The heterogeneous reactions of SO2 in the presence
of NO2 and C3H6 on TiO2 were investigated with the aid
of in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS)
under dark conditions or with UV–Vis irradiation. Sulfate formation with or
without the coexistence of NO2 and/or C3H6 was analyzed with
ion chromatography (IC). Under dark conditions, SO2 reacting alone resulted in sulfite
formation on TiO2, while the presence of parts per billion (ppb) levels of NO2
promoted the oxidation of SO2 to sulfate. The presence of
C3H6 had little effect on sulfate formation in the heterogeneous
reaction of SO2 but suppressed sulfate formation in the heterogeneous
reaction of SO2 and NO2. UV–Vis irradiation could significantly
enhance the heterogeneous oxidation of SO2 on TiO2, leading to
copious generation of sulfate, while the coexistence of NO2 and/or
C3H6 significantly suppressed sulfate formation in experiments
with UV–Vis lights. Step-by-step exposure experiments indicated that
C3H6 mainly competes for reactive oxygen species (ROS), while
NO2 competes with SO2 for both surface active sites and ROS.
Meanwhile, the coexistence of NO2 with C3H6 further resulted
in less sulfate formation compared to introducing either one of them
separately to the SO2–TiO2 reaction system. The results of this
study highlighted the complex heterogeneous reaction processes that take
place due to the ubiquitous interactions between organic and inorganic
species and the need to consider the influence of coexisting volatile organic compounds (VOCs) and other
inorganic gases in the heterogeneous oxidation kinetics of SO2.
Introduction
Atmospheric aerosol pollution has attracted widespread attention in recent
years because of its adverse effects on human health, visibility, and climate
(Thalman et al., 2017; Davidson et al., 2005; Pöschl, 2005). In many
developing countries, such as China and India, high concentrations of
SO2, NOx, and volatile organic compounds (VOCs) coexist in the
atmosphere (Zou et al., 2015; Liu et al., 2013; Yang et al., 2009) and
result in “complex atmospheric pollution” (Yang et al.,
2011) and heavy haze events. Sulfate was found to play important roles in
the occurrence of these haze events (Zhang et al., 2011; Z. R. Liu et al.,
2017) due to both its high mass concentration in fine particles
(PM2.5) and its strong hygroscopicity. Rapid formation of sulfate was
frequently observed in haze episodes in China, in which heterogeneous
reactions played important roles (He et al., 2014; Zhang et al., 2006; Ma
et al., 2018). However, the mechanism of the heterogeneous reaction process
as well as its contribution to sulfate formation in complex atmospheric
pollution remains uncertain (Yang et al., 2018; Ma et al., 2018; Wang et
al., 2018; Yu and Jang, 2018). These uncertainties are considered to be the
main reason for the inaccuracy of sulfate simulation in air quality models
(Wang et al., 2014b; Zheng et al., 2015; Yu and Jang, 2018).
About 1000 to 3000 Tg of mineral aerosols are emitted into the atmosphere
every year (Dentener et al., 1996; Shen et al., 2013; Jaoui et al., 2008)
and provide abundant surface area for the heterogeneous oxidation of
SO2. The heterogeneous uptake of SO2 can form bisulfite
(HSO3-) or sulfite
(SO32-) on γ-Al2O3
and sulfate (SO42-) on MgO (Goodman
et al., 2001b). Similarly, SO2 can be converted into sulfite, bisulfite,
or sulfate on mineral dust such as metal oxides (Zhang et al., 2006),
calcite, and Chinese loess (Usher et al., 2002). The
heterogeneous reaction of SO2 on mineral dust can be promoted by
gaseous oxidants. For example, SO2 could be oxidized into sulfate by
O3 on the surface of CaCO3 particles (Li et al., 2006; Zhang et
al., 2018). Similar results were obtained when introducing H2O2
into the heterogeneous oxidation system (Capaldo et al., 1999; Jayne
et al., 1990). NO2 can also promote the heterogeneous oxidation of
SO2. In our previous studies, it was found that SO2 was oxidized
to sulfate on γ-Al2O3 in the presence of NO2 and
O2, while it was only converted to sulfite in the absence of them
(Ma et al., 2008). Therefore, NO2 was proposed to act as a catalyst
in the oxidation of SO2 by O2, in which the intermediates observed
in the spectra, i.e., nitrogen tetroxide (N2O4), might play an
important role (Ma et al., 2008). This synergistic effect between
SO2 and NO2 was further observed on many other mineral oxides such
as CaO, α-Fe2O3, ZnO, MgO, α-Al2O3, and
TiO2 (Liu et al., 2012; Ma et al., 2017; Zhao et al., 2018; Yu et al.,
2018). These effects were confirmed in smog chamber studies and field
observations of heavy haze in China, and they were proposed to be an important
reason for the rapid growth of sulfate in haze events (He et al., 2014; Ma
et al., 2018; Wang et al., 2014a; Chu et al., 2016). Heterogeneous oxidation
of SO2 may also be affected by the coexistence of organic compounds.
Pre-adsorption of acetaldehyde (CH3CHO) was found to suppress the
heterogeneous reaction of large amounts of SO2 on the surface of
α-Fe2O3 (Zhao et al., 2015), while HCHO was
proposed to react with SO32- and
generate hydroxymethanesulfonate (HMS) in the northern China winter haze
period (Moch et al., 2018; Song et al., 2019). Wu et al. (2013) found that the synergistic effects
between formic acid (HCOOH) and SO2 in the heterogeneous reaction on
hematite provide a new source of sulfate.
UV illumination can affect both the properties of particles and
heterogeneous reactions on them (Nanayakkara et al., 2012; Cwiertny et
al., 2008; George et al., 2015). The photooxidation of SO2 in the
presence of mineral dust may represent an important pathway for generating
sulfate aerosols (Park et al., 2017; Yu and Jang, 2018). TiO2, an
n-type semiconductor material, has been widely used for studying
heterogeneous photochemical reactions (Chen et al., 2012).
TiO2 can be excited by UV light (λ<387nm), resulting
in electrons and holes that can react with O2 and H2O and
produce ⚫O2- and
⚫OH, respectively. These reactive oxygen species (ROS),
primarily ⚫O2- and
⚫OH, can participate in the heterogeneous oxidation of
SO2 on TiO2 (Chen et al., 2012). Shang et al. (2010) studied the heterogeneous reaction of SO2 on TiO2
particles using in situ diffuse reflectance infrared fourier transform spectroscopy
(DRIFTS) and observed that SO2 was oxidized to sulfate on TiO2
with UV illumination while remaining as sulfite under dark conditions. Our
recent study showed that O2 and H2O have contrary roles in the
photooxidation of SO2 on TiO2, where surface water exhibits a
competition effect in the reaction of SO2 due to the occupation of
surface OH (Ma et al., 2019). Besides H2O, the coexistence of
organics may also suppress the formation of sulfate due to competition with
SO2 for reactive oxygen species. For example, Du et al. (2000) studied the photocatalytic reaction of SO2 in the presence
of heptane (C7H16) and found that the formation of sulfate was
suppressed.
Despite these studies involving the heterogeneous oxidation of SO2
under various conditions, the effects of coexisting pollutants on the
heterogeneous oxidation of SO2 under both dark and illuminated
conditions need further investigation. Meanwhile, the interactions between
organic and inorganic species in these heterogeneous processes at low
concentrations are not fully understood. In this study, we focus on the
effects of coexisting NO2 and propene (C3H6) on the
heterogeneous oxidation of SO2 on TiO2 under both dark and
illuminated conditions with in situ DRIFTS. In order to better study the effects of
NO2 and C3H6 on the heterogeneous oxidation in a relatively
complex oxidation system (with coexistence of multiple gases, in both dark
and illuminated conditions), we chose TiO2 due to the fact that it is a
semiconductor material and a well-known photocatalyst. TiO2 has been
widely reported to be present in airborne particulate matter (PM)
(Chen et al., 2012). Although TiO2 represents only a
relatively small portion of the mass of PM and is less abundant than CaO,
Fe2O3, or MgO, the TiO2 particles are expected to provide
important surfaces for heterogeneous photocatalysis of atmospheric gases due
to their high photocatalytic activity, especially with the growing
application of TiO2 in human activities (Chen et al.,
2012). Propene is selected as a representative VOC since it is the most
abundant alkene compound in the atmosphere, and it coexists with NOx in
vehicle exhaust emissions (Wang et al., 2016a). Propene is widely used as
an accelerator in photochemical reactions in some smog chamber studies
(Jang and Kamens, 2001; Song et al., 2007). The relatively simple
oxidation products and well understood oxidation mechanism of propene are
also helpful in explaining our experimental results. Propene is selected
also due to the high vapor pressure of its oxidation products, which
normally do not generate condensed organic aerosol (Odum et
al., 1996). However, we must point out that the heterogeneous reactivity
depends greatly on the properties of the mineral oxides, such as acid–base
nature or redox properties (Tang et al., 2016; Yang et al., 2016, 2019), while different VOCs may also have quite different heterogeneous
and photochemical reactivity. Investigating these processes on different
mineral dust and authentic dust particles with different types of VOCs is
needed in future studies. Rather than UV lights, a xenon light is used in
this study to better simulate the solar ultraviolet radiation on the earth's
surface. Generally, our study could be helpful for gaining a better
understanding of the heterogeneous formation of sulfate under complex air
pollution conditions, in which abundant SO2, NOx, VOCs, and
mineral dust coexist in the atmosphere.
Experimental sectionMaterials
TiO2 (Degussa P25) used in this study is a typical commercially
available material, which contains 75 % anatase and 25 % rutile. It has
been widely used in laboratory studies due to its good photocatalytic
properties. The surface area of the material in this study was 50.50 m2g-1, measured by an ASAP2010 BET apparatus with multipoint
Brunauer–Emmett–Teller (BET) analysis. The average particle diameter was
about 20 nm, determined by transmission electron microscopy (H-7500, Hitachi
Inc.). For gases, N2 (99.999 % purity, Beijing Huayuan) and O2
(99.999 % purity, Beijing Huayuan) were introduced as synthetic air (80 % N2 and 20 % O2) in this study, while SO2 (5.9 ppm in
N2, Beijing Huayuan), NO2 (3.9 ppm in N2, Beijing Huayuan),
and C3H6 (5.9 ppm in N2, Beijing Huayuan) were used as
reactant gases.
Experimental methodsIn situ DRIFTS
In situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) spectra were recorded on a Nicolet Nexus 670 Fourier transform infrared spectroscope (FTIR) equipped with a
mercury cadmium telluride (MCT) detector, scanning from 4000 to 650 cm-1 at a resolution of 4 cm-1 for 100 scans. Before each
experiment, the oxide sample was finely ground and placed into a ceramic
crucible in the in situ chamber. Then the sample was pretreated at 503 K and
atmospheric pressure for 120 min to remove adsorbed species in 100 mLmin-1 synthetic air. All the spectra are presented in the Kubelka–Munk
(K-M) scale to improve the linearity of the dependence of signal intensity
upon concentration (Armaroli et al., 2004). The UV–Vis irradiation
was acquired with 500 W xenon light (CHF-XM35, Beijing Changtuo) and was
introduced into the DRIFTS reaction cell via a UV optical fiber. The
intensity of UV–Vis irradiation was measured as 478 µWcm-2 by a
UV meter (Photoelectric Instrument Factory of Beijing Normal University).
The wavelengths of the UV–Vis irradiation were measured to be in the range
of 300–800 nm by a fiber-optic spectrometer (BLUE-Wave-UVNb, Stellar Net
Inc., USA), as shown in Fig. S1 in the Supplement. The
spectrum of the UV–Vis irradiation is comparable to the spectrum of solar
irradiation on the earth surface, and therefore we think the UV–Vis
irradiation used in this study may represent the conditions in the real
atmosphere.
To investigate heterogeneous sulfate formation in complex atmospheric
pollution, in situ DRIFTS was used to analyze the products on particle surfaces in
the reactions under different conditions. Two series of in situ DRIFTS experiments
were carried out in this study. For the heterogeneous reaction of SO2
under different gas conditions, the TiO2 sample was initially flushed
with the synthetic air at a total flow rate of 100 mLmin-1 for 2 h.
The temperature was 303 K and the relative humidity was less than 1 % in
all experiments. Then the background spectra were recorded when they showed
little change with time. After that, gas reactants, such as 200 ppb SO2, 200 ppb NO2, and 200 ppb C3H6, were
introduced to the gas flow and then passed through the reaction chamber for
12 h. These experiments were carried out under both dark and with UV–Vis
irradiation conditions. The other series of experiments were step-by-step
exposure experiments for further investigation of the effects of NO2
and C3H6 on the heterogeneous oxidation of SO2 with UV–Vis
irradiation. The concentrations of reactants in the step-by-step exposure
experiments were changed from 200 ppb to 200 ppm to strengthen the signals
of the products. These step-by-step exposure experiments all included three
steps, namely first exposing the particles to NO2, C3H6, or
both for 2 h; flushing with air for 1 h; and finally exposing them to
SO2 for 2 h.
IC
Sulfate products on the powders after the in situ DRIFTS study were also measured
quantitatively using ion chromatography (IC). The powders were firstly
weighed and placed in 8 mL transparent glass jars. After adding 5 mL of
ultrapure water (specific resistance ≥18.2MΩcm-1)
containing about 1 % formaldehyde (50 µL) to inhibit the oxidation of
sulfite to sulfate, the samples were then extracted by sonication at 303 K
for 120 min. After a standing time of 120 min, the obtained
supernatant was passed through a 0.22 µm PTFE membrane filter and was then
analyzed using a Wayee IC-6200 ion chromatograph equipped with a Thermo
AS14 analytical column. An eluent of 3.5 mMNa2CO3 was used at a
flow rate of 0.8 mLmin-1.
Dynamic changes in the in situ DRIFTS spectra of the TiO2 sample as
a function of time at 303 K in a flow of 20 % O2+80 % N2 with 200 ppb SO2 under dark conditions (a) and with UV–Vis light (b); with 200 ppb SO2+200 ppb NO2 under dark conditions (c) or
with UV–Vis light (d); with 200 ppb SO2+200 ppb C3H6
under dark conditions (e) or with UV–Vis light (f); with 200 ppb SO2+200 ppb NO2+200 ppb C3H6+ under dark conditions (g)
or with UV–Vis light (h).
Vibrational frequencies of chemisorbed species formed on TiO2.
Surface species Frequencies (cm-1)ReferencesSO32-/HSO3-monodentate sulfite1098, 1078, 1052Liu et al. (2012), Nanayakkara et al. (2012)SO42-state of aggregation1344Nanayakkara et al. (2012)bidentate1290Yang et al. (2005)bridging1177, 1141Chen et al. (2007)NO3-bridging1611, 1246Goodman et al. (2001b), Underwood et al. (1999), Hadjiivanov and Knözinger (2000)bidentate1584, 1284Hadjiivanov and Knözinger (2000)monodentate1503, 1453Piazzesi et al. (2006)HNO31682Goodman et al. (2001a)COO-1585, 1541Busca et al. (1987), Idriss et al. (1995), Rachmady and Vannice (2002a), Mattsson and Österlund (2010)-CH31452, 1379Busca et al. (1987)-CH1361Rachmady and Vannice (2002b)-CHO1745Liao et al. (2001)H2Obending vibration1626Goodman et al. (1999)OHisolated bicoordinated (on Ti atoms)3690Primet et al. (1971)H bonded3631Tsyganenko and Filimonov (1973) Ferretto and Glisenti (2003)OHadsorbed water3456, 3310, 3190Tarbuck and Richmond (2006)ResultsHeterogeneous reaction of SO2 under different conditionsHeterogeneous reaction of SO2 on TiO2
DRIFTS spectra for heterogeneous reaction of 200 ppb SO2 on TiO2
under dark conditions or with UV–Vis irradiation are shown in Fig. 1, while
the vibrational frequencies of chemisorbed species formed on the surface of
TiO2 are listed in Table 1. In the dark experiment, the reaction
products on the surface of TiO2 were mainly sulfite. As shown in Fig. 1a, the positive bands observed at 1098, 1078, and 1052 cm-1 can be
assigned to monodentate sulfite (Hug, 1997; Peak et al., 1999). Negative
peaks at 3691 and 3630 cm-1 were attributed to hydroxyl on TiO2
(Primet et al., 1971; Tsyganenko and Filimonov, 1973; Ferretto and
Glisenti, 2003). These negative peaks were observed in all the reaction
systems in this study, as shown in Fig. 1, which is consistent with previous
studies (Nanayakkara et al., 2012; Ma et al., 2019). The loss of surface
hydroxyl groups from the surface upon adsorption of SO2 implies that
surface OH groups were involved in the reaction of SO2 on TiO2
under both dark and UV–Vis irradiation conditions.
With UV–Vis light illumination, SO2 was oxidized on TiO2 and
resulted in abundant sulfate species, as shown in Fig. 1b. The main bands
in the 1400–1100 cm-1 region became more apparent with increasing
exposure time. The spectra in this region were assigned to sulfate in
different coordination modes, including aggregation at 1344 cm-1,
bidentate at 1290 cm-1, and bridging sulfate at 1177 and 1141 cm-1
(Hug, 1997; Peak et al., 1999; Fu et al., 2007). With UV–Vis illumination,
TiO2 can be excited by UV light (λ<387nm), and then the
photogenerated electrons and holes can react with H2O and O2 to
produce additional ROS (primarily ⚫O2- and ⚫OH), which oxidize more
SO2 to sulfate on TiO2 than that produced under dark conditions
(Shang et al., 2010; Chen et al., 2012). The sharp band at 1626 cm-1
and the broad bands with maxima at 3316 and 3190 cm-1 in Fig. 1b
can be assigned to the bending vibration and stretching modes of molecularly
adsorbed water. Surface water can be formed in the heterogeneous reaction of
SO2 (Nanayakkara et al., 2012; Zhang et al., 2006) or via enhanced
adsorption of water due to the increased hygroscopicity induced by sulfate
(Ma et al., 2019). Although the RH was controlled at less than 1 % in
our experiments, water cannot be entirely removed in the introduced gas
flows. In Fig. 1, there is a positive correlation between the signal
intensities of the adsorbed water and sulfite/sulfate among different
experimental systems.
Heterogeneous reaction of SO2 and NO2 on TiO2
As reported in previous studies, the presence of NO2 can promote the
heterogeneous oxidation of SO2 (Ma et al., 2008, 2017; Liu et al., 2012), which was also investigated in this study under both dark and
illuminated conditions. The spectra regarding the reaction of 200 ppb SO2 and 200 ppb NO2 on TiO2 under dark conditions are shown
in Fig. 1c. Sulfite, sulfate, and nitrate species were observed in this
reaction system. Specifically, the bands at 1361 and 1346 cm-1 were
assigned to aggregated sulfate; bands at 1163 and 1115 cm-1 were
related to bridging sulfate; and bands at 1074 and 1010 cm-1 were
ascribed to monodentate sulfite (Liu et al., 2012; Yang et al., 2017, 2018). The other bands in the 1620–1370 and 1300–1240 cm-1
regions were due to nitrate species, including bridging nitrate (1611, 1246 cm-1), bidentate nitrate (1584, 1284 cm-1), and monodentate nitrate
(1503, 1453 cm-1) (Goodman et al., 2001a; Ma et al., 2010). The
consumption of OH groups (negative peaks at 3691 and 3630 cm-1) and
formation of water (3310, 3191, and 3341 cm-1) on the particle surface
were also observed. These results indicated that SO2 can be partially
oxidized to sulfate in the presence of NO2 under dark conditions, which
is consistent with previous studies (Ma et al., 2008; Liu et al., 2012),
in spite of much lower concentration levels of SO2 and NO2 being
used in this study.
The spectra of TiO2 exposed to 200 ppb SO2 and 200 ppb NO2
simultaneously with UV–Vis irradiation were recorded and shown in Fig. 1d.
The bands at 1629, 1584, and 1503 cm-1 were related to nitrate species
while the bands at 1344, 1284, 1177, and 1141 cm-1 were
associated with sulfate species. Compared to the dark experiment of SO2
and NO2 in Fig. 1c, more sulfate species were generated with UV–Vis
irradiation, which might be due to the fact that UV–Vis irradiation
significantly promotes sulfate formation by generating additional active
species (Shang et al., 2010; Chen et al., 2012) as in the reaction of
SO2 alone.
Heterogeneous reaction of SO2 and C3H6 on TiO2
To investigate the heterogeneous reaction with the coexistence of inorganic
and organic gases on TiO2, propene was chosen as a representative
volatile organic compound, and its effect on the heterogeneous oxidation of
SO2 was studied. Under dark conditions, the in situ spectra after introduction
of 200ppbSO2+200ppbC3H6 were recorded and are shown in
Fig. 1e. No distinguishable products were observed except for the bands at
1074 and 1048 cm-1, which were assigned to monodentate sulfite.
Compared to the reaction of SO2 alone, the coexistence of
C3H6 had no apparent effect in this dark experiment. With UV–Vis
irradiation, the sulfate bands are between 1360 and 1100 cm-1 with peaks at
1343, 1289, 1244, 1177, and 1139 cm-1 increasing with reaction time, as
shown in Fig. 1f. Compared to the reaction of SO2 alone with UV–Vis
irradiation, similar peaks in spectra were obtained for the SO2+C3H6
reaction but the intensities were lower.
Heterogeneous reaction of SO2, NO2, and C3H6 on
TiO2
In order approximate the complexity of the real atmosphere, we investigated
the heterogeneous reaction of SO2, NO2, and C3H6 on
TiO2. Figure 1g and h show the dynamic changes in the spectra after
introducing these three gases together on TiO2 under dark conditions
and with UV–Vis irradiation, respectively. The concentrations of SO2,
NO2, and C3H6 were all 200 ppb. The reaction of
SO2/NO2/C3H6 on TiO2 included both the
SO2/NO2 reaction (Fig. 1c and d) and the
SO2/C3H6 reaction (Fig. 1e and f) under dark conditions
and with UV–Vis irradiation, respectively. Thus, the products included
sulfite, nitrate, and some sulfate under dark conditions, while mainly
sulfate and nitrate with UV–Vis irradiation.
Sulfate formation and the influence of NO2 and C3H6
To obtain the area of an individual band for quantitative analysis, a
curve-fitting procedure was used employing Lorenz and Gaussian curves based
on the second-derivative spectrum to deconvolute overlapping bands. An
example of the analysis for the bands in Fig. 1b, with a correlation
coefficient of 0.992, is shown in Fig. S2 in the Supplement.
The band at 1070 cm-1 is attributed to sulfite, while the bands at 1140, 1178,
1240, 1292, and 1346 cm-1 are attributed to sulfate. To avoid
interference by nitrate species and other surface products in reactions with
the presence of NO2, the peaks at 1198–1135 cm-1 were chosen for
calculation of the sulfate K-M integrated area.
Integrated absorbance of the sulfate band (1198–1135 cm-1)
observed during the reaction of 200 ppb SO2, 200 ppb SO2+200 ppb NO2, 200 ppb SO2+200 ppb C3H6, 200 ppb SO2+200 ppb NO2+200 ppb C3H6 in dark experiments (a) and
experiments with UV–Vis light (b).
The K-M integrated areas of bridging sulfate in the four reaction systems:
(1) SO2; (2) SO2+C3H6; (3) SO2+NO2; and (4) SO2+NO2+C3H6 in the dark and with UV–Vis light are
shown in Fig. 2a and b, respectively. In the dark experiments, no
apparent sulfate was generated in the reaction of SO2 alone. The
presence of C3H6 had no discernible effect on the formation of
sulfate in dark experiments. The presence of NO2 promoted the oxidation
of SO2 on TiO2, with the result that mostly sulfate was yielded
from the reaction of SO2+NO2. The presence of NO2 seemed to
induce the generation of some ROS, which oxidize S(IV) to S(VI) on TiO2
(Ma et al., 2008, 2017; Liu et al., 2012). The detailed
mechanism for this effect has not been fully explored and will be discussed
later. It has also been proposed that aqueous oxidation of SO2 by
NO2 (as an oxidizing agent) contributed to significant sulfate
formation in haze events (Wang et al., 2016b; Cheng et al., 2016). This
reaction should not be the main pathway in the reaction systems in this
study since the experiments were carried out under dry conditions
(RH<1 %), although water can still exist, as we mentioned
earlier. When SO2 was introduced into the cell with NO2 and
C3H6 together, sulfate formation was less than that in the
reaction of SO2+NO2, probably due to the competition between
SO2 and C3H6 for the ROS due to NO2. In the UV–Vis
irradiation experiments, on the contrary, both NO2 and C3H6
had a distinct suppressing effect on the sulfate formation compared to the
individual reaction of SO2. The opposite effect of NO2 on
sulfate formation relative to dark experiments may be explained by the
different influence of NO2 on the oxidation capacity in the
heterogeneous photooxidation, compared to dark experiments. In dark
experiments, the contribution of NO2 to the oxidation capacity is
predominantly due to the limited availability of ROS, while it becomes of
lesser importance when surface ROS are continuously generated in the
experiments with UV–Vis irradiation. What is more, the nitrate formation from
oxidation of NO2 might block some surface reactive sites, and
therefore resulted in less sulfate formation in the reaction of
SO2+NO2 than that of SO2 alone with UV–Vis irradiation. To
further probe and analyze the total amounts of sulfate in different systems,
the samples after reaction in the different experiments were also analyzed
by IC. The results, which are shown in Fig. 3, are consistent with the
results derived from integrated peak areas in Fig. 2. Since formaldehyde was
added to inhibit the oxidation of sulfite to sulfate in the solution, there
is a possibility that HMS would be generated in the solution and be measured
as sulfate (Moch et al., 2018). However, the possible interference by HMS
in the measurement of sulfate by IC will not influence our conclusions on
the effects of NO2 and C3H6, since the K-M integrated area of
sulfate in the In situ DRIFTS spectra were also compared. Despite the different
yields of sulfate under different atmospheres, the presence of UV–Vis
irradiation always increased sulfate formation significantly. We also
observed that the promotion effect of UV–Vis irradiation on the
heterogeneous oxidation of SO2 was most significant for the individual
reaction of SO2, while it became less noticeable under more complex
pollution, i.e., in the presence of NO2 and some VOCs.
Ion chromatography results of the amounts of sulfate (product per
unit mass divided by surface area of sample) formed on the surface of TiO2 after
reaction with SO2, SO2+NO2, SO2+C3H6, and
SO2+C3H6+NO2 in experiments under dark conditions or
with UV–Vis light. Since formaldehyde was added to inhibit the oxidation of
sulfite to sulfate in the solution, there is a possibility that HMS would be
generated in the solution and would be measured as sulfate.
In situ DRIFTS spectra of surface products on TiO2 in the
step-by-step exposure experiments with irradiation: (a) exposure to 200 ppm NO2 for 2 h (black lines), after purging 1 h (blue line), and then to
200 ppmSO2 for 2 h (red lines); (b) exposure to 200 ppm C3H6
for 2 h (black lines), after purging 1 h (blue line), and then to 200 ppmSO2 for 2 h (red lines); (c) exposure to 200 ppm NO2+200 ppm C3H6 for 2 h (black lines), after purging 1 h (blue line), and
then to 200 ppmSO2 for 2 h (red lines).
Step-by-step experiments with UV–Vis irradiation and related mechanisms
In the step-by-step experiments, the spectra for TiO2 exposure to 200 ppm NO2 after the first step are shown by the black lines in Fig. 4a.
The nitrate bands at 1611, 1586, 1507, 1288, and 1241 cm-1 increased in
intensity. When the NO2 was cut off, the particles were purged with air
for 1 h, and the spectrum was recorded as the blue line in Fig. 4a. Air
purging did not noticeably change the spectra, except that the nitrate band
at 1611 cm-1 shifted to 1637 cm-1 due to the absorption of water
(Ma et al., 2010), indicating a relatively steady adsorption of
nitrate species. Then the NO2-preadsorbed TiO2 particles were
exposed to SO2 in the third step, marked by red lines in Fig. 4a. A
new band at 1168 cm-1 assigned to sulfate appeared and the bands at
1350–1200 cm-1 became broader due to the formation of sulfate.
Meanwhile, the nitrate bands at 1586 and 1507 cm-1 decreased in
intensity and even disappeared. The possible reason might be either the
replacement of nitrite by sulfate from SO2 heterogeneous photooxidation
(Park et al., 2017) or the photolysis of nitrate
(Ye et al., 2017).
In the 200 ppm C3H6 presaturated experiment, which is shown in
Fig. 4b, after C3H6 was introduced into the reaction cell for 2 h, intense bands at 1582, 1541, 1452, 1379, and 1361 cm-1 were
observed. These principal bands are assigned to carboxylate (-COO: 1582,
1541 cm-1) methyl (-CH3: 1452, 1379 cm-1), and methyne (-CH:
1361 cm-1) (Busca et al., 1987; Idriss et al., 1995).
Based on the above bands, the main products could be deemed to be formate
and acetate species. After stopping the flow of C3H6 and flushing
the cell with synthetic air for 1 h, the band areas of surface products were
reduced, indicating that these species from C3H6 were not stable
and could be removed easily from the surface. The subsequent introduction of
SO2 into the system resulted in sulfate formation, as seen by the bands
in the 1380–1050 cm-1 region. Introducing NO2 and C3H6
together before SO2 resulted in both nitrate and organic species on
TiO2, as shown in Fig. 4c. It is interesting that some distinct new
bands were observed when the surface was exposed to
NO2+C3H6, such as the bands at 1750, 1682, and 1524 cm-1, which could be assigned to CH2O (Liao et al.,
2001), HNO3 (Goodman et al., 2001a), and COO groups
(Mattsson and Österlund, 2010), respectively. This may indicate
some interaction between NO2 and C3H6 and a possible
influence of C3H6 on nitrate formation, as well as NO2 on
C3H6 oxidation in the heterogeneous photooxidation.
Integrated absorbance of the sulfate band (1168 cm-1) for the
illuminated reactions with UV–Vis light of 200 ppmSO2 (black, solid),
200 ppmSO2 on a 200 ppm C3H6-presaturated surface (blue,
dashed), 200 ppm SO2+200 ppm NO2 (red, solid), 200 ppmSO2
on a 200 ppm NO2-presaturated surface (green, dashed), 200 ppm SO2+200 ppm C3H6 (blue, solid), 200 ppm SO2+200 ppm NO2+200 ppm C3H6 (pink, solid), and 200 ppmSO2 on a
200 ppm NO2+200 ppm C3H6-presaturated surface (purple,
dashed).
Figure 5 compares the K-M integrated areas of bridging sulfate (1168 cm-1) formed during these step-by-step experiments under different
conditions. Compared to the reaction with SO2 alone, the pre-adsorption
of C3H6 on TiO2 did not have any apparent influence. This is
consistent with the supposition that the formate and acetate species from
heterogeneous oxidation of C3H6 might be easily removed from the
surface. Since introducing C3H6 with SO2 together suppressed
sulfate formation in the heterogeneous photooxidation, while pre-adsorption
of C3H6 had little influence, C3H6 is proposed to
compete with SO2 for ROS rather than surface reactive sites in the
heterogeneous photooxidation. Instead, the pre-adsorption of NO2 on
TiO2 suppressed the formation of sulfate, which might have resulted
from the different absorption status of the oxidation products of NO2
and C3H6. Compared to the experiment introducing NO2 and
SO2 simultaneously, sulfate formation was more inhibited with
pre-adsorption of NO2 in the first hour, while sulfate formation in
these two cases became similar after 1.5 h duration. This may indicate that
NO2 suppressed sulfate formation, mainly due to the competition between
SO2 and NO2 for surface reactive sites. Compared to the individual
reaction of SO2, both pre-adsorption of NO2 and introducing
NO2 simultaneously suppressed sulfate formation from the beginning of
the heterogeneous photooxidation. It is interesting that pre-adsorption with
NO2+C3H6 resulted in much less sulfate formation compared
to the pre-adsorption of NO2 or C3H6, as well as the reaction
of SO2+NO2+C3H6. Although the detailed reason for
this phenomenon was not discovered in this study, a possible reason might be
that the oxidation products from NO2 and C3H6 blocked some
reactive sites on TiO2 and suppressed sulfate formation in
heterogeneous photooxidation, since NO2 and C3H6 were cut off
after pre-adsorption and ROS were expected to be generated on TiO2 with
UV–Vis irradiation. According to the DRIFTS spectra in Fig. 4c, besides
nitrate, aldehydes (1750 cm-1) and carboxylic acids (1524 cm-1)
were also observed on TiO2 after pre-adsorption with NO2+C3H6.
DiscussionDark reactions
The heterogeneous oxidation of SO2 on TiO2 has been investigated
by many previous studies. The following mechanisms for SO2 adsorption
on TiO2 surfaces have been proposed in previous studies
(Nanayakkara et al., 2012):
1Ti-OH+SO2→Ti-OSO2H,22Ti-OH+SO2→Ti2-SO3⋅H2O,3Ti-O2-+SO2→Ti-SO32-.
These adsorption processes result in the conversion of SO2 to sulfite
(S(IV)) on the surface. It has been demonstrated that coexisting NO2
can induce the generation of some ROS, which oxidize S(IV) to S(VI) on
mineral oxides (Ma et al., 2008, 2017; Liu et al., 2012). There
were several possible responsible ROS proposed in previous studies, although
the detailed mechanism has not yet been fully explored. One possible ROS is
N2O4, which can undergo hydrolysis to N(III) and N(V) species
(Liu et al., 2012; Finlayson-Pitts et al., 2003; Li et al., 2018). These
reactive nitrogen species can oxidize S(IV) to S(VI) (Wang et al.,
2016b; Li et al., 2018).
42Ti-NO2→Ti2-N2O4,5N2O4(ad)→NO+NO3-⟶H2OHNO3+HONO.
Besides N2O4, NO2 may also react directly with surface OH and
form HNO3 on TiO2 (C. Liu et al., 2017). The HNO3
generated through this pathway may also contribute to the oxidation of S(IV)
to S(VI). It has also been proposed that aqueous oxidation of SO2 by
NO2 (as an oxidizing agent) contributed to significant sulfate
formation in haze events (Wang et al., 2016b; Cheng et al., 2016). This
aqueous reaction should not be significant in the reaction systems of this
study due to the limited amount of water under low RH conditions (<1 % RH).
When C3H6 was introduced together with NO2, sulfate formation
was less than that in the reaction of SO2+NO2, probably due to
the reaction between C3H6 and the reactive nitrogen species. The
detailed mechanism was not explored in this study. The following reactions
may take place in this process.
62NO+NO3-+Ti-C3H6→H3CCHO+HCHO+2NO+NO2-,7NO+NO3-+HCHO→HCOOH+NO+NO2-,8NO+NO3-+H3CCHO→H3CCOOH+NO+NO2-.
Heterogeneous reactions between NO2 and organics can also lead to
nitro-organics on hexane soot (Kwamena and Abbatt, 2008; Al-Abadleh and
Grassian, 2000), which may also occur on the surface of TiO2, and
these products blocked some reactive sites for sulfate formation.
Light reactions
With UV illumination, TiO2 can be excited by UV light (λ<387nm), then the photogenerated electrons and holes can react
with H2O and O2 to produce additional ROS (primarily
⚫O2- and ⚫OH),
and oxidize more SO2 to sulfate on TiO2 than that produced under
dark conditions (Shang et al., 2010; Chen et al., 2012). The detailed
mechanism was summarized by Chen et al. (2012)
and references therein:
9TiO2+hν(λ<387nm)→e-h+→e-+h+,10O2+e-→⚫O2-,11H2O+h+→⚫OH+H+.
Then the SO2 can react with these ROS and promote the formation of
sulfate (Shang et al., 2010):
12Ti-SO2+⚫O2-→Ti-SO3+O-,13Ti-SO3+H2O→Ti-H2SO4,14Ti-SO32-+2⚫OH→Ti-SO42-+H2O.
In the UV–Vis irradiation experiments, NO2 had a distinct suppressing
effect on the sulfate formation compared to the individual reaction of
SO2. Rather than resulting in ROS formation and oxidation of S(IV) to S(VI) in dark experiments, the main reaction of NO2 with the surface ROS
resulted in nitrate and nitrite formation in experiments with UV–Vis
irradiation (Ndour et al., 2008; Yu and Jang, 2018).
15Ti-NO2+⚫OH→Ti-HONO2,16Ti-NO2+⚫O2-→Ti-NO2-+O2.
The nitrate or nitrite generated from the oxidation of NO2 might block
some surface reactive sites, since in the step-to-step experiments the
pre-adsorption of NO2 on TiO2 also suppressed the formation of
sulfate and resulted in similar sulfate formation to that in the experiment
introducing NO2 and SO2 simultaneously. The competition between
SO2 and NO2 for surface reactive sites might be the main reason
for the fact that the coexistence of NO2 with SO2 resulted in
decreased sulfate formation with UV–Vis irradiation in this study. Although
Gen et al. (2019) found that photolysis of
nitrate enhanced sulfate formation in wet aerosols, this mechanism may not
be applied in this study since the reaction system is quite different from
their study. The ROS, which oxidize S(IV) to S(VI), are mainly
⚫O2- and ⚫OH
in the presence of UV–Vis irradiation rather than the photolysis of nitrate.
C3H6 also had a distinct suppressing effect on sulfate formation.
Similar to NO2, C3H6 will react with surface ROS.
C3H6⟶⚫OHRCHO⟶⚫OHRCOOH⟶⚫OHCO2+H2O,
where R represents H or an alkyl group. These gaseous products in the
photooxidation of C3H6 do not seem to block surface reactive
sites, which can explain why the pre-adsorption of C3H6 on
TiO2 did not show an obvious suppressing effect on the formation of
sulfate in the step-by-step experiment.
When C3H6 and NO2 were introduced simultaneously into the
reaction system together with SO2, both competed for ROS with SO2
and therefore resulted in the lowest formation of sulfate among the
heterogeneous reactions. Besides, in the step-by-step experiments, the
pre-adsorption of C3H6 and NO2 on TiO2 suppressed sulfate
formation significantly, which indicated that lots of reactive sites for
SO2 oxidation might be blocked by these oxidation products in
pre-adsorption with UV–Vis irradiation. Karagulian et al. (2009) found that nitrite can induce the
photooxidation of VOCs on airborne particles and produce organic nitrates
and carbonyl compounds. Thus, the formation of organic nitrates may be an
important factor to suppress the formation of sulfate due to the blocking
effect.
Conclusions and environmental implications
Based on the experimental results obtained in this study, we propose the
following possible mechanisms for the reaction of SO2 in the presence
of NO2 and C3H6 under conditions close to those in the real
atmosphere. Under dark conditions at 303 K, SO2 could hardly react on
the particle surface and only a few sulfite-like species formed. With
reaction time increasing, the adsorption sites on the surface became
saturated with sulfite and prevented SO2 from adsorbing on the
particles further. Coexisting NO2 could enhance the heterogeneous
formation of sulfate with much lower concentrations (200 ppb) relative to
previous studies (∼100 ppm) (Ma et al., 2008; Liu et al.,
2012; Zhao et al., 2018). The presence of C3H6 had little effect on
sulfate formation in the heterogeneous reaction of SO2 but suppressed
sulfate formation in the heterogeneous reaction of SO2 and NO2,
because C3H6 could react ROS generated in the adsorption of
NO2. When irradiation was introduced into the system, the ROS such as
⚫OH and ⚫O2-
could initiate photocatalytic oxidation of S(IV) species to sulfate. Sulfate
formation was suppressed significantly with the coexistence of NO2
and/or C3H6 in the presence of UV–Vis light. The formation of
nitrate, carbonyl compounds, and organic nitrate consumed both available ROS
and surface reactive sites.
These results indicated that heterogeneous oxidation of SO2 might be
influenced by the coexisting inorganic and organic gas pollutants under
complex pollution conditions due to the competition for ROS and active
surface sites among them. In this study, only one VOC was investigated,
but the heterogeneous oxidation of various VOCs has been reported in
previous studies (Niu et al., 2017; Du et al., 2000). When a VOC and
SO2 coexist, the competition for ROS and surface reactive sites between
the VOC and SO2 is likely to suppress sulfate formation in the
heterogeneous reactions, such as that observed for the presence of
CH3CHO on α-Fe2O3 in dark experiments
(Zhao et al., 2015), the presence of C7H16 on TiO2
with UV–Vis irradiation (Du et al., 2000), and the presence of
C3H6 on TiO2 under dark condition or with UV–Vis irradiation
in this study. Due to the different properties of the oxidation products,
the influence of coexisting VOCs might be different for different VOC
species and on different mineral dusts. Some coexisting VOCs, such as HCOOH
on α-Fe2O3 (Wu et al., 2013),
and HCHO in aerosol water (Moch et al., 2018; Song et al., 2019) might
enhance sulfate formation. These results highlighted the very complex
heterogeneous reaction processes that take place under complex air pollution
conditions due to the ubiquitous interactions between organic and inorganic
species. For better estimation of heterogeneous sulfate formation, the
kinetics of the heterogeneous oxidation of SO2 must be developed with
consideration of the influence of coexisting VOCs and other inorganic gases.
Data availability
All the data related to this paper may be requested from the corresponding
author: qxma@rcees.ac.cn.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-14777-2019-supplement.
Author contributions
QM, BC, and HH designed the study. YW, WY, and BC carried out the experiments.
BC, WY, JM, and QM analyzed the data with input from all co-authors. BC and
YW wrote the paper with contribution from YL, JM, WY, and PZ on the editing
of the paper.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Multiphase chemistry of secondary aerosol formation under severe haze”. It is not associated with a conference.
Acknowledgements
This work was supported by the National Key R&D Program of China
(2018YFC0506901), National Natural Science Foundation of China (41877304,
21876185, 91744205), the National Research Program for Key Issues in Air
Pollution Control (DQGG0103), and the Youth Innovation Promotion
Association, CAS (2018060, 2018055, 2017064).
Financial support
This research has been supported by the National Key R&D Program of China (grant no. 2018YFC0506901), the National Natural Science Foundation of China (grant nos. 41877304,
21876185, 91744205), the National Research Program for Key Issues in Air Pollution Control (grant no. DQGG0103), and the Youth Innovation Promotion Association, CAS (grant nos. 2018060, 2018055, 2017064).
Review statement
This paper was edited by Jingkun Jiang and reviewed by three anonymous referees.
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