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Atmospheric Environment 67 (2013) 63e72

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Atmospheric Environment

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Heterogeneous reactions of gaseous hydrogen peroxide on pristine and acidicgas-processed calcium carbonate particles: Effects of relative humidity andsurface coverage of coating

Yue Zhao, Zhongming Chen*, Xiaoli Shen, Dao HuangState Key Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China

h i g h l i g h t s

< First kinetic study of H2O2 uptake on acidic gas-aged mineral dust.< H2O2 uptake on calcium carbonate is highly related to acidic gas aging.< H2O2 favors sulfate formation on mineral dust particularly at high RH.< Atmospheric aging of mineral dust should be considered in atmospheric models.

a r t i c l e i n f o

Article history:Received 5 March 2012Received in revised form29 October 2012Accepted 31 October 2012

Keywords:Atmospheric hydrogen peroxideHeterogeneous reactionsCalcium carbonateMineral dustAtmospheric aging

* Corresponding author.E-mail address: zmchen@pku.edu.cn (Z. Chen).

1352-2310/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.atmosenv.2012.10.055

a b s t r a c t

Atmospheric aging appears to alter physical and chemical properties of mineral dust aerosol and thus itsrole as reactive surface in the troposphere. Yet, previous studies in the atmosphere have mainly focusedon the pristine surfaces of mineral dust aerosol, and the reactivity of aged mineral dust toward atmo-spheric trace gases is poorly recognized. This work presents the first laboratory investigation ofheterogeneous reactions of gaseous hydrogen peroxide (H2O2), an important atmospheric oxidant, on thesurfaces of HNO3 and SO2-processed calcium carbonate particles as surrogates of atmospheric mineraldust aged by acidic trace gases. It is found that the processing of the calcium carbonate particles withHNO3 and SO2 has a strong impact on their reactivity toward H2O2. On HNO3-processed particles, thepresence of nitrate acts to either decrease or increase H2O2 uptake, greatly depending on RH and surfacecoverage of nitrate. On SO2-processed particles, the presence of surface sulfite appears to enhance theintrinsic reactivity of the mineral particles due to its affinity for H2O2, and the uptake of H2O2 increasessignificantly relative to the pristine particles, in particular at high RH. The mechanisms for heterogeneousreactions of H2O2 with these processed particles are discussed, as well as their potential implications ontropospheric chemistry. The results of our study suggest that the reactivity of mineral dust aerosoltoward H2O2 and maybe other trace gases is markedly dependent on the chemical composition andcoverage of the coatings as well as ambient RH, and thus will vary considerably in different polluted airmasses.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Particulate matter (PM) plays an important role in atmosphericenvironment and climate change (Sokolik and Toon, 1996; Buseckand Posfai, 1999; Seinfeld and Pandis, 2006). Mineral dust aero-sol, emitted from the arid and semiarid regions with an estimatedannual flux of 1000e3000 Tg, constitutes one of the largest fractionof atmospheric PM (Dentener et al., 1996). As a reactive surface in

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the global troposphere, mineral dust aerosol can interact witha variety of trace gases during atmospheric transport, and thus hasthe potential to influence atmospheric chemistry (Dentener et al.,1996; Bauer et al., 2004; Zhu et al., 2010). Laboratory studies haveshown that heterogeneous reactions of trace gases including HNO3,oxides of nitrogen and sulfur, and volatile organic compounds onmineral dust and its components generally lead to the formation ofsome low-volatile and/or nonvolatile products, such as nitrate,sulfate, and oxidized organics, on particle surfaces (Usher et al.,2003a; Crowley et al., 2010; Kolb et al., 2010). Field studies havealso observed that atmospheric mineral dust, through heteroge-neous uptake of acidic gases and organic compounds, often

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e7264

accumulates nitrate, sulfate, and also organic coatings (Buseck andPosfai, 1999; Zhuang et al., 1999; Falkovich et al., 2004; Matsukiet al., 2005; Sullivan et al., 2007; Li and Shao, 2009; Formentiet al., 2011). These coating materials have been expected to alterphysical and chemical properties of mineral dust aerosol and thusits role as a reactive surface in the downstream atmosphere.Therefore, it is necessary to investigate the reactivity of mineraldust aerosol aged by interacting with atmospheric trace gases.However, up to date, few studies on atmospheric heterogeneouschemistry have focused on the aged mineral aerosol particles.Available laboratory studies have shown that the reactive uptake ofozone on mineral oxide particles depended on the coatings underdry conditions (Usher et al., 2003b); the irradiation of surfacenitrate on aluminum oxide particle surface can significantly yieldNO and NO2 (Schuttlefield et al., 2008); and NO2-reacted aluminaparticles can lead to thermal and photochemical oxidation ofadsorbed organics (Raff et al., 2011). Obviously, information on thereactivity of aged mineral aerosols toward atmospheric trace gasesis still limited. In addition, little is know about the associatedkinetics under humid conditions that are more relevant to the realtroposphere.

Hydrogen peroxide (H2O2) is an important atmospheric oxidant,playing an important role in secondary sulfate formation and thecycle of HOx (OH and HO2) radicals, which dominates the oxidativecapacity of the atmosphere (Finlayson-Pitts and Pitts, 2000; Reevesand Penkett, 2003; Hua et al., 2008). Field studies have shown thatheterogeneous reactions on ambient mineral aerosol (De Reuset al., 2005) and urban aerosol (He et al., 2010) seem to be animportant sink for gaseous H2O2. Accordingly, recent severallaboratory studies have explored the kinetics and mechanisms ofthe reactions of H2O2 with the pristine surface of mineral aerosol(Pradhan et al., 2010a,b; Zhao et al., 2011; Wang et al., 2011).However, the reactivity of agedmineral dust aerosol toward H2O2 isstill unclear.

The objective of this work is to study the effect of atmosphericaging on the reactivity of mineral dust aerosol by investigatingH2O2 uptake onmineral particles that are processed with HNO3 andSO2, two of the most common acidic gases in the atmosphere.Calcium carbonate (CaCO3) was used as substrate due to its abun-dance in atmospheric mineral dust and its affinity for acidic tracegases. For example, calcite (CaCO3) mineral, which can exist as pureparticles or aggregate with other mineral phase in the atmosphere,can make up as much as 20e30% of the total dust loading (Kandleret al., 2009; Díaz-Hernández et al., 2011). Additionally, CaCO3particles are well known to be highly reactive toward HNO3 andnitrogen oxides (Usher et al., 2003a; Crowley et al., 2010). Fieldmeasurements have shown that the nitrate on dust, in most cases,is closely associated with Ca-containing dust, which is most likelycalcite (Zhuang et al., 1999; Sullivan et al., 2007; Li and Shao, 2009).In particular, single particle studies have suggested that calciteparticles are frequently covered by visible calcium nitrate coatingsmainly through the reactionwith HNO3 (Krueger et al., 2004; Li andShao, 2009). Although calcite particles are much less reactivetoward the less acidic SO2, field observations have also suggestedthe involvement of calcite in atmospheric processing of SO2(Matsuki et al., 2005; Díaz-Hernández et al., 2011). Thus, calciumcarbonate, to various degrees, can be used as a real or model dustsample to understand the evolution of the reactivity of mineral dustaerosol upon atmospheric aging.

The present study explored H2O2 uptake on HNO3 and SO2-processed calcium carbonate particles as a function of relativehumidity (RH) and surface coverage of coatings (to simulatedifferent degree of atmospheric aging). By employing the sameexperimental method to investigate H2O2 uptake on both pristineand processed particles, we are able to probe the effects of the

processing on the reactivity of mineral particles toward H2O2. It isfound that the reactivity of calcium carbonate particles towardH2O2 is dramatically modified due to the HNO3 and SO2-processing,with a striking dependence on the RH and coverage of coating.

2. Experimental

2.1. Materials

The calcium carbonate powder (Alfa Aesar, 99.5%, averageparticle size around 6.6 mm)was used as substrate in this study. TheBrunauereEmmetteTeller (BET) surface area of calcium carbonatesamples was measured to be 1.4 m2 g�1 using a Micromeritics ASAP2010 BET apparatus. Commercial H2O2 solution (SigmaeAldrich,50 wt%) was processed to generate gaseous H2O2. ConcentratedHNO3 solution (65e68 wt%, Beijing Chemical Factory) was used togenerate gaseous HNO3. SO2 gas (29.4 � 10�6 mol mol�1 N2,National Research Center for Standard Substances, China) was alsoused as received.

2.2. Instrumentation

Fig. 1 shows schematic diagram of the experimental set-up thatconsists of three main components: pumping and gas handlingsystem, reactor for gaseparticle interaction, and H2O2 collectionand detection system.

2.2.1. Generation of gaseous H2O2

The commercial 50 wt% H2O2 solution was concentrated ina bubbler by bubbling 100 standard cubic centimeter per minute(sccm) dry N2 through it at 277 � 0.2 K for several days. A fewmilliliters of the concentrated H2O2 solution were then transferredto another bubbler at 277 � 0.2 K and a dry N2 flow of 20 sccmwasbubbled through the solution. The resulting N2 flow was thenbalanced with 380 sccm of synthetic air (20% O2 þ 80% N2) beforebeing introduced into the reactor. The 400 sccm balanced airthen gives an initial gaseous H2O2 concentration of1.3 � 1014 molecules cm�3 for uptake experiments on pristine andprocessed calcium carbonate particles. For gaseous H2O2 concen-tration measurement, the 400 sccm H2O2-containing air flowbypasses the reactor, directly mixed with 2.25 slm dry N2 anddrawn into the thermostatically controlled H2O2 collector, followedby the determination with a high-performance liquid chromatog-raphy (HPLC) instrument. The details on H2O2 collection and HPLCanalysis can be seen in Section 2.2.2. All of the tubing for thismeasurement was made of Teflon.

2.2.2. Gaseparticle interactionThe gaseparticle interactions including the processing of

mineral particles with acidic gases and H2O2 uptake on processedparticles were investigated in the same reactor shown in Fig. 1. Thereactor (length 15 cm, ID 3.3 cm) is a quartz tube with two infraredwindows made of ZnSe (Infrared Analysis Corp., USA). The(23 � 0.2) mg calcium carbonate powder was evenly pressed ontoa 250-mesh stainless steel circular grid to form a circular solidcoating, which was then mounted in the cylindrical quartz reactor.The CaCO3 coating on the grid has a thickness of 0.14 mm anda diameter of 2.5 cm (area of 4.9 cm2), which covers about 75% ofthe total grid area of 6.5 cm2. The reactant-containing synthetic airwas introduced into the reactor at a flow rate of 400 sccm. Asa result, the main gas flow would pass through the grid via theuncovered area around the solid coating, and a part of the gaswould diffuse through the samples via the interstitial spacebetween the particles. During the reactions, a Fourier transforminfrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific)

Fig. 1. Schematic diagram of experimental set-up. MFC, mass flow controller.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e72 65

equipped with a mercuryecadmium-telluride detector was used tomonitor the reaction progress. All spectra were collected in thetransmission mode in the frequency range of 4000e400 cm�1 andat a resolution of 4 cm�1, and 64 scans were averaged for eachspectrum corresponding to a time resolution of 39 s. All of the gaseparticle interactions were performed at 298 � 1 K.

2.2.2.1. Processing of mineral particles with HNO3 and SO2.Previous studies (Al-Hosney and Grassian, 2005; Li et al., 2006;Vlasenko et al., 2006; Santschi and Rossi, 2006; Prince et al., 2007;Liu et al., 2008a) have shown that HNO3 and SO2 can irreversiblyreact with CaCO3 to form surface-bound nitrate and sulfite species,respectively, and the presence of water can favor both reactions.Thus, to ensure relatively large reactivity of the particles towardHNO3 and SO2, the humidity of the airflowwas controlled at 45% RHfor HNO3eCaCO3 reaction and 80% RH for SO2eCaCO3 reaction. TheRH was measured at the outlet of the reactor using a commercialhygrometer (Vaisala HMT100) with the uncertainty of �1.7%.Before exposure to HNO3 or SO2, the particle samples were evac-uated at room temperature for at least 60 min to remove thephysisorbed impurities as much as possible using an Oil RotaryVane Vacuum Pump (GLD-N201, ULVAC NINGBO Co., Ltd.) withPumping speed of 240 L min�1. The pressure at evacuation wasabout 6 Pa. In order to evaluate the effects of the aging degree onthe reactivity of both mineral particles, different coverages ofsurface coatings were derived by varying the reaction time.

The surface coverage of coatings wasmeasuredwith themethodas follows: in the sulfite case, the processed particles were exposedto gaseous H2O2 on a much longer timescale than that for a typicalkinetic run to make sure of the complete conversion of sulfite intosulfate, which was also monitored using T-FTIR. The sulfate onparticle samples was then extracted with sonication using 10 mlMilli-Q water (Millipore, USA). In the nitrate case, as calcium nitrateproduct is quite soluble, the nitrate on processed particle sampleswas extracted without sonication using 10 ml Milli-Q water, whichenables to simultaneously extract the molecularly adsorbed H2O2.

To verify the reliability of the extraction of nitrate, control experi-ments, where nitrate was extracted by sonication, were also per-formed. The resulting solutionwas then filtered and analyzed usingan ion chromatography (IC) instrument (Dionex ICS2000, USA),which is equipped with a Dionex AS 11 analytical column anda conductivity detector. The nitrate concentration in unsonicatedsolutions is analyzed to agree very well with that in sonicatedsolutions (the discrepancy is less than 1%). Assuming surface nitrateand sulfate species are stable during the whole process of H2O2uptake, the amount of nitrate and sulfate determined using IC wasconsidered to be identical to that of nitrate and sulfite on processedparticles. The coverage of coatings was then derived by normalizingthe amount of surface ions with the BET surface area of the particlesamples.

In this study, the surface coverage of nitrate and sulfite onprocessed CaCO3 particles was determined to be 20e195 � 1018 molecules m�2 and (7.0 � 0.3) � 1018 molecules m�2.Using the density of a closely related crystalline solid, namelyCa(NO3)2 and CaSO4, we can derive the molecular amount ofa nitrate and sulfite monolayer, N* in molecules cm�2, via thefollowing equation:

N* ¼ ðNAr=MÞ23 (1)

where NA is Avogadro’s number, r and M is the density andmolecular weight of Ca(NO3)2 and CaSO4, respectively. Given thedensity of pure calcium nitrate (2.53 g cm�3) and calcium sulfate(2.61 g cm�3), we obtained 4.4�1014 and 5.1�1014molecules cm�2

for a nitrate and sulfite monolayer. Thus, surface nitrate and sulfiteformed on processed CaCO3 particles corresponds to 4.5e44monolayers and 1.4 monolayers, respectively.

2.2.2.2. H2O2 uptake experiments. After reaction with HNO3 or SO2,the calcium carbonate particles covered with multilayers of nitrateand sulfite were exposed to gaseous H2O2. T-FTIR spectra of theparticles were recorded upon H2O2 uptake. Each spectrum was

0.0

2.0

4.0

6.0

8.0

0 200 400 600 800 1000time (s)

{H

2O

2} ( ×

10

18

mo

lec

ule

s m

-2

)

75%RH 45%RH 3%RH

75%RH 45%RH 3%RH

Fig. 2. Wall loss and total loss (uptake by particles plus wall loss) of gas phase H2O2 inthe presence of nitrate-coated calcium carbonate particles as a function of relativehumidity. Surface coverage of nitrate, around 75 � 1018 molecules m�2; [H2O2],1.3 � 1014 molecules cm�3. Empty marker, wall loss; filled marker, total loss. All thedata are normalized by the BET surface area of particles. Error bars represent one-standard deviation of individual measurements.

Table 1The loss percentage of gas-phase H2O2 in the reactor at different relative humidity(RH).

Surface Coverage(monolayers)

3% RH 25% RH 45% RH 75% RH

RW e 5% 5% 8% 15%P-CaCO3 þ RW e 35% 15% 15% 20%N-CaCO3 þ RW 4.5 13% 17% 24% 33%

13e17 12% 20% 26% 37%31e35 e 24% 28% 47%44 10% e 29% e

S-CaCO3 þ RW 1.4 36% 37% 42% 55%

Note: RW, reactor wall; P-CaCO3, pristine CaCO3; N-CaCO3, nitrate-coated CaCO3; S-CaCO3, sulfite-coated CaCO3.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e7266

referenced to the spectrum of particles saturated with H2O2-freesimulated air. At the exit of the reactor, the loss of gas-phase H2O2in airflow was determined with the method described in Section2.2.3. The effect of water on H2O2 uptake was investigated by per-forming similar experiments over a wide range of RH (3e75%). Aseries of control experiments were also performed in the absence ofa sample on grid under conditions similar to H2O2 uptake experi-ments to probe the wall losses of H2O2.

2.2.3. H2O2 collection and detectionAfter exiting the reactor, the 400 sccm airflow, mixed with 2.25

standard liter per minute (slm) N2, was drawn into a thermostati-cally controlled scrubbing coil collector, which was maintained at277 � 0.2 K, by a vacuum pump. In order to make the airflow passthrough the reactor freely and to avoid the pressure higher than1 atm, the flow rate of the pump was controlled at 2.70 slm. Thepressure in the reactor was measured to be 0.95 atm. The 1 mMH3PO4 solution (prepared by 85% aqueous solution for HPLC,SigmaeAldrich), used as the stripping solution, was delivered intothe collector by a peristaltic pump at a rate of 0.2 ml min�1 tocollect the gas-phase H2O2. The resulting solution was thenimmediately analyzed with an HPLC instrument described below.The collection efficiency of the coil for H2O2 was determined to be�98% (Hua et al., 2008), and further details about this collectionsystem can be seen in our previous work (Hua et al., 2008).

H2O2 was determined using an HPLC instrument (Agilent 1200,USA) equipped with a fluorescent detector, with post-columnderivation involving the hemin-catalyzed oxidation of H2O2 toa fluorescent derivative by hydroxyphenylacetic acid. Furtherdetails about the method can be also found in our previous work(Hua et al., 2008).

3. Results and discussion

3.1. Kinetic analysis and data uncertainty evaluation

In the present study, the loss of H2O2 in the gas phase wasmeasured as H2O2-containing airflow exited the reactor. Asdescribed in the experimental section, two kinds of measurementswere carried out: one was in the presence of a sample on grid to gettotal loss of gas-phase H2O2 and one in the absence of a sample ongrid to obtain the wall loss of H2O2. Fig. 2 shows the H2O2 walllosses and an example of total losses of gas phase H2O2 in thepresence of nitrate-coated CaCO3 particles with surface coveragearound 75 �1018 molecules m�2 as a function of RH (3e75%). It canbe observed that at 3% RH, the wall loss in the quartz reactor wasabout 5% of gas-phase H2O2; at 45% RH the wall loss was about 8%;but at 75% RH, this loss was up to about 15%. However, in thepresence of nitrate-coated CaCO3 particles, the total losses of gas-phase H2O2 were determined to be 14% at 3% RH, 25% at 45% RH,and 37% at 75% RH, which is significantly larger than the corre-sponding wall losses. An evidently larger total loss of H2O2 in thepresence of a sample relative to that in the absence of a sample (i.e.,the H2O2 wall loss) was also observed under other conditions (seeTable 1). Therefore, we are able to accurately measure the amountof H2O2 taken up by the particles by subtracting the wall loss fromtotal loss of gas-phase H2O2. The wall losses (% value) listed inTable 1 are instantaneous values within the first 8e10 min. Notethat at a certain RH, the wall losses appear to keep constant withinthe first 8e10 min and then decrease gradually due to the satura-tion of the reactor wall (see Fig. 2). Therefore, the wall losses givenhere represent the upper limit.

For a heterogeneous process, the uptake reactivity of the parti-cles is usually characterized in terms of the uptake coefficient (g),which is defined as the rate of trace gases taken up by the particles

(R¼ d{C}/dt) divided by the rate of collisions of trace gases with theparticle surface (Z):

g ¼ dfCg=dtZ

(2)

Z ¼ 14As½C�

ffiffiffiffiffiffiffiffiffiffi8RTpMC

s(3)

where {C} is the uptake of trace gases by particle surfaces, Z iscollision frequency of trace gases with the particle surface, [C] is theinitial trace gas concentration, MC is the molecular weight of tracegases, and As is the available surface area of the particle samples.Apparently, accurate estimation of the surface area available for theuptake is indispensable to measure the true uptake coefficients.However, for a powdered sample, this is almost always the majorsource of the uncertainty in the uptake kinetics. Different estima-tions of available surface area can lead to a large gap (several ordersof magnitude) between the values of uptake coefficients for thesame reaction (Crowley et al., 2010). In this study, we performedthe uptake experiments on both pristine and processed particlesusing the same experimental approach, the same gas-phase H2O2concentration, and the same particle mass. If the uptake of H2O2 is

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e72 67

not dominated by its mass-transport in the gas phase, we canderive the relative uptake coefficient of H2O2 on processed particlesto that on pristine particles (i.e., gprocessed/gpristine) using equation(4) as an approximation:

gprocessedgpristine

¼ ðdfCg=dtÞprocessed=ZprocessedðdfCg=dtÞpristine=Zpristine

¼ RprocessedRpristine

$ZpristineZprocessed

(4)

When Zpristine is equal to Zprocessed, the relative uptake coeffi-cients (gprocessed/gpristine) can be derived by measuring uptake rate(R) of both cases. As a result, gprocessed/gpristine is independent of theavailable surface area of the particles.

For gas uptake by particles, a resistor model has been widelyused to express the measured uptake as a sum of terms due to gasphase diffusion (Gdiff) to the particle surface and subsequent surfacechemistry (gtrue) (Pöschl et al., 2007).

1gexp

¼ 1Gdiff

þ 1gtrue

(5)

Gdiff ¼ Knð1þ KnÞ0:75þ 0:28 Kn

(6)

Kn ¼ 6Dg

udp(7)

where Gdiff is normalized rate of gas phase diffusion, Kn is theKnudsen number, Dg is the gas phase diffusion coefficient of tracegas, dp is particle diameter, and u is themean velocity of trace gas. Ifthe uptake is fast, a pressure gradient for trace gas may be formednear the particle surface. In this case, the experimental measureduptake coefficient (gexp) may underestimate the true uptake coef-ficients (gtrue) due to gas phase diffusion effect. Given the gas-phasediffusion coefficient for H2O2 (0.153 cm2 s�1 in air at 296 K)(McMurtrie and Keyes, 1948; Ivanov et al., 2007) and the sizedistribution of CaCO3 particles of 0.3e20 mm (with a mean value of6.6 mm) as measured with a laser particle sizer (MasterSizer 2000,Malvern), Gdiff is calculated to be 0.014e1.27 (with a mean value of0.043). It was reported that the true uptake coefficient, gtrue, ofH2O2 on pristine mineral particles is on the order of 10�4e10�3

(Pradhan et al., 2010a,b), much smaller than Gdiff, thus the gasphase diffusion of H2O2 is not a limited factor for its uptake onpristine CaCO3 particles. The reactivity of nitrate or sulfite coatedCaCO3 toward H2O2 appears to increase by several factors relative tothe pristine particles (see Section 3.3 and 3.4). The largestenhancement of the reactivity is observed for sulfite-coated parti-cles at 75% RH, i.e., a factor of 10, giving gtrue on the order of 10�3e

10�2. Even in this case, the gas phase diffusion of H2O2 only hasa minor limitation on its uptake, and gexp only slightly underesti-mates gtrue (w17%). As the uptake of H2O2 on both pristine andprocessed CaCO3 is not dominated by mass-transport, the relativeuptake coefficients (gprocessed/gpristine) can be approximately esti-mated using equation (4).

In addition, to derive a relative uptake coefficient, the effectiveBET surface of the pristine and processed CaCO3 particles should beidentical within experimental uncertainty. In this study, the size ofpristine CaCO3 particles is in the range of 0.3e20 mm with a meanvalue of 6.6 mm. Using this size distribution and assuming a spher-ical shape for CaCO3 particles, the geometric surface area of theCaCO3 particles (AG) was calculated to be1.02 m2 g�1, slightly lowerthan the BET surface area of the particles (ABET ¼ 1.4 m2 g�1). As forHNO3 and SO2-processed CaCO3, about 4.5e44 monolayers ofnitrate and 1.4monolayers of sulfite are formed on processed CaCO3

particles, respectively. Given the thickness of a monolayer of nitrate(0.47 nm) and sulfite (0.44 nm) (d2 ¼ 1/N*), the thickness of nitrateand sulfite coating on processed CaCO3 is estimated to be 2�20 nmand 0.6 nm respectively. Calcium nitrate coating is known to bevery hygroscopic, and is expected to increase in thickness underhumid conditions. Based on the hygroscopic growth factor ofa calcium nitrate particle with dry diameter of 89 nm, that is, 1.3 at25% RH, 1.4 at 45% RH, and 1.6 at 75% RH (Gibson et al., 2006), thethickness of aqueous nitrate layer on processed CaCO3 is estimatedto be 2.6e26 nm at 25% RH, 2.8e28 nm at 45% RH, and 3.2e32 nmat 75% RH. Compared to calcium nitrate, calcium sulfate is relativelyhydrophobic, the thickness of the sulfate coating thus is not ex-pected to significantly increase under humid conditions. Consid-ering the micron size of the CaCO3 particle, the nitrate and sulfitecoatings on processed CaCO3 do not seem to have a significantimpact on the effective surface area of the particles.

Overall, the relative uptake coefficients of H2O2, gprocessed/gpris-

tine, can be derived from the relative uptake rate Rprocessed/Rpristine,within the uncertainties estimated above. The uptake rate (R) wasdetermined from the linear fit to the time-dependent uptake datawithin the first 8 min of exposure. At least three individualexperiments were averaged to get H2O2 uptake rate under eachexperimental condition. Errors are given as one-standard deviation(�1s) of individual measurements under the same experimentalconditions.

3.2. H2O2 uptake on pristine calcium carbonate particles

The uptake of H2O2 on pristine calcium carbonate particles wasdetermined as base cases for the subsequent measurements onprocessed particles. It is apparent from Fig. 3a that the measureduptake rate of H2O2 on pristine calcium carbonate particlesdramatically decreases with increasing RH, implying a competitionbetween H2O2 and water molecules for surface active sites. It isreported that the pristine surface of calcium carbonate is generallycovered by a layer of hydroxyl and bicarbonate groups (Stipp et al.,1994), which are most likely the reactive sites for H2O2 uptake onthe particles. However, under higher humidity a thin water filmgrows on this reactive layer and thus inactivates the particlesurface. Similar negative correlation between H2O2 uptake and RHwas also observed for pristine TiO2 (Pradhan et al., 2010a) and a-Al2O3 (Zhao et al., 2011) particles where the uptake of H2O2 is alsodetermined by the quantity of available surface active sites, andthus the occupation of surface sites by adsorbedwater acts to retardH2O2 uptake.

In addition to the uptake rate, the contributions of the chemicaldecomposition as well as molecular adsorption to the uptake ofH2O2 on pristine calcium carbonate particles were also measured.As shown in Fig. 3b, H2O2 molecules impinging on the pristinesurface of calcium carbonate can decompose dramatically,accounting for about 85e90% of the total H2O2 uptake. Thisdecomposition is more pronounced than that on pristine a-Al2O3particles where the decomposed H2O2 contributes to 70e80% of thetotal uptake, probably due to the higher alkalinity of the carbonatesurface relative to alumina surface as alkali conditions appear tofavor H2O2 decomposition (Do et al., 2009).

3.3. H2O2 uptake on HNO3-processed calcium carbonate particles

Fig. 4 shows H2O2 uptake on HNO3-processed calcium carbonateparticles as a function of RH (3e75%) and surface nitrate coverage(20e195 � 1018 molecules m�2, which corresponds to 4.5e44 monolayers). The uptake of H2O2 on pristine calciumcarbonate at varying RHwas also given for comparison. It is evidentthat H2O2 uptake on calcium carbonate coated with multilayers

0.0

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0 40 80 120 160 200

SCN ( × 1018

molecules m-2

)

R (

× 1

015

mo

lec

ule

s m

-2

s-1

)

3% RH

25% RH

45% RH

75% RH

0.0

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0 40 80 120 160 200

SCN ( × 1018

molecules m-2

)

pro

ce

ss

ed/

pris

tin

e

3% RH

25% RH

45% RH

75% RH

a

b

Fig. 4. H2O2 uptake rates (a) and relative uptake coefficients (b) on nitric acid-processed calcium carbonate particles as a function of surface nitrate coverage (SCN)and relative humidity (RH). The uptake of H2O2 on pristine calcium carbonate surfaceswas also given for comparison. R, uptake rate; gprocessed/gpristine, relative uptake coef-ficient on processed particles versus pristine particles. Error bars represent one-standard deviation.

0.0

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4.0

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10.0

0 20 40 60 80relative humidity (%)

R (

× 1

015

mo

lec

ule

sm

-2 s

-1

)

0.0

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6.0

8.0

3 25 45 75

relative humidity (%)

{H

2O

2} ( ×

10

18

mo

lec

ule

s m

-2

)

molecularly adsorbed

decomposed

a

b

Fig. 3. H2O2 uptake on pristine calcium carbonate particles as a function of relativehumidity. a, uptake rates of H2O2, error bars represent one-standard deviation ofindividual measurements. b, the amount of the molecularly adsorbed and decomposedH2O2 after 15 min of exposure.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e7268

nitrate is distinctly different from that on pristine calciumcarbonate particles and strongly dependent on RH and surfacenitrate coverage. At 3% RH, the presence of nitrate on calciumcarbonate particles leads to a 30e85% decrease in uptake reactivityas its surface coverage increases from 20 � 1018 to195 � 1018 molecules m�2, whereas at higher RH, surface nitratecoatings considerably promote H2O2 uptake, yielding an enhance-ment in H2O2 uptake about 20e60% at 25% RH, a factor of 1e3 at45% RH, and a factor of 3e8 at 75% RH relative to pristine particles.It was reported that calcium nitrate coating on processed calciumcarbonate particles undergoes deliquescence at w10% RH (Kruegeret al., 2004; Liu et al., 2008b). Thus, at RH below 10% (e.g., 3%), thepresence of solid nitrate on calcium carbonate occupies the surfaceactive sites (i.e., hydroxyl and bicarbonate groups) and greatlydepresses the reactivity of particles toward H2O2, resulting in thedecrease of H2O2 uptake. However, at higher RH (e.g., 25%, 45%,75%), compared to pristine calcium carbonate surface, which iscovered by a thin water film under humid conditions, an aqueouslayer forms on processed calcium carbonate as a result of the

deliquescence of the nitrate coating. As surface coverage of nitrateincreases, this aqueous layer grows in thickness. Given the negli-gible solubility limitation of H2O2 in concentrated nitrate solution(for example, the effective Henry’s law constant of H2O2 in sodiumnitrate solution with concentrations of 6 M is up to1.1 � 105 M atm�1, slightly lower than 1.3 � 105 M atm�1 in purewater, at 292 K (Chung et al., 2005)), the growing thickness ofaqueous nitrate layer acts to drive the uptake of the soluble H2O2.Additionally, we can also see from Fig. 4 that H2O2 uptake onnitrate-coated particles appears to increase with increasing RH,opposite to what occurs on pristine calcium carbonate particles.This also reflects that H2O2 uptake on nitrate-coated calciumcarbonate particles is controlled by the content of surface liquid-like water, whereas on pristine particles H2O2 uptake is driven byavailable surface reactive sites.

After H2O2 uptake, the molecularly adsorbed H2O2 by processedparticles was extracted using 10 ml cold ultrapure water (at 277 K),followed by analysis with HPLC. As listed in Table 2, the amount of

Fig. 5. T-FTIR spectra of the oxidation of sulfite into sulfate on SO2-processed calciumcarbonate particles exposed to gaseous H2O2. a, 3% RH; b, 45% RH. The spectra areoffset for clarity.

Table 2The amount of molecularly adsorbed H2O2 and its proportion to the total uptake ofH2O2 on the surface of nitrate-coated calcium carbonate particles after 15 min ofexposure at different relative humidity (RH) and surface thickness of nitrate.

Surface nitrate thickness (monolayers)

0 4.5 13e17 31e35

[H2O2]ads 25% RH 1.9 � 0.4 2.0 � 0.3 4.5 � 1.0 7.6 � 1.245% RH 2.1 � 0.6 2.7 � 0.7 7.8 � 0.7 16.2 � 1.575% RH 2.2 � 0.3 5.3 � 1.0 17.2 � 1.7 26.1 � 3.2

f ads 25% RH 0.12 � 0.03 0.12 � 0.02 0.23 � 0.05 0.35 � 0.0545% RH 0.15 � 0.06 0.17 � 0.04 0.32 � 0.03 0.39 � 0.0475% RH 0.17 � 0.03 0.26 � 0.05 0.43 � 0.02 0.45 � 0.06

Note: [H2O2]ads represents the amount of molecularly adsorbed H2O2 on the surfaceof particles (�1017molecules m�2); f ads represents the proportion of [H2O2]ads to thetotal uptake of H2O2 on the surface of particles.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e72 69

molecularly adsorbed H2O2 on processed calcium carbonateparticles largely increases with RH and surface coverage of nitrate.This is consistent with growing aqueous layer on particle surfacethat promote H2O2 uptake. However, the molecularly adsorbedH2O2 on particles accounts for less than 50% of the total H2O2uptake, suggesting that H2O2 molecules, after adsorption intoaqueous layer, can also undergo significant decomposition. Inorder to evaluate the role of aqueous calcium nitrate layer in thedecomposition of H2O2, we have performed an experiment inwhich a mixed solution of 1 M of calcium nitrate and 0.1 M H2O2 atroom temperature was prepared (the concentrations of H2O2 andcalcium nitrate are similar to those estimated on nitrate-coatedCaCO3 used in our study), and the solution was measured every15 min. The result shows that the concentration of H2O2 in thesolution has no obvious change within 60 min, implying thestability of H2O2 in this solution. Therefore, we can speculate thatthe decomposition of H2O2 would not occur within the bulksolution of the aqueous calcium carbonate layer. A probablescenario is that H2O2 molecules can diffuse through the aqueouslayer and react at the liquidesolid CaCO3 interface wherea hydration layer as represented by Ca(OH)(HCO3) exists (Stippet al., 1994). Alternatively, the aqueous nitrate solution oncalcium carbonate particles may not be distributed uniformly (Al-Hosney and Grassian, 2005), leaving bare surface that acts to reactwith H2O2. Given that increasing coverage of water seems tostabilize H2O2 ( _Zegli�nski et al., 2006; Zhao et al., 2011), and thatbare surface would decrease with increasing of aqueous layers, thedecomposition of H2O2 on processed calcium carbonate particles isexpected to be less efficient at higher RH and surface coverage ofnitrate (see Table 2).

3.4. H2O2 uptake on SO2-processed calcium carbonate particles

It has been shown that the reaction of SO2 with calciumcarbonate particles produces surface-coordinated sulfite and/orbisulfite species (Al-Hosney and Grassian, 2005; Li et al., 2006;Santschi and Rossi, 2006; Prince et al., 2007). As an efficient oxidantfor sulfate formation in cloud water and on ice surfaces (Finlayson-Pitts and Pitts, 2000; Clegg and Abbatt, 2001), H2O2 may also havethe potential to oxidize S(IV) species to S(VI) on the surface ofmineral particles, via the reactions:

SO2�3ðadsÞþ H2O2ðadsÞ/SO2�

4ðadsÞþ H2O (R1)

HSO�3ðadsÞþ H2O2ðadsÞ/HSO�

4ðadsÞþ H2O (R2)

The SO2-processed calcium carbonate particles are thereforeexpected to have distinctly different reactivity toward H2O2compared to the HNO3-processed particles. In this study, a fixed

coverage of sulfite, i.e., (7.0 � 0.3) � 1018 molecules m�2 (corre-sponding to 1.4 monolayers), was derived for SO2-processedcalcium carbonate particles.

From T-FTIR spectra (Fig. 5), we found that sulfite on processedcalcium carbonate particles are apparently converted into sulfateupon exposure to gaseous H2O2, and the adsorbed water canpromote this conversion. In order to further evaluate the impact ofSO2-processing on the reactivity of calcium carbonate particles, theuptake of H2O2 on SO2-processed particles were also measured asa function of RH. From Fig. 6, we can see that at 3% RH, there is noclear difference in H2O2 uptake on SO2-processed and pristineparticles. This suggests that although pre-reaction with SO2consumes surface bicarbonate and hydroxyl groups, the sulfiteprovides new active sites that can react with H2O2. Notably, athigher RH (i.e., 25%, 45%, and 75%), the presence of sulfite on pro-cessed calcium carbonate enhances H2O2 uptake by a factor of 3e10relative to that on the pristine particles, revealing that surfacesulfite is more reactive toward H2O2 than carbonate active sites inparticular at high RH.

Our results suggest that the presence of water appears topromote the oxidation of sulfite to sulfate, leading to increase inH2O2 uptake of on SO2-processed particles. The reason is probably

SO2-processed

HNO3-processed

14.6

15.0

15.4

15.8

16.2

13.2 13.4 13.6 13.8 14.0 14.2

log ([H2O2]/molecules cm-3

)

log

( R

/ m

ole

cu

les

m-2

s-1

)

Fig. 7. Double-logarithmic plot of H2O2 uptake rate on processed calcium carbonateparticles versus initial gas-phase H2O2 concentration. Surface coverage is about150 � 1018 molecules m�2 for nitrate and 7.0 � 1018 molecules m�2 for sulfite. All theexperiments were performed at 45% RH. Error bars represent one-standard deviation.

0.0

4.0

8.0

12.0

16.0

0 20 40 60 80relative humidity (%)

R (

× 1

015

mo

lec

ule

sm

-2 s

-1

)

processed

pristine

0

2

4

6

8

10

0 20 40 60 80relative humidity (%)

pro

ce

ss

ed/

pri

stin

e

a

b

Fig. 6. H2O2 uptake rates (a) and relative uptake coefficients (b) on SO2-processedcalcium carbonate particles as a function of relative humidity. The uptake of H2O2 onpristine calcium carbonate was also displayed for comparison. R, uptake rate; gprocessed/gpristine, relative uptake coefficient on processed particles versus pristine particles.Error bars represent one-standard deviation.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e7270

that adsorbed water under humid conditions may favor theconversion of surface sulfite to bisulfite via the hydrolysis, and thebisulfite ion, which is more acidic than the sulfite ion, is expected tobe more reactive toward H2O2.

3.5. Effect of varying H2O2 concentration on the uptake

The effect of varying gas-phase H2O2 concentration on theuptake should be considered. Here, we investigated the uptake ofH2O2 on processed calcium carbonate particles over the H2O2

concentration range of (1.9e12.6) � 1013 molecules cm�3 (0.8e5.1 ppmv) at 45% RH. As shown in Fig. 7, the double-logarithmicplot of the uptake rates of H2O2 on HNO3 and SO2-processedcalcium carbonate versus gas-phase H2O2 concentrations givesa slop of 0.96 � 0.07 and 0.98 � 0.05, respectively, reflecting a first-order reaction of H2O2 with processed particles. These first-orderreaction rates suggest that the uptake coefficient of H2O2 is inde-pendent of its gas-phase concentrations. Therefore, we suggest thatthe uptake coefficient ratios (gprocessed/gpristine) determined in this

study may be applicable to a lower H2O2 concentration that is morerelevant to the atmosphere.

4. Conclusions and atmospheric implications

In this study, we investigated heterogeneous reactions of H2O2on HNO3 and SO2-processed calcium carbonate particles, as proxiesof ambient mineral dust aged by acidic trace gases, as a function ofRH and surface coverage of coatings. Our results suggest that thereactivity of mineral dust toward H2O2 can be strongly modifieddue to atmospheric aging. For example, in the typical RH range(20e80%) of the troposphere, calcium carbonate particlesaccumulates a coating of sulfite at coverage of 1.4 monolayerswill exhibit an enhanced H2O2 uptake by a factor of 3e10 relativeto the pristine particles. Alternatively, when calcium carbonateparticles are coated with multilayers nitrate, which was frequentlyobserved in the atmosphere especially for Ca-containing dustparticles, the uptake of H2O2 will decrease by 30e85% at 3% RHwhereas increase by 20e60% at 25% RH, a factor of 1e3 at 45% RH,and a factor of 3e8 at 75% RH relative to pristine particles, assurface coverage of nitrate increases from 4.5 to 44 monolayers.Therefore, the role of mineral dust as a sink for atmospheric H2O2 isintimately related to the chemical composition and coverage ofsurface coating as well as ambient RH, and hence will varyconsiderably in different polluted air masses.

In addition, it is found that there exists a considerable amount ofmolecularly adsorbed H2O2 on nitrate-coated mineral particlesunder humid conditions. Given the important role that H2O2 playsin sulfate formation in atmospheric aqueous phase, efficientoxidation of sulfite species by the molecularly adsorbed H2O2 onnitrate-coated particles can be expected. This may be able to partlyexplain the correlation between nitrate and sulfate in ambient agedmineral particles (Matsuki et al., 2005). It has been reported that O3could promote the oxidation of S(IV) species to S(VI) on mineraldust particles (Ullerstam et al., 2002; Li et al., 2006). Our resultssuggest that sulfite formed on mineral particles can also be easilyoxidized to sulfate by H2O2. It becomes clear that in cloud dropletsoxidation of S(IV) by H2O2 can be more significant than that by O3(Finlayson-Pitts and Pitts, 2000), whereas the relative importanceof H2O2 and O3 in the oxidation of SO2 on mineral dust needs to befurther evaluated.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e72 71

We note that the HNO3 and SO2-processed calcium carbonateparticles are simple models for ambient aged mineral dust, whichhas more complicated physicochemical properties (e.g., composi-tion and mixing state). However, our results presented here dosuggest that mineral dust aged in the atmosphere has distinctreactivity relative to the freshly emitted mineral particles andbehaves differently with respect to the uptake of trace gases. Tobetter understand the role of mineral dust in atmospheric chem-istry, the effects of atmospheric aging should be considered in theatmospheric models.

Acknowledgments

The authors gratefully thank the National Natural ScienceFoundation of China (grants 40875072 and 21077003) and the StateKey Laboratory of Environment Simulation and Pollution Control(special fund) for financial support.

References

Al-Hosney, H.A., Grassian, V.H., 2005. Water, sulfur dioxide and nitric acidadsorption on calcium carbonate: a transmission and ATR-FTIR study. PhysicalChemistry Chemical Physics 7, 1266e1276.

Bauer, S.E., Balkanski, Y., Schulz, M., Hauglustaine, D.A., Dentener, F., 2004. Globalmodeling of heterogeneous chemistry on mineral aerosol surfaces: influence ontropospheric ozone chemistry and comparison to observations. Journal ofGeophysical Research 109, D02304. http://dx.doi.org/10.1029/2003jd003868.

Buseck, P.R., Posfai, M., 1999. Airborne minerals and related aerosol particles: effectson climate and the environment. Proceedings of the National Academy ofSciences of the United States of America 96, 3372e3379.

Chung, M.Y., Muthana, S., Paluyo, R.N., Hasson, A.S., 2005. Measurements of effec-tive Henry’s law constants for hydrogen peroxide in concentrated salt solutions.Atmospheric Environment 39, 2981e2989.

Clegg, S.M., Abbatt, J.P.D., 2001. Oxidation of SO2 by H2O2 on ice surfaces at 228 K:a sink for SO2 in ice clouds. Atmospheric Chemistry and Physics 1, 73e78.

Crowley, J.N., Ammann, M., Cox, R.A., Hynes, R.G., Jenkin, M.E., Mellouki, A.,Rossi, M.J., Troe, J., Wallington, T.J., 2010. Evaluated kinetic and photochemicaldata for atmospheric chemistry: volume V e heterogeneous reactions on solidsubstrates. Atmospheric Chemistry and Physics 10, 9059e9223.

De Reus, M., Fischer, H., Sander, R., Gros, V., Kormann, R., Salisbury, G.,Dingenen, R.V., Williams, J., Zollner, M., Lelieveld, J., 2005. Observations andmodel calculations of trace gas scavenging in a dense Saharan dust plumeduring MINATROC. Atmospheric Chemistry and Physics 5, 1787e1803.

Dentener, F.J., Carmichael, G.R., Zhang, Y., Leliveld, J., Crutzen, P.J., 1996. Role ofmineral aerosol as a reactive surface in the global troposphere. Journal ofGeophysical Research 101, 22869e22889.

Díaz-Hernández, J.L., Martín-Ramos, J.D., López-Galindo, A., 2011. Quantitativeanalysis of mineral phases in atmospheric dust deposited in the south-easternIberian Peninsula. Atmospheric Environment 45, 3015e3024.

Do, S.H., Batchelor, B., Lee, H.K., Kong, S.H., 2009. Hydrogen peroxide decompositionon manganese oxide (pyrolusite): kinetics, intermediates, and mechanism.Chemosphere 75, 8e12.

Falkovich, A.H., Schkolnik, G., Ganor, E., Rudich, Y., 2004. Adsorption of organiccompounds pertinent to urban environments onto mineral dust particles.Journal of Geophysical Research 109, D02208. http://dx.doi.org/10.1029/2003JD003919.

Finlayson-Pitts, B.J., Pitts, J.N., 2000. Chemistry of the Upper and Lower Atmosphere.Academic Press, New York, pp. 306e315.

Formenti, P., Schutz, L., Balkanski, Y., Desboeufs, K., Ebert, M., Kandler, K., Petzold, A.,Scheuvens, D., Weinbruch, S., Zhang, D., 2011. Recent progress in understandingphysical and chemical properties of African and Asian mineral dust. Atmo-spheric Chemistry and Physics 11, 8231e8256.

Gibson, E.R., Hudson, P.K., Grassian, V.H., 2006. Physicochemical properties ofnitrate aerosols: implications for the atmosphere. Journal of Physical ChemistryA 110, 11785e11799.

He, S.Z., Chen, Z.M., Zhang, X., Zhao, Y., Huang, D.M., Zhao, J.N., Zhu, T., Hu, M.,Zeng, L.M., 2010. Measurement of atmospheric hydrogen peroxide and organicperoxides in Beijing before and during the 2008 Olympic Games: chemical andphysical factors influencing their concentrations. Journal of GeophysicalResearch 115, D17307. http://dx.doi.org/10.1029/2009JD013544.

Hua, W., Chen, Z.M., Jie, C.Y., Kondo, Y., Hofzumahaus, A., Takegawa, N., Chang, C.C.,Lu, K.D., Miyazaki, Y., Kita, K., Wang, H.L., Zhang, Y.H., Hu, M., 2008. Atmospherichydrogen peroxide and organic hydroperoxides during PRIDE-PRD006, China:their concentration, formation mechanism and contribution to secondaryaerosols. Atmospheric Chemistry and Physics 8, 6755e6773.

Ivanov, A.V., Trakhtenberg, S., Bertram, A.K., Gershenzon, Y.M., Molina, M.J., 2007.OH, HO2, and ozone gaseous diffusion coefficients. Journal of Physical Chem-istry A 111, 1632e1637.

Kandler, K., Schütz, L., Deutscher, C., Ebert, M., Hofmann, H., Jäckel, S., Jaenicke, R.,Knippertz, P., Lieke, K., Massling, A., Petzold, A., Schladitz, A., Weinzierl, B.,Wiedensohler, A., Zorn, S., Weinbruch, S., 2009. Size distribution, massconcentration, chemical and mineralogical composition and derived opticalparameters of the boundary layer aerosol at Tinfou, Morocco, during SAMUM2006. Tellus B 61, 32e50.

Kolb, C.E., Cox, R.A., Abbatt, J.P.D., Ammann, M., Davis, E.J., Donaldson, D.J.,Garrett, B.C., George, C., Griffiths, P.T., Hanson, D.R., Kulmala, M., McFiggans, G.,Posch, U., Riipinen, I., Rossi, M.J., Rudich, Y., Wagner, P.E., Winkler, P.M.,Worsnop, D.R., O’ Dowd, C.D., 2010. An overview of current issues in the uptakeof atmospheric trace gases by aerosols and clouds. Atmospheric Chemistry andPhysics 10, 10561e10605.

Krueger, B.J., Grassian, V.H., Cowin, J.P., Laskin, A., 2004. Heterogeneous chem-istry of individual mineral dust particles from different dust source regions:the importance of particle mineralogy. Atmospheric Environment 38, 6253e6261.

Li, W.J., Shao, L.Y., 2009. Observation of nitrate coatings on atmospheric mineraldust particles. Atmospheric Chemistry and Physics 9, 1863e1871.

Li, L., Chen, Z.M., Zhang, Y.H., Zhu, T., Li, J.L., Ding, J., 2006. Kinetics and mechanismof heterogeneous oxidation of sulfur dioxide by ozone on surface of calciumcarbonate. Atmospheric Chemistry and Physics 6, 2453e2464.

Liu, Y., Gibson, E.R., Cain, J.P., Wang, H., Grassian, V.H., Laskin, A., 2008a. Kinetics ofheterogeneous reaction of CaCO3 particles with gaseous HNO3 over a widerange of humidity. Journal of Physical Chemistry A 112, 1561e1571.

Liu, Y.J., Zhu, T., Zhao, D.F., Zhang, Z.F., 2008b. Investigation of the hygroscopicproperties of Ca(NO3)2 and internally mixed Ca(NO3)2/CaCO3 particles bymicro-Raman spectrometry. Atmospheric Chemistry and Physics 8, 7205e7215.

Matsuki, A., Iwasaka, Y., Shi, G., Zhang, D., Trochkine, D., Yamada, M., Kim, Y.-S.,Chen, B., Nagatani, T., MiyazawaL, T., Nagatani, M., Nakata, H., 2005. Morpho-logical and chemical modification of mineral dust: observational insight intothe heterogeneous uptake of acidic gases. Geophysical Research Letters 32,L22806. http://dx.doi.org/10.1029/2005GL024176.

McMurtrie, R.L., Keyes, F.G., 1948. A measurement of the diffusion coefficient ofhydrogen peroxide vapor into air. Journal of American Chemical Society 70,3755e3758.

Pöschl, U., Rudich, Y., Ammann, M., 2007. Kinetic model framework for aerosol andcloud surface chemistry and gas-particle interactions: part 1 e general equa-tions, parameters, and terminology. Atmospheric Chemistry and Physics 7,5989e6023.

Pradhan, M., Kalberer, M., Griffiths, P.T., Braban, C.F., Pope, F.D., Cox, R.A.,Lambert, R.M., 2010a. Uptake of gaseous hydrogen peroxide by submicrometertitanium dioxide aerosol as a function of relative humidity. EnvironmentalScience and Technology 44, 1360e1365.

Pradhan, M., Kyriakou, G., Archibald, A.T., Papageorgiou, A.C., Kalberer, M.,Lambert, R.M., 2010b. Heterogeneous uptake of gaseous hydrogen peroxide byGobi and Saharan dust aerosols: a potential missing sink for H2O2 in thetroposphere. Atmospheric Chemistry and Physics 10, 7127e7136.

Prince, P.A., Kleiber, P., Grassian, V.H., Young, M.A., 2007. Heterogeneous interac-tions of calcite aerosol with sulfur dioxide and sulfur dioxideenitric acidmixtures. Physical Chemistry Chemical Physics 9, 3432e3439.

Raff, J.D., Szanyi, J., Finlayson-Pitts, B.J., 2011. Thermal and photochemical oxidationof self-assembled monolayers on alumina particles exposed to nitrogen dioxide.Physical Chemistry Chemical Physics 13, 604e611.

Reeves, C.E., Penkett, S.A., 2003. Measurements of peroxides and what they tell us.Chemical Reviews 103, 5199e5218.

Santschi, C., Rossi, M.J., 2006. The uptake of CO2, SO2, HNO3 and HCl on CaCO3 at300K: mechanism and the role of adsorbed water. Journal of Physical ChemistryA 110, 6789e6802.

Schuttlefield, J., Rubasinghege, G., El-Maazawi, M., Bone, J., Grassian, V.H., 2008.Photochemistry of adsorbed nitrate. Journal of American Chemical Society 130,12210e12211.

Seinfeld, J.H., Pandis, S.N., 2006. Atmospheric Chemistry and Physics: From AirPollution to Climate Change, second ed. Wiley, New York, pp. 1054e1091.

Sokolik, I.N., Toon, O.B., 1996. Direct radiative forcing by anthropogenic airbornemineral aerosols. Nature 381, 681e683.

Stipp, S.L.S., Eggleston, C.M., Nielsen, B.S., 1994. Calcite surface structure observed atmicrophotographic and molecular scales with atomic force microscopy (AFM).Geochimica et Cosmochimica Acta 58, 3023e3033.

Sullivan, R.C., Guazzotti, S.A., Sodeman, D.A., Prather, K.A., 2007. Direct observationsof the atmospheric processing of Asian mineral dust. Atmospheric Chemistryand Physics 7, 1213e1236.

Ullerstam, M., Vogt, R., Langer, S., Ljungstrom, E., 2002. The kinetics and mechanismof SO2 oxidation by O3 on mineral dust. Physical Chemistry Chemical Physics 4,4694e4699.

Usher, C.R., Michel, A.E., Grassian, V.H., 2003a. Reactions on mineral dust. ChemicalReviews 103, 4883e4939.

Usher, C.R., Michel, A.E., Steca, D., Grassian, V.H., 2003b. Laboratory studies ofozone uptake on processed mineral dust. Atmospheric Environment 37,5337e5347.

Vlasenko, A., Sjogren, S., Weingartner, E., Stemmler, K., Gäggeler, H.W., Ammann, M.,2006. Effect of humidity on nitric acid uptake to mineral dust aerosol Particles.Atmospheric Chemistry and Physics 6, 2147e2160.

Wang, W.G., Ge, M.F., Sun, Q., 2011. Heterogeneous uptake of hydrogen peroxide onmineral oxides. Chinese Journal of Chemical Physics 24, 515e520.

Y. Zhao et al. / Atmospheric Environment 67 (2013) 63e7272

Zhao, Y., Chen, Z.M., Shen, X.L., Zhang, X., 2011. Kinetics and mechanisms ofheterogeneous reaction of gaseous hydrogen peroxide on mineral oxide parti-cles. Environmental Science and Technology 45, 3317e3324.

Zhu, S., Butler, T., Sander, R., Ma, J., Lawrence, M.G., 2010. Impact of dust ontropospheric chemistry over polluted regions: a case study of the Beijingmegacity. Atmospheric Chemistry and Physics 10, 3855e3873.

Zhuang, H., Chan, C.K., Fang, M., Wexler, A.S., 1999. Formation of nitrate and non-sea-salt sulfate on coarse particles. Atmospheric Environment 33, 4223e4233.

_Zegli�nski, J., Piotrowski, G.P., Pieko�s, R., 2006. A study of interaction betweenhydrogen peroxide and silica gel by FTIR spectroscopy and quantum chemistry.Journal of Molecular Structure 794, 83e91.