Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber

JES-00930; No of Pages 7 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 6 ) XX X–XXX

Available online at www.sciencedirect.com

ScienceDirect www.elsevier.com/locate/jes

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Yueyue Chen1 , Jia Liu1 , Jing Shang⁎, Tong Zhu

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College of Environmental Science and Engineering, Peking University, Beijing 100081, China

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Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber

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AR TIC LE I NFO

ABSTR ACT

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Article history:

The reaction of HCHO with Beijing winter's real ambient particulate matter (PM) inside a 14

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Received 21 April 2016

3.3 m3 Teflon Chamber was conducted in this study. NO2, O3 and H2O gases were removed 15

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Revised 8 August 2016

from the ambient aerosol before entering into the chamber. The decays of HCHO were 16

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Accepted 22 August 2016

monitored (acetylacetone spectrophotometry method) during the reactions at different PM 17

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Available online xxxx

number concentrations (Na) and relative humidities (RHs), and the formed particulate 18 Q5

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Keywords:

10%–15%, the decay rate of HCHO in the chamber was higher with the existence of PM from 20

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Formaldehyde

relatively clean days (with number concentration (Na) < 200,000 particle/L, 0.35–22.5 μm) 21

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Particulate matter

compared to dirty days (Na > 200,000 particle/L, 0.35–22.5 μm). When RH increased to 30%– 22

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Chamber

45%, PM can hardly have significant influences on the decay of HCHO. The formations of 23

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Formate

formate on the reacted PM were consistent with the HCHO decay rates at different ambient 24

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Volatile organic compounds

PM Na and RH conditions. This is a first study related to the “real” ambient PM reacted with 25

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formate was detected by IC and XPS techniques. The results showed that when RH was 19

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HCHO and suggested that in the clean and low RH days, PM could be an effective medium 26 27

for the conversion of HCHO to formate.

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© 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 28

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Introduction

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Volatile organic compounds (VOCs), key constituents in the atmosphere, play important roles in the formation and aging of fine particulate matter (PM). It can react with oxidizing gases (e.g., ozone) and radicals (e.g., OH and HO2 radicals), contributing to the growth of fine PM. VOCs may even go through heterogeneous reactions with substances on PM, which enhances particle growth and changes the chemical composition of particles as well as the physical properties, e.g., hygroscopicity and optical properties (Liggio et al., 2005). Formaldehyde (HCHO) is the most abundant carbonylcontaining VOCs in the atmosphere, and has a strong reducibility (Possanzini et al., 2002; Liu et al., 2015). HCHO in the atmosphere

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Published by Elsevier B.V. 29

comes from the oxidation of organic compounds or the direct emitting from combustion and industrial sources (Seinfeld and Pandis, 1998). The reaction with OH radicals, photolysis, and dry or wet deposition are well known as its removal pathways. Recently, the heterogeneous processes between HCHO and aerosols, e.g., sulfate aerosols (Jayne et al., 1996; Iraci and Tolbert, 1997), mineral dust aerosols (Carlos-Cuellar et al., 2003; Xu et al., 2006, 2010; Hatch et al., 2007; Sassine et al., 2010), and organic aerosols (Li et al., 2011), were found to contribute to the destruction of HCHO, providing additional sinks for HCHO (Hatch et al., 2007). Among those researches, mineral dust aerosols were explored most, such as SiO2, TiO2, α-Al2O3 and α-Fe2O3. The adsorptions of HCHO by α-Al2O3, α-Fe2O3 and TiO2 were found to be irreversible and formate was revealed as the most important

⁎ Corresponding author. E-mail: [email protected] (Jing Shang). 1 These two authors contributed equally to this paper.

http://dx.doi.org/10.1016/j.jes.2016.08.018 1001-0742/© 2016 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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1.1. Reaction chamber set-up

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Schematic of the indoor chamber is illustrated in Fig. 1. It is a 3.3-m3 Teflon bag (Plastic research institute of Shanghai, China) with a side length of 1.5 m, located on the third floor in the building of the College of Environmental Sciences and Engineering in Peking University, Beijing, China (116.3°E, 40.0°N). Experiments performed in the chamber were mainly conducted on winter days from Nov. 2013 to March 2015. An air blower (FLS-11; Fulanshi, China) with a flow of 41.6 L/min was used to introduce ambient PM into the chamber. Before entering into the chamber, the gas flow of ambient air was passed through three serial denuders filled with silica gel, KOH and KI (Analytical reagent; Tianjin Zhiyuan Chemical Reagent, Tianjin, China) sequentially. These denuders decreased 56%, 90% and 67% the levels of water vapor, NO2 and O3 in ambient air, respectively (Liu et al., 2015). PM concentration in the chamber was kept relatively constant by varying introduction time. Each time before experiment, the chamber went through a cyclic cleaning by particle-free clean air generated from ambient air where a particle filter (Oil-X Evolution efficient compressed air filter, ACS010BBMX; Parker Domnick Hunter, UK) was used to remove ambient PM along with the above described denuders. RH in the chamber was controlled by bubbling water system with clean particle-free air. An RH of 10%–15% is adopted in most of the chamber experiment but a higher RH of 30%–45% was used when estimating the impact of RH changes. A thermohygrometer (HD 2301.0, Delta OHM, Italy) with a sensor probe (HP 474 AC, Delta OHM, Italy) was used to measure RH and temperature in the chamber. All the chamber experiments were conducted under the temperature of 288 K. A Shade cloth was used to shade the chamber from sunlight radiation. Wall effects of the chamber for PM and HCHO were both evaluated before all the chamber experiments, where ambient PM or HCHO was introduced into the chamber respectively (the latter is described later in Section 1.2). In the reaction experiment, HCHO was introduced into the chamber after PM and the reaction timing started after a 10–15 min mixing time. The number concentration of PM in chamber was recorded and the loss rate of PM was calculated. For HCHO, the decay

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1. Materials and methods

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and abundant product during the reaction (Carlos-Cuellar et al., 2003; Xu et al., 2006), which may affect the hygroscopicity and 71 optical properties of particles (Hatch et al., 2007). In addition, 72 meteorological factors, e.g., temperature, relative humility (RH) 73 and light radiation, also affect the heterogeneous reactions of 74 Q8 VOCs (Shen et al., 2013). 75 However, the reaction of formaldehyde with ambient PM 76 has not been studied yet. Monocomponent particles, though 77 frequently used in current studies, cannot reflect the situation 78 in real atmosphere. Ambient PM usually goes through an 79 aging process in the atmosphere and become multicompo80 nent and complicated, making it different from the simulated 81 particles in laboratory experiment (Shen et al., 2013). Addi82 tionally, under different pollution conditions, ambient PM 83 tends to appear with different chemical compositions, which 84 will directly affect its reactivity in the atmosphere and its 85 physiochemical characteristics change afterwards (Khalizov 86 Q9 et al., 2010; Sun et al., 2013a, 2013b). 87 Beijing, the political center and one of the megacities in 88 northern China, has been suffering from heavy air pollution 89 during the last two decades (Chen et al., 2007). According to 90 the data from Beijing Environmental Statement, the annual 91 average mass concentration of PM2.5 in Beijing was 89.5 and 92 85.9 μg/m3 in 2013 and 2014 respectively (Sun et al., 2015), over 93 2 times of annual average of 35 μg/m3 in the National Ambient 94 Air Quality Standard. The occurrence frequency of severe air 95 pollution was quite different in different seasons in Beijing 96 throughout the year. Winter, with heating process and special 97 weather conditions, has typically heavier air pollution than in 98 other seasons in Beijing (Sun et al., 2013a, 2013b). The 99 compositions of PM on those polluted days are also changing 100 in different seasons, e.g., PM in winter pollution period tends 101 to possess larger mass fraction of secondary organics (Liu 102 Q10 et al., 2012; Cheng et al., 2015). 103 Facing with the above problems, this study conducted the 104 reaction of HCHO and ambient PM gathered from different 105 polluted-level days in the winter of Beijing via an indoor 106 environmental chamber. This is the first work using “real” 107 ambient PM as interface to investigate the potential impact 108 that complex ambient PM may have on the decay of HCHO. 109 The decay rate of HCHO, the generated formate on the 110 particles, the surface characteristics and the changes of 111 element content on particles were analyzed and the impact 112 of humidity on the process was explored.

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Fig. 1 – Schematic of the indoor environmental chamber. Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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rate was calculated according to its concentrations varying over time (see Section 1.4).

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1.2. HCHO generation and detection

158 Q12 A pyrolysis method according to Xu et al. (2006) was adopted 159

to generate gaseous HCHO. Powdery paraformaldehyde (puri160 ty 95%; J&K Scientific, Beijing, China) was placed in self-made 161 glassware and heated under a temperature of 343 K in water 162 bath. A constant flow of N2 (0.08 L/min) was introduced into 163 the glassware to carry high-purity gaseous HCHO into the 164 chamber. The initial concentration levels of HCHO in chamber 165 were controlled by varying the time of gas introduction and 166 Q13 measured by acetylacetone spectrophotometry (Wang et al., 167 2011). In the acetylacetone spectrophotometry method, a 168 constant flow of HCHO (0.16 L/min) from the chamber was 169 absorbed by deionized water (10 mL) in a porous sieve-plate 170 tube, followed by measuring the absorbance at 412 nm with a 171 spectrophotometer (T6 UV–Vis; Purkinje, China) after the 172 introduction of acetylacetone and a boiling water bath. The 173 concentration of HCHO in the chamber was derived according 174 to the absorbance and a standard curve. The standard curve 175 was obtained at the beginning of the reaction experiment, as 176 shown in Eq. (1),

1.4. HCHO decay rate evaluation

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To estimate the impact of ambient PM on the decay of HCHO, it is necessary to examine the net decay rate of HCHO without the existence of ambient PM first, i.e., the wall loss of HCHO in the chamber. Particle-free clean air and HCHO were introduced into the chamber successively. After a mixing time of 15 min, the concentration of HCHO was measured in every 15 min. The decay of HCHO in chamber fits a Pseudo first-order kinetics equation,

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C ¼ C0 e–kt0

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where, C (ppm) and C0 (ppm) stand for the concentration of HCHO at t moment and initial concentration, k0 is the decay rate of HCHO without ambient PM existing in the chamber. When evaluating the influences of ambient PM on the decay rate of HCHO, ambient PM was introduced into the chamber with cleaned ambient air following the introduction of HCHO. Then the HCHO concentration was measured. In this case, k0 in Eq. (2) is replaced by kp, which means the decay rate of HCHO with ambient PM existing.

where, A denotes the absorbance of HCHO at 412 nm and C is the concentration (ppm) of HCHO.

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1.3. Particle monitoring, collection and characterization

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Before and after reaction, several methods are used to measure the PM concentration, element and organo-functional group components of PM and formate concentration on the surface of PM. The particle number concentration (particle/L, hereinafter, Na) and mass concentration (mg/m3 or μg/m3; hereinafter, Ca) were both monitored by a portable aerosol spectrometer Grimm1.108 (Grimm Aerosol Technik, GmbH & Co. KG, Ainring, Germany) at a flow rate of 1.2 L/min. Employing a semiconductor laser as light source, Grimm1.108 measures the number of particles per unit volume of air via light scattering technology. It can measure particles with an optical size range from 0.35 to 22.5 μm and concentration range of 1–2,000,000 particle/L or mass concentration range of 0.001–100 mg/m3. Ambient PM in the chamber after reaction was collected at a flow rate of 37.5 L/min by quartz filters (pore diameter 0.45 μm, 47 mm; Pall Life Sciences, USA) placed within a Teflon holder. Quartz filters were heated above 823.15 K for 6 hr and then balanced for over 24 hr under constant temperature and humidity before sampling. Filters loaded with PM were stored at 253.15 K refrigerator in dark for follow-up chemical characterizations. Element components and energy bands of PM were analyzed by X-ray photoelectron spectroscopy (XPS; Axis Ultra, Kratos Analytical Ltd., USA) and Casa XPS software was used to perform data analysis. The concentration of formate on PM surface was measured by ion chromatography (IC; ICS-2000, Dionex, USA), where deionized water was used to extract PM from filters and also as the eluent in IC.

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2. Results and discussion
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2.1. Decay rate of HCHO in the chamber

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2.1.1. Wall loss effect of HCHO in the chamber

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As mentioned above, the decay rate of HCHO (k0) with different C0 ranging from 2 to 22 ppm under the RH of 10%–15% was measured. Here, one of a typical HCHO concentration versus time curve is illustrated in Fig. 2 at that C0 is 22 ppm and k0 is 0.0121/min. Considering the wall loss effect of HCHO in the chamber, k0 was measured in every chamber experiment and (kp − k0) is calculated and used to evaluate the impact of ambient PM on the decay rate of HCHO (see Section 2.1.2).

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2.1.2. Impact of ambient PM on the decay rate of HCHO

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Under a constant initial concentration of HCHO around 10 ppm, the impact of ambient PM on the decay of HCHO in the chamber was evaluated by calculating (kp − k0) for PM number concentration (Na) varying from 10,000 to 1,060,000 particle/cm3 in the chamber (Fig. 3). The larger the value is above zero, the bigger the impact is. Given the huge discrepancy on PM number concentrations and to illustrate the data in a clearer way, log(Na), the logarithm of number concentration of PM, was used to identify PM for different polluted days. In Fig. 3, a negative correlation between (kp − k0) and log(Na) was revealed. Additionally, (kp − k0) is above zero when log(Na) is less than 5.3, hence log(Na) = 5.3 (corresponding Na = 200,000) can be regarded as a cutting line. Hereinafter, days with log(Na) above 5.3 is described as relatively dirty days (DD) and days with log(Na) below 5.3 is described as relatively clean days (CD). A further significant analysis was made by SPSS. Paired-sample t-test was adopted after logarithm transformation and normal distribution test of the data. The test result shows that significant differences exist between kp and k0 when log(Na) is less than 5.3 (significance level p-value = 0.014 < 0.05), while the opposite is not (significance level p-value = 0.804 > 0.05). Hence, the data in Fig. 3

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Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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2.2. Particulate formate formation

Fig. 5. Results shown in Fig. 5a, b were from the experiment conducted on Nov. 6th, 2015 when Na = 12,589 (logNa = 4.1), while that in Fig. 5c and d were from the one conducted on Oct. 9th, 2015 when Na = 398,107 (logNa = 5.6). Both Fig. 5a, b show XPS spectra of PM before HCHO exposure while Fig. 5c, d are after HCHO exposure. Quantitative evaluations of the PM surface functional groups were performed by analyzing the C1s bond. Generally, peaks at approximately 284.81–285.03, 286.42–286.73, 287.81–288.03 and 288.60–289.55 eV were attributed to hydrocarbons (C\C, C\H), ethers (C\O), carbonyl group (C_O) and carboxylate groups (O\C_O), respectively (Singh et al., 2014). As shown in Fig. 5, four peaks at 284.8, 285.6, 287.0 and 288.8 eV were observed in each figure after curve fittings of the C1s spectra. More detailed results are displayed in Table 1 where A0 and Af are the peak-area fractions of the functional groups before and after HCHO exposure, respectively, and (Af − A0)/A0 indicates the net growth of functional groups. As is revealed, the C/O ratio of PM from CD and DD declines from 1.84 to 0.24 and 1.15 to 0.16, respectively, which means a larger fraction of oxygen-containing groups on PM after HCHO exposure. Concretely, the fraction of C_O and O\C_O on PM increased after the interaction with HCHO in the chamber, while the fraction of C\H and C\O declined. Other studies, though using different types of particle samples such as black carbon or biochar, found similar variation tendency for O\C_O functional groups on particles after aging (Cheng et al., 2006; Singh et al., 2014). The results suggest that ambient PM probably went through an aging process in the chamber with the presence of HCHO. Additionally, for PM from DD, the fraction of C_O and O\C_O components was found to hold larger mass percentages but increase less after HCHO exposure, comparing to that of PM from CD (Fig. 5, Table 1). This result coincides with the ones found in Figs. 3 and 4. These phenomena indicate that PM from DD experienced atmospherically aging process resulting in its higher carboxylate compounds but lower reactivity with HCHO, while PM from CD maybe more reactive to HCHO and promotes the transfer of HCHO to formate.

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2.4. Effect of relative humidity

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Xu et al. (2006) have studied the heterogeneous reaction of 266 formaldehyde with α-Al2O3 by using in-situ diffuse reflectance 267 infrared Fourier transform spectroscopy. Their study con268 firmed that HCHO can be oxidized firstly to dioxymethylene 269 by reactive oxygen species and then formate. In this study, the 270 mass percent of formate on PM was measured by IC and 271 recorded as P0 and Pf, respectively before and after HCHO 272 exposure. Fig. 4 denotes the growth of formate measured on 273 Q17 PM after exposure to HCHO. As revealed in Fig. 4, (Pf − P0) 274 declines as log(Na) increases, which means that less formate 275 is found on PM from DD than from CD after HCHO exposure. 276 Comparing Figs. 3 and 4, that relation of (Pf − P0) and log(Na) 277 shares similar trend with that of (kp − k0) and log(Na). 278 Therefore, it may be stated that PM from CD accelerates the 279 decay of HCHO, resulting in larger generation of formate.

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Fig. 4 – The increment of formate production after HCHO exposure for PM with different Na (RH: 10%–15%, temperature: 288 K). PM: particulate matter; RH: relative humility.

demonstrates that PM from CD contributes more to the decay of HCHO, while PM from DD seems to slow down the decay of HCHO.

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Fig. 2 – Variation of HCHO concentrations with time in the chamber without PM at C0 = 22 ppm, RH: 10%–15%, temperature: 288 K. PM: particulate matter; RH: relative humility.

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Elemental components of PM from days with different Na before and after HCHO exposure were analyzed by XPS. Two typical results from CD and DD respectively are illustrated in

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Fig. 3 – Decay rate of HCHO in the presence of PM with different Na at RH: 10%–15%, temperature: 288 K. PM: particulate matter; RH: relative humility.

Except for RH of 10%–15%, a higher RH of 30%–45% was also 323 tested. By comparing the results of relatively high RH (30%– 324

Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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Fig. 5 – XPS spectra of PM from CD (log Na = 4.1) before (a) and after (b) interaction with HCHO; XPS spectra of PM from DD (log Na = 5.6) before (c) and after (d) interaction with HCHO. RHs are all of 10%–15%, and temperature of 288 K. XPS: X-ray photoelectron spectroscopy; PM: particulate matter; CD: clean days; DD: dirty days; RH: relative humility.

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For (Af − A0)/A0 (%), positive value indicates the increase of corresponding functional group content and the vise versa. PM: particulate matter; CD: clean days; DD: dirty days.

45%) with low RH (10%–15%), the effect of RH on the decay of HCHO and on the formation of formate on PM after HCHO 327 exposure was explored. As for HCHO decay rate (kp − k0) 328 shown in Fig. 6, the high level of RH has different impacts on 329 (kp − k0) considering days with different Na levels. When RH is 330 increased to 30%–45%, both the inhibiting effect of PM from 331 DD and the acceleration effect of PM from CD on HCHO decay 332 become retarded, nearly losing effects. This was similar 333 with the results of other studies about mineral aerosol dust Q21334 Q20 (Sassine et al., 2003; Zhang et al., 2006; Singh et al., 2014). For 335 instance, Sassine et al. (2010) observed that the increase of RH 336 from 6% to 70% first enhanced and then at around 30% slowed 337 down the HCHO uptake on particles. It can be explained by the 338 competitive adsorption between water and HCHO or the 339 Q22 decreasing amount of ROS on particle surface (Zhang et al., 340 2006). As for the formate content, RH in this study showed a 341 similar effect as it on the decay of HCHO. As is shown, the 342 increase of RH from 10%–15% to 30%–45% generally reduces

(Pf − P0). Moreover, (Pf − P0) declines more for PM from days with smaller Na when RH increases. But, as log(Na) becomes larger than 5.3, (Pf − P0) may even increase slightly when RH rises to 30%–45%. That, except for data deviation, might be attributed to that higher RH facilitate the uptake of water for formic acid on PM surface, leading to an increase in formate formation (Paulot et al., 2011). Besides, Beijing PM is more hygroscopic at conditions of high PM concentrations during winter (Cheng et al., 2015), which probably further promotes the uptake of water (Fig. 7).

3. Conclusions

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Fig. 6 – The impact of ambient PM with different number concentrations on HCHO's decay rate under different RH. PM: particulate matter; RH: relative humility.

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In this study, ambient PM was introduced into a Teflon chamber to investigate the “real” atmospheric particles' interaction with HCHO. In the condition of RH 10%–15%, PM from Beijing winter relatively CD can promote the decay rate of HCHO, which was further confirmed by the enhanced formation of formate and carbonyl group on the reacted particles; while PM from relatively DD shows no acceleration for the decay rate of HCHO with slightly increased amount of formate and carbonyl group formation. Additionally, when RH in the chamber increased from 10%–15% to 30%–45%, PM hardly presents obvious impact on the decay rate of HCHO, which is also independent of its Na value. The enhancement of RH generally reduced formate mass fraction on PM from CD. This study provided, so far as we know, a first exploration of reaction between HCHO and real atmospheric PM in a chamber. The enhancement of HCHO decay rates after PM introduction in chamber matched well with that of formate and carbonyl groups formation. These results indicate that ambient PM in relatively clean and low RH days could act as a medium for the conversion of HCHO to formate and the vice versa.

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Jia et al., 2011 Liu, 2015 Proctor and Sherwood, 1982 Seinfeld and Pandis, 2006 Wang and Lu, 2004

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Acknowledgments

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Fig. 7 – The increment of formate on PM from days with different Na after HCHO exposure under different. PM: particulate matter.

This work was supported by the National Natural Science 384 Foundation of China (nos. 41421064, 21577003, 21277004). 385

Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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Please cite this article as: Chen, Y., et al., Simulated reaction of formaldehyde and ambient atmospheric particulate matter using a chamber, J. Environ. Sci. (2016), http://dx.doi.org/10.1016/j.jes.2016.08.018

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