Introduction
Organic aerosol, typically a large fraction of fine particles, contains thousands or more of organic compounds and contributes to visibility reduction,
photochemical smog, climate change and adverse health effects (Novakov
and Penner, 1993; Goldstein and Galbally, 2007; Jimenez et al., 2009; Pöschl
and Shiraiwa, 2015). A significant component of organic aerosol is secondary
organic aerosol (SOA) formed from the gas-phase oxidation of volatile
organic compounds (VOCs) followed by partitioning of products into particles
or from heterogeneous reactions of VOCs with particles (Hallquist et al.,
2009; Zhang et al., 2015). Dicarboxylic acids (DCAs) are abundant and
ubiquitous constituents in SOA and can be effective tracers for the
oxidative processes leading to the formation of SOA (Kawamura and
Ikushima, 1993; Ervens et al., 2011; Wang et al., 2012; Cheng et al., 2013).
DCAs normally have high water solubility and low vapor pressure. Thus, they
play important roles in controlling the hygroscopic properties of organic
aerosols (Prenni et al., 2003; Ma et al., 2013) and activating cloud
condensation nuclei (Booth et al., 2009). The primary emissions
of DCAs from anthropogenic sources in urban areas are minor (Huang and
Yu, 2007; Stone et al., 2010), and they are mainly derived from secondary
oxidation of VOCs and subsequent intermediates (Ho et al.,
2010; Myriokefalitakis et al., 2011). High concentrations of DCAs have been
observed in biomass burning plumes (Kundu et al., 2010; Kawamura et al.,
2013), with more than 70 % of DCAs produced from photochemical oxidation of
water-soluble organic compounds, and only a small contribution from direct
biomass burning (BB) emission (van Pinxteren et al., 2014).
The production of DCAs through photochemical reactions has been reported in
many field studies via the analysis of the diurnal and seasonal variations
in DCA (dicarboxylic acid) (Kawamura and Ikushima, 1993; Kawamura and Yasui, 2005; Aggarwal and
Kawamura, 2008; Pavuluri et al., 2010; Ho et al., 2011; Wang et al., 2017), but
the mechanism of DCA formation is still not well understood. Oxalic acid is
usually the most abundant DCA observed in the field (Kawamura et al.,
2004, 2010; Ho et al., 2007). A number of ground-based and
airborne field studies have found a close correlation between oxalic acid
and sulfate in ambient particles and cloud droplets, relating aqueous-phase
chemistry to the formation of oxalic acid in aerosols and cloud droplets
(Yao et al., 2002, 2003; Yu et al., 2005; Sorooshian et al.,
2006, 2007a, b; Miyazaki et al.,
2009; Wonaschuetz et al., 2012; Wang et al., 2016). In recent years, several
model and laboratory studies have suggested that the aqueous-phase oxidation of
highly water-soluble organics like glyoxal, methylglyoxal and glyoxylic acid
can efficiently produce oxalic acid in aerosol particles and cloud droplets
(Lim et al., 2010; Myriokefalitakis et al., 2011; Ervens et al., 2014; Yu et
al., 2014; McNeill, 2015). Recent stable carbon isotope studies and field
observations have also suggested that oxalic acid forms through aqueous-phase reactions (Wang et al., 2012; Cheng et al., 2015). However, the
formation process of oxalic acid in ambient aerosols is still associated
with great uncertainty due to the oxidation rates of precursors and oxidant
levels in photochemistry and aqueous-phase chemistry, which needs to be
further studied.
Online measurements of the size distribution of oxalic-acid-containing
particles and the mixing state of oxalic acid with other compounds in
aerosols are useful for examining the formation and evolution of oxalic acid
and SOA particles. Sullivan and Prather (2007) investigated the diurnal
cycle and mixing state of DCA-containing particles in Asian aerosol outflow
using aerosol time-of-flight mass spectrometry (ATOFMS) and proposed the
formation of DCA on Asian dust. In addition, Yang et al. (2009) measured
oxalic acid particles in Shanghai and proposed that in-cloud processes and
heterogeneous reactions on hydrated aerosols contributed to the formation of
oxalic acid (Yang et al., 2009). While the formation mechanism of oxalic
acid, especially in urban areas, is still not clear, online measurements of
the mixing state of oxalic acid provide a powerful tool to better understand
the formation of oxalic acid in aerosol particles and cloud droplets.
The Pearl River Delta (PRD) region has distinct meteorological seasonality
under the influence of the Asian monsoon system, which brings air from the
ocean in spring and summer and carries polluted air from northern China in
autumn and winter. Strong photochemical activity occurs in summer under the
condition of high temperature and relative humidity, and in winter high
loadings of particles from northern cities are favorable for the occurrence
of haze episodes (Bi et al., 2011; Zhang et al., 2013, 2014).
Here we present the seasonal field measurements of the mixing state of
oxalic-acid-containing particles using a single-particle aerosol mass
spectrometer (SPAMS) at a rural supersite of the PRD region. The seasonal
characteristics of oxalic acid particles and mixing state with secondary
species were investigated to explore the formation mechanisms of oxalic acid
and aging process of SOA.
Methods
Aerosol sampling
Particles were sampled using a SPAMS at the Guangdong atmospheric supersite (22.73∘ N, 112.93∘ E), a rural
site at Heshan city (Fig. S1 in the Supplement). The supersite is surrounded by farmland
and villages, with no local industrial or traffic emissions. Ambient
aerosols were sampled using the SPAMS through a 2.5 m long copper tube with 0.5 m
of the sampling inlet located above the top of the building. The measurement
period was from 18 July to 1 August in 2014 and from 27 January to 8 February
in 2015. Real-time PM2.5 mass concentration was simultaneously
measured by a TEOM monitor (series 1405, Thermo Scientific), and hourly
concentrations of O3 were measured by an O3 analyzer (model 49i,
Thermo Scientific). The local meteorological data including temperature,
relative humidity and visibility were measured on the rooftop of the
building. The average temperature during the field study was 29.5 ∘C in summer and 14.1 ∘C in winter and the average relative
humidity was 71.7 and 63 % in summer and winter, respectively.
SPAMS
Real-time measurements of single atmospheric particles were used
by Prather and co-workers in the 1990s using aerosol time-of-flight mass
spectrometry (ATOFMS) (Prather et al., 1994; Noble and Prather, 1996). Based
on the same principle, the SPAMS
developed by Guangzhou Hexin Analytical Company was applied to field
measurements of single particles in the current work. The details of the
SPAMS system were introduced previously (Li et al., 2011). Briefly,
aerosol particles are sampled into the vacuum pumped aerodynamic lens of the
SPAMS through an electro-spark-machined 80 µm critical orifice at
a flow rate of 75 mL min-1. The individual particles with terminal
velocity are introduced to the sizing region. The velocity of each single-particle is detected by two continuous laser beams (diode Nd:YAG, 532 nm)
with a space of 6 cm. The velocity is then used to calculate the single
particle aerodynamic diameter and provide the precise timing of the firing of
a 266 nm laser used to induce desorption and ionization (Nd:YAG laser,
266 nm). The energy of the desorption–ionization 266 nm laser was 0.6 mJ
and the power density was kept at about
1.06 × 108 W cm-2 during both
sampling periods. The 266 nm laser generates positive and negative ions that
are detected by a Z-shaped bipolar time-of-flight mass spectrometer. The size
range of the detected single particles is 0.2 to 2 µm. Polystyrene
latex spheres (nanosphere size standards, Duke Scientific Corp., Palo Alto)
of 0.22–2.0 µm diameter were used for size calibration.
Data analysis
The size and chemical composition of single particles detected by SPAMS were
analyzed using the Computational Continuation Core (COCO) toolkit based on the MATLAB software. Particles were clustered into
several groups using the neural network algorithm (ART-2a) to group particles
into clusters with similar mass spectrum features. The ART-2a parameters used
in this work were set to a vigilance factor of 0.8, a learning rate of 0.05
and a maximum of 20 iterations. We collected 516 679 and 767 986 particles
with both positive and negative mass spectra in summer and winter,
respectively. A standard solution of oxalic acid was prepared with pure
oxalic acid (H2C2O4, purity: 99.99 %, Aladdin Industrial
Corporation) and atomized to aerosols. After drying through two silica gel
diffusion driers, pure oxalic acid particles were directly introduced into
the SPAMS. The positive and negative mass spectra of oxalic acid are shown in
Fig. S2. Based on the mass spectra of pure oxalic acid and previous ambient
measurements using ATOFMS (Silva and Prather, 2000; Sullivan and Prather,
2007; Yang et al., 2009), HC2O4- (m/z -89) is selected as
the ion peak for oxalic acid containing particles. In this work, oxalic acid
particles are identified if the peak area of m/z -89 was larger than
0.5 % of the total signal in the mass spectrum. With this threshold,
13 109 and 20 504 of oxalic-acid-containing particles were obtained in
summer and winter separately, accounting for 2.5 and 2.7 % of the total
detected particles. The percentage of oxalic-acid-containing particles in
total particles in this work was comparable to the reported value in the
urban area of Shanghai (3.4 %) (Yang et al., 2009). However, these
percentages are in general much lower than those reported in cleaner
environments such as the western Pacific Ocean where oxalic acid was found in
up to 1–40 % of total particles due to little anthropogenic influences
(Sullivan and Prather, 2007).
The oxalic acid containing particles are classified into eight types in the
following order: elemental carbon (EC), organic carbon (OC), elemental and
organic carbon (ECOC), BB, heavy metal (HM), secondary
(Sec), sodium-potassium (NaK) and dust. Different types of particles are
identified according to characteristic ion markers and dominant chemical
species (Table S1 in the Supplement): (1) particles containing abundant
carbon clusters like ±12[C]+/-, ±24[C2]+/- and
±36[C3]+/- with a relative peak area of more than 0.5 % are
classified as EC type; (2) any remaining particles containing abundant
signals of 27[C2H3]+, 43[C2H3O]+ and
hydrocarbon clusters with a relative peak area of more than 0.5 % are
classified as OC type; (3) any remaining particles containing signals of
±12[C]+/-, ±24[C2]+/-, 37[C3H]+ and
43[C2H3O]+ with a relative peak area of more than 0.5 % are
classified as ECOC type; (4) any remaining particles containing abundant
signals of 39[K]+ (peak area > 1500) with a relative peak area of
-59[C2H3O2]- and -73[C3H5O2]-
of simultaneously more than 0.5 % are classified as BB type; (5) any
remaining particles containing signals of 55[Mn]+, 56[Fe]+,
63/65[Cu]+, 64[Zn]+ and 208[Pb]+ with a relative peak area of more
than 0.5 % are classified as HM type; (6) any remaining particles
containing abundant signals of 18[NH4]+ (peak area > 50),
-62[NO3]- (peak area > 100) and -97[HSO4]- (peak
area > 100) are classified as Sec type; (7) any remaining particles
containing abundant signals of 23[Na]+ (peak area > 1500) and related
species are classified as NaK type; (8) any remaining particles containing
signals of 40[Ca]+, 56[CaO]+ and related species are classified
as dust type. The rules for oxalic acid particle classification in the
current work have been reported in previous studies (Sullivan and Prather,
2007; Yang et al., 2009; Zhang et al., 2013; Li et al., 2014).
Inorganic ions and in situ pH (pH<inline-formula><mml:math id="M54" display="inline"><mml:msub><mml:mi/><mml:mi mathvariant="normal">is</mml:mi></mml:msub></mml:math></inline-formula>)
Water-soluble inorganic ions and trace gases were determined by an online
analyzer for monitoring aerosols and gases (MARGA, model ADI 2080, Applikon
Analytical B. V. Corp., the Netherlands), with a PM2.5 sampling inlet at
1 h resolution from 18 July to 1 August in 2014. The principle and
instrumental design has been described in detail elsewhere (ten Brink et al.,
2007; Du et al., 2011; Behera et al., 2013; Khezri et al., 2013). Standard
solutions containing all detected ions were injected into MARGA before and
after the field measurement. The liquid water content and the concentration
of H+ in particles are calculated using the ISORROPIA II model (Nenes et
al., 1998, 1999; Fountoukis and Nenes, 2007). We choose stable mode and
reverse type in the ISORROPIA model to calculate the concentration of H+
and the liquid water content in this work. The in situ pH (pHis)
of particles is calculated through the following equation:
pHis=-logαH+=-logγH+×nH+×1000/Va,
in which nH+ is the concentration of H+ (mol m-3)
and Va is the volume concentration of the H2O
(cm3 m-3), while γH+ is the activity
coefficient of H+ (Xue et al., 2011; Cheng et al., 2015). The
temporal variation in pHis of ambient PM2.5 particles is
presented in Fig. S3 and demonstrated that 97 % of particles were acidic
in summer.
Results and discussion
Seasonal variation in oxalic-acid-containing particles
The clustered 48 h back trajectories of air masses arriving in Heshan during
the sampling period are shown in Fig. S4. In summer, air masses at 500 m
levels above the ground were mainly from the ocean and rural areas with less
influence of human activity, while in winter air masses were directly from
the urban areas of Guangzhou and Foshan, indicating a strong influence from
anthropogenic emissions. The temporal variations in the total detected
particles and oxalic acid containing particles in summer and winter are shown
in Fig. 1. The total particles had similar trends with the mass concentration
of ambient PM2.5, suggesting that the counts of total particles detected
by SPAMS can be representative of PM2.5 mass concentration during the
whole sampling period. The oxalic acid (C2-containing) particles, in
general, exhibited distinct diurnal peaks from 28 July to 1 August, while
they showed different temporal trends in winter. The relative abundance of
oxalic acid particles in all of the sampled particles (C2 / total
ratio) had the same variation with the abundance of oxalic acid particles in
summer, especially in the period of 28 July–1 August (Fig. 1). In winter,
however, particle counts and relative abundance of oxalic acid had different
temporal changes, except in 30 January and 5–8 February, when the count and relative
abundance of oxalic acid particles simultaneously had a sudden increase.
Temporal variations in total detected particles and oxalic-acid-containing particles during the whole sampling period in Heshan, China:
(a)
hourly variations in PM2.5 mass concentration, total detected particle
counts, oxalic acid containing particles, ratio of oxalic-acid-containing / total particles and major types of oxalic-acid-containing
particles; (b) variation patterns of relative abundance of major types of
oxalic-acid-containing particles.
![]()
The averaged positive and negative ion mass spectra of oxalic-acid-containing particles is investigated in summer and winter: (a) summer
positive, (b) summer negative, (c) winter positive and (d) winter negative. The
color bars represent each peak area corresponding to a specific fraction in
individual particles.
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The oxalic-acid-containing particles were clustered into eight groups, and
they altogether accounted for 89.6 and 95.1 % of total oxalic acid
particles in summer and winter, respectively. Table 1 shows that in summer
HM particles contributed 31.3 % to total oxalic acid
particles, followed by Sec (19.2 %) and BB (13 %) particles. However,
in winter BB particles were the most abundant and accounted for
24.2 % of the oxalic-acid-containing particles, followed by EC and HM
particles. In addition, carbonaceous particles, including EC, OC, ECOC and BB,
accounted for 28.1 % of oxalic acid particles in summer and 59.8 % in
winter, indicating the different seasonal characteristics of oxalic acid
particles. The temporal variations in eight groups of oxalic acid particles
in summer and winter are illustrated in Fig. 1. In summer HM particles
(orange color) and total oxalic acid particles exhibited similar diurnal
patterns, suggesting a possible connection between the production of oxalic
acid and the transition metals (e.g., Fe, Cu) (Zhou et al., 2015). Although
Sec, BB and EC particles showed similar diurnal patterns with total
oxalic acid particles, the concentrations of these types of particles were
generally lower than HM particles. In winter, diurnal variation of oxalic
acid particles was not obvious but a sharp increase, accompanied by the
increase in BB, EC and Sec particles, was observed on 8 February.
Summary of major groups of oxalic-acid-containing particles in
summer and winter in PRD, China.
Particle type
Summer
Winter
(18 Jul–1 Aug 2014)
(27 Jan–8 Feb 2015)
Count
Percentage, %
Count
Percentage, %
EC
1473
11.2
3161
15.4
ECOC
41
0.3
2233
10.9
OC
473
3.6
1922
9.4
BB
1702
13.0
4953
24.2
HM
4104
31.3
3124
15.2
Sec
2511
19.2
2192
10.7
NaK
303
2.3
17
0.1
Dust
1139
8.7
1888
9.2
Abbreviations of major particle types: elemental carbon (EC),
elemental and organic carbon (ECOC), organic carbon (OC), biomass burning
(BB), heavy metal (HM), secondary (Sec), sodium-potassium (NaK), and dust
(Dust).
(a) Mixing state of oxalic acid with sulfate, nitrate and
ammonium in oxalic-acid-containing particles; (b) linear correlation
between NH4+-containing oxalic acid particles and the total oxalic
acid particles in summer; (c) linear correlation between
NH4+-containing oxalic acid particles and the total oxalic acid
particles in winter. Abbreviations: C2-NH4+ represents
the NH4+-containing oxalic acid particles and C2-SO42-
and C2-NO3- follow the same pattern.
![]()
The averaged positive and negative ion mass spectra of oxalic acid containing
particles are shown in Fig. 2. The positive ion spectrum of oxalic acid
particles in summer was characterized by high fractions of metal ion peaks,
including 23[Na]+, 27[Al]+, 39[K]+, 55[Mn]+,
56[Fe]+, 63/65[Cu]+, 64[Zn]+, and 208[Pb]+ and
carbonaceous marker ions at m/z 27[C2H3]+,
36[C3]+, 43[C2H3O/C3H7]+, and
48[C4]+ (Fig. 2a). The negative ion spectrum of oxalic acid
particles in summer was characterized by the strong intensity of secondary
ions, including m/z -46[NO2]-, -62[NO3]-,
-79[PO3]-, -80[SO3]-, -96[SO4]-, and
-97[HSO4]-, as well as carbon clusters of -24[C2]-,
-36[C3]-, and -48[C4]- and BB markers of
-59[C2H3O2]- and -73[C3H5O2]-
(Fig. 2b) (Zauscher et al., 2013). More carbonaceous clusters, i.e.,
27[C2H3]+, 29[C2H5]+, 36[C3]+,
37[C3H]+, 43[C2H3O]+, 48[C4]+,
51[C4H3]+, 55[C4H7]+, 60[C5]+,
63[C5H3]+, 65[C5H5]+ and
74[C2H2O3]+, 77[C6H5]+, were observed in
the positive ion spectrum of oxalic acid particles in winter (Fig. 2c) than
in summer. The negative ion spectrum of oxalic acid particles in winter (Fig. 2d) contained a large number of secondary ions, similar to those found in
summer, and a more intense signal of nitric acid
(-125[HNO3NO3]-), suggesting an acidic nature of oxalic acid
particles in winter.
The mixing state of oxalic acid particles with sulfate, nitrate and ammonium
(SNA) was investigated through the percentage of SNA-containing oxalic acid
particles in total oxalic acid particles (Fig. 3). Oxalic acid was found to
be internally mixed with sulfate and nitrate during both sampling periods
with 93 and 94 % in summer respectively, and both with 98 % in winter
(Fig. 3a). However, the NH4+-containing oxalic acid particle
(C2-NH4+) only accounted for 18 % of total oxalic
acid particles in summer but this fraction increased to 71 % in winter,
and linear correlation between C2-NH4+ particles and
total oxalic acid particles showed better linear regression (r2=0.98) in
winter than summer, indicating a general mixing state of NH4+
with oxalic acid in winter. Aqueous-phase production of SO42-
has been studied well and the linear correlation between oxalic acid and
SO42- has been used to study the production of oxalic acid
through aqueous-phase reactions (Yu et al., 2005; Miyazaki et al., 2009;
Cheng et al., 2015). In our work, oxalic acid and
C2-SO42- displayed good correlations in summer and
winter (both r2=0.99), which suggests a common production route of
oxalic acid and sulfate, likely aqueous-phase reactions.
Figure 4 shows the unscaled size-resolved number distributions of the eight
types of oxalic acid particles. Oxalic acid mainly existed in 0.4 to
1.2 µm particles during the entire sampling period but exhibited
different peak modes for each particle type in summer and winter. In summer,
major types of oxalic acid particles showed distinct peak mode at different
size diameters. EC and Sec particles peaked at 0.5 µm,
followed by BB particles at 0.55 µm, then HM particles
at 0.6 µm, and OC particles at 0.7 µm. The
difference in peak mode suggests the possible different chemical evolution
process for each type of oxalic-acid-containing particle. However, in winter,
oxalic acid particles showed broader size distribution from 0.5 to
0.8 µm for all particle types. Oxalic acid particles of all types
were generally larger in winter than summer, possibly due to condensation and
coagulation of particles during aging of oxalic acid particles in winter.
Unscaled size-resolved number distributions of major types of
oxalic acid particles in summer and winter.
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Temporal variations in O3 concentrations, oxalic acid
particles, malonic acid particles and heavy metal oxalic acid
particles during the entire sampling period in Heshan, China.
![]()
The averaged digitized positive and negative ion mass spectra of
heavy metal oxalic-acid-containing particles in summer.
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Photochemical production of oxalic acid in summer
In summer oxalic acid particles showed peaks in the afternoon especially from
28 July to 1 August, which was in agreement with the variation pattern of the
O3 concentration (Fig. 5), indicating a strong association of oxalic
acid formation with photochemical reactions. Malonic acid is another product
of photochemical oxidation of organic compounds (Kawamura and Ikushima, 1993;
Wang et al., 2012; Meng et al., 2013, 2014). In our campaign,
malonic-acid-containing particles had diurnal trends similar to oxalic acid
particles and O3 concentration. As the dominant particle type, HM
particles had variation patterns identical to total oxalic acid particles.
They are characterized by highly abundant metal ion peaks like 55[Mn]+,
56[Fe]+, 63/65[Cu]+, 64[Zn]+ and 208[Pb]+, as well as
secondary ion peaks of -46[NO2]-, -62[NO3]-,
-80[SO3]-, -96[SO4]- and -97[HSO4]- in
the negative spectrum in summer (Fig. 6). In order to investigate the
photochemical formation of oxalic acid in summer, the diurnal variations in
O3, oxalic acid particles, HM particles and pHis of ambient
particles averaged from 28 July to 1 August 2014 are shown in Fig. 7. The
concentration of O3 increased after 09:00 and peaked at
17:00 (all times hereafter in Beijing time, UTC + 8), while oxalic acid particles and HM particles both increased after
10:00 and showed two peaks at 15:00 and 19:00. The pHis of
ambient particles ranging from -1.42 to 4.01 indicated an acidic
environment, and the temporal trends in RH, inorganic ions and H+ (aq)
in aerosols are shown in Fig. S5. The oxidation of glyoxal and glyoxylic acid
by ⚫OH has been identified as an important pathway of oxalic
acid production by field and laboratory studies (Ervens et al., 2004; Ervens
and Volkamer, 2010; Wang et al., 2012, 2015). In summer, strong photochemical
activity and high O3 concentrations in the afternoon lead to more
production of dicarbonyls and aldehydes (e.g., glyoxal and methylglyoxal)
from VOCs (Myriokefalitakis et al., 2011), which increases the precursors of
oxalic acid. The aqueous-phase oxidation of glyoxal can take place in both
clouds and wet aerosols (Lim et al., 2010). However, the lower yield of
oxalic acid from glyoxal in wet aerosols compared to in clouds has been
reported in previous chamber experiments due to the formation of a
substantial amount of high-molecular-weight products such as oligomers in
aerosol-related concentrations (Carlton et al., 2007; Tan et al., 2009).
These findings may explain the lower peak of oxalic acid particles at
15:00 compared to that at 19:00. In addition, the precursors of oxalic acid such as glyoxylic
acid have a higher reaction rate with ⚫OH in high-pH solutions
according to previous studies (Ervens et al., 2003; Herrmann, 2003; Cheng et
al., 2015), and in this work the increase in pHis was observed as
the enhancement of oxalic acid particles in the afternoon (Fig. 7), which
suggests an efficient oxalic acid production from the oxidation of
precursors.
The diurnal variations in O3 concentration, oxalic acid
particles, HM group particles and in situ pH (pHis) from 28
July to 1 August in 2014.
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The diurnal variations in peak area of iron (m/z = 56) and
oxalic acid (m/z = -89) in the HM oxalic acid particles from
28 July to 1 August 2014.
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The temporal variations in peak area of nitrate, sulfate, and oxalic
acid and the relative acidity ratio (Rra) in carbonaceous
oxalic acid particles in winter.
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The photochemical pattern of HM particles similar to O3 and
total oxalic acid particles implies a possible participation of metal ions in
the formation process of oxalic acid. The modeling studies from Ervens et al.
(2014) suggest that oxalic acid production from glyoxal and glyoxylic acid in
the aqueous phase significantly depends on ⚫OH availability
(Ervens et al., 2014). The main sources of aqueous-phase ⚫OH
in cloud droplets include direct uptake from the gas phase (Jacob, 1986),
ozone photolysis by UV and visible light at the air–water interface (Anglada
et al., 2014) and also aqueous-phase chemical reactions (Gligorovski et al.,
2015). For the last kind of source, ⚫OH radicals could be
generated through Fenton or Fenton-like reactions and photolysis of
H2O2, NO3-, NO2- and chromophoric dissolved
organic matter (CDOM) (Badali et al., 2015; Ervens, 2015; Herrmann et al.,
2015; Tong et al., 2016). Given that the SPAMS cannot be used to quantify the
concentrations of iron ions and H2O2, we will investigate the
relative contribution of different source ⚫OH radicals to the
formation of oxalic acid and show results in our follow-up studies.
The oxalic acid loss through the photolysis of iron oxalato complexes is a
significant sink according to field measurements and model simulations
(Sorooshian et al., 2013; Weller et al., 2014; Zhou et al., 2015).
Considering the high abundance of iron in oxalic acid particles in the
current work (Fig. 6), the photolysis of iron oxalato complexes could have
played an important role in the diurnal variation in oxalic acid particles.
Because the mass concentration of Fe (III) and oxalic acid could not be
obtained using a SPAMS, the diurnal variations in peak area of iron
(m/z=56) and oxalic acid (m/z=-89) were used to investigate the role of
iron in the net production of oxalic acid in the HM particles from 28 July to
1 August 2014 (Fig. 8). Interestingly, the peak area of iron exhibited
opposite trends to the peak area of oxalic acid from 04:00 to 11:00. As the
peak area of Fe increased from 1565 to 29 920 from 04:00 to 07:00, the peak
area of oxalic acid decreased from 6052 to 3487 accordingly. From 08:00 to 11:00,
the peak area of Fe had a very low value of 1168, but the peak area of oxalic
had a very high value of 5538. In addition, the peak area of iron exhibited a
high value of 138 199 at 14:00, while the peak area of oxalic acid showed a
lower peak of 7687 at 14:00 and a higher peak of 11 879 at 19:00 with an
extremely low abundance of iron. Above opposite variation patterns of iron
and oxalic acid in iron-rich HM particles during the photochemical
activity period from 05:00 to 19:00 strongly indicated that photolysis of
iron oxalato complexes could be an efficient sink of oxalic acid.
The influence from traffic emissions was investigated through the diurnal
variations in total EC particles and NO2 (Fig. S6). The EC
particles increased from 12:00 to 21:00, which had the same variation as total
oxalic acid, but NO2 followed the rush hour pattern, with two peaks from
05:00 to 08:00 and from 18:00 to 21:00. Traffic emission is not expected to
have a large contribution to oxalic acid in this study. The wind speed was
low during the whole day (Fig. S6), especially between 09:00 and 18:00, which
provided a stagnant environment for the increase in oxalic acid produced from
photochemical processes.
The comprehensive study of oxalic acid particle increase on
8 February 2015: (a) the digitized positive and negative ion mass
spectrum of oxalic acid particles during the haze episode; (b) linear
regression between total oxalic acid particles and organosulfate-containing
oxalic acid particles (m/z∼-155).
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Formation process of oxalic acid in winter
Despite lower O3 concentrations and photochemical activity in winter,
oxalic acid was still prevalent in carbonaceous particles, especially BB particles. While oxalic acid was found to be internally mixed with sulfate
and nitrate both in summer and winter, the nitric acid was only observed in
oxalic acid particles in winter, indicating a strongly acidic nature of
oxalic acid particles in winter. Considering a possible connection of oxalic
acid production with the acidic environment, the temporal concentrations of
oxalic acid, sulfate and nitrate were investigated through their peak areas
in the carbonaceous type oxalic acid particles, including EC, OC, ECOC and BB
types in Fig. 9. The peaks of m/z -62[NO3]- and -97[HSO4]- represent nitrate and sulfate, respectively. Nitrate,
sulfate and oxalic acid showed very similar variation patterns in winter,
suggesting a close connection of the formation of oxalic acid with the
existence of nitrate and sulfate. Although nitric acid was found in the
oxalic acid particles, the acidity of the oxalic acid particles was not
estimated since the real-time concentration of inorganic ions was not
available during the sampling period in winter. Instead, the relative acidity
ratio (Rra), defined as the ratio of total peak areas of nitrate
and sulfate to the peak area of ammonium (m/z 18[NH4]+), was used
(Denkenberger et al., 2007; Pratt et al., 2009). The Rra of carbonaceous
type oxalic acid particles ranged from 7 to 114, with an average value of 25
(Fig. 9), indicating an intensely acidic environment of carbonaceous type
oxalic acid particles in winter. Several studies have reported the formation
of oxalic acid through the oxidation of glyoxal and related precursors in
the acidic aqueous phase (Carlton et al., 2006, 2007; Tan et al.,
2009). Although the influence of different particle acidity on the oxidation
process of glyoxal still needs evaluation, the moderate acidic environment is
favorable for the production of oxalic acid from the oxidation of glyoxal
(Herrmann, 2003; Ervens and Volkamer, 2010; Eugene et al., 2016). In this
work the acidic environment of the carbonaceous type oxalic acid particles
and similar variation patterns among oxalic acid, sulfate and nitrate may
suggest a relationship between the degradation of organic precursors and the
acidic chemical process. However, the temporal change in Rra did
not follow a similar trend as the peak area of oxalic acid in most particles,
possibly due to the multistep formation of oxalic acid influenced by many
factors such as precursors, liquid water content and ion strength (Carlton et
al., 2007; Cheng et al., 2013, 2015).
The sharp increase in oxalic acid particles on 8 February 2015 (Fig. 1) was
selected as a typical haze episode to investigate the formation processes of
oxalic acid in winter. During the haze episode, the 48 h back trajectory analysis
showed air masses that originated from the urban areas of Guangzhou and
Foshan cities (Fig. S4), indicating strong influence on organic precursors from
anthropogenic emissions. Oxalic acid particle types were dominated by BB
(23.2 %), followed by EC (22.0 %) and Sec (15.1 %)
(Table 2). Carbonaceous particles including EC, ECOC, OC and BB accounted for
61.6 % of the total oxalic acid particles. The mass spectra of oxalic
acid particles were characterized by many hydrocarbon clusters of
27[C2H3]+, 29[C2H5]+, 37[C3H]+,
43[C2H3O]+, 51[C4H3]+,
55[C4H7]+, 63[C5H3]+, 65[C5H5]+,
74[C2H2O3]+, and 77[C6H5]+, and carbon
clusters of 36[C3]+, 48[C4]+, and 60[C5]+ on
a positive mass spectrum, while the negative mass spectrum was characterized by
elemental carbon clusters like -24[C2]-, -36[C3]-,
and
-48[C4]-; biomass burning markers of
-59[C2H3O2]- and -73[C3H5O2]-; and
secondary species including -42[CNO]-, -46[NO2]-,
-62[NO3]-, -79[PO3]-, -80[SO3]-,
-96[SO4]-, and -97[HSO4]- (Fig. 10a).
The abundance of major particle types in total oxalic-acid-containing particles during the haze episode in winter (8 February 2015).
EC
ECOC
OC
BB
Sec
HM
Dust
Other
Count
1250
604
326
1320
856
377
814
132
Percentage, %
22.0
10.6
5.7
23.2
15.1
6.6
14.3
2.3
As the precursor of oxalic acid, glyoxal has the potential to react with
sulfuric acid to produce organosulfates through acid-catalyzed nucleophilic
addition according to laboratory and chamber studies (Surratt et al., 2007;
Galloway et al., 2009). The negative ion of
-155([C2H3O2SO4]-) has been identified as the
marker ion of organosulfates derived from glyoxal in chamber and field
measurements using ATOFMS (Surratt et al., 2008; Hatch et al., 2011). The
formation of organosulfates from glyoxal requires an acidic aqueous
environment, which can be used as a marker of the acidic aqueous-phase aging
process of organic compounds. The temporal trend of organosulfate-containing
oxalic acid particles in winter is shown in Fig. S7, which exhibited a
similar pattern to the total oxalic acid particles during the whole sampling
period in winter. The percentage of organosulfate-containing oxalic acid
particles in total oxalic acid particles ranged from 0 to 16.4 %, with the
highest ratio observed in the haze episode (8 February). The linear regression
between total oxalic acid particles and organosulfate-containing oxalic acid
particles in the haze episode is exhibited in Fig. 10b, and the robust correlation
(r2=0.81) between them suggests that oxalic acid and organosulfate may
share similar formation processes. Based on the discussion above, the
degradation of carbonaceous species associated with acidic aqueous-phase
chemical reactions may have an important contribution to the formation of
oxalic acid during the haze episode in winter. Similar particle types and mass
spectra of oxalic-acid-containing particles during the haze episode and the whole
sampling period in winter were observed, which suggests the acidic aqueous-phase chemical processing of organic precursors as a potential source for
oxalic acid.