Heterogeneous Reaction of NO2 on Caco3 Particles T

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This discussion paper is/has been under review for the journal Atmospheric Chemistry Heterogeneous and Physics (ACP). Please refer to the corresponding ﬁnal paper in ACP if available. reaction of NO2 on CaCO3 particles T. Zhu et al.

Kinetics and mechanisms of Title Page heterogeneous reaction of NO on CaCO Abstract Introduction 2 3 Conclusions References surfaces under dry and wet conditions Tables Figures

H. J. Li, T. Zhu, D. F. Zhao, Z. F. Zhang, and Z. M. Chen J I State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China J I

Received: 7 January 2009 – Accepted: 30 January 2009 – Published: 17 March 2009 Back Close Correspondence to: T. Zhu ([email protected]) Full Screen / Esc Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version

Interactive Discussion

7115 Abstract ACPD Calcium nitrate (Ca(NO3)2) was observed in mineral dust and could change the hygroscopic and optical properties of mineral dust significantly due to its strong water solu- 9, 7115–7154, 2009 bility. The reaction of calcium carbonate (CaCO3) with nitric acid (HNO3) is believed 5 the main reason for the observed Ca(NO3)2 in the mineral dust. In the atmosphere, Heterogeneous the concentration of nitrogen dioxide (NO ) is orders of magnitude higher than that of 2 reaction of NO2 on HNO ; however, little is known about the reaction of NO with CaCO . In this study, 3 2 3 CaCO3 particles the heterogeneous reaction of NO2 on the surface of CaCO3 particles was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined T. Zhu et al. 10 with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) under wet and dry conditions. Nitrate formation was observed in both conditions, and Title Page nitrite was observed under wet conditions, indicating the reaction of NO2 on the CaCO3 surface produced nitrate and probably nitrous acid (HONO). Relative humidity (RH) in- Abstract Introduction fluenced both the initial uptake coefficient and the reaction mechanism. With RH<52%, 15 surface –OH was formed through dissociation of the surface adsorbed water via oxy- Conclusions References gen vacancy, thus determining the reaction order. With RH>52%, a monolayer of water Tables Figures formed on the surface of the CaCO3 particles, which reacted with NO2 as a first order reaction, forming HNO3 and HONO. The initial uptake coefficient γ0 was determined to be (1.66±0.38)×10−7 under dry conditions and up to (0.84±0.44)×10−6 under wet J I 20 conditions. Considering that NO2 concentrations in the atmosphere are orders of mag- J I nitude higher than those of HNO3, the reaction of NO2 on CaCO3 particle should have Back Close similar importance as that of HNO3 in the atmosphere and could also be an important source of HONO in the atmosphere. Full Screen / Esc

1 Introduction Printer-friendly Version

Interactive Discussion 25 It has been estimated that 1000–3000 Tg of mineral aerosols are emitted into the atmosphere annually (Jonas et al., 1995). The main components of mineral dust include 7116 quartz, feldspar, carbonate (e.g. calcite, dolomite) and clay. Each of these components provides surfaces and reactants for heterogeneous reactions in the atmosphere. Many ACPD gas phase species in the atmosphere could also condense or adsorb onto mineral 9, 7115–7154, 2009 dust during long-range transport to impact atmospheric chemistry and climate change 5 (Chen, 1985; Quan, 1993; Carmichael et al., 1996; Zhang et al., 2000). Modeling studies suggested that approximately 40% of nitrate formation is associated with mineral Heterogeneous

aerosols (Dentener et al., 1996). Aerosol samples taken in East Asia showed a good reaction of NO2 on correlation between nitrate and calcium (Zhuang et al., 1999; Song and Carmichael, CaCO3 particles 2001; Sullivan et al., 2007). T. Zhu et al. 10 In fresh Asian mineral aerosols, calcium is found primarily in the form of calcium carbonate (CaCO3) (Song et al., 2005; Okada et al., 2005). The observed associa- tion of nitrate with calcium in mineral dust samples after long range transport suggests Title Page that after exposure to nitrogen oxides in polluted region, calcium carbonate in mineral dust had been converted to calcium nitrate, which in turn can change the composition Abstract Introduction 15 and morphology of calcium-containing aerosols and enhance their hygroscopicity and the rates of heterogeneous reactions, thus inﬂuence the atmospheric chemistry. Cal- Conclusions References

cium nitrate can also alter the optical properties of aerosols to impact radiation, cloud Tables Figures formation, and global climate change. Laboratory studies have demonstrated that nitric acid (HNO3) can react with CaCO3 J I 20 to form calcium nitrate (Fenter et al., 1995; Goodman et al., 2000; Hanisch and Crow- ley, 2001; Krueger et al., 2003a, b). The measured uptake coefficient accounting for J I −4 the BET area of the samples was determined to be (2.5±1)×10 for HNO3 on CaCO3 Back Close under dry condition (Goodman et al., 2000). Johnson et al. (2005) determined the initial −3 uptake coefficient of HNO3 on CaCO3 to be (2±0.4)×10 . The net reaction probability Full Screen / Esc 25 of HNO3 on CaCO3 particles was found to increase with increasing relative humidity, from 0.003 at RH=10% to 0.21 at 80% (Liu et al., 2008a). Vlasenko et al. (2006) re- Printer-friendly Version ported an uptake coefficient of HNO3 on CaCO3 to be 0.11 at 33% relative humidity and a humidity dependence on Arizona Test Dust. Interactive Discussion Compared to nitric acid, little is known about the reaction of NO2 on CaCO3. The 7117 uptake coefficients of NO2 on mineral oxide particles, such as Al2O3, Fe2O3, TiO2, have been determined to be on the order of 10−7 (Underwood et al., 1999); this is ACPD consistent with the results of the reaction of NO on mineral dust samples from the 2 9, 7115–7154, 2009 Cape Verde Islands (Ullerstam et al., 2003). 5 No literature report about the uptake coefficient of NO2 on CaCO3 has been found. Considering that the concentration of NO2 in the atmosphere is at least an order of Heterogeneous magnitude higher than that of HNO3, the reaction of CaCO3 particles with NO2 may reaction of NO2 on comprise an important atmospheric reaction that lead to the formation of calcium nitrate CaCO3 particles during the long range transport of mineral dust. This reaction could be even more T. Zhu et al. 10 important in East Asia, where industrial regions with high NO2 emissions are located in the path of long range transport of Asian Dust. To understand the importance of the heterogeneous reaction of NO on mineral dust, 2 Title Page it is important to measure the uptake coefficient of NO2 on CaCO3 particles. Similar to the reaction of HNO3 on CaCO3, the reaction of NO2 on CaCO3 may also change Abstract Introduction 15 under the influence of surface water. Therefore, it is interesting to study the reaction Conclusions References mechanisms and to measure the uptake coefficient of the reaction of NO2 on CaCO3 particles under dry and wet conditions. Tables Figures In this study, the reactions of NO2 on CaCO3 surfaces were investigated in situ using a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in combination J I 20 with X-ray photoelectron spectroscopy (XPS), ion chromatography (IC), and scanning electron microscopy (SEM); initial uptake coefficients were measured and the reaction J I mechanisms were studied under different relative humidity (RH). Back Close

Full Screen / Esc 2 Experimental

The infrared (IR) spectrum of the particle surface was investigated on line with DRIFTS Printer-friendly Version

25 during the reaction. The reactor of DRIFTS is a vacuum reaction chamber (Model HVC- Interactive Discussion DR2) surrounding a Harrick Scientific diffuse reflectance accessory (Model DRA-2CS) located in the sampling compartment of a Nicolet Nexus FTIR Spectrometer equipped 7118 with a mercury cadmium telluride (MCT) detector. Data from 128 scans, 4 cm−1 reso- lution, taken over about 80 s were averaged for one spectrum. ACPD Besides online infrared spectroscopy measurements, for a number of selected ex- 9, 7115–7154, 2009 periments, the reaction products on the particle surface and extractable soluble com- 5 ponents were also measured off line using X-ray photoelectron spectroscopy (XPS) and ion chromatography (IC), respectively. The morphological changes were observed Heterogeneous using SEM. Further details of the instruments have been described previously (Li et reaction of NO2 on al., 2006). CaCO3 particles CaCO (>99.999%, Alfa Aesar) powder was prepared by grinding and the size of 3 T. Zhu et al. 10 the grinded particle was about 1–10 µm with a mean value of 5.6 µm, as measured with a laser particle sizer (MasterSizer 2000, Malvern). The specific geometric surface 2 −1 area of the CaCO3 particles was calculated to be 0.37 m g for cubic shape particles. Title Page About 20 mg of the sample was placed in the sample holder, pressed flat using a glass slide, and stored in a desiccator as previously described (Li et al., 2006). Water Abstract Introduction 15 vapor was produced and gases were mixed using the apparatus displayed in Fig. A1. Conclusions References All tubes and surfaces were made of inert glass or Teflon with the exception of the stainless steel in situ reactor. Concentrations of NO2 entering the reactor were adjusted Tables Figures by mixing a NO2 standard gas (710 ppm in N2, Center of National Standard Material Research) with pure N2 (>99.999%) using two mass flow controllers (FC-260, Tylan, J I 20 Germany). In the reactor, the NO2 diffused to the particle surface and reacted with J I CaCO3. The formation of the reaction products was monitored using DRIFTS. Humidity was regulated by mixing dry nitrogen with water vapor by bubbling through two glass Back Close grits in pure water (Millipore Corporation, Billerica, MA, USA). Humidity was monitored using a temperature sensor (PT100, Vaisala, Vantaa, Finland) and a humidity sensor Full Screen / Esc 25 (HM1500, Vaisala). The volumetric BET (Brunauer, Emmett and Teller model) surface area of particles Printer-friendly Version was measured with an ASAP2010 BET apparatus (Micromeritics Co., USA). The spe- 2 −1 Interactive Discussion cific surface area, As, was determined to be 4.91 m g , this is 13.3 times the specific geometric surface area of cubic shape CaCO3 particles with a size of 5.6 µm. 7119 After the reactions, the absolute numbers of nitrate and nitrite ions formed during the reaction were determined with IC. The reacted CaCO3 particles were sonicated in ACPD 10 mL of pure water (Millipore Corporation), and then the filtered solution was analyzed 9, 7115–7154, 2009 using an IC (Dionex DX-500 system), which was equipped with an Ionpac AS-11HC-4 5 mm analytical column and a conductivity detector. Details of analyzing conditions can be found in a previous paper (Li et al., 2006). Heterogeneous

The XPS spectra were taken on an AXIS-Ultra instrument from Kratos Analytical reaction of NO2 on using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low–energy electron CaCO3 particles flooding for charge compensation. To compensate for surface charges effects, binding T. Zhu et al. 10 energies were calibrated using C1s hydrocarbon peak at 284.80 eV. The data were converted into VAMAS file format and imported into CasaXPS software package for manipulation and curve-fitting. Title Page

Abstract Introduction 3 Results Conclusions References 3.1 The influence of water vapor Tables Figures 15 Calcite is the main form of CaCO3 in the environment and its (104) crystal plane has the lowest energy state (de Leeuw and Parker, 1998). Both laboratory and computer J I modeling studies have demonstrated that at certain relative humidity, a hydrated layer J I on CaCO3 could be dissociated by oxygen vacancy to form stable –OH that remained surface-bound even in a high vacuum (Stipp et al., 1996; McCoy and LaFemina, 1997; Back Close 20 de Leeuw and Parker, 1998). The results of Kuriyavar et al. (2000) and Stipp (1999) Full Screen / Esc demonstrated that the dissociation of water could produce surface hydroxyl on CaCO3 + − − + + surfaces. Species including H , OH , HCO3 , Ca(OH) , and Ca(HCO3) can exist on the surfaces of calcite and alter its surface properties (Thompson and Pownall, 1989). Printer-friendly Version XPS analysis of the CaCO3 particle sample showed two peaks of O1s at 531.3 eV and Interactive Discussion 25 533.1 eV (Fig. A2), corresponding to the oxygen atoms of CaCO3 and –Ca(OH) (Stipp, 1999). The oxygen atoms of –Ca(OH) accounted for 3.4–3.6% of the total number of 7120 atoms. The presence of –Ca(OH) implied that water can have a major effect on the CaCO3 surface properties. Therefore, understanding water adsorption on surfaces of ACPD CaCO particles is very important. 3 9, 7115–7154, 2009 From the FTIR spectra of the CaCO3 particle samples, we integrated the area of −1 5 the stretching vibration of –OH from 3644 to 2983 cm and plotted it as a function of RH at 296 K (Fig. 1, left axis); it demonstrated that the proportion of surface bound Heterogeneous water increased with RH. The experimental data were fitted using the three-parameter reaction of NO2 on equation of BET adsorption to determine the equivalent number of molecular layers of CaCO3 particles water, and we concluded that a monolayer of water formed on the CaCO surface at 3 T. Zhu et al. 10 52% RH (Fig. 1, right axis). This suggests that the mechanisms of the heterogeneous reactions on the surface CaCO3 particles could be different when RH is lower or higher than 52%. − Title Page The NO3 production rate on the CaCO3 particle samples can be expressed as: Abstract Introduction − m q n d{NO3 }/dt = k{CaCO3} [H2O] [NO2] (1) Conclusions References 15 where {x} denotes the concentration of surface species (e.g. nitrate ions and active Tables Figures site on CaCO3), [x] denotes the concentration of species in gas phase, and m, n, and q are the reaction order for CaCO3, NO2 and H2O, respectively. At the initial stage of the − reaction, the number of NO3 ions formed on the surface of CaCO3 is small compared J I to the surface active site, {CaCO3}, which can be considered constant. At a constant − J I 20 NO2 concentration, the production rate of NO3 is only a function of [H2O]. The slope of − double-log line of the NO3 production rate versus the water concentration should give Back Close the reaction order with respect to H2O, q. 15 −3 Full Screen / Esc At a constant NO2 concentration of 6.88×10 molecules cm , the double-log curve − of the NO3 production rate versus the water concentration is shown in Fig. 2. The Printer-friendly Version 25 reaction order of H2O(g) was −0.44 when RH<50%, and 0.20 when RH>50%. This indicates that at low RH, H2O(g) inhibits the formation of surface nitrate but at high RH Interactive Discussion it had very little influence on the formation of surface nitrate. This result is consistent with the observation that at low RH, the formation of the active site on the surface of the 7121 CaCO3, e.g. –OH, involved the adsorbed water and the existence of adsorbed water can alter the reactivity of the particle surface (Elam et al. 1998; Hass et al., 1998; Eng ACPD et al., 2000; Hass et al., 2000). Meanwhile, water could easily adsorb onto the surface 9, 7115–7154, 2009 –OH, and compete with the adsorption of NO2 (Stumm, 1992). At RH>52%, water 5 layer forms on the surface of CaCO3 and changes the reaction on the calcite surface from gas-solid to gas-liquid reaction. Thus, the water vapor plays an important role in Heterogeneous the heterogeneous reactions of NO2 with CaCO3. reaction of NO2 on CaCO3 particles 3.2 Reaction under dry conditions T. Zhu et al. Experiments under dry conditions were carried out by first heating the CaCO3 at 623 K 10 for 2 h, thus eliminating nearly all of the adsorbed water. A mixture of NO2 and N2 without water vapor was admitted at 296 K. Title Page Figure 3 shows typical IR spectra of CaCO when reacted with NO under dry con- 3 2 Abstract Introduction ditions for 0 to 100 min. Nitrate was clearly observed on the CaCO3 surface. The −1 peak at 816 cm was assigned to the out-of-plane bending of nitrate ν2. The peak Conclusions References −1 15 at 1043 cm was assigned to the symmetric stretching of nitrate ν1 and the peak at −1 Tables Figures 748 cm was assigned to the in-plane bending of nitrate ν4. Peaks at 1295, 1330, and −1 1350 cm were assigned to the asymmetric stretching of nitrate ν3. The ν3 mode of J I nitrate on the CaCO3 surface has many peaks; this is different from the nitrate ions in Ca(NO3)2 that were attributable to the adsorbed nitrate of different coordination (mon- J I 20 odentate, bidentate, and bridging), while the nitrate ions did not form on the CaCO3 Back Close surface (Borensen et al., 2000). The asymmetric stretching of adsorbed nitric acid has been reported to be at Full Screen / Esc 1710/1680 cm−1 (Nakamoto, 1997; Finlayson-Pitts et al., 2003). In our study, to con- firm the vibration frequency of the adsorbed HNO3, calcium sulfate (CaSO4) particles Printer-friendly Version 25 were exposed to gas phase HNO3. The IR absorption peaks of adsorbed HNO3 were observed at 1670, and 1720 cm−1; therefore, we can assign the peaks at 1670 and Interactive Discussion −1 1703 cm to adsorbed HNO3, this suggests that HNO3 was formed when NO2 re- 7122 acted with CaCO3. The peaks at 1630 and 3520 cm−1 were assigned to the bending and asymmet- ACPD ric stretching of crystal hydrate water. Compared with the vibration frequencies of −1 −1 9, 7115–7154, 2009 the crystallized water in Ca(NO3)2·4H2O at 1640 cm and 3490 cm , a shift of both 5 peaks occurred, due to the decreased hydrogen bonding. The asymmetric stretching shifted to the higher wave number, and the bending shifted to the lower wave num- Heterogeneous −1 ber. The peaks at 3140 and 3330 cm were assigned to surface adsorbed water, both reaction of NO2 on peaks decreased as the reaction proceeded until the peaks for crystal hydrate water at CaCO3 particles 1630 cm−1 and 3520 cm−1 appeared. 15 −3 T. Zhu et al. 10 The XPS spectrum of CaCO3 particle that reacted with 1.06×10 molecules cm NO2 for 626 min (Fig. A3a) showed the peak of nitrogen of +5 valence at 407.3 eV, and confirmed the formation of nitrate. The formation of nitrate was further confirmed by IC Title Page analysis of reacted CaCO3 particles (Fig. A3b). However, no nitrite was detected with both XPS and IC, this is probably due to the reason that HONO formed in the reaction Abstract Introduction 15 was released into the gas phase and carried away by N2. Conclusions References Figure 4 shows the formation of nitrate on CaCO3 through the reaction of NO2 as a function of time. The amount of nitrate formed is represented by the integrate ab- Tables Figures −1 sorbance (IA) of the IR peak area between 1077 and 1013 cm . As the reaction proceeded, the nitrate concentration on the calcite surface increased and the rate of J I 20 nitrate formation increased with higher NO2 concentrations. The nitrate formation pro- cess could be divided into three stages: stage I lasted from the initiation of the reaction J I until I reached 0.3, stage II was the transition stage when the increase of I was slow- A A Back Close ing down between 0.3 and 0.4, and stage III was the growth stage when IA>0.4 and the increase rate of IA reached to a constant. Full Screen / Esc − 25 The amount of nitrate ions formed on the particles {NO3 } was determined with a −1 linear relationship between IA in the range of 1073–1013 cm and the amount of nitrate Printer-friendly Version determined by ion chromatography {NO−}: 3 Interactive Discussion − f I {NO3 } = × A (2) 7123 The conversion factor f was found to be independent of reaction time and NO2 concentration as long as the experiment was completed at a stage when the absorp- ACPD tion of the nitrate band was still growing. For our study, f was calculated to be 8 9, 7115–7154, 2009 1.89×10 molecules/IA. − 5 Using f , the production rate of nitrate d{NO3 }/dt was calculated, and the reaction order of NO2 on CaCO3 particles was obtained from the slope of the double-log plots Heterogeneous − of d{NO3 }/dt versus NO2 concentrations (Fig. 5). In stage I the reaction order of reaction of NO2 on NO2 was 1.63±0.23 and in stage II 0.41±0.55. This means that the nitrate formation CaCO3 particles with respect to NO2 was a second-order reaction in the initial stage (I) and a zero-order T. Zhu et al. 10 reaction in the transition stage (II).

3.3 Reaction under wet conditions Title Page Experiments of CaCO reaction with NO under wet conditions were carried out using 3 2 Abstract Introduction non-heat-treated CaCO3 particles at 296 K with RH=60–71%. Figure 6 shows the typical IR spectra of CaCO3 when NO2 entered the reactor under wet conditions, at Conclusions References 15 reaction times from 0 to 60 min. −1 Tables Figures The peak at 1048 cm was assigned to the symmetric stretching ν1 of nitrate, the −1 −1 peak at 1344 cm to the asymmetric stretching ν3 of nitrate. The peak at 1251 cm −1 J I was assigned to the asymmetric stretching ν3 of nitrite. The peak at 1688 cm was as- −1 signed to the adsorption peak of HNO3 and the peak at 1636 cm to the bending of the J I −1 20 surface adsorbed water; the peak at 1472 cm appeared in the range of peaks for the Back Close asymmetric stretching of carbonate and was assigned to the carbonate vibration peak. Compared to the corresponding peak observed under dry conditions, these peaks had Full Screen / Esc all shifted more or less. We attributed these shifts to the changed environment caused by increased surface adsorbed water under wet conditions. Printer-friendly Version −1 25 The peaks at 1525 and 1540 cm were in the range of asymmetric stretching of bicarbonate and were assigned to the bicarbonate vibration peaks (Al-Hosney and Interactive Discussion Grassian 2004; Al-Abadleh et al., 2005). Negative peaks at 1472, 1525, and 1540 cm−1

7124 were similar to the negative peaks from the reaction of HNO3 with CaCO3 (Fig. A5). − This indicated that the intermediate products HCO3 and H2CO3 were formed during the ACPD reaction of NO with CaCO . The result is consistent with the findings of Al-Hosney and 2 3 9, 7115–7154, 2009 Grassian (2004) that H2CO3 was the intermediate product when HNO3, SO2, HCOOH 5 and CH3COOH reacted with CaCO3. The formation of nitrate and nitrite was further confirmed by the XPS spectrum Heterogeneous 15 −3 of CaCO3 particles reacting with 1.06×10 molecules cm of NO2 for 30 min at reaction of NO2 on RH=80±2%, which demonstrated the existence of N of +5 valence and +3 valence CaCO3 particles as shown in Fig. A4a, and the formation of nitrate and nitrite was shown in IC chro- T. Zhu et al. 10 matogram of Fig. A4b. To obtain the change rate of each peak, Gaussian peak-fitting was used within the spectral range of 1750–1150 cm−1 (Fig. 7a). Figure 7b shows the integrate absorbance Title Page of nitrate peak at 1344 cm−1 increased during the reaction while that of the carbonate −1 peaks at 1472, 1525, and 1540 cm decreased with time, indicating that nitrate was Abstract Introduction 15 produced when CaCO3 was consumed. −1 −1 Conclusions References The nitrite ion peak at 1251 cm and HNO3 peak at 1688 cm increased initially and subsequently decreased, implying that the decrease of nitrite was related to the Tables Figures decrease of HNO3. It was inferred that the increase of adsorbed nitric acid caused the release of nitrite in the form of gaseous HONO which led to the decreased of both −1 J I 20 nitrite ion and nitric acid. The adsorbed water peak at 1636 cm exhibited a linear increase initially but later reached a plateau, demonstrating that the CaCO3 particle J I surface was gradually saturated with adsorbed water. Back Close Figure 8 shows the nitrate formation as a function of time during the reaction of CaCO3 with NO2 under wet (RH=60–71%) conditions. The integrated absorbance Full Screen / Esc −1 25 was in the range of 1077–1013 cm . Unlike the reaction under dry conditions, under wet conditions, the nitrate on the CaCO3 surface increased linearly with time but did Printer-friendly Version not become saturated during the experimental period, and the nitrate formation rate Interactive Discussion increased with the increasing of NO2 concentrations. 8 Using the conversion factor f =1.89×10 molecules/IA, the integrate absorbance of 7125 the nitrate peak in the range of 1077–1013 cm−1 was converted to the number of nitrate − ACPD ions formed on CaCO3 particle surface {NO3 }. The reaction order of NO2 was obtained − from the slope of the double-log plot of the nitrate production rate d{NO3 }/dt versus 9, 7115–7154, 2009 the NO2 concentration (Fig. 9). The slope of the curve was 0.94±0.10; this means that 5 under wet conditions, the NO reaction on CaCO particles is first order with respect 2 3 Heterogeneous to NO2. reaction of NO2 on 3.4 Uptake coefficient CaCO3 particles

Reactive uptake coefficient or reaction probability, γ, is defined as the ratio of reactive T. Zhu et al. gas-surface collision rate to the total gas-surface collision rate. The reactive uptake 10 coefficient of NO2 by the surface of the CaCO3 particles is Title Page

dN(NO2)/dt γ = (3) Abstract Introduction Z

1 Conclusions References Z = 4 Aωn (4) Tables Figures where N(NO2) is the number of reactive NO2 collisions with the surface, Z is the total gas-surface collision rate of NO2 on the surface of CaCO3, A is the surface area of J I 15 CaCO3 particles (BET surface area was used here), ω is the mean speed of the gas molecule, and n is the concentration of gas phase species. The rate of reactive colli- J I sions can be obtained from the production rate of nitrate d{NO3 − }/dt, the reactive Back Close uptake coeﬃcient then become: 2d NO− /dt Full Screen / Esc γ = 3 (5) Z Printer-friendly Version 20 Previous studies show that the reaction of NO2 with particles was through N2O4 Interactive Discussion (Finlayson-Pitts et al., 2003), then the reactive NO2 loss rate in the gas phase is twice of the nitrate formation rate.

7126 − The nitrate formation rate d{NO3 }/dt could be calculated from the curve of nitrate formation as a function of time, and the nitrate formation rate in the initial stage of the ACPD reaction was used for calculating the initial uptake coefficient γ . Table 1 lists the initial 0 9, 7115–7154, 2009 uptake coefficient of NO2 on CaCO3 particles under both dry and wet (RH=60–71%) −7 5 conditions. The initial uptake coefficient under dry conditions was (1.09±0.25)×10 , while under wet conditions, it was slightly lower, with a value of (0.63±0.33)×10−7. To Heterogeneous reaction of NO on further investigate the influence of RH on the uptake of NO2 on CaCO3 particle, the 2 initial uptake coefficient of the reaction was determined as a function of RH. Figure 10 CaCO3 particles shows the results. As RH increased, γ decreased initially with increasing RH; at 0 T. Zhu et al. 10 RH=45–53%, γ0 showed a turning point and jumped to a higher value at RH=53%; with RH>53%, γ0 started to increase slightly with the increasing RH. The change of γ0 with RH demonstrates the influence of water vapor on the reaction Title Page mechanism of NO2 on CaCO3 particles. When RH<53%, H2O was competing with Abstract Introduction NO2 molecules for reactive sites on the surface of CaCO3 particles, the initial uptake 15 coefficient of NO was therefore decreasing with increasing RH. 2 Conclusions References As described in the above section, at RH=52±2%, a monolayer of adsorbed water was formed on the surface of the CaCO3 particles, hence with RH>52%, the reaction of Tables Figures NO2 on CaCO3 particles was controlled by the reaction of NO2 with surface condensed water on the CaCO3 particles. This may explain why the increase of RH had little J I 20 influence on γ0 when RH was larger than 52%. The formation of a water layer on J I CaCO3 particles suggests that the reaction mechanism of NO2 on CaCO3 became different from that when RH was lower than 52%. Back Close It is puzzling to see from both Table 1 and Fig. 10 that under dry and wet condi- −7 −7 Full Screen / Esc tions, γ0 were varied within a narrow range of 0.6×10 –1.2×10 . Mertes and Wah- 25 ner (1995) have measured a lower limit of the mass accommodation coefficient of NO2 on aqueous surfaces of α≥2×10−4 at 278 K surface temperature. This suggests that Printer-friendly Version at RH>52%, with the formation of water layer on the surface of CaCO3 particles, γ0 −7 Interactive Discussion of NO2 on CaCO3 particles should have a value much higher than 10 . The possible reason for a lower γ0 value is that we used the specific BET surface area of CaCO3 7127 2 −1 particles (4.91 m g ) to calculate γ0, while at RH>52%, the water layer formed could make the surface of CaCO3 particles smoother with less pores, and the available sur- ACPD face area for the reaction should be close to the geometric surface area of a cubic 2 −1 9, 7115–7154, 2009 shape particle. Using 0.37 m g as the specific geometric surface area of CaCO3 par- −6 5 ticle samples, from Table 1 we calculated γ at RH=60–71% to be (0.84±0.44) ×10 ; 0 Heterogeneous this should be the upper limit of γ0 and is about ten times higher than γ0 at RH<52%, reaction of NO2 on when BET surface area was used. The uptake coefficient for HNO3 on CaCO3 was de- −4 CaCO3 particles termined to be at a level of 10 (e.g. Goodmann et al., 2000). Considering that NO2 concentrations in the atmosphere is orders of magnitude higher than that of HNO3, at T. Zhu et al. 10 RH>52%, NO2 may have similar importance of HNO3 in the heterogeneous reactions with CaCO3 particle in the atmosphere. Title Page

4 Discussion Abstract Introduction

We concluded from the above results that water vapor had a major inﬂuence on the Conclusions References heterogeneous reaction mechanism of NO with CaCO . The reaction mechanism was 2 3 Tables Figures 15 diﬀerent before and after a monolayer of water was formed. Prior to the formation of the monolayer of water on CaCO3 surface, –OH was produced on the calcite surface via the dissociation of water by oxygen vacancy and this seems to be the rate determining J I

factor for the reaction. After formation of a monolayer of water, surface condensed J I water participated in the reaction. We discuss the reaction mechanisms for both of Back Close 20 these two conditions below.

Full Screen / Esc 4.1 Mechanism under dry conditions

Under dry conditions, the reaction mechanism proceeded as shown in Fig. 11a. At low Printer-friendly Version

RH, there was still some water vapor remaining and the surface adsorbed water on the Interactive Discussion

7128 CaCO3 surface equilibrated with water vapor, as shown in Reaction (R1): ACPD k1 H2O(g) H2O(a) (R1) k−1 9, 7115–7154, 2009

Surface-adsorbed water existed on the CaCO3 surface despite heating at 623 K for 2 h. The adsorbed water on the CaCO3 surface was dissociated by an oxygen vacancy Heterogeneous 5 site (O) to form the active site S·(OH)H that was stable even at high temperature and reaction of NO2 on high vacuum. When the surface-adsorbed water increased, the surface active site CaCO3 particles converted to adsorbed-water due to the action of the hydrogen bond: T. Zhu et al. k2 site (O) + 2H2O(a) S · (OH)H + H2O(a) (R2) k−2

Finlayson-Pitts et al. (2003) found NO2 was likely to form nitrogen dioxide dimmers Title Page 10 (nitrogen tetraoxide, N2O4) at high concentrations, then N2O4 was adsorbed to the Abstract Introduction particle surface. Conclusions References k3 2NO (g) N O (g) (R3) 2 k 2 4 −3 Tables Figures

k4 N2O4(g) N2O4(a) (R4) k−4 J I

N2O4 reacted with the surface active site forming adsorbed nitric acid HNO3 and nitrous J I 15 acid HONO, as illustrated in Reaction (R5): Back Close

k5 N2O4(a) + S · (OH)H −→ HNO3(a) + HONO(g) (R5) Full Screen / Esc Previous studies showed that HONO was unlikely to adsorb onto the calcite surface, but rather being released into the gas phase. Although we could not detect gas phase Printer-friendly Version HONO, we did detect adsorbed HNO with DRIFTS. 3 Interactive Discussion

7129 Surface adsorbed water was in equilibrium with water vapor, see Eq. (6), where [x] indicates the concentration of gas phase species and {x} denotes the concentration of ACPD surface adsorbed species and active sites. 9, 7115–7154, 2009 k1 {H2O} = [H2O] = K 1 [H2O] (6) k−1 Heterogeneous

5 At the initial stage, surface-adsorbed water increased with RH, and the surface active reaction of NO2 on site S·(OH)H was converted to adsorbed water. CaCO3 particles

k2 T. Zhu et al. {S · (OH)H} = {site(O)}{H2O} = K 2 {site(O)}{H2O} (7) k−2

NO2 was in equilibrium with N2O4 in the gas phase, Title Page k 3 2 2 Abstract Introduction [N2O4] = [NO2] = K 3[NO2] (8) k−3 Conclusions References 10 No adsorbed N2O4 was observed on the particle surface, suggesting N2O4 could be treated as an intermediate in the reaction: Tables Figures d{N O } 2 4 = k [N O ] − k {N O } − k {N O }{S · (OH)H} = 0 (9) dt 4 2 4 −4 2 4 5 2 4 J I

− J I Assuming the HNO3 formed was quickly converted to NO3 on the surface of CaCO3, Then Back Close − d{NO3 } d{HNO3} = = k {N O }{S · (OH)H} Full Screen / Esc 15 dt dt 5 2 4 (10)

Combining Eqs. (6, 7, 8, 9, and 10), one can get Printer-friendly Version

− d{NO } K K K k k [NO ]2{site(O)}[H O] Interactive Discussion 3 = 1 2 3 4 5 2 2 dt k−4+K 1K2k5{site(O)}[H2O] (11)

7130 Equation (11) shows that, at the initial stage of the reaction, the amount of surface active site was relative large and formation of HNO3 on the CaCO3 surface was a ACPD second-order reaction with respect to NO ; this is consistent with the measured reac- 2 9, 7115–7154, 2009 tion order of 1.63±0.23 obtained from Fig. 5.

5 4.2 Mechanism under wet conditions Heterogeneous reaction of NO on > 2 In the above section, we showed that at RH 52%, water condensed on CaCO3 forming CaCO particles a monolayer of adsorbed water. Here, the wet conditions refer to the conditions when 3 RH>52%. The reaction mechanism under wet conditions was illustrated by the general T. Zhu et al. scheme in Fig. 11b. NO2 was adsorbed on CaCO3 to form adsorbed NO2(a):

k6 Title Page 10 NO2(g) NO2(a) (R6) k−6 Abstract Introduction Previous studies have demonstrated that the disproportionation reaction of NO2 with surface adsorbed water was a ﬁrst-order reaction with the adsorption of NO2 being Conclusions References the rate-limiting step (Svensson et al., 1987; Jenkin et al., 1988). The nitrite and nitrate formed on the particle surface was attributed to the reaction of surface-adsorbed Tables Figures 15 water with NO2; i.e., adsorbed NO2(a) reacted with condensed water H2O(a) to form adsorbed HNO3(a) and gaseous HONO(g). J I k 7 J I H2O(a) + 2NO2(a) −→ HNO3(a) + HONO(g) (R7) At the initial stage, HONO dissolved in adsorbed water to form nitrite. As the concentra- Back Close tion of HNO3(a) increased, the surface pH decreased and nitrite was escaped from the Full Screen / Esc 20 surface as gaseous HONO. Meanwhile, HNO3(a) reacted with CaCO3 to form H2CO3 and Ca(NO3)2. Printer-friendly Version

k8 2HNO3(a) + CaCO3(s) −→ Ca(NO3)2(s) + H2CO3(a) (R8) Interactive Discussion

7131 Afterward, H2CO3 decomposed in adsorbed water to form H2O and CO2, resulting in − 2− the decrease of HCO3 and CO3 on the surface. ACPD 9, 7115–7154, 2009 H2CO3(a) −→ H2O(g) + CO2(g) (R9)

Ca(NO3)2 has strong hygroscopic property and the Ca(NO3)2 particles were in the form Heterogeneous 5 of solution droplets at RH>10% (Liu et al., 2008b); this caused the broadened peak for adsorbed water in the IR spectrum. reaction of NO2 on CaCO3 particles Ca(NO3)2(s) + nH2O(a) Ca(NO3)2 · nH2O (R10) T. Zhu et al. Since the adsorption of NO2 was the rate-limiting step and HNO3(a) was the intermediate product, the rate of nitrate production in the presence of excess surface water 10 could be expressed as the following. Title Page

d{NO−} 3 1 k [NO ] Abstract Introduction dt = 2 6 2 (12) Conclusions References

Equation (12) shows the surface nitrate formation with respect to NO2 as a ﬁrst order Tables Figures reaction; this is consistent with the experimental result of 0.94±0.10 and results in the literature. J I

J I 15 5 Conclusions Back Close The initial uptake coeﬃcient of NO2 on CaCO3 particles γ0 was found to change with Full Screen / Esc increasing relative humidity. Using BET surface area, γ0 under dry conditions (RH∼0%) was determined to be (1.09±0.25)×10−7, while under “wet” conditions (RH=60–71%), it was slightly lower, with a value of (0.63± 0.33)×10−7. Our results show that relative Printer-friendly Version

20 humidity had a major influence on the surface properties of CaCO3 particles by not only Interactive Discussion influencing the initial uptake coefficient but also by changing the reaction mechanism. At RH<52%, surface –OH produced via the dissociation of surface-adsorbed water 7132 by an oxygen vacancy on the CaCO3 surface was the rate determining factor in the reaction. Before surface –OH was depleted, the reaction was second order in NO2. At ACPD RH>52%, a water layer of more than a monolayer was formed and NO reacted with 2 9, 7115–7154, 2009 water on the surface of CaCO3 to produce HNO3 and HONO; at this stage, the reaction 5 was the first order in NO2. At RH>52%, the water layer formed could make the surface smoother with less Heterogeneous

pores, and the surface area should be much smaller than the BET surface area. Using reaction of NO2 on 2 −1 0.37 m g as the specific geometric surface area of CaCO3 particle samples, we CaCO3 particles calculated γ at RH=60–71% to be (0.84±0.44)×10−6; this is about ten times higher 0 T. Zhu et al. 10 than that under dry conditions and could be used as the upper limit of γ0. Considering −4 the uptake coefficient for HNO3 on CaCO3 is at a level of 10 and NO2 concentrations in the atmosphere is orders of magnitude higher than that of HNO3, at RH>52 %, NO2 Title Page should have similar importance of HNO3 in the heterogeneous reactions with CaCO3 Abstract Introduction particle in the atmosphere, and the NO2 reaction with water adsorbed on CaCO3 could 15 also be an important source of HONO in the atmosphere. Further efforts to detect Conclusions References HONO formed in the reaction of NO2 on CaCO3 particles are under way. Acknowledgements. This work was supported by the National Basic Research Priorities Pro- Tables Figures gram (Grant No. 2002CB410802) and National Natural Science Foundation of China (Grant No. 40490265). J I

J I 20 References Back Close Al-Abadleh, H. A., Al-Hosney, H. A., and Grassian, V. H.: Oxide and carbonate surfaces as environmental interfaces: the importance of water in surface composition and surface reactivity, Full Screen / Esc J. Mol. Catal. A – Chem., 228, 47–54, 2005. Al-Hosney, H. A. and Grassian V. H.: Carbonic acid: An important intermediate in the surface Printer-friendly Version 25 chemistry of calcium carbonate, J. Am. Chem. Soc., 126, 8068–8069, 2004. Boerensen, C., Kirchner, U., Scheer, V., Vogt, R., and Zellner, R.: Mechanism and kinetics of Interactive Discussion the reactions of NO2 or HNO3 with alumina as a mineral dust model compound, J. Phys. Chem. A, 104, 5036–5045, 2000. 7133 Carmichael , G. R., Zhang ,Y., Chen, L. L., Hong, M. S., and Ueda, H.: Seasonal variation of aerosol composition at Cheju Island, Korea, Atmos. Environ., 30, 2407–2416, 1996. ACPD Chen, L. Q.: Long range atmospheric transport of China desert aerosol to north Pacific Ocean, Acta Oceanol. Sin., 7, 554–560, 1985 (in Chinese). 9, 7115–7154, 2009 5 de Leeuw, N. H. and Parker, S. C.: Surface structure and morphology of calcium carbonate polymorphs calcite, aragonite, and vaterite: an atomistic approach, J. Phys. Chem. B, 102, 2914–2922, 1998. Heterogeneous Dentener, F. J., Carmichael, G. R., Zhang, Y., Lelieveld, J., and Crutzen, P. J.: Role of min- reaction of NO2 on eral aerosol as a reactive surface in the global troposphere, J. Geophys. Res. Atmos., 101, CaCO3 particles 10 22869–22889, 1996. Elam, J. W., Nelson, C. E., Cameron, M. A., Tolbert M. A., and George, S. M.: Adsorption T. Zhu et al. of H2O on a single-crystal alpha-Al2O3(0001) surface, J. Phys. Chem. B, 102, 7008–7015, 1998. Eng, P. J., Trainor, T. P., Brown, Jr., G. E., Waychunas, G. A., Newville, M., Sutton, S. R., and Title Page 15 Rivers, M. L.: Structure of the hydrated alpha-Al2O3 (0001) surface, Science, 288, 1029– 1033, 2000. Abstract Introduction Fenter, F. F., Caloz, F., and Rossi, M. J.: Experimental-Evidence for the Efficient Dry Deposition of Nitric-Acid on Calcite, Atmos. Environ., 29, 3365–3372, 1995. Conclusions References Finlayson-Pitts, B. J., Wingen, L. M., Sumner, A. L., Syomin, D., and Ramazan, K. A.: The het- Tables Figures 20 erogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism, Phys. Chem. Chem. Phys., 5, 223–242, 2003. Goodman, A. L., Underwood, G. M., and Grassian, V. H.: Heterogeneous Reaction of NO2: J I Characterization of Gas-Phase and Adsorbed Products from the Reaction, 2NO2(g)+H2O(a) J I f HONO(g)+HNO3(a) on Hydrated Silica Particles, J. Phys. Chem. A, 103, 7217–7223, 1999. 25 Goodman, A. L., Underwood, G. M., and Grassian, V. H.: A laboratory study of the heteroge- Back Close neous reaction of nitric acid on calcium carbonate particles, J. Geophys. Res. Atmos., 105, 29053–29064, 2000. Full Screen / Esc Hanisch, F. and Crowley, J. N.: Heterogeneous reactivity of gaseous nitric acid on Al2O3, CaCO3, and atmospheric dust samples: A Knudsen cell study, J. Phys. Chem. A, 105, 3096– Printer-friendly Version 30 3106, 2001. Hass, K. C., Schneider, W. F., Curioni, A., and Andreoni, W.: The chemistry of water on alumina Interactive Discussion surfaces: Reaction dynamics from first principles, Science, 282, 265–268, 1998. Hass, K. C., Schneider, W. F., Curioni A., and Andreoni, W.: First-principles molecular dynamics 7134 simulations of H2O on alpha-Al2O3 (0001), J. Phys. Chem. B, 104, 5527–5540, 2000. Jenkin, M. E., Cox, R. A., and Williams D. J.: Laboratory Studies of the kinetics of formation of ACPD nitrous-acid from the thermal-reaction of nitrogen-dioxide and water-vapor, Atmos. Environ., 22, 487–498, 1988. 9, 7115–7154, 2009 5 Johnson, E. R., Sciegienka, J., Carlos-Cuellar, S., and Grassian, V. H.: Heterogeneous Uptake of Gaseous Nitric Acid on Dolomite (CaMg(CO3)2) and Calcite (CaCO3) Particles: A Knud- sen Cell Study Using Multiple, Single, and Fractional Particle Layers, J. Phys. Chem. A, 109, Heterogeneous 6901–6911, 2005. reaction of NO2 on Jonas, P. R., Charlson, R. J., Rodhe, H., Anderson, T. L., Andreae, M. O., Dutton, E., Graf, CaCO3 particles 10 H., Fouquart, Y., Grassl, H., Heintzenberg, J., Hobbs, P. V., Hofmann, D., Hubert, B., et al.: Aerosols, in: Climate Change 1994: Radiative Forcing of Climate Change, edited by: T. Zhu et al. Houghton, J. T., Meira Filho, L. G., Bruce, J., Lee, H., Callander, B. A., Haites, E., Harris, N., and Maskell, K., Cambridge University Press, Cambridge, UK, 127–162, 1995. Krueger, B. J., Grassian, V. H., Iedema, M. J., Cowin, J. P., and Laskin, A.: Probing heteroge- Title Page 15 neous chemistry of individual atmospheric particles using scanning electron microscopy and energy-dispersive X-ray analysis, Anal. Chem., 75, 5170–5179, 2003a. Abstract Introduction Krueger, B. J., Grassian, V. H., Laskin, A., and Cowin, J. P.: The transformation of solid atmospheric particles into liquid droplets through heterogeneous chemistry: Laboratory insights Conclusions References into the processing of calcium containing mineral dust aerosol in the troposphere, Geophys. Tables Figures 20 Res. Lett., 30, 1148, doi:10.1029/2002GL016563, 2003b. Kuriyavar, S. I., Vetrivel, R., Hegde, S. G., Ramaswamy, A. V., Chakrabarty, D., and Mahapatra, S.: Insights into the formation of hydroxyl ions in calcium carbonate: temperature dependent J I FTIR and molecular modelling studies, J. Mater. Chem., 10, 1835–1840, 2000. J I Li, H. J., Zhu, T., Ding, J., Chen, Q., and Xu, B. Y.: Heterogeneous reaction of NO2 on the 25 surface of NaCl particles, Sci. China, Ser. B Chem., 49, 371–378, 2006. Back Close Liu, Y., Gibson, E. R., Cain, J. P., Wang, H., Grassian, V. H., and Laskin, A.: Kinetics of Heterogeneous Reaction of CaCO3 Particles with Gaseous HNO3 over a Wide Range of Full Screen / Esc Humidity, J. Phys. Chem. A, 112, 1561–1571, 2008a. Liu, Y. J., Zhu, T., Zhao, D. F., and Zhang, Z. F.: Investigation of the hygroscopic properties Printer-friendly Version 30 of Ca(NO3)2 and internally mixed Ca(NO3)2/CaCO3 particles by micro-Raman spectrometry, Atmos. Chem. Phys., 8, 7205–7215, 2008b, http://www.atmos-chem-phys.net/8/7205/2008/. Interactive Discussion McCoy, J. M. and LaFemina, J. P.: Kinetic Monte Carlo investigation of pit formation at the CaCO3(10(1)over-bar4) surface-water interface, Surf. Sci., 373, 288–299, 1997. 7135 Mertes, S. and Wahner, A.: Uptake of Nitrogen Dioxide and Nitrous Acid on Aqueous Surfaces, J. Phys. Chem., 99, 14000–14006, 1995. ACPD Nakamoto, K.: Infrared and Raman Spectra of Inorganic and Coordination Compounds Part A, New York, John Wiley & Sons, 221–247, 1997. 9, 7115–7154, 2009 5 Okada, K., Qin, Y., and Kai, K.: Elemental composition and mixing properties of atmospheric mineral particles collected in Hohhot, China, Atmos. Res., 73, 45–67, 2005. Quan, H.: Discussion of the transport route of dust storm and loess aerosol from Northwest Heterogeneous China at the high altitude, Environ. Sci., 14, 60–65, 1993 (in Chinese). reaction of NO2 on Song, C. H. and Carmichael, G. R.: Gas-particle partitioning of nitric acid modulated by alkaline CaCO3 particles 10 aerosol, J. Atmos. Chem., 40, 1–22, 2001. Song, C. H., Maxwell-Meier, K., Weber, R. J., Kapustin, V., and Clarke, A.: Dust composition T. Zhu et al. and mixing state inferred from airborne composition measurements during ACE-Asia C130 Flight #6, Atmos. Environ., 39, 359–369, 2005. Stipp, S. L. S., Gutmannsbauer, W., and Lehmann, T.: The dynamic nature of calcite surfaces Title Page 15 in air, Am. Mineral., 81, 1–8, 1996. Stipp, S. L. S.: Toward a conceptual model of the calcite surface: Hydration, hydrolysis, and Abstract Introduction surface potential, Geochim. Cosmochim. Acta, 63, 3121–3131, 1999. Stumm, W.: Chemistry of the solid-water interface: processes at the mineral-water and particle- Conclusions References water interface in natural systems, New York, John Wiley & Sons, 168 pp., 1992. Tables Figures 20 Sullivan, R. C., Guazzotti, S. A., Sodeman, D. A., and Prather, K. A.: Direct observations of the atmospheric processing of Asian mineral dust, Atmos. Chem. Phys., 7, 1213–1236, 2007, http://www.atmos-chem-phys.net/7/1213/2007/. J I Svensson, R., Ljungstrom, E., and Lindqvist, O.: Kinetics of the Reaction between Nitrogen Dioxide and Water Vapor, Atmos. Environ., 21, 1529–1539, 1987. J I 25 Thompson, D. W. and Pownall, P. G.: Surface Electrical-Properties of Calcite, J. Colloid Inter- Back Close face Sci., 131, 74–82, 1989. Ullerstam, M., Johnson, M. S., Vogt, R., and Ljungstrom,¨ E.: DRIFTS and Knudsen cell study Full Screen / Esc of the heterogeneous reactivity of SO2 and NO2 on mineral dust, Atmos. Chem. Phys., 3, 2043–2051, 2003, http://www.atmos-chem-phys.net/3/2043/2003/. Printer-friendly Version 30 Underwood, G. M., Miller, T. M., and Grassian, V. H.: Transmission FT-IR and Knudsen Cell Study of the Heterogeneous Reactivity of Gaseous Nitrogen Dioxide on Mineral Oxide Parti- Interactive Discussion cles, J. Phys. Chem. A, 103, 6184–6190, 1999. Vlasenko, A., Sjogren, S., Weingartner, E., Stemmler, K., Gaggeler,¨ H. W., and Ammann, M.: 7136 Effect of humidity on nitric acid uptake to mineral dust aerosol particles, Atmos. Chem. Phys., 6, 2147–2160, 2006, http://www.atmos-chem-phys.net/6/2147/2006/. ACPD Zhang, D. Z., Shi, G. Y., Iwasaka, Y., and Hu, M.: Mixture of sulfate and nitrate in coastal atmosphere aerosol :individual particle studies in Qingdao (36 degrees 040 N, 120 degrees 9, 7115–7154, 2009 0 5 21 E), China, Atmos. Environ., 34, 2669–2679, 2000. Zhuang, H., Chan, C. K., Fang, M., and Wexler, A. S.: Formation of nitrate and non-sea-salt sulfate on coarse particles, Atmos. Environ., 33, 4223–4233, 1999. Heterogeneous reaction of NO2 on CaCO3 particles T. Zhu et al.

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Table 1. Initial uptake coeﬃcient of NO2 on CaCO3 particles under dry and wet conditions. The Title Page RH was measured at 296 K. Abstract Introduction 15 −3 γ ± σ −7 replicate time [NO2] (10 molecule cm ) 0 2 (10 ) RH (%) Conclusions References 14 6.90–16.84 1.09±0.25 dry 5 4.58–11.40 0.63±0.33 60–71 Tables Figures

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20 Monolayers Conclusions References 0.6 (3644-2983cm 0.4 Tables Figures 10

0.2 J I Integrated absorbance of H Integrate absorbance of H 0.0 0 J I 020406080100 Back Close Relative Humidity(%) Full Screen / Esc Fig. 1. Isotherm adsorption curve of water vapor on CaCO particles at 296 K. IR absorbance Figure 1. Isotherm adsorption curve of water vapor on CaCO3 3 particles at 296 K. IR Printer-friendly Version of the stretching vibration of –OH of adsorbed water is integrated between 3644 and 2983 cm−1. absorbance of the stretching vibration of –OH of adsorbed water is integrated between A monolayer of water was formed at 52% RH, based on the calculation using the BET equation Interactive Discussion (ﬁlled3644 black and square 2983 cm indicates–1. A monolayer the integration of water area was of water;formed at denotes52% RH, the based number on the of single- molecule water layers). calculation using the BET equation (■ indicates the integration area of water; ○ 7139 denotes the number of single-molecule water layers).

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38.5 39.0 39.5 40.0 40.5 41.0 41.5 J I ln{[H O]/molecule cm-3} Back Close 2 Full Screen / Esc − Fig. 2. Double-log curve of NO3 production rate versus [H2O]. The reaction order with respect to H2O(g) was −0.44 at RH<50%, and was– 0.20 at RH>50%. Printer-friendly Version Figure 2. Double-log curve of NO3 production rate versus [H2O]. The reaction order Interactive Discussion with respect to H2O(g) was –0.44 at RH < 50%, and was 0.20 at RH > 50%.

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J I Fig.Figure 3. TypicalTypical IR spectra IRspectra of CaCO3 particles of CaCO after reactionparticles with NO after2 at 296°K reaction and reaction with times NO of 0at to 100 296 min. K RH and was reaction <10%, NO2 16 –3 3 2 concentration was 1.22×10 molecule cm . 16 −3 Back Close times of 0 to 100 min. RH was <10%, NO2 concentration was 1.22×10 molecule cm . Full Screen / Esc

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0.8 9, 7115–7154, 2009 4.81×11015 0.7 8.82×11015 Heterogeneous 16 reaction of NO2 on 0.6 1.06×110 CaCO3 particles 1.22×11016 0.5 T. Zhu et al. III 0.4 II Title Page 0.3 Abstract Introduction I 0.2 Conclusions References Integrate absorbance Integrated absorbance 0.1 Tables Figures

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Fig. 4. Nitrate formation as a function of time during the reaction of CaCO3 with NO2. Full Screen / Esc The nitrate concentration is represented by the integrated absorbance of the peak area −1 15 betweenFigure 1077 4. Nitrate and 1013formation cm . as Thea functi NOonconcentration of time during was the changedreaction of from CaCO 4.813 ×with10 to 2 Printer-friendly Version 1.22×1016 molecule cm−3. The data points and the error bars are the average value and the NO2. The nitrate concentration is represented by the integrated absorbance of the peak standard deviation of three duplicate experiments. –1 Interactive Discussion area between 1077 and 1013 cm . The NO2 concentration was changed from 4.81×1015 to 1.22 ×1016 molecule m-3. The data points and the error bars are the average value and the standard deviation7142 of three duplicate experiments. ACPD

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Fig. 5. Double-log curves of nitrate-producing rate versus NO2 concentration. The data points and error bars are the average values and standard deviations of three replicate experiments. Figure 5. Double-log curves of nitrate-producing rate versus NO2 concentration. The Printer-friendly Version The reaction order was of NO2 1.63±0.23 in stage I, and 0.41±0.55 in transition stage II. data points and error bars are the average values and standard deviations of three Interactive Discussion

replicate experiments. The reaction order was of NO2 1.63 ± 0.23 in stage I, and 0.41 ± 0.55 in transition stage II. 7143 20 ACPD 9, 7115–7154, 2009 1344 Heterogeneous 15 1636 60min reaction of NO on 50min 2 CaCO particles 40min 3 30min T. Zhu et al. 10 20min 1048 16min 12min Title Page 1688 8min Abstract Introduction Kubelka-Munk 5 4min 0min 1251 Conclusions References

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Figure 6. Typical IR spectrum for CaCO3 reacted with NO2 under wet condition at Fig. 6. Typical IR spectrum for CaCO3 reacted with NO2 under wet condition at reaction time Printer-friendly Version of 0 to 60 min.reaction time of 0 to 60 min. Interactive Discussion

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16 16 −3 Fig. 7. (a) IR spectrumFigure 7. (a) ofIR CaCOspectrum3 ofparticles CaCO3 particles after after reaction reaction with with1.06 1.06×10×10 moleculesmolecules cm of Printer-friendly Version -3 NO2 for 60 min aftercm of Gaussian NO2 for 60 peak min ﬁtting.after Gaussian(b) Change peak f initting. integrated (b) Change absorbance in integrated of each peak Interactive Discussion with reaction time.adsorbance of each peak with reaction time.

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Full Screen / Esc Fig. 8. Nitrate formation as a function of time during the reaction of CaCO3 with NO2 at 60– 71%RH.Figure The 8. nitrateNitrate concentrationformation as isa functi representedon of time by theduring integrated the reaction absorbance of CaCO of3 the with peak −1 15 area between 1077 and 1013 cm . The NO2 concentration was changed from 4.58×10 to Printer-friendly Version 1.14NO×10216 atmolecule 60–71% cm RH.−3. TheThe datanitrate points concentrati and theon error is barsrepresented are the averageby the valueintegrated and the –1 Interactive Discussion standardabsorbance deviation of the of three peak duplicatearea between experiments. 1077 and 1013 cm . The NO2 concentration was changed from 4.58×1015 to 1.14 ×1016 molecule m-3. The data points and the error bars are the average value and the standard7146 deviation of three duplicate experiments. ACPD

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Fig. 9. Double-log plot of the nitrate production rate versus the NO2 concentration. The reaction was at 296 K and RH=66±2%. The data points and error bars are the average values and Printer-friendly Version standardFigure deviations 9. Double-log of two plot or three of the replicate nitrate experiments.production rate versus the NO2 concentration. The reaction was at 296°K and RH = 66 ± 2%. The data points and error bars are the Interactive Discussion average values and standard deviations of two or three replicate experiments. 7147 ACPD 9, 7115–7154, 2009

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Fig. 10. Initial uptake coeﬃcient of NO2 on CaCO3 at diﬀerent RH, the NO2 concentration was 6.88×1015 molecules cm−3 for uniform format. The horizontal error bars are the errors of RH Full Screen / Esc measurements.Figure 10. Initial uptake coefficient of NO2 on CaCO3 at different RH, the NO2 15 3 Printer-friendly Version concentration was 6.88×10 molecules/cm . The horizontal error bars are the errors of

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7148 ACPD 9, 7115–7154, 2009 N O 2NO 2 4 2 HONO H2O HO- HO…NO2 O2N…NO2 HONO H2O H…ONO HNO3 Heterogeneous H2O H2O reaction of NO2 on CaCO3 Initial stage CaCO3 particles

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Figure 11. Schematics of the mechanism of NO2 reaction on the surface of CaCO3 Printer-friendly Version Fig. 11. Schematicsparticles of theunder mechanism(a) dry and (b) wet conditions. of NO2 reaction on the surface of CaCO3 particles under (a) dry and (b) wet conditions. Interactive Discussion

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80000 Heterogeneous CO O1s 3 reaction of NO2 on 70000 531.3 CaCO3 particles 60000 T. Zhu et al.

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Intensity(CPS) Conclusions References Ca(OH) 20000 533.1 Tables Figures

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Fig. A2. XPS spectrum of O1s at the surface of CaCO3 particles. Printer-friendly Version Figure A2. XPS spectrum of O1s at the surface of CaCO3 particles

Interactive Discussion

7151 ACPD

120000 O 1s 9, 7115–7154, 2009 (a) 531.3eV 100000 Heterogeneous 80000 Ca 2p3 reaction of NO2 on 60000 346.9eV CaCO particles Ca 2p1 3 C 1s_a N 1s 40000 350.4eV 284.8eV 407.3eV T. Zhu et al. Intensity(CPS) C 1s_c 20000 289.4eV

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Conclusions References 10 NO - 3 8 (b) Tables Figures

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Figure A3. Typical XPS spectrum (a) and IC chromatogram (b) of CaCO3 particle Interactive Discussion Fig. A3. Typical XPS spectrum (a) and15 IC chromatogram-3 (b) of CaCO3 particle after reacted 15 after reacted with−3 1.06×10 molecules cm NO2 for 626 min. with 1.06×10 molecules cm NO2 for 626 min. 7152 ACPD 5800 N(+5) (a) 9, 7115–7154, 2009

5600 N(+3) Heterogeneous

5400 reaction of NO2 on CaCO3 particles Intensity(CPS) 5200 T. Zhu et al.

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Abstract Introduction 8 NO - 2 NO - 3 Conclusions References (b) 6 Tables Figures s)

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Figure A4. XPS spectrum (a) and IC chromatogram (b) analysis of CaCO3 particles Interactive Discussion Fig. A4. XPS spectrum (a) and IC chromatogram16 -3 (b) analysis of CaCO3 particles after reaction after reaction with 1.06×10 molecules cm of NO2 at RH = 80±2% for 30min. The with 1.06×1016 molecules cm−3 of NO at RH=80±2% for 30 min. The dashed line in (a) was dashed line in (a) was the Lorentzian2 peak fit. the Lorentzian peak ﬁt. 7153 ACPD 9, 7115–7154, 2009

0.4 Heterogeneous _ reaction of NO on 1340 NO 2 H O 3 2 CaCO3 particles 0.3 1650 40min T. Zhu et al. 30min 0.2 20min HNO 10min Title Page 3 0.1 0min 1709 Abstract Introduction

Absorbance Conclusions References 0.0 Tables Figures -0.1 1540 1460 J I + CO H CO 2- -0.2 3 3 J I 1700 1600 1500 1400 1300 1200 -1 Back Close Wavenumber(cm ) Full Screen / Esc

Figure A5. Typical IR spectrum of CaCO3 after reacted with HNO3 for 0 to 40 min. Fig. A5. Typical IR spectrum of CaCO3 after reacted with HNO3 for 0 to 40 min. Printer-friendly Version

Interactive Discussion

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