Mixing state of oxalic acid containing particles in the rural area of Pearl 1 River Delta , China : implication for seasonal formation mechanism of 2 Secondary Organic Aerosol ( SOA )

The formation of oxalic acid and its mixing state in atmospheric particulate matter (PM) were studied using a single particle aerosol mass spectrometer (SPAMS) in the summer and winter of 2014 in Heshan, a supersite in the rural area of the Pearl River Delta (PRD) region in China. Oxalic acid-containing particles accounted for 2.5 % and 2.7 % in total detected ambient particles in summer and winter, respectively. Oxalic acid was measured in particles classified as elemental carbon (EC), organic carbon (OC), elemental and organic carbon (ECOC), biomass burning (BB), heavy metal (HM), secondary (Sec), sodium-potassium (NaK) and dust. Oxalic acid was found predominantly mixing with sulfate and nitrate during the whole sampling period, likely due to aqueous phase reactions. In summer, oxalic acid-containing particle number and ozone concentration followed a very similar trend, which may reflect the significant contribution of photochemical reactions to oxalic acid formation. Furthermore, favorable in-situ pH (2–4) conditions were observed, which promote Fenton like reactions for efficient production of •OH in HM type particles. A mechanism in which products of photochemical oxidation of VOCs partitioned into the aqueous phase of HM particles, followed by multistep oxidation of •OH through Fenton like reactions to form oxalic acid is proposed. In wintertime, carbonaceous type particles contained a substantial amount of oxalic acid as well as abundant carbon clusters and biomass burning markers. The general existence of nitric acid in oxalic acid-containing particles indicates an acidic environment during the formation process of oxalic acid. Organosulfate-containing particles well correlated with oxalic acid-containing particles during the episode, which suggests the formation of oxalic acid is closely associated with acid-catalyzed reactions of organic precursors.


Introduction
Organic aerosol, typically a large fraction of fine particles, contains more than thousands 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;Poschl 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, so 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 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).Even though high concentrations of DCAs have been observed in air masses influenced by biomass burning (Kundu et al., 2010;Kawamura et al., 2013), the primary source of DCAs is still not clear (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 of DCA (Kawamura and Ikushima, 1993;Kawamura and Yasui, 2005;Aggarwal and Kawamura, 2008;Pavuluri et al., 2010;Ho et al., 2011), but the mechanism of DCAs formation is still not well understood.Oxalic acid is usually the most abundant DCA observed in the field (Kawamura et al., 2004;Ho et al., 2007;Kawamura et al., 2010).
Several studies have found a tight correlation between oxalic acid and sulfate in ambient particles, implying that aqueous chemistry leads to the formation of oxalic Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1081, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.acid in aerosols and cloud droplets (Yao et al., 2002;Yao et al., 2003;Yu et al., 2005;Sorooshian et al., 2007;Miyazaki et al., 2009).In recent years, many model and laboratory studies suggest 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 exact formation pathways of oxalic acid in ambient particles are still unknown due to the complexity of meteorological condition and the temporal resolution limitations of conventional filter sampling studies and bulk chemical analysis.
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 to examine the formation and evolution of oxalic acid and SOA particles.Sullivan and Prather 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 (Sullivan and Prather, 2007).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).So far the formation mechanism of oxalic acid especially in urban areas is still not clear.Online measurements of the mixing state of oxalic acid provides a powerful context 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 episode (Bi et al., 2011;Zhang et al., 2013;Zhang et al., 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) in a rural supersite of the PRD region.The seasonal characteristic 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.

Aerosol sampling
Particles were sampled using a single particle aerosol mass spectrometer (SPAMS) at the Guangdong Atmospheric Supersite (22.73N, 112.93E), a rural site at Heshan city (Figure S1).The supersite is surrounded by farm land and villages, with no local industrial or traffic emissions.Ambient aerosols were sampled to the SPAMS through a 2.5m long copper tube with 0.5m of the sampling inlet located above the top of the building.The measurement period was from July 18 to August 1 in 2014, and from January 27 to February 8 in 2015.Real-time PM 2.5 mass concentration was simultaneously measured by a TEOM monitor (series 1405, Thermo scientific), and hourly concentrations of O 3 were measured by an O 3 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 has been demonstrated 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 single particle aerosol mass spectrometer (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 have been 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 80μm Atmos.Chem. Phys. Discuss., doi:10.5194/acp-2016-1081, 2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.critical orifice at a flow rate of 75 ml min -1 .The individual particles with a 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, 266nm).The energy of the desorption/ionization 266 nm laser was 0.6 mJ and the power density was kept at about 1.6×10 8 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 0.22-2.0μm diameter were used for size calibration.

Data analysis
The size and chemical composition of single particles detected by SPAMS were analyzed using the 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 (H 2 C 2 O 4 , purity: 99.99%, Aladdin Industrial Corporation) and atomized to aerosols.After drying through two silica gel diffusion driers, pure oxalic acid particles were directly introducing into the SPAMS.The positive and negative mass spectra of oxalic acid are shown in Figure S2.
Based on the mass spectra of pure oxalic acid and previous ambient measurements by ATOFMS (Silva and Prather, 2000;Sullivan and Prather, 2007;Yang et al., 2009) (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 ΙΙ model (Nenes et al., 1998(Nenes et al., , 1999;;Fountoukis and Nenes, 2007).The in-situ pH (pH is ) of particles is calculated through the following equation: where n H + is the concentration of H + (mol m -3 ) and V a is the volume concentration of the H 2 O (cm 3 m -3 ) , while  H + is the activity coefficient of H + (Xue et al., 2011;Cheng et al., 2015).The temporal variation of pH is of ambient PM 2.5 particles is presented in Figure S3, and demonstrated that 97% of particles were acidic in summer.

Seasonal variation of oxalic acid containing particles
The clustered 48 hr back trajectories of air masses arriving in Heshan during the sampling period are shown in Figure S4.In summer, air masses at 500m 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 urban areas of Guangzhou and temporal trends in winter.The relative abundance of oxalic acid particles in all of the sampled particles (C 2 /total ratio) had the same variations with the abundance of oxalic acid particles in summer, especially in the period of July 28 -August 1 (Figure 1).In winter, however, particle counts and relative abundance of oxalic acid had different temporal changes except Jan 30 and February 5-8, when the count and relative abundance of oxalic acid particles simultaneously had a sudden increase.
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 heavy metal (HM) type particles contributed 31.3% to total oxalic acid particles, followed by the Sec (19.2%) and BB type (13%).However, in winter BB type particles were the most abundant and accounted for 24.2% of the oxalic acid-containing particles, followed by EC and HM type.Besides, carbonaceous type particles including EC, OC, ECOC and BB accounted for 28.1% of oxalic acid particles in summer and 59.8% in winter, indicating the seasonal different characteristics of oxalic acid particles.The temporal variations of eight groups of oxalic acid particles in summer and winter are illustrated in Figure 1.In summer HM type particles (purple) and total oxalic acid particles exhibited similar diurnal patterns, suggesting a possibly connection between the production of oxalic acid and the transition metals (e.g.Fe, Cu) (Sorooshian et al., 2013).Although Sec, BB and EC type particles showed similar diurnal patterns with total oxalic acid particles, the concentrations of these type particles were generally lower than HM type particles.In winter diurnal variation of oxalic acid particles was not obvious but a sharp increase, accompanied by the increase of BB, EC and Sec type particles, was observed on February 8.The mixing ratios of oxalic acid particles with sulfate, nitrate and ammonium (SNA) were investigated through the relative abundance of SNA-containing oxalic acid particles in total oxalic acid particles (Figure 3).Oxalic acid was found to be internally mixed with sulfate and nitrate during both sampling periods with mixing ratio of 93% and 94% in summer respectively, and both 98% in winter (Figure 3 a).
However, the mixing ratio of NH 4 + with oxalic acid was only 18% in summer but increased to 71% in winter.Linear correlations between NH 4 + -containing oxalic acid particles (C 2 -NH 4 + ) and total oxalic acid particles are depicted in Figure 3, with better linear regression (r 2 =0.98) in winter than summer.The low mixing ratio of NH 4 + in oxalic acid particles in summer indicated that the presence of oxalic acid in NH 4 + -poor particles.Aqueous phase production of SO 4 2-has been studied well and the linear correlation between oxalic acid and SO 4 2-has been used to study the production of oxalic acid through aqueous phase reactions (Yu et al., 2005;Miyazaki et  2009; Cheng et al., 2015).In our work, oxalic acid and C 2 -SO 4 2-displayed good correlations in summer and winter (both r 2 =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 diameter.EC and Sec type particles peaked at 0.5 μm, followed by BB type particles at 0.55 μm, then HM type particles at 0.6 μm, and OC type particles at 0.7 μm.The difference of peak mode suggests possibly different chemical evolution process for each type oxalic acid-containing particles.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.

Photochemical production of oxalic acid in summer
In summer oxalic acid particles showed peaks in the afternoon especially from July 28 to August 1, which was in agreement with the variation pattern of the O 3 concentration (Figure 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;Meng et al., 2014).In our campaign, malonic acid containing particles had in acidic solutions has been considered as a substantial source of •OH (Fenton, 1894;Dunford, 2002;Herrmann et al., 2015).The high abundance of metal ions in oxalic acid particles may be an indication of possible Fenton reactions in the acidic aqueous phase of acidic particles (pH<5, Figure S3), although we cannot exclude the possibility of gas phase condensation of oxalic and malonic acids onto HM particles.
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).The modeling studies from Ervens et al. (2014) suggest that oxalic acid production from glyoxal and glyoxylic acid in aqueous phase significantly depends on •OH availability (Ervens et al., 2014).
While the partition of •OH from gas to aqueous phase is limited by its low Henry's law constant (K H,OH =30 M atm -1 ) and short lifetime of •OH in the gas phase (Hanson et al., 1992), the main sources of aqueous •OH are from the photolysis of H 2 O 2 , NO , and chromophoric dissolved organic matter (CDOM) (Yu et al., 2014;Badali et al., 2015;Gligorovski et al., 2015;Tong et al., 2016).Among these sources the photolysis of H 2 O 2 through Fenton reactions involving the catalysis of transition metal ions like Fe 2+/3+ , Cu +/2+ and Mn 2+/3+ is an efficient source of •OH (Deguillaume et al., 2005;Herrmann et al., 2005;Ervens et al., 2014).The •OH formation process through Fenton reactions can be expressed as (Ervens, 2015): The actual chemical process is far more complex and involves iron oxides and iron-complexes, thus in the current work we focus on the potential availability of •OH from Fenton reactions and the impact on the oxidation process of organic precursors.
In order to investigate the photochemical aqueous phase formation of oxalic acid in summer, the diurnal variations of O 3 , oxalic acid particles, HM group particles and pH is of ambient particles averaged from July 28 to August 1, 2014 are shown in dark and light radiation conditions, only photo-Fenton reactions had significant contribution to the enhancement of oxalic acid particles in the current work.This was possibly due to the diurnal variation of pH is , since Fenton reactions strongly depend on the pH of the aqueous phase (Gligorovski et al., 2015).When pH<1, Fe 2+ is directly oxidized by H 2 O 2 to Fe 3+ with no production of •OH (Barb et al., 1951;Kremer, 2003), and the most favorable pH value for Fenton reaction is between 2.5 and 5 (Deguillaume et al., 2005).In the current work the pH is of ambient particles ranged from -1.42 to 4.01, and the influences of pH is from RH and inorganic ions are discussed in Figure S5.Strongly acidic particles were observed during the whole day with high pH is at 6:00 and after 12:00.Although pH is was around 2 at 6:00, only a few oxalic acid-containing particles were observed during this period due to low abundance of HM particles.Oxalic acid-containing particles were found to increase from 12:00 to 21:00, which was attributed to increased organic precursors from VOCs oxidation and enhanced •OH production from Fenton reactions under pH is at 1-4.The number concentration of oxalic acid particles peaked at 19:00 instead of during the strong photochemical activity period in the afternoon; this was possibly due to the efficient degradation of oxalic acid from the complex of Fe(oxalate) 2 - (Sorooshian et al., 2013;Zhou et al., 2015).On the other hand, photolysis of Fe(oxalate) 2 -can contribute to 99% of the overall degradation of oxalic acid (Weller et al., 2014).
Although the enhanced •OH production from photo-Fenton reactions was favorable for the formation of oxalic acid from 12:00 to 18:00, we speculate that a high degradation rate of oxalic acid by iron complexation resulted in a lower net production of oxalic acid than at 19:00.
Based on above discussions, detailed mechanism for oxalic acid formation in acidic aqueous phase of particles is proposed for our field observations (Figure 8).In and Cu potentially yield more •OH in acidic aqueous phase, then hydrated dicarbonyls and aldehydes can be oxidized by •OH to glyoxylic acid and finally to oxalic acid (Wang et al., 2012).Recently Ma et al. (2015) had studied the Fe-containing particles in the PRD and found Fe-containing particles are more efficient at generating •OH in summer than winter (Ma et al., 2015), which supports the enhanced •OH production in HM type particles in this work.A large amount of Fe related particles are emitted from steel industries in the North China Plain and metals like V, Zn, Cu and Pb from electronic manufacturing (Cui and Zhang, 2008;Dall'Osto et al., 2008).These metals contribute significantly to haze episodes (Moffet et al., 2008;Li et al., 2014), which possibly increases the formation of SOA by yielding more OH participating the heterogeneous and aqueous reactions.

Formation process of oxalic acid in winter
Despite lower O 3 concentrations and photochemical activity in winter, oxalic acid particles were still prevalent in carbonaceous particles, especially BB type particles.The sharp increase of oxalic acid particles on February 8, 2015 (Figure 1) was selected as a typical episode to investigate the formation processes of oxalic acid in winter.
During the episode, the 48 hr back trajectory analysis showed air masses that originated from the urban areas of Guangzhou and Foshan city (Figure 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%) type ( identified in oxalic acid particles not only in the episode but also during the entire sampling period in winter, indicating a strongly acidic nature of oxalic acid particles in winter. 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([C 2 H 3 O 2 SO 4 ] -) 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 organosulfate derived from glyoxal requires acidic aqueous environment of particles, and herein is used as an indicator of acid-catalyzed ageing process of organic compounds.The temporal variation of organosulfate (m/z=-155) containing particles during the entire sampling period in Heshan, China is shown in Figure S6.During the episode, oxalic acid particles had moderate linear correlation with organosulfate particles (Figure 9b).
Based on the above discussion, the degradation of carbonaceous species associated with acid-catalyzed reactions may have a significant contribution to the formation of oxalic acid during the episode in winter.Similar particle types and mass spectra of oxalic acid-containing particles during the episode and the whole sampling period in winter were observed, which suggest the acid-catalyzed oxidation of organic precursors as a potential source for oxalic acid.

Summary and conclusions
Oxalic acid containing particles were measured by a single particle aerosol mass spectrometer (SPAMS) in the summer and winter of 2014 in Heshan, China.They accounted for 2.5% and 2.7% of the total detected ambient particles.In summer heavy metal-containing particles were the largest group of particles containing oxalic acid with a fraction of 31.3% followed by Sec type (19.2%), while in winter BB type was the dominant group with a percentage of 24.2%.More than 90% of oxalic acid particles were internally mixed with sulfate and nitrate during the whole sampling period.Only 18% of oxalic acid particles contained ammonium in summer, which

Table list:
Table 1.Summary of major groups of oxalic acid-containing particles in summer and winter in PRD, China.
Table 2.The abundance of major particle types in total oxalic acid-containing particles during the episode in winter (2/8/2015).
Foshan, indicating a strong influence from anthropogenic emissions.The temporal Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2016-1081,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.variations of the total detected particles and oxalic acid containing particles in summer and winter are shown in Figure 1.The total particles had similar trends with the mass concentration of ambient PM 2.5 , suggesting that the counts of total particles detected by SPAMS can be representative of PM 2.5 mass concentration during the whole sampling periods.The oxalic acid (C 2 -containing) particles, in general, exhibited distinct diurnal peaks from July 28 to August 1, while they showed different Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2016-1081,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.The averaged positive and negative ion mass spectra of oxalic acid containing particles are shown in Figure 2. The positive ion spectrum of oxalic acid particles in summer was characterized by high fractions of metal ion peaks including 23[Na] were observed in the positive ion spectrum of oxalic acid particles in winter (Figure2 c) than in summer.The negative ion spectrum of oxalic acid particles in winter (Figure2 d) contained a large amount of secondary ions, similar to those found in summer, and a more intense signal of nitric acid NO 3 ] -) , suggesting an acidic nature of oxalic acid particles in winter.

Figure 7 .
Figure 7.The concentration of O 3 increased after 9:00 and peaked at 17:00, while oxalic acid particles and HM group particles both increased after 10:00 and showed two peaks at 15:00 and 19:00.The prominent photochemical feature of oxalic acid particles suggested a close association of photochemical reactions with oxalic acid production.Although •OH production from Fenton reactions can both occur under summer strong photochemical activity and high O 3 concentrations in the afternoon leads to more production of reactive radicals such as •OH and HO 2 • , which promote the oxidation of VOCs to dicarbonyls and aldehydes (e.g.glyoxal and methylglyoxal), followed by a subsequent partitioning into the aqueous phase of particles(Myriokefalitakis et al., 2011).Acidic particles containing transition metals like Fe Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2016-1081,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.
Atmos.Chem.Phys.Discuss., doi:10.5194/acp-2016-1081,2016 Manuscript under review for journal Atmos.Chem.Phys.Published: 21 December 2016 c Author(s) 2016.CC-BY 3.0 License.increased to 71% in winter.In summer oxalic acid and O 3 concentration exhibited similar diurnal variations, indicating a substantial contribution of photochemical reactions to oxalic acid formation.The favorable in-situ pH and the dominance of transition metal ions in oxalic acid particles suggests an enhanced production of •OH from Fenton like reactions.A mechanism involving the photochemical production of VOCs via efficient aqueous phase reactions with enhanced •OH to oxalic acid was proposed.In winter carbonaceous type particles including EC, OC, ECOC and BB groups accounted for 59.8% of oxalic acid particles and increased to 61.6% in the episode.Nitric acid and organosulfate were found to co-exist in oxalic acid-containing particles in the winter, which suggests a close association with acid-catalyzed reactions.Acid-catalyzed oxidation of organic precursors is a potential contribution for the formation of oxalic acid in winter.The current study also indicates that SPAMS can be a robust tool for exploring the formation and transformation processes of SOA, contributing to the improvement of global climate modeling and the development of effective air pollution mitigation strategies.

Figure 1 .
Figure 1.Temporal variations of total detected particles and oxalic acid containing particles during whole sampling periods in Heshan, China: (a) hourly variations of PM 2.5 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.

Figure 2 .
Figure 2. 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, (d) winter negative.The color bars represent each peak area corresponding to specific fraction in individual particles.

Figure 3 .
Figure 3. (a) Mixing state of oxalic acid with sulfate, nitrate and ammonium in oxalic acid-containing particles; (b) Linear correlation between NH 4 + -containing oxalic acid particles and the total oxalic acid particles in summer; (c) Linear correlation between NH 4 + -containing oxalic acid particles and the total oxalic acid particles in winter.Abbreviations: C 2 -NH 4 + represents the NH 4 + -containing oxalic acid particles, and same expressions for C 2 -SO 4 2-and C 2 -NO 3 -.

Figure 4 .
Figure 4. Unscaled size-resolved number distributions of major types of oxalic acid particles in summer and winter.

Figure 5 .
Figure 5. Temporal variations of O 3 concentrations, oxalic acid particles, malonic acid particles and heavy metal type of oxalic acid particles during the entire sampling period in Heshan, China.

Figure 6 .
Figure 6.The averaged digitized positive and negative ion mass spectra of heavy metal type of oxalic acid-containing particles in summer.

Figure 7 .
Figure 7.The diurnal variations of O 3 concentration, oxalic acid particles, HM group particles and in-situ pH (pH is ) from July 28 to August 1 in 2014.

Figure 8 .
Figure 8.The formation process of oxalic acid in the aqueous phase of particles in summer: the red steps are enhanced by photochemical activities in the current study.

Figure 9 .
Figure 9.The comprehensive study of oxalic acid particles increase on Feb 8, 2015: (a) The digitized positive and negative ion mass spectrum of oxalic acid particles during the episode; (b) Linear regression between oxalic acid particles and organosulfate particles (m/z -155).

Figure 2 .
Figure 2. 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, (d) winter negative.The color bars represent each peak area corresponding to specific fraction in individual particles.

Figure 4 .
Figure 4. Unscaled size-resolved number distributions of major types of oxalic acid particles in summer and winter.Abbreviations of major particle types: elemental carbon (EC), organic carbon (OC), elemental and organic carbon (ECOC), biomass burning (BB), heavy metal (HM), secondary (Sec), sodium and potassium (NaK) and dust.

Figure 5 .
Figure 5. Temporal variations of O 3 concentrations, oxalic acid particles, malonic acid particles and heavy metal type of oxalic acid particles during the entire sampling period in Heshan, China.

Figure 6 .Figure 7 .
Figure 6.The averaged digitized positive and negative ion mass spectra of heavy metal type of oxalic acid-containing particles in summer.

Figure 8 .
Figure 8.The formation process of oxalic acid in the aqueous phase of particles in summer: the red steps are enhanced by photochemical activities in the current study.

Figure 9 .
Figure 9.The comprehensive study of oxalic acid particles increase on Feb 8, 2015: (a) The digitized positive and negative ion mass spectrum of oxalic acid particles during the episode; (b) Linear regression between oxalic acid particles and organosulfate particles (m/z -155).

Table 1 .
Summary of major groups of oxalic acid-containing particles in summer and winter in PRD, China.