Heterogeneous Reaction of HCOOH on NaCl Particles at Different

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Heterogeneous Reaction of HCOOH on NaCl Particles at Different Relative Humidities Kaihui Xia, Shengrui Tong, Ying Zhang, Fang Tan, Yi Chen, Wenqian Zhang, Yu-Cong Guo, Bo Jing, Maofa Ge, Yao Zhao, Khalid A. Alamry, Hadi M Marwani, and Suhua Wang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02790 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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The Journal of Physical Chemistry

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Heterogeneous Reaction of HCOOH on NaCl Particles at Different Relative Humidities

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Kaihui Xia,1,2,3 Shengrui Tong,3,* Ying Zhang,3,4 Fang Tan,3,4 Yi Chen,3,4 Wenqian Zhang,3,4

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Yucong Guo,3 Bo Jing,3 Maofa Ge,3,4,* Yao Zhao,5 Khalid A. Alamry,6 Hadi M. Marwani,6

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and Suhua Wang1,2,6,7,*

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1

7

China

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2

University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China

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3

State Key Laboratory for Structural Chemistry of Unstale and Stable Species, CAS

1

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui, 230031, P. R.

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Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry,

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Chinese Academy of Sciences, Beijing, 100190, P. R. China

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University of Chinese Academy of Sciences, Beijing 100049, P. R. China

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5

Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese

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Academy of Sciences, Beijing 100190, P. R. China

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6

16

Arabia

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7

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102206, P. R. China

Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi

School of Environment and Chemical Engineering, North China Electric Power University, Beijing

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1 ACS Paragon Plus Environment

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ABSTRACT:

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The contribution of volatile organic acids to chloride depletion still remains unclear under ambient

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conditions in the coast and land. In this work, the heterogeneous reaction of HCOOH on the NaCl

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surface at a series of RHs was investigated using diffuse reflectance infrared Fourier transform

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spectroscopy (DRIFTS). The formate was found to be formed on NaCl surface under dry and wet

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conditions, accompanied with the corresponding chloride depletion. The adsorbed HCOOH and the

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formation of formate on NaCl surface decreased with increasing RH below 30% RH. The adsorbed

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HCOOH decreased while the formation of formate increased with enhanced RH at 45-70% RH. The

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variation in the formation of formate with RH suggests that chloride depletion may undergo similar

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changes. Additionally, the mechanism and kinetics for uptake of HCOOH on NaCl surface at various

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RHs were discussed and analyzed. Our results highlight the role of heterogeneous chemistry of

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volatile organic acid in the chloride depletion of NaCl in the coast and land.

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1. Introduction

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Sea salt aerosols (SSAs), the common component in the atmospheric environment, are yielded

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by wave action on the whole oceans.1, 2 The influences of SSAs on global climate include

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direct effects by scattering light and indirect effects by serving as cloud condensation nuclei

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(CCN) in the troposphere.3, 4 SSAs could also affect ozone chemistry and oxidative capacity

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by releasing halogen compounds such as HCl, Cl2, and ClNO in the atmosphere.5-12 The

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reaction of SSAs with acidic species can release volatile HCl and produce corresponding salt,

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which has a well-known mechanism as illustrated by R15-7, 12-16:

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NaCl (aq, s) + HA (aq, g) ⟶ NaA (aq, g) + HCl (g)

(R1)

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where HA represents acidic species such as H2SO4 and HNO3. This reaction could induce

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chloride depletion of NaCl which is a major component of SSAs.5, 13, 15, 17 Sea salt aerosol is a

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complicated system containing many components such as MgCl2, NaCl, sulfate, nitrate, and

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organics.12, 18-20 NaCl, the most abundant component of sea salt, has been extensively used as

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surrogate for sea salt aerosols in the studies on the interactions with trace gases such as NO2,

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H2SO4, and HNO3.8, 9, 21-27 For example, HNO3, a volatile inorganic acid, can displace the

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chloride of NaCl to generate gaseous HCl under both dry (in the absence of water vapor) and

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wet conditions (in the presence of water vapor ).14, 16 The condensed phase H2SO4 could

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substitute chloride in NaCl.5, 13, 15 However, the chloride depletion due to the influence of

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inorganic species cannot fully explain the whole chloride deficit in the atmospheric aerosols.6,

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12, 17

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Combined field measurements and laboratory studies, Laskin et al. reported the surprising

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chloride depletion triggered by the reactions of condensed phase organic acids (such as

Therefore, there should be other pathways for chloride depletion in the atmosphere.



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malonic acid, citric acid, and malic acid) with NaCl in mixed droplets upon dehydration.12

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Peng et al. also found the NaCl droplets mixed with dicarboxylic acids could undergo chloride

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depletion and form corresponding organic salts upon dehydration, which significantly

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influence the hygroscopic growth of NaCl.19, 28, 29 Recent field measurements have suggested

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that volatile organic acids such as formic acid (HCOOH) and acetic acid are potentially

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important contributors to chloride depletion.6, 17 However, Laskin et al. confirmed the slight

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chloride depletion of NaCl droplets mixed with acetic acid during the dehydration process.12

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For volatile organic acids, the reaction R1 is likely not favored in the aqueous droplets since

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the evaporation of organic acids is comparable to that of HCl from droplets.12 Thus, the

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heterogeneous reaction of volatile organic acids with solid NaCl may play a role in the

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chloride depletion.

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HCOOH is a typical volatile organic acid, which is emitted from vehicle exhaust,

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biomass and fossil fuel burning,30, 31 soils,32 vegetation,33 phytoplankton,34 and the secondary

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formation by the oxidation of VOCs in the atmosphere 35, 36. HCOOH could affect the acidity

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of rainwater and even influence the formation of CCN in the troposphere.37-40 The

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concentration of HCOOH can be as high as ppb scale in the coast and inland.35, 36, 41 As a

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result, formic acid is an important trace gas in the atmospheric environment. According to

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Braun et al.’s and Zhao et al.’s work, formic acid has been considered as an important

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contributor to chlorine loss of sea salt particles.6, 17

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The previous studies have found the potential role of water vapor in the heterogeneous

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reactions of trace gases with NaCl.21-23, 25, 42 For example, the formation rate of nitrate from

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reactions between NO2 and NaCl crystal was significantly enhanced in the presence of water 4 ACS Paragon Plus Environment

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vapor relative to the conditions of absence of water vapor.22, 23, 25 The similar phenomenon

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also appears for the reaction of SO2 with mixtures of NaCl and MgCl2·6H2O.21 SO2 uptake on

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mixtures of NaCl and MgCl2·6H2O in the absence of water vapor is considerably lower than

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that under wet condition (50% RH).21, 43 In addition, density functional theory (DFT) indicates

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that the surface reconstruction induced by adsorbed water is essential for the heterogeneous

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reaction of gas HNO3 with solid NaCl.42 Therefore, RH effect likely plays a great role in the

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heterogeneous reaction of NaCl particles with trace gases.

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The concentrations of SSAs in the coast and inland are determined to be from 1 to 20

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µg/m3.7,

44-47

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potentially important contributor to chloride depletion.6, 17 The relative humidity in coast and

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inland can even be less than 20% RH.48, 49 Thus, the mixed particles of NaCl with formic acid

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can be expected during the transport processes from the sea to inland at the lower RH. In this

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work, DRIFTS was applied to investigate the heterogeneous reactions of HCOOH with NaCl

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under various RHs (97%, Alfa Aesar) with N2 (>99.99%, Beijing Tailong Electronics Co., Ltd.) in the

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glass bottle. The absolute pressure transducer (MKS 627B range from 0 to 1000 torr) was

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used to monitor the partial pressure of the mixed gas. Synthetic air was prepared by 20%

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volume percent of O2 (>99.99%, Beijing Tailong Electronics Co., Ltd.) and 80% volume

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percent of N2.

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2.2. Experimental Methods

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FTIR (Nicolet FTIR Spectrometer iS50) spectra for the heterogeneous reactions of HCOOH

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on the surface of NaCl powders were recorded by in situ DRIFTS (Model CHC-CHA-3,

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Harrick

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mercury-cadmium-telluride (MCT) detector. The DRIFTS used here have been described in

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detail in our previous studies and the structure of DRIFTS was shown in Figure S1(a).51-55

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The core of the system was the reaction chamber with an identical dimension steel sample

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holder. The thermocouple and temperature sensor were coupled with the sample holder to

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control and monitor the sample temperature, respectively. In this work, the stainless steel

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holder with 10 mm in diameter and 0.5 mm in depth was loaded with the samples of

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approximate 45 mg, and the geometric surface area of samples was 7.85 × 10-5 m2. The IR

Scientific

Corp.)

equipped

with

a

liquid-nitrogen-cooled


narrow

band

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spectra with spectral range from 4000 to 650 cm-1 were acquired at a resolution of 4 cm-1 and

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100 scans in a time of 40 s.

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the spectrum data during the reaction processes.

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The OMNIC software was used to automatically record all

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To minimize errors, all the experimental steps for the sample powder in the reaction

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chamber at different RH were performed consistently. Firstly, in order to remove adsorbed

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water and impurities, the samples were heated to 573 K for 120 min and then cooled to 298 K

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by purging of N2 purging. Secondly, the sample powders were exposed to synthetic air with

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aimed RH for about 60 min to achieve water adsorption equilibrium on the sample surface.

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Then the IR spectra of the powders were collected as the background spectra at the

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corresponding RH. Therefore, the background spectra contained the corresponding adsorbed

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water. Finally, HCOOH gas was introduced into the reaction chamber at a total flow rate of

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400 ml min-1 and then infrared spectra were recorded as a function of reaction time. As a

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result, the positive and negative absorption bands could indicate the formation and losses of

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species on the surface of the samples during the reaction process. The gaseous HCOOH was

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mixed with synthetic air to reach the desired concentration ((5.20 ± 0.02) × 1014 molecules

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cm-3) in this system. The gas flow was regulated and monitored by a mass flow controller

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(Beijing Sevenstar Electronics Co., Ltd.). The aimed RHs from dry condition to 70% RH

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were adjusted by changing the flow rates of dry N2 and humidified N2 which was yielded by

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dry N2 through water in a plastic bottle. The study under dry conditions can help to deduce the

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mechanism under wet conditions. The temperature and RH were monitored by a commercial

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temperature and humidity system detector (HMT330, Vaisala) with an uncertainty of ± 1 K

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and ± 1 % in temperature and RH measurement, respectively. The schematic diagram of the 7 ACS Paragon Plus Environment


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experimental setup was shown in Figure S1 (b). All reaction experiments were repeated at

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least three times.

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TOF-SIMS (ION-TOF, GmbH, Munster, Germany) equipped with a 30 keV pulsed Bi3+

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liquid metal gun as a primary ion source was applied to further identify the surface species

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formed through NaCl surface exposed to HCOOH gas for 120 min at different RHs. For the

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TOF-SIMS experiment, the 0.5 mol L-1 NaCl solution was nebulized and deposited on the

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silicon wafer in this work.12, 56 Then, the silicon wafer was put into the reaction chamber of

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DRIFTS to react with HCOOH at aimed RH. All the steps were the same as the experiments

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of DRIFTS. After reaction, the silicon wafer was carried out and put into the TOF-SIMS to

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further confirm the surface product. All the data were processed by SurfaceLab 6 software

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ver6.3.

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3. Results and discussion

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3.1. Surface Products at Various RH

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The typical in situ DRIFTS spectra for reactions of NaCl particles with HCOOH ((5.20 ± 0.02)

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× 1014 molecules cm-3) under dry condition (RH60%) accompanied with the increased coverage of adsorbed water, similar to previous

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studies.8, 74 Thus, the multilayers of water can be formed on the surface of packed NaCl

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powder.

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The effects of RH on the reaction of HCOOH with NaCl particle are shown in Figure 3

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(a). Absorption lines were recorded after a reaction time of 120 min at various RH. At RH

Heterogeneous Reaction of HCOOH on NaCl Particles at Different

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