 3.7 ± 1.9 µg sm-3 (Fig. 2a), which were lower than that was measured in
2010 (2.7 µg sm-3 in 2012 vs. 5.4 µg sm-3 in 2010 for
the same WSIs in July and August) at the QSS (Xu et al., 2014a), probably
because of the low frequency of dust storm events in 2012. Indeed, mass
concentration of calcium was more than 4 times lower in 2012 (0.27 µg sm-3) than that in 2010 (1.2 µg sm-3). Overall, sulfate was
the main contributor (on average 36 %) to the aerosol mass concentrations
during the observation period (Fig. 2a), similar to previous observations in
the northern QXP (Li et al., 2013; Xu et al., 2014a; Zhang et al., 2014b).
The mass concentration of sulfate was also lower in 2012 (1.4 µg sm-3) than that in 2010 (2.7 µg sm-3). Major cations
– ammonium (10 %), calcium (6.6 %), sodium (1.5 %), magnesium
(0.5 %), and potassium (0.3 %) – and anions – sulfate (36.2 %), nitrate
(16.9 %), chloride (0.9 %) – together accounted for 45–88 % (mean = 73 %)
of the total aerosol mass. The ion balance, expressed as the ratio
of the equivalent concentration (µeq sm-3) cation to that of anion
(C / A), is shown in Fig. 2b. The mean C / A ratio was 0.97, which was close to
1, further indicating that the contribution of mineral dust was negligible
since carbonate and bicarbonate were not measured in the IC analyses. The
average EC concentration was 0.09 µg sm-3 during the observation
period, and its mean contribution to the total PM2.5 mass was 2.6 %.
The average (±1σ) concentrations of OC and TON were 0.66
(±0.43) and 0.24 (±0.16) µg sm-3, respectively. The
OC and EC concentrations found in this study were lower than those found in
the summer of 2010 at Qinghai Lake (1.6 and 0.4 µg sm-3), which is
also in the northern part of the QXP. The concentrations were different
probably because Qinghai Lake is located at a lower elevation
(3200 m a.s.l.;
Li et al., 2013) than the QSS (4180 m a.s.l.) and is subjected to more
emissions from the boundary layer.
The correlation coefficients (r) between all of the chemical species are
shown in Table 1, and the strong correlations (r≥0.75) are shown in
bold. In general, strong correlations were found between Na+ and
Ca2+, Mg2+, and Cl-, which are representative of primary
species in mineral salts, and among secondary ions such as SO42-,
NO3-, and NH4+. In addition, the good correlations
between secondary ions (SO42- and NO3-) and K+ and
Mg2+ might arise from acid replacement on mineral particles. The facts
that WSOC and OC tightly correlate (r=0.97) and that WSOC accounts for
79 % of the OC indicate that a majority of OC in fine particles was
secondary at the QSS (more discussions are given in Sect. 3.3).
Correlation coefficients (Pearson's r) between the water-soluble
inorganic ions, organic carbon (OC), elemental carbon (EC), water-soluble
organic carbon (WSOC), and total organic nitrogen (TON) concentrations
(n=19). Values that indicate a strong correlation (i.e., r≥0.75) are
in bold.
Na+
NH4+
K+
Mg2+
Ca2+
Cl-
SO42-
NO3-
EC
WSOC
OC
NH4+
0.28
K+
0.38
0.87
Mg2+
0.84
0.51
0.63
Ca2+
0.80
0.04
0.23
0.79
Cl-
0.85
0.32
0.41
0.83
0.77
SO42-
0.57
0.88
0.86
0.77
0.47
0.58
NO3-
0.41
0.81
0.77
0.67
0.35
0.42
0.83
EC
-0.22
0.23
0.22
-0.26
-0.28
-0.22
0.10
0.03
WSOC
-0.04
0.17
0.12
-0.10
0.10
-0.03
0.16
0.17
0.59
OC
-0.01
0.16
0.14
-0.05
0.17
-0.01
0.18
0.23
0.56
0.97
TON
-0.10
0.81
0.61
0.16
-0.16
-0.08
0.65
0.67
0.51
0.45
0.43
Chemically resolved size distributions
The size distributions of all species are shown in Fig. 3, and the sum of
the species presents a prominent accumulation mode peaking at a MOUDI stage of
0.32–0.56 µm and a coarse mode peaking at 1.8–3.2 µm. In the
accumulation mode, sulfate dominated (39 %) the composition of particles
in the size range of 0.18–1 µm, with OC and EC accounting for 24 %
and 5.0 %, respectively, followed by TON (9.4 %), NH4+
(7.9 %), NO3- (3.9 %), Cl- (2.8 %), K+ (1.3 %),
and Ca2+ (0.6 %). However, OC dominated (36 %) the composition of
particles smaller than 0.18 µm with sulfate contributing 26 %. In
the coarse mode, nitrate was the main contributor, accounting for 23 % of
the particle mass in the size range 1.8–5.6 µm, followed by OC
(17 %), sulfate (16 %), TON (12 %), Ca2+ (11 %), Cl-
(8.4 %), and EC (2.2 %); the rest of the species in total accounted for
3.7 % at this size range. TON and OC were important contributors of
particle mass over the whole size range (0.056–18 µm).
(a) Average size distributions of the mass concentrations of water-soluble inorganic ions (SO42-, NO3-, Cl -,
NH4+, Ca2+, Mg2+, and K+), total organic nitrogen
(TON), elemental carbon (EC), and organic carbon (OC). (b) The fractional
contributions of individual species to total mass in different size bins.
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The different size distributions of different species suggested that they
had different sources and/or have undergone atmospheric transformation
processes. The species that were relatively abundant in the accumulation
mode aerosols were mainly secondary species, such as ammonium, sulfate, and
OC, while the species that were relatively abundant in the coarse mode
aerosols were mainly primary mineral ionic species, such as Ca2+,
Na+, and Cl-. Nitrate was closely associated with dust particles
as a result of its formation through the reactions of HNO3 gas with
carbonate salts (such as calcite and dolomite) (Sullivan et al., 2009) which
could form Ca(NO3)2 and Mg(NO3)2 (Li and Shao, 2009). As
shown in Fig. 4, the equivalent balances of water-soluble species in
different size modes indicate that the accumulation mode particles were
somewhat acidic (with the linear regression slope of [NH4++
Ca2++ Mg++ K+] vs. [SO42-+ NO3-]
being 0.6) and that the coarse mode particles were almost neutral (the slope
was 0.999), similar to the results observed at Mount Hua in 2009 (Wang et al.,
2013). The sizes of WSIs in this study were smaller than that observed at other
sites, e.g., Hong Kong, where the accumulation mode was at 1–1.8 µm
and the coarse mode was at 3.2–5.6 µm (Xue et al., 2014). The
smaller mode size at the QSS was probably because of the lower specific
humidity at the QSS.
(a) Average equivalent concentrations water-soluble inorganic
species in individual size bins. The vertical dashed line indicates the
boundary between the accumulation mode and the coarse mode. (b) Scatter plot
that compares the equivalent concentrations of cations (i.e., NH4+
+ Ca2++ Mg2++ K+) and the equivalent concentrations
of anions (i.e., SO42-+ NO3-) in the accumulation
mode and coarse mode particles, respectively.
![]()
The OC and TON species in the coarse mode probably came from soil organic
matter or were formed by the condensation of volatile organic gases on
mineral dust. The EC and OC species reaching maximum concentrations in the
accumulation mode, consistent with other results, indicating the occurrence
of aging and aqueous processing of particles (Yu and Yu, 2009).
Relationship between OC, EC, and WSOC
Scatter plots of (a) the organic carbon (OC) concentration against
the elemental carbon (EC) concentration and (b) the water-soluble organic
carbon concentration (WSOC) against the OC colored by the sampling date.
![]()
The relationship between OC and EC concentrations can provide useful
insights into the origin of carbonaceous aerosols, because particles from
different sources have different OC / EC ratios. The correlation between EC
and OC concentrations at the QSS was statistically significant (r2= 0.4, n=19), with the slope of 3.29 and the intercept of 0.23 (Fig. 5a and
b). The OC / EC ratios ranged from 2.8 to 26.4 with an average of 7.6, which
is higher than those observed from Chinese urban sites (1 to 4) (Cao et al.,
2003). However, the OC / EC ratio at the QSS was similar to the ratios found
at remote sites in western China (Fig. S1) such as Qinghai Lake (6.0 ± 3.9 in PM2.5) during the summer of 2010 (Li et al., 2013), Muztagh Ata
mountains (11.9 in total suspended particles) during the summers of 2004–2005
(Cao et al., 2009), and Akdala (12.2 in PM10) during July 2004 and
March 2005 (Qu et al., 2009). The high OC / EC ratios observed at the QSS
suggest the formation of secondary OC during the long-range transport. The
chemical characteristics of particulate organics can be further evaluated
using the WSOC / OC ratio, which can be used to assess the aging of organic
species. High WSOC / OC ratios (> 0.4) have been found in aged
aerosols because a significant portion of the OC can be oxidized to WSOC
(Ram et al., 2010). A tight correlation between WSOC and OOA was observed
previously in Tokyo, indicating that OOA and WSOC have very similar chemical
characteristics (Kondo et al., 2007). Field studies have shown that the OOA
factors derived from multivariate analysis AMS organic aerosol mass spectra
are generally representative of SOA (Zhang et al., 2005, 2011). The WSOC and
OC concentrations in the QSS were strongly positively correlated (r2=0.97)
with a slope of 0.79. This slope is higher than those in Chinese
urban sites (0.3–0.6) (Pathak et al., 2011) and at remote sites in western
China, such as Qinghai Lake (0.42) during the summer of 2012 (Li et al.,
2013) and the Himalayas (0.26–0.51) (Rengarajan et al., 2007; Ram et al., 2010;
Shrestha et al., 2010). However, it is similar to the average (±1σ)
ratio of WSOC to OOA (0.88 ± 0.29) in Tokyo (Kondo et al.,
2007). The high OC / EC ratios, tight WSOC and OC correlation, and the high
WSOC / OC ratios found in the aerosol particles from the QSS can be regarded
as a solid evidence for the formation of SOA in the QXP region.
Spectra characteristics of the WSOM
Average water-soluble organic carbon (WSOM) mass spectrum and mass
concentration fraction (pie charts) colored by the contributions of six ion
categories and elements (C, O, H, and N) for PM2.5 filter samples.
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The average MS for the WSOM in PM2.5 is shown in Fig. 6. The major
spectral features are the high mass fraction of m/z 44 (f44) (mainly
CO2+, 94.6 %), m/z 18 (H2O+), and m/z 28 (CO+). The
f44 peaks were almost identical (∼ 20 %) in all filter
samples, and in two samples, QSS3 and QSS18, the contributions of f44 peaks
were particularly high, at 27.4 and 27.3 %, respectively. It has
previously been suggested that CO2+ ion in the HR-ToF-AMS MS is
typically associated with the presence of carboxylic acids (Takegawa et al.,
2007), which can be the oxidation products of organic species through
heterogeneous and homogeneous chemical processes (fragmentation) (Jimenez et
al., 2009). This can be supported by the low intensity fragments in the
higher m/z range (m/z > 50), which were probably caused by the
fragmentation of organic species during oxidation and conversion into small
organic acids (increased f44). The similarities between the mass spectra of
the acidified and untreated QSS1 sample (r2=0.98; Fig. S4) ruled
out the potential influence of carbonate salts on the intensity of the
CO2+ peak. The O / C ratio, an oxidation degree index, has a mean
value of 1.16 and a range of 0.93–1.66 (Fig. 6a). The mean OM / OC ratio of
WSOM in our filter samples was 2.75, which contrasts strongly with the range
of 1.6–2.2 found in other studies, indicating that secondary organic
aerosol made important contributions to the aerosols in our study. The H / C
ratio was also relatively high, at 1.89. The high O / C and H / C ratios were
caused not only by the high contribution of CO2+ ions but also by
the high contribution from H2O+, which can be produced by the
fragmentation of acidic species. Carbon oxidation state (OSc) values are
more robust and less variable than measured H / C and O / C ratios, so we
calculated the OSc values (2× O / C–H / C). The mean OSc for the
filter samples was 0.4 ranging between 0.1 and 1.2, similar to the OSc
values of diacids and multi-functional acids (Canagaratna et al., 2015). The
elemental composition of the WSOM in the filter samples was C (36 %), O
(56 %), H (6 %), and N (2 %). The mass spectrum was composed of
HyO1+ (25 %), CxHyO1+ (22 %),
CxHyO2+ (23 %), and CxHy+ (25 %)
ions, indicating that oxygenated functional groups were predominant in the
WSOM. The high contribution from HyO1+ ions could be
generated from diacids and alcohols (Canagaratna et al., 2015).
The high oxidation state of organics at the remote area of the northern QXP has
been suggested in a previous study at Qinghai Lake in 2010 (Li et al.,
2013). We sought further evidence for the high oxidation state of organics
in our samples by checking the volatility of OC, which normally has a
reverse relationship with the degree of oxidation (Huffman et al., 2009).
The OC mass fractions that evaporated at different temperature steps were
12 % at 120 ∘C (OC1), 26 % at 250 ∘C (OC2), 39 %
at 450 ∘C (OC3), and 23 % at 550 ∘C (OC4).
The large fractions of OC at high temperature suggested its
low volatility. Indeed, the volatility measurements performed using the TD
system on the QSS1 sample indicated that the organic species were less
volatile than ammonium and nitrate and more volatile than sulfate (Fig. 7a).
However, the remaining fraction of organics was higher than sulfate at
180–200 ∘C (Fig. 7b). The volatility distribution of the WSOM of
the QSS1 sample was similar to that of LV-OOA in samples from urban sites
(Huffman et al., 2009). Note that the thermal profile of sulfate in our
sample is similar to that of ammonium sulfate but nitrate appears to be less
volatile than ammonium nitrate based on the laboratory study of Huffman et
al. (2009); a possible reason is that the nitrate in filter aerosol was
mainly present in the form or organic nitrate or metal nitrate. The ratio of
NO+ vs. NO2+ in mass spectrum of the QSS filter samples indeed
shows higher values than that of ammonium nitrate which usually resulted
from the organic nitrate or metal nitrate (Farmer et al., 2010) (Fig. S6).
This is consistent with the fact that a significant fraction of nitrate in
these samples was likely associated with metals such as calcium and sodium
(Fig. 2).
Thermal profiles of organic aerosol, sulfate, nitrate, and ammonium
in PM2.5 from the QSS based on (a) the mass fraction of remaining aerosol
compared to ambient temperature; (b) the composition of non-refractory
aerosol materials at given TD temperatures.
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The AMS spectra showed that organic species in PM2.5 collected at the QSS
were on average more oxidized than low-volatility oxygenated organic aerosol
factors (e.g., LV-OOA, which had an O / C ratio of 0.5–1) determined using
PMF analyses of urban aerosols (Aiken et al.,
2008; Ng et al., 2010). The higher level of oxidation of organic species in
our samples was probably caused by intense photo-chemistry because of the
stronger solar radiation in free troposphere and/or aqueous-phase reactions
in cloud droplets and particulate water phase during the upslope transport
of air mass from the lowlands. Indeed, the highly oxidized organic MS
observed in this study is similar to that of oxidized organic matter
reported in the study of Lee et al. (2012) (r2=0.95; Fig. S5),
during which filter samples collected at a mountain site were oxidized in
the laboratory using a photochemical reactor. The potentially strong
oxidizing environment at the QSS was another important factor in producing
the highly oxidized organic species. For example, the photochemical
reactions on snow/ice are suspected to have released reactive gaseous
species, including H2O2, HONO, and OH (Grannas et al., 2007).
Estimation of secondary organic aerosol concentrations
The EC-tracer method, which estimates SOC concentration based on the
relationship of POC and EC, has been widely used, although significant
uncertainty may arise due to the usage of assumed (OC / EC)primary
ratios. In this study, a value of 2.0 is used to represent the
(OC / EC)primary ratio at the QSS, which is commonly used to estimate SOA
mass (Chow et al., 1996). In addition, this value is similar to the minimum
OC / EC ratios that have been used for estimation of SOA concentration at
remote sites in China, such as Mount Heng (2.2) (Zhou et al., 2012) and
Mount Tai (2.19) (Wang et al., 2012). The minimum OC / EC (2.8) in the
PM2.5 samples at the QSS is higher than this value, which suggests the
filter samples were likely always a mixture of POA and SOA due to the fact
that each filter was collected for 3 days.
(a) and (b) estimated concentrations of primary and secondary
organic carbon and their percent contributions to total OC in PM2.5
from the QSS; (c) and (d) estimated primary and secondary organic aerosol mass
concentrations and percent contributions to total OA mass.
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The average concentration of SOC at the QSS during the summer was 0.44 ± 0.37 µg sm-3,
on average accounting for 64 % (29–92 %) of the total OC (Fig. 8a and b). The average concentration of SOA
was 1.27 ± 0.88 µg sm-3, on average accounting for 80 %
(62–96 %) of the total OA (Fig. 8c and d). These results indicate that
secondary aerosols were dominant at the QSS during the summer. The SOC and
SOA contributed relatively little to PM2.5 mass during the late sampling
period, and this is consistent with drier condition and thus fewer chances of
aqueous processing during this period. The SOC contribution at the QSS is
consistent with those (36–52 %) at the Himalayas during summer. For
example, Ram et al. (2008) found, using the EC-tracer method, that SOC
contributed 52 % of the aerosols at Manora Peak during summer. The
estimated SOC contribution is also consistent with the results at Mount Heng
(54 % of the total OC) and at Mount Tai (57 and 71 % of total OC in
spring and summer, respectively) (Wang et al., 2012; Zhou et al., 2012).
Conclusions
An intensive study was conducted at a high elevation remote site (QSS) in
the northern part of the QXP to characterize the chemical compositions of
PM2.5 during the summer of 2012 – a period that was influenced by a
strong exchange of air masses between boundary layer and free troposphere.
The average ± 1σ mass concentration of PM2.5 species,
which include WSIs (SO42-, NO3-, NH4+,
Ca2+, Na+, Cl-, Mg2+, and K+), OC, EC, and TON, was
3.7 ± 1.9 µg sm-3. Mineral dust appeared to be a minor
PM2.5 component during this study. SO42- was a main
contributor to the PM2.5 mass (36 %), followed by OC (18 %),
NO3- (17 %), NH4+ (10 %), Ca2+ (6.6 %), TON
(6.4 %), and EC (2.6 %). The size distribution of the particles
presented a bimodal distribution with a prominent accumulation mode
(0.32–0.56 µm) and a coarse mode (1.8–3.2 µm). Sulfate, OC,
and EC dominated the accumulation mode (contributing ∼ 70 %
of the mass), while nitrate was a main contributor to the coarse mode
(contributing 23 %), followed by OC (17 %), sulfate (16 %), TON
(12 %), Ca2+ (11 %), Cl- (8.4 %), and EC (2.2 %).
Stoichiometry analysis indicated that submicrometer particles were on
average acidic, whereas coarse particles were mostly neutral. The facts that
OC / EC ratios were high (2.8–26.4) and that a major fraction of OC was
water soluble (79 %) suggest an important contribution of secondary OC to
PM2.5 composition at the QSS. Indeed, chemical characterization using
HR-ToF-AMS showed that WSOM was mainly composed of highly oxygenated organic
species. For example, the average AMS spectrum of WSOM was dominated by
HyO1+ (24 %), CxHyO1+ (22 %),
CxHyO2+ (22 %), and CxHy+ (26 %) ions
and its O / C and OM / OC ratios were 1.16 and 2.75, respectively. These results
suggest that organic species became highly oxidized during long-range
transport from lowland to elevated mountain areas and/or locally in the
northeastern region of QXP through intense photochemical and aqueous-phase
processing in the free troposphere. The estimated SOA on average accounted
for 80 % (range 62–96 %) of the PM2.5 mass, which is higher than
those reported from the Himalayas previously. Given that dry and wet deposition
of aerosol particles strongly influences the chemical composition of snow
and glaciers, our results may shed lights on the coupling between
atmospheric chemistry and cryospheric chemistry in the northern QXP region.