Introduction
Mineral dust particles constitute a substantial fraction of atmospheric
aerosol mass and play various roles in atmospheric physics and chemistry (Bi
et al., 2011; Dentener et al., 1996; Fu et al., 2009; Sokolik and Toon, 1996;
Tegen et al., 1996). Dust particles at their source areas are mainly composed
of quartz, clays, micas, feldspars, carbonates (primarily calcite,
CaCO3) and other minor minerals (Usher et al., 2003). While suspended,
they may be altered by the uptake of gases and smaller particles and by
surface reactions. Laboratory studies have demonstrated the formation of
sulfate and nitrate on dust particles upon exposure to reactive gases such as
NOx, HNO3, NO3, N2O5 and SO2 (Usher et al.,
2003). The formation of salts on the particles can enhance the solubility of
the particles, lower their effective deliquescence relative humidity (RH) and
alter their size and physical state in association with atmospheric
conditions (Semeniuk et al., 2007). These changes in turn feed back into the
activities of dust particles in various chemical and physical processes in
the atmosphere (Bauer and Koch, 2005), such as the enhancement of
bioavailable iron (Meskhidze, 2003) and the removal of acidic gases in the
atmosphere (Dentener et al., 1996; Dong et al., 2016; Zhang and Carmichael,
1999).
Field studies have shown different results regarding the formation of sulfate
and nitrate on dust particles, and some results are contradictory. Many
studies reported substantial sulfate and nitrate on the surface of Asian dust
particles after the particles were transported over long distances in the
atmosphere (Cao et al., 2003; Huang et al., 2010; Li and Shao, 2009; Mori,
2003; Nie et al., 2012; Nishikawa et al., 1991; Qi et al., 2006; Ro et al.,
2005; Sun et al., 2010, 2004; Y. Wang et al., 2005, 2007; Wu and Okada, 1994;
Zhao et al., 2011). In contrast, an early study of Zhang and Iwasaka (1999)
found that sulfate and nitrate were rarely formed on Asian dust particles
that had been transported over a long distance in inland China after about
2 days. A study at the Taklimakan Desert pointed out that in some cases the
content of sulfate in dust particles might not change even when the particles
traveled over a long distance. The dust particles contained substantial
sulfate (∼ 4 % by mass), which was from the surface soil (Wu et al.,
2012). It was also found that, for a dust plume lofted from the surface by a
synoptic mid-latitude cyclone, the plume did not mix significantly with
adjacent air parcels polluted by anthropogenic sources; the dust plume and
the polluted air were separated as two air parcels by the cold front
associated with the cyclone (Bates et al., 2004; Tsai et al., 2014; Wang et
al., 2013; Zhang et al., 2005). Some measurements of the chemical composition of
long-distance-transported dust particles have also shown that most of the
dust particles were not altered chemically and were externally mixed with
species produced in the air via gas-to-particle reactions, such as sulfate
and nitrate (Denjean et al., 2015; Song et al., 2005). These results leave us with a
question: why are there so many different rates of sulfate and nitrate
production during dust transport to polluted areas?
In April 2014, we collected a series of atmospheric particle samples during a
cyclone-induced dust storm at the eastern edge of the Tengger Desert, which
is one of the most significant sources of Asian dust (J. Wang et al., 2012;
Zhang et al., 2003). We also collected a series of samples at Xi'an, a large
city in northwestern China when a dust storm from the Tengger Desert passed
there after traveling about 6 h following a cold front. In this study, we
compare the concentrations and mass fractions of sulfate and nitrate in the
samples at the two sites and examine the production of nitrate and sulfate on
desert dust particles after the particles were transported from the desert to
the populated area in an attempt to understand the chemistry and aging on
dust.
Summary of weather conditions during the sample collection.
Samples
Sampling time
Pressure
Temperature
RH
Wind
ID
(BSTc)
(hPa)
(∘C)
(%)
Direction
Speed (m s-1)
Tengger Desert (24 April 2014)
T1
06:32–08:32
870.2–872.5
9.5–11.0
30–35
NW
5.7–13.6
T2
08:42–10:42
872.4–874.7
8.0–9.4
31–37
NW
5.6–11.4
T3
10:51–12:51
875.2–876.7
5.5–7.7
31–39
NW
6.0–14.6
T4a
13:02–15:03
876.2–878.1
-1.4–4.8
40–97
NW
5.8–12.3
Xi'an (1 May 2014)
X1
07:16–09:16
965.6–968.1
17.6–19.6
63–72
W
0–1.8
X2
09:20–10:20
968.2–969.7
19.5–21.7
45–67
NW
0–2.5
X3
10:22–11:22
970–971.7
21.2–22.2
39–50
NW
0.5–3.2
X4b
11:27–12:27
971.7–972.7
20.1–21.3
40–43
NW
0.6–3.4
X5
12:28–14:28
972.7–973.5
20.0–20.7
38–42
NW
0.4–2.9
X6
14:38–16:38
972.4–973.2
19.0–21.6
38–47
NW
0–5.4
X7
16:43–19:43
972.5–973.1
18–21.8
38–48
NW
0–3.0
a Not suitable for comparison because of snow and
the results from
this sample were excluded for further analysis.b Not available for analysis because the collection system was
blown down by wind.c Beijing standard time (8 h prior to GMT).
Particle collection and analysis
The observation site at the Tengger Desert was located at an active sand
dune at a location called Tonggunao'er along the northeastern rim of the
desert (38.79∘ N, 105.38∘ E; Fig. 1). The
closest village, with a population less than 200, is about 5 km to the east
of the site and the nearest city is Bayan Hot (Inner Mongolia Autonomous
Region, China) about 35 km to the east of the site (Fig. S1 in the Supplement). Anthropogenic
pollutants from the village and the city may arrive at the site if the wind
direction is from the east. Backward trajectories of air masses (Fig. S2)
and the simulation (Fig. S3a) of the online Chemical Weather Forecasting
System (CFORS; developed with open access online by NIES and Kyushu University,
Japan: http://www-cfors.nies.go.jp/~cfors/index-j.html)
showed that a dust storm was induced by a synoptic-scale, mid-latitude
cyclone in the southwestern part of Mongolia on 23 April 2014. The
resulting dust plume was then transported southeastward and passed the
sampling site on the morning of 24 April (Fig. S3a).
Observations were carried out between 06:30 BST (Beijing standard time: GMT + 08:00) and 15:00 BST on 24 April 2014. Particles were collected using a
homemade filter pack sampling system, which consisted of one Teflon front
filter for collecting particulate matter and one back filter for the
collection of gas-phase species. The flow rate of 16.7 L min-1 was controlled with a mass flow controller (SmartTrak 50; Sierra).
The filters were changed every 2 h, and the collection of the fourth
sample was stopped when it started to snow (Table 1). Field blank filters
were prepared and obtained by mounting filters in a sampling system in a
similar way to the particle collection for 2 h without pumping air.
Right after the sample collection of each filter, the filter was put into a
polystyrene petri dish, which was in turn sealed in a plastic bag and stored
in a refrigerator at -1 ∘C until subsequent analyses.
Meteorological conditions including surface pressure, temperature, relative
humidity, wind speed and wind direction were monitored with a weather
tracker (Kestrel 4500; Kestrelmeters). The cold front passed the site
between 04:00 and 04:30 BST on 24 April, which was characterized by a
rapid decrease in relative humidity, a sudden change in the wind direction
from south to north, an increase in wind speed and a gradual increase in
pressure (Fig. 2a). Therefore, all samples were collected after the passage
of the cold front. This sample collection ensured that mineral particles
collected on the filters were dominated by dust particles from the desert,
and possible influence of anthropogenic pollutants from the village or the
city was suppressed.
Location of the sampling sites. Also shown is the Chinese Loess
Plateau.
Surface pressure, temperature, relative humidity (RH) and wind
during the sampling periods at the desert site (a: 24 April 2014
04:00–15:00 BST) and at the Xi'an site (b: 1 May 2014
04:00–12:00 BST). Also shown are the front passage (shaded bars) and the
durations of sample collections (Table 1).
Xi'an (34.22∘ N, 108.87∘ E) lies in central China,
approximately 700 km from the Tengger Desert (Fig. 1). The observation in
Xi'an was carried out on the roof of a building (10 m above the ground) on the
campus of the Institute of Earth Environment. The institute is located in the
southwestern area of the city, and its surroundings are residences, streets and
office buildings. There are no large, continuous sources of anthropogenic
pollutants such as factories or agriculture fields near the institute.
Previous studies at this site have revealed that the local pollutants are
mainly from traffic and the particulate pollutants mainly include particles
from vehicle engines, road dust and construction dust, all of which have
been characterized by contents of crustal elements, sulfate, organic matter,
nitrate and ammonium (R.-J. Huang et al., 2014; Zhang et al., 2011).
Particles were collected on 1 May 2014 when dust-loading air passed Xi'an.
Backward trajectories of air masses (Fig. S2) and the CFORS simulation (Fig. S3b) showed that, similar to the dust storm observed at the Tengger Desert,
this dust storm was also induced by a cyclone in the southwestern part of
Mongolia and moved southeastward (Fig. S3b). It passed the site of the
Tengger Desert on the evening of 30 April and arrived at Xi'an on the
morning of 1 May. Particle collection was carried out between 07:00
and 19:00 BST on 1 May. The same sampling system and the same type
filters as those used for the particle collection at the desert site were
used. Samples were collected at a time interval of 1 or 2 h. In total, seven samples were obtained from the
dust storm approach to its dissipation (Table 1). Each sample filter was put into a polystyrene petri dish, which was in
turn sealed in a plastic bag and stored in a refrigerator at -1 ∘C
until subsequent analyses. Meteorological conditions were monitored by the
Kestrel 4500 weather tracker. The sudden decrease in relative humidity
showed that the arrival of the cold front of the cyclone occurred between
09:30 and 11:30 BST (Fig. 2b). Therefore, the first sample was collected
before the arrival of the cold front, the second and third samples were
collected in the frontal air, and the fourth, fifth and sixth samples were
collected after the passage of the cold front. Unfortunately, the fourth
sample was not available for analysis because the collection system was
blown down by wind when this sample was collected. The concentrations of
SO2 and NO2 were measured by using a UV fluorescence analyzer
(Ecotech; model EC9850) and a chemiluminescence analyzer (Thermo; model 42i)
and were recorded in units of ppb at a time interval of 5 min. The
lowest detection limit was approximately 0.04 ppb for SO2 and 0.4 ppb
for NO2. Since the dust occurring in Xi'an had the same source and a
transport route similar to that at the desert site (Figs. S2, S3), the
comparisons between the samples collected at the desert site and the Xi'an
site can show changes in dust particles during the transport although the
samples were not from the same dust event.
Teflon-membrane filter samples were equilibrated in a temperature- and
relative-humidity-controlled environment (20–23 ∘C and 35–45 %
RH, respectively) before gravimetric analysis to minimize particle
volatilization and aerosol–liquid water interferences. Filters were weighed
before and after sampling using an ME5-F electronic microbalance (Sartorius;
Göttingen, Germany) with a sensitivity of ±0.001 mg. The precision of
multiple weighings for unexposed and exposed filters was smaller than
±0.010 and ±0.015 mg, respectively. Filters were exposed to a
low-level radioactive source (500 pCi of polonium-210) before sample
weighing to remove static charge. Mass concentrations were calculated with
the difference in weight before and after sampling and the volume of sampled
air. Half of each filter was analyzed to quantify the water-soluble components
in particles. Each sampled filter was initially wetted with 200 µL
of ethanol. Water-soluble components were extracted by ultrasonic agitation in
10 mL of distilled water. The extraction solution was filtered with 0.45 mm
pore size microporous membranes, and then the filtrate solution was stored
at 4 ∘C until subsequent analyses. An ion chromatograph (Dionex
DX-600) was used to quantify sulfate (SO42-), nitrate
(NO3-), chloride (Cl-), sodium (Na+), potassium
(K+), ammonium (NH4+), calcium (Ca2+) and magnesium
(Mg2+) in the solution. Calibration curves were constructed from the
peak areas of the chromatograms, which were produced from a series of mixed
standards. Samples and blanks collected at both sites were analyzed with
replicates and surrogates following standard lab protocols. The relative
uncertainty in the mass percentage of sulfate and nitrate, according to the
surrogates, was less than 10 %.
Concentration (Conc., in µg m-3) of TSP [M], sulfate
[SO42-], nitrate [NO3-] and ammonia [NH4+] at the
desert site in the dust episode. Also included are the relative amounts (R.
M., in %) of the three ions in TSP.
Samples
[M]
[SO42-]
[NO3-]
[NH4+]
Conc.
R. M.
Conc.
R. M.
Conc.
R. M.
T1
4754
59
1.2
5.9
0.12
0.27
< 0.01
T2
4487
54
1.2
5.1
0.11
0.22
< 0.01
T3
3481
39
1.1
3.8
0.11
ND∗
ND
Ave.
4241 ± 672
51 ± 10
1.2 ± 0.1
5.0 ± 1.1
0.12 ± 0.11
0.16 ± 0.14
< 0.01
∗ Not detected.
An energy dispersive X-Ray fluorescence (EDXRF) spectrometry (Epsilon 5
EDXRF; PANalytical B. V., the Netherlands) was used to quantify elements in
the samples of the remaining parts of sample filters. Five crustal elements
(K, Ca, Ti, Mn, Fe and Ba) and two common anthropogenic trace elements (Zn
and Pb) were quantitatively determined. Analytical uncertainties, as checked
by parallel analysis of the NIST standard reference material (SRM-2683),
were at or less than 10 % for the detected elements.
Results and discussion
Sulfate
In the dust plume at the Tengger Desert, the concentration of sulfate ranged
from 39 to 59 µg m-3. The relative amount of sulfate in the dust
samples, i.e., the mass fraction of sulfate in the samples, was between
1.1 and 1.2 % and the average was 1.2 % (Table 2). These values were
close to the levels of the relative mass ratios of sulfate in TSP or
PM10 in samples collected under dust conditions at the Gobi Desert,
which were reported in previous studies (Table S1).
The concentration of SO42- at Xi'an varied in a large range as the
front of the dust-loading cyclone was approaching, passing and leaving (Fig. 3). It was 17 µg m-3, contributing 4 % of the aerosol mass in the
prefrontal air, i.e., before the dust arrival. As the front was passing, the
concentration decreased rapidly. In the postfrontal air, the concentration
was 3.8 µg m-3 right after the front passage, 3.5 µg m-3
2 h after the passage and 3.4 µg m-3 4 h after the
passage. The average in the postfrontal air was 3.5 µg m-3. The
relative amounts of sulfate in these samples were 0.9, 1.1 and
1.8 %, respectively, and the average was about 1.3 %. In comparison
to that in the prefrontal air, the concentration in the postfrontal air
was very small and approximately constant, although the relative amount
increased gradually as the front left.
Concentrations of mass, SO42-, NO3-, NH4+
and Ca2+ at the Xi'an site during the dust passage. Data for samples between
11:27 and 12:27 BST are not included since the sampler was blown down by
wind. The relative amounts (R. M.: the ratios of the ion concentrations to
the total mass concentration in percentage) of these ions are also
illustrated.
In cyclones moving eastward across northern China, the source and
consequently the compositions of major particles in the air before and after
cold fronts are different although they are in the same cyclones (Hu et al.,
2016; Niu et al., 2011). Prefrontal air usually moves slowly from the south
or southwest directions toward the north or northeast and is usually warm
and humid. Particles in prefrontal air originate mainly from local or
regional areas and they are usually dominated by anthropogenic pollutants (Li
et al., 2012). The postfrontal air moves more rapidly from the north or northwest
direction and is usually cold and dry. Particles in postfrontal air are
lifted by the cold fronts of the cyclones at the places they pass, and
long-distance-transported dust particles are usually the majority if the
cyclones have caused dust storms at the arid and semiarid areas in
northwestern China (S. Wang et al., 2005). The cold fronts are the boundaries
between the local or regional anthropogenically polluted air and the
long-distance-transported air because the movement of air on a synoptic scale
is approximately adiabatic; i.e., the air is hardly mixed with
the thermodynamically different air it meets, although some small-scale mixing
might occur in the front. Aerosol particles at the time of front
passage should be of both local origin and transported from long distances. The rapid decrease in sulfate with the passage of the cold
front at Xi'an was consistent with the increase in long-distance-transported
dust.
Mass ratios of Ca, Fe, Ti, Mn, Ba, Zn and Pb to Fe in the samples at
the two sites.
Samples
Ca / Fe
K / Fe
Ti / Fe
Mn / Fe
Ba / Fe
Zn / Fe
Pb / Fe
Tengger Desert (24 April 2014)
T1
1.47
0.54
0.084
0.023
0.013
0.003
0.0014
T2
1.47
0.55
0.082
0.023
0.013
0.0023
0.0011
T3
1.57
0.57
0.086
0.024
0.012
0.002
0.0009
Xi'an (1 May 2014)
X1∗
NA
NA
NA
NA
NA
NA
NA
X2
1.86
0.66
0.084
0.028
0.012
0.037
0.009
X3
2.16
0.63
0.087
0.039
0.008
0.010
0.004
X5
1.76
0.62
0.089
0.045
0.018
0.003
0.0009
X6
1.44
0.63
0.092
0.031
0.015
0.003
0.0008
X7
1.80
0.68
0.089
0.024
0.022
0.003
0.0009
∗ No enough sample for analysis.
NH4+ is one of the major water-soluble species in aerosol
particles and can be remarkably enhanced by anthropogenic emissions. We
found that its concentration was close to the lowest detection limit in dust
samples at the desert site. This fact makes NH4+ a good indicator
for examining the influence of local or regional anthropogenic particles on
the samples observed at Xi'an. The variation in NH4+ during the
sampling period is also shown in Fig. 3. With the increase in long-distance-transported dust particles, NH4+ decreased remarkably as the
front was passing the site, and was very low in the frontal air. In the
postfrontal air, the NH4+ concentration was lower than the detection
limit in the first sample and increased slightly in the second and third
samples. These results indicate that the composition of particles in the
postfrontal air was close to the state of dust particles at the desert
areas, whereas the composition was gradually affected by local emissions
afterwards. Zn and Pb are two common anthropogenic trace elements in urban
air. Their ratios to Fe in the dust samples in the postfrontal air were much
lower than those in the prefrontal air and very close to those in the desert
air (Table 3), further suggesting the limited influence of pollution on
desert dust particles in the postfrontal air.
The relative amount of SO42- in dust samples at the desert, 1.1 to
1.2 % in mass, was similar to or even larger than the relative amount at
Xi'an (the relative amount in the first sample in the postfrontal air was
0.9 %). This result indicates that sulfate was rarely produced on dust
particles during particle traveled from the desert to the distant urban
area. Heterogeneous reactions involving SO2 on mineral particles were the
major processes for sulfate production on the particles. The conversion of
SO2 to sulfate by heterogeneous reactions on particles is much more
efficient under humid conditions than under dry conditions (Usher et al.,
2003). The relative humidity was less than 40 % during the dust storm
episode in Xi'an (Fig. 2b), which did not favor the formation of sulfate on
the surface of mineral components (X. Huang et al., 2014). Moreover, the cold
air lofting the dust storm particles was from arid or semiarid areas in the
southwestern part of Mongolia where SO2 emissions are usually weak
(Fig. S4). The concentration of SO2 in the postfrontal air was nearly
close to the detection limit (Fig. S5), which was much smaller than the
concentration in the prefrontal air (∼ 20 ppb). Therefore, sulfate was
hardly formed on the dust particles due to the lack of SO2 and the dry
condition. The postfrontal air had passed some populated areas between the
desert and Xi'an where anthropogenic SO2 emissions were usually observed
due to human activities (Wang et al., 2011). However, the postfrontal air did
not pick up any accumulated air pollutants on the way. Anthropogenic
pollutants that might have been taken into the air were those freshly emitted
at the moment of the air passage. Such pollutants should not have a
considerable influence on the dust. Otherwise, (1) the movement of the
dust-loading air should not have been an adiabatic process, which is the reason for
the cold front occurrence when arriving at Xi'an, (2) the front should have
disappeared, (3) some NH4+ should have been present and (4) the sulfate
content in the samples at Xi'an should be larger than in the desert sample.
The vertical thermodynamic structure near the surface at the two sites
became more stable when dust occurred (Fig. S6; from the homepage of
Atmospheric Soundings at the University of Wyoming,
http://weather.uwyo.edu/upperair/sounding.html), indicating that the
dust plume layer established at the dust source was not mixed with air of
different chemical (gas and particulate phase) composition from above during
the advection.
There was a small amount of sulfate in the dust samples at Xi'an. The
concentration was much smaller than that in the dust samples at the desert
area. However, the relative level of sulfate in the total aerosol mass in the
postfrontal air samples (0.91–1.75 %) was close to and even smaller than
that at the desert area (1.19 % on average). This result means that the
sulfate in the dust samples at Xi'an was very likely one of the original
components of the dust particles, i.e., the so-called soil-derived sulfate in
desert dust. It has been found that dust at desert areas contains substantial
soil-derived sulfate (Abuduwaili et al., 2008; Sun et al., 2010; X. Wang et
al., 2012; Wu et al., 2012; Yabuki et al., 2005; Zhang et al., 2009), and
sulfate was confirmed in long-distance-transported dust in downstream areas
in a small number of studies (Wang et al., 2014, 2007; Q. Wang et al., 2016;
Wu et al., 2012). For these reasons, we consider the sulfate detected in the
dust samples right after the cold front to be mainly from the desert areas as
soil-derived sulfate rather than sulfate produced by chemical conversions on
the particle surface when the particles floated in the air.
Nitrate
At the desert site, the NO3- concentration in dust samples was 4–6 µg m-3 and the average was 5 µg m-3. The relative
amount of NO3- ranged between 0.11 % and 0.12 %, and the
average was 0.12 %. At the Xi'an site, similar to SO42-, the
concentration of NO3- varied in a large range as the dust-loading
cyclone passed, with the concentration high in the prefrontal air and low in
the postfrontal air. Right after the passage of the cold front (the first
sample in the postfrontal air), the concentration of NO3- was 0.9 µg m-3 and it occupied 0.2 % of the aerosol mass. These values
were close to the levels of the relative mass ratios of nitrate in TSP or
PM10 in samples collected under dust conditions at the Gobi Desert,
which were reported in previous studies (Table S1).
The relative amount in this sample was about twice that in the desert samples
although it was the lowest in the samples at the Xi'an site, indicating that
nitrate was likely produced on dust particles during their travel to Xi'an.
We assume that the removal of dust particles from the dust plume was
independent of the chemical components of dust particles. This assumption is
reasonable because the dust plume was relatively dry during its movement from
the desert area to the urban area and the settling of dust particles under
such conditions depends on particle size only (Zhang, 2008). The
concentration of nitrate in dust samples at the desert site was used as a
referential value for the part of nitrate originating from the desert areas
in the samples at Xi'an areas. The results show that nitrate production on
the dust particles was 1.0–1.1 ng µg-1 of dust
(approximately 0.1–0.11 % in mass) during the dust movement from the
desert to Xi'an. Since the dust plume took approximately 6 h to move from
the desert to Xi'an, this increase in nitrate was equivalent to a production
rate of 4–5 ng nitrate µg-1 of dust per day. Note that this
rate should be the maximum rate because not all the nitrate could have been
produced on dust particles and the increase in the relative amount of nitrate
during the movement of a dust plume from the desert to Xi'an could have been
the consequence of possible differences in the removal rates of dust
particles and nitrate-containing particles. The estimated rate is much
smaller than that in polluted urban air in which secondary nitrate usually
accounts for 2–6 % of aerosol loading (Wang et al., 2003) and the
production rate is 20–60 ng nitrate µg-1 day-1 if we
consider the residence time of particles in polluted air to be 24 h.
In general, nitrate on dust particles is produced on the surface via
heterogeneous conversions or the uptake of HNO3, which is formed via
homogeneous reactions in the atmosphere. Model studies have shown that the
latter is the major route for nitrate formation on dust particles, and the
contribution of the former route, in particular under dry conditions, is very
small (Fairlie et al., 2010; Song and Carmichael, 2001). Desert soil hosts
the most abundant natural nitrate minerals
on the earth (Walvoord, 2003). Nitrate in desert soil can be reduced to
NOx through microbiological denitrification (Hartley and Schlesinger,
2000) and abiotic thermal decomposition (McCalley and Sparks, 2009). A
background-like nitrate, which is about 2–8 µg m-3 and
assumed to be in the form of nitric acid, has been found in desert air (Wu et
al., 2014). Such HNO3 could be absorbed and transformed into nitrate on
dust particles during dust movement from the source region to Xi'an.
A first-order chemical kinetic equation was used to estimate the uptake of
HNO3 during transport:
ct=c0e-kt,
where ct is the concentration of HNO3 at transport time t, c0
is the initial concentration of HNO3 in the air mass and k is the first-order rate at which the gaseous precursor is taken up by dust. The reaction
rate can be calculated as
k=14vHNO3γHNO3AP,
where vHNO3 is the mean molecular speed of HNO3 and is
taken as 3.0 × 104 cm s-1 (Fairlie et al., 2010);
γHNO3 is the reactive uptake coefficient for HNO3
on dust particles. Measurements in laboratory experiments showed that the
uptake coefficient of HNO3 on mineral dust ranged from
5.2 × 10-5 (Underwood et al., 2001) to 5 × 10-3
(Song et al., 2007). The coefficient was a function of relative humidity
(Vlasenko et al., 2006) and estimated to be ∼ 2 × 10-4
under experimental conditions of 30–40 % relative humidity (Fairlie et
al., 2010). AP (cm2 cm-3) is the total surface area of
dust particles and is determined by the loading of dust particles and their
specific surface area. Surface area analysis has shown the specific area of
Gobi dust to be approximately 110 cm2 mg-1 (Underwood et al.,
2001). The dust load by mass (in µg m-3) initially at the
Tengger Desert for the dust plume observed at Xi'an is estimated to be
1187 µg m-3 based on the dust load of the first sample of
postfrontal air (415.4 µg m-3) and the relationship between
dust concentration and their transport distance as suggested by Mori et
al. (2002).
The relative amounts (%) of nitrate, sulfate and ammonia in TSP
or PM2.5 in postfrontal air during dust storms at downwind Chinese
cities.
Site
SO42-
NO3-
NH4+
Remarks
References
Xi'an
0.91
0.22
NA
TSP (1 May 2014)
This study
Xi'an
1.05
0.21
0.09
TSP (9 March 2013)
Wang et al. (2014)
Beijing
0.75
0.08
0.05
TSP (22 March 2002)
Zhao et al. (2007)
Shanghai
2.7
1.4
0.7
PM2.5 (20–21 March 2010)
Wang et al. (2013)
The reaction rate for HNO3 gas uptake on mineral dust was
1.96 × 10-4 s-1. The concentration of HNO3 after 6 h
reduced to ∼ 1 % of the initial concentration, indicating that
almost all HNO3 could be transformed into nitrate on the dust particles
during their transport. As described above, nitrate produced on the dust
samples was 0.1–0.11 %, indicating that the concentration of
NO3- increased by 1.2–1.3 µg m-3. To produce this
amount of nitrate, the HNO3 concentration in the dust-loading air should
be approximately 1.2–1.3 µg m-3 on average. Unfortunately,
there are no data on nitric acid in the air over the Tengger Desert for
further comparison. The concentration of nitrate (including particulate and
gaseous phases) at the Taklimakan Desert in northwestern China was
2–8 µg m-3 (Wu et al., 2014), which was in the same range as
found that for the nitrate concentration in the dust at our Xi'an site.
Intercomparisons and implications
There have been studies on dust-associated sulfate and nitrate in aerosol
particles downwind of areas in mainland China (Table 4). Zhao et al. (2007) investigated the evolution of air pollutants when an extremely strong
dust storm from the Gobi Desert near the China–Mongolia border passed Beijing and
confirmed the rapid decrease in sulfate and nitrate after the passage of the
dust-associated cold front. The relative amounts of sulfate and nitrate in
the postfrontal air were 0.77 and 0.08 %, respectively, which were
close to the relative amounts of the salts we observed at the desert site
and the Xi'an site in this study. Wang et al. (2014) reported hourly sulfate and
nitrate in aerosols at Xi'an during a dust storm period on 9 March 2013.
The dust storm also originated from the Gobi Desert in the southwestern part of
Mongolia, similar to the dust cases in this study. After the passage of
the cold front, the relative amounts of sulfate and nitrate were 1.05
and 0.21 %, respectively, which were also very close to the results of
this study.
A number of studies in China reported that dust particles significantly
enhanced the formation of sulfate and nitrate when dust plumes advected over
urban areas (Li and Shao, 2009; Li et
al., 2014; Nie et al., 2012; Qi et al., 2006; Sheng et al., 2003; Wang et
al., 2013; Zhao et al., 2011, 2007), which is very different from the
conclusions of this study. We carefully examined the available
meteorological records for the dust episodes in those studies. In cases of
cold-front-associated dust (Li et al., 2014; Qi et
al., 2006; Sheng et al., 2003; Zhao et al., 2007), the samples used in those
studies were collected repeatedly in pre-fixed time periods without a
careful consideration of the time of dust arrival. That means the samples
were not separately collected from defined prefrontal air and postfrontal
air masses, and some samples used for analysis included particles from both
prefrontal air and postfrontal air. The results from such samples would show
the presence of substantial sulfate and nitrate. However, the sulfate and
nitrate must have been contributed by particles in prefrontal air, which
should be from local or regional areas and abundant in sulfate and nitrate.
In a recent publication about the passage of a dust storm over Beijing with
a high time resolution, online records clearly demonstrated the separation of
the prefrontal pollutants and the postfrontal dust plume (Hu et
al., 2016), further indicating the necessity of separating particles in
prefrontal air and postfrontal air for an accurate description of salt
origin in dust samples.
In addition, mineral ions or crustal elements (e.g., the Ca2+ ion or
elemental Al) have frequently been applied as the indicators of mineral dust
in studies (Nie et al., 2012; Zhao et al., 2007). Samples with the highest
loading of mass or crustal compositions (e.g., Ca2+, Al) were frequently
regarded as samples of long-distance-transported dust. However, the
samples in those studies were actually collected from the front air, as we
described above, and were a mixture of long-distance-transported dust
particles and locally and regionally originated aerosols. For example, in
the study of Zhao et al. (2007), the mineral to TSP (total suspended particulate
matter) ratios in samples of the highest TSP loading were significantly
lower than those in samples collected after the occurrence of maximum aerosol
loading, indicating that the samples at the highest TSP moment were not dust
particles from desert areas only. The indicators of mineral crustal ions or
elements (e.g., the Ca2+ ion or elemental Al) may actually cause large
uncertainties in explaining origins and sometimes even
misunderstandings about the origins of aerosol particles. The reason is that,
in addition to road dust and construction dust, coal burning is a major source of
mineral components in aerosols in China and emits particulate matter that is
abundant in mineral elements such as Si, Al, Ca and Fe (Chen et al., 2012).
Coal burning emissions have been proven to cause significant air pollution in
China (Cao et al., 2005; Wang et al., 2015; H. Wang et al., 2016).
Anthropogenic pollutants are usually present in prefrontal air. If only the
presence of substantial mineral elements such as Ca, Al or Si is used as the
indicator of the occurrence of mineral dust particles from desert areas,
anthropogenic pollutants such as road dust and particles emitted from coal
burning will be categorized as desert dust particles. Such an
examination would lead to a result indicating the occurrence of substantial sulfate-
and nitrate-containing dust particles in samples.
In many cases, dust was observed in cyclonic disturbances with weak fronts
or without fronts (Cao et al., 2003; Nie et al., 2012; Wang et
al., 2013). Cold fronts could not be confirmed clearly at places of dust
arrival in those studies. The cases usually occur in the southern parts of
China after the cold and dry air from the north lose their adiabatic state,
and the postfrontal air arriving at such places has, to an extent, mixed
with the local and regional air. Samples of aerosol particles after dust
arrival at such places contain both long-distance-transported dust particles
and locally or regionally emitted pollutants. For example, Wang et al. (2013) reported the occurrence of nitrate and sulfate in particles during
two extreme dust storms in Shanghai, a megacity in eastern China, from 20 to
22 March 2010 and from 26 to 27 April 2010. Weather charts (see Fig. 8) showed the consecutive transport of anthropogenic air masses and
dust storm plumes to Shanghai during the dust periods either with a cold
front arrival (the previous case; equivalent to dust case with front in this
study) or by the stimulation of a cold front event even though the front did
not extend to Shanghai (the latter case; dust case without front). The
relative amounts of sulfate and nitrate in samples at Shanghai during this
period were 2.7 and 1.4 % in the previous case and 9 and 5.9 %
in the latter case. Anthropogenic sulfate and nitrate in particles from the
local and regional areas in the latter case would have appeared in the
samples although they were not produced on dust particles from desert areas.
To further examine the situation of previously reported sulfate and nitrate
formation on dust storm particles at populated areas in eastern China, we
investigated all published papers we found on this subject and carefully
checked the records of sample collection and the available meteorological
conditions when the samples were collected in those studies from the papers
and official public websites for historical meteorological records. The
papers were separated into three groups according to the records and
meteorological conditions, as mentioned above. The first group includes
papers in which the sample collection records were vague, and we are unable
to make clear it clear whether the samples were dominated by desert dust particles or
contained a large fraction of locally emitted particles (Table S2a). It is
uncertain whether the sulfate and nitrate reported in those papers were really
on desert dust particles or not. The second group includes papers in which
the sample collection was started before the arrival of the fronts of
dust-loading cyclones or the fronts had disappeared and front-associated
dust-loading air had mixed with locally polluted air (Table S2b). That means
the samples in the studies contained not only long-distance-transported
desert dust particles but also locally emitted mineral particles, such as
road dust, construction dust and fly ashes. In such samples, the sulfate and
nitrate must have been substantially contributed by locally emitted mineral
particles, as we discussed above. The third group includes papers in which
the results from samples of locally or regionally originated particles in
prefrontal air and from samples of long-distance-transported desert dust
particles in postfrontal air can be identified (Table S2c). The production
of sulfate and nitrate in the postfrontal dust samples was all very limited,
and the production in prefrontal samples was significant, which is
consistent with our results in this study.
Conclusion
Dust particles were collected at the Tengger Desert and Xi'an during two
dust storm periods. Meteorological records showed that these two dust storms
originated from the same source region and had similar transport routes. The
comparison of sulfate and nitrate of dust aerosol at the two places
indicated that the production of sulfate and nitrate on dust
particles following cold fronts was limited when the dust moved from the
desert to populated areas. The adiabatic process of the dust-loading air
movement was most likely the reason for the absence of sulfate formation,
and the uptake of HNO3 was responsible for the small nitrate production.
The significant sulfate and nitrate in dust storm periods in China reported in
previous studies did not link to
reactions on the dust surface for the most part. They were likely from locally emitted and
urban mineral particles, in addition to soil-derived sulfate. The major
reason is that in those studies the air from which the samples were
collected had been significantly influenced by local emissions. Without a
proper evaluation of the contribution of sulfate and nitrate in the samples
by locally emitted and urban mineral particles, i.e., nondesert mineral
particles, it is not safe to attribute all the detected sulfate and nitrate
to production on dust storm particles.
The results of this study are from the comparison of dust particles in two
dust storms: one at the dust source area and another at an urban area after
long-distance transport. Although the thermodynamic structure of the
dust-loading air in the two cases was similar and comparable, data from
multiple cases of the same dust storms at desert areas and downwind populated
areas are needed to make the present conclusions more accurate and
confident. In addition, the conclusions were derived for front-associated
dust storm particles. The adiabatic nature of the postfrontal air during its
long-distance movement kept the air dry and hardly polluted by accumulated
anthropogenic pollutants from the areas it passed. There are other types
of airborne dust particles in China, such as floating dust. The movement of
air-loading floating dust is usually slow and not adiabatic, and the air
is usually well mixed with locally emitted pollutants, which is very
different from postfrontal air. It can be expected that floating dust
particles could be more frequently changed to sulfate and nitrate carriers
via surface chemical reactions than front-associated dust storm particles in
urban areas. However, how to separate the sulfate and nitrate produced on
floating dust particles from those produced on locally emitted mineral
particles is still a big challenge in field observations because floating
dust particles and locally emitted mineral particles coexist in urban air
when floating dust occurs.