The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on

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The Role of Iron-Bearing Minerals in NO2 to HONO Conversion on Soil Surfaces Mulu A. Kebede,† David L. Bish,‡ Yaroslav Losovyj,§ Mark H. Engelhard,∥ and Jonathan D. Raff*,†,§ †

School of Public and Environmental Affairs, ‡Department of Geological Sciences, and §Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States ∥ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *

ABSTRACT: Nitrous acid (HONO) accumulates in the nocturnal boundary layer where it is an important source of daytime hydroxyl radicals. Although there is clear evidence for the involvement of heterogeneous reactions of NO2 on surfaces as a source of HONO, mechanisms remain poorly understood. We used coated-wall flow tube measurements of NO2 reactivity on environmentally relevant surfaces (Fe (hydr)oxides, clay minerals, and soil from Arizona and the Saharan Desert) and detailed mineralogical characterization of substrates to show that reduction of NO2 by Fe-bearing minerals in soil can be a more important source of HONO than the putative NO2 hydrolysis mechanism. The magnitude of NO2-to-HONO conversion depends on the amount of Fe2+ present in substrates and soil surface acidity. Studies examining the dependence of HONO flux on substrate pH revealed that HONO is formed at soil pH < 5 from the reaction between NO2 and Fe2+(aq) present in thin films of water coating the surface, whereas in the range of pH 5−8 HONO stems from reaction of NO2 with structural iron or surface complexed Fe2+ followed by protonation of nitrite via surface Fe−OH2+ groups. Reduction of NO2 on ubiquitous Fe-bearing minerals in soil may explain HONO accumulation in the nocturnal boundary layer and the enhanced [HONO]/[NO2] ratios observed during dust storms in urban areas.

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INTRODUCTION Daytime photolysis of nitrous acid (HONO) accumulated in the nocturnal boundary layer releases a pulse of hydroxyl (OH) radicals that initiates ozone (O3) and secondary organic aerosol formation in the lower atmosphere.1−5 Despite the pivotal role played by HONO in atmospheric chemistry, the exact mechanisms responsible for its formation are unclear.6,7 Vertical gradients of HONO observed at night (with lower concentrations aloft) suggest that ground-level processes generate HONO.8−14 In the past, HONO formation has been attributed to deposition of NO2 and subsequent hydrolysis in thin films of water coating boundary layer surfaces according to reaction 1.15−17

which are slow and may not support the buildup of relevant levels of HONO. Aerosol or boundary layer substrates comprised of TiO2, humic acid, or soot present in surfaces are known to promote NO2-to-HONO conversion. In addition, displacement of deposited nitrite by strong acids generated during photooxidation events are another source of HONO during the daytime.37 However, these mechanisms depend on sunlight to generate the reductant or the strong acid.37−43 Lee et al. proposed that ferrous iron (Fe2+) present in cloudwater could reduce NO2, although they concluded that the low Fe2+ concentrations present in this medium would render this reaction negligible in the troposphere.34 Although iron is only a minor component of aqueous cloud droplets,36,44 it is the fourth most abundant element in Earth’s crust, making it a major component of soil and wind-blown mineral dust.45 In soil, iron oxide abundance ranges from less than 0.1% to over 50%,46 and 5−6% by weight of global Fe occurs in sedimentary rocks.47 The abundance of iron in mineral dust aerosol from various source locations has been shown to be 3−5% by mass.45,48 Iron is also a common component of urban infrastructure, for example, in bricks or as

surface

2NO2 (g) + H 2O(l) ⎯⎯⎯⎯⎯→ HONO(aq) + HNO3(aq) (1)

Laboratory18−23 and computational studies24−29 have evaluated pathways for this reaction, although the mechanism has proven difficult to confirm in the field. The first step is dimerization of NO2, which is favored when NO2 concentrations are high, although it was proposed that the reaction may occur at lower concentrations at the air−water interface and in the presence of certain anions.30−33 It has been argued that under atmospherically relevant partial pressures of NO2, deposition of gas-phase NO2 to aqueous surfaces is controlled by mass transport and the aqueous reaction rate,34−36 both of © 2016 American Chemical Society

Received: Revised: Accepted: Published: 8649

April 18, 2016 June 24, 2016 July 13, 2016 July 13, 2016 DOI: 10.1021/acs.est.6b01915 Environ. Sci. Technol. 2016, 50, 8649−8660

Environmental Science & Technology

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EXPERIMENTAL SECTION Mineral Substrates. Table 1 provides a list of soils and clay minerals used in this study along with their relevant properties.

rust coatings. Amorphous nanoparticles (100 m2/g) components of soil, making it an effective sorbent even if its bulk abundance is low.51−54 It is well-known that the Fe2+/Fe3+ redox couple drives many biogeochemical cycles and determines the fate and toxicity of pollutants.47,55−57 Weathering of primary Fe-bearing minerals under oxidizing conditions leads to the formation of secondary pedogenic Fe (hydr)oxides that are associated with a shift in the Fe2+/Fe3+ ratio in favor of trivalent iron. However, reductive dissolution of Fe3+ minerals means that a nonnegligible amount of Fe2+ will be present under oxic conditions.47 The presence of Fe (hydro)oxides also influences the point of zero charge of the soil and hence the proton donating capacity of soil surfaces.58−60 Therefore, it is reasonable to assume that Fe2+ in soil plays an important role in initiating redox reactions of NO2, in addition to determining whether HONO outgasses from soil or is retained as nitrite (NO2−). In the mechanism shown in reactions 2−5, NO2 is reduced to NO2− by ferrous iron (Fe2+) in an electron transfer reaction that is facilitated by adsorbed water.61,62 Subsequent protonation of nitrite yields HONO (aqueous pKa = 3).63,64

Table 1. Description of Soil and Clay Substrates and Selected Properties substrate Az-1 Az-2 Az-3 Sah-1 Sah-2 NAu-2 NAu-2a SWy-2 PFl-1 SA-K

surface

NO2 (g) ⎯⎯⎯⎯⎯→ NO2 (surf)

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description Arizona desert soil Arizona desert soil Arizona desert soil Saharan desert soil Saharan desert soil nontronite oxalate-treated nontronite Namontmorillonite palygorskite Sigma-Aldrich kaolinite

native pHa

wt % Fe (bulk)b

wt % Fe (surface)c

BET surface aread,e (m2g−1)

5.39

1.47

1.9

4.2 [2.0]

9.41

1.56

0.3

3.1 [2.2]

8.96

1.20

1.3

3.2 [2.0]

8.65

0.41

1.5

1.9 [1.3]

8.74

0.75

2.1

2.3 [1.2]

7.91 4.04

23.9f

21 8.6

29 [26]

8.85

2.35f

1.1

32

8.47 5.76

2.98f −

3.5 0.6

136 23

a

1:2 substrate/water (w/v). bFrom ICP-MS analysis. cXPS analysis. from N2 adsorption isotherm. eValues in brackets are surface area following extraction with oxalic acid. fFrom Ref 113. d

surface

NO2 (surf) + Fe2 + ⎯⎯⎯⎯⎯→ NO−2 + Fe3 +

(3)

NO−2 + H+ ⇌ HONO(surf)

(4)

HONO(surf) ⇌ HONO(g)

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Soils from the Sahara Desert were collected in Egypt in the vicinity of the Giza pyramid complex (Sah-1; 29.98, 31.13) and the western outskirts of 6th of October City (Sah-2, 29.89, 30.88). Three soil samples from Arizona were collected from relatively undisturbed areas near the cities of Prescott (Az-1; 34.37, -112.29), Phoenix (Az-2; 33.12, -111.84), and Tucson (Az-3; 32.24, -111.06). Soil samples were collected in January, 2012, from the top 5 cm of soil at each site. The clay minerals nontronite (NAu-2), Na-rich montmorillonite (SWy-2), and palygorskite (PFl-1) were purchased from the Clay Minerals Society Source Clay Repository (Purdue University, West Lafayette, IN), and kaolinite (SA-K) was purchased from Sigma-Aldrich. Prior to use, soil and clay mineral samples were broken up and sieved through a 125 μm cutoff stainless steel sieve. Iron oxides used in this study are from the following sources and used as received: Magnetite (Fe3O4/γ-Fe2O3, Inframat Advanced Materials), maghemite (γ-Fe2O3, SigmaAldrich), goethite (α-FeOOH, Alpha Aesar), and hematite (αFe2O3, Alpha Aesar). Note that the “magnetite” (Fe3O4/γFe2O3) sample is ∼70% γ-Fe2O3 and 30% Fe3O4 in solid solution, as determined by powder X-ray diffraction (XRD) analysis using the unit-cell parameter. The average crystal size in Fe3O4/γ-Fe2O3, γ-Fe2O3, α-FeOOH, and α-Fe2O3 samples were determined from transmission electron microscopy (TEM) to be 20, 37, 52, and 75 nm, respectively; the size for Fe3O4/γ-Fe2O3 is consistent with the coherent diffracting domain size determined through Rietveld refinement, 11 nm. The surface areas for the Fe3O4/γ-Fe2O3, γ-Fe2O3, α-FeOOH, and α-Fe2O3 powders, 99, 36, 22, and 98 m2 g−1, respectively, were determined from Brunauer−Emmett−Teller (BET) analysis of N2-adsorption isotherms using an ASAP 2020 Accelerated Surface Area and Porosimetry Analyzer (Micrometrics). The surface area of untreated mineral dust and oxalic

Reactions 2−5 are assumed to take place on a soil surface, which may consist of bare mineral surface or adsorbed water depending on ambient relatively humidity (RH). In reaction 3 Fe2+ and Fe3+ exist as aqueous cations, complexed to a surface, or within a mineral structure. Factors influencing the efficiency of reactions 2−5 within the soil environment include the total iron content, mineralogy, Fe speciation, pH, and the reactive surface area in the soil substrate. To our knowledge, the role of Fe2+ in reducing NO2 has not been considered in studies of HONO formation on soil surfaces. Indeed, previous investigations of NO2 uptake onto iron oxide surfaces have attributed the appearance of HONO to reaction 1.65 The objective of this work is to test the hypothesis that Fe2+ plays an important role in HONO formation on soil surfaces by characterizing the formation of HONO following uptake of atmospherically relevant levels of NO2 onto a range of Fecontaining surfaces found in the environment. Experiments were conducted using a coated-wall flow tube38,42 coupled to a chemical ionization mass spectrometer (CIMS) for detection of HONO. Substrates included iron oxide particles (magnetite (Fe3O4), maghemite (γ-Fe2O3), goethite (α-FeOOH), and hematite (α-Fe2O3)), Fe-bearing clay minerals, and soil samples collected from Arizona and the Sahara Desert. Particular attention was paid to investigating how variables such as Fe2+ abundance, mineralogical composition, and pH influence the amount of HONO formed. The results are compared with findings from laboratory and field studies, and the environmental implications of these findings in the context of heterogeneous HONO sources are discussed. 8650

DOI: 10.1021/acs.est.6b01915 Environ. Sci. Technol. 2016, 50, 8649−8660

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tubing and tube fittings were used throughout, and all glass surfaces with the exception of the substrate-coated tubes were coated with a hydrophobic and inert perfluorinated polymer film (Fluoropel PFC 801A, Cytonix Corp.) to reduce unwanted heterogeneous reactions on apparatus surfaces. Control experiments were carried out in the absence of powder substrates to quantify background NO2-to-HONO conversion on flow tube walls; uptake was found to be negligible under the conditions of our experiments.

acid-treated mineral dust samples is tabulated in Table 1. A full description of XRD and X-ray photoelectron spectroscopy (XPS) analysis of substrates is provided in the Supporting Information (SI). Flow Tube Studies. Formation of HONO from the reactive uptake of NO2 on the various substrates was measured using a jacketed (296 K) horizontal flow tube (100 × 2 cm i.d.) equipped with a movable injector and attached to a CIMS.60,66,67 The inner surface of a 6.5 cm long Pyrex tube was coated with a thin film of the substrate and positioned inside the flow tube. Coatings were prepared as follows: Slurries were made from powder substrates (10 mL comprised of 1:1 (w/w) substrate/liquid) using either water (all soils, clay minerals, and SA-K mixtures) or a 1:3 (v/v) water/ethanol solution (for Fe oxides only) as the liquid phase. Aqueous solutions were prepared using nanopure water (18.2 MΩ·cm; Milli-Q Integral). The substrate slurry pH was measured using an OrionStar A211 pH meter (Thermo Scientific). The pH of all substrates was measured as recommended by Hendershot et al. using 1:2 soil/water (v/v) slurries.68 In some cases, the pH of the slurries was adjusted using H2SO4 (Fluka Analytical) and NaOH (EMD Chemicals) solutions. The substrate slurries were dripped onto the inner wall of the cylindrical glass tube and dried under gentle heat (∼40 °C) and a flow of nitrogen for 1−3 min. Coated tubes were stored in a chamber under a flow of air (1 L min−1 at 50% RH) for up to 1 h until they were ready to be used in an experiment. The resulting film was homogeneous to the eye, yielding a dry weight of 0.30 ± 0.02 g (for clay minerals, Fe oxides, and γ-Al2O3) and 1.0 ± 0.1 g (for soil samples) over the entire length of the tube. Previous studies have shown that uptake efficiency of trace gases on powder substrates increases until a critical substrate mass is reached, at which point uptake efficiency is constant and independent of substrate mass.69,70 Our substrate dry weight was chosen to be well within this mass-independent region to best represent an infinitely thick substrate layer typical of a soil surface. During an experiment, the coated tubes were positioned in the jacketed flow tube housing and equilibrated with a flow of air (50% RH, flow of 1.43 L min−1) for 30 min. RH was set by adjusting the flow of dry and humidified (water bubbler) ultrahigh purity air that was monitored with an in-line RH and temperature probe (Vaisala, HMT130). A constant flow of NO2 (Matheson Tri-Gas) diluted in ultrahigh purity air was established with the injector placed downstream of the coated tube such that none of the substrate coated surface was exposed to the gas flow. Reactions were initiated by retracting the movable injector to a position well behind the coated segment of the flow tube; this allowed for complete mixing and to establish a laminar flow of NO2 before it interacted with the coated film. The Reynolds number for these experiments was calculated to be 164, well within the laminar regime. Gas phase HONO was measured by CIMS using SF5¯ as the reagent ion.66,71,72 A more detailed description of this technique is available in the SI. The flux of HONO from the substrate surfaces was calculated from Flux = ((Css − C0) νa)/(SBET m). The variable Css is the steady-state concentration of HONO achieved following exposure of substrates to HONO, C0 is the background concentration of HONO in each experiment prior to retracting the flow injector, νa is the flow of NO2 and carrier gas in the reactor, SBET is the BET surface area of the substrate, and m is the mass of the substrate coating exposed to NO2. All experiments were done under strict exclusion of light. Teflon

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RESULTS AND DISCUSSION Evidence for Fe2+-Mediated Surface Redox Chemistry. Figure 1A provides a comparison of the HONO flux arising when 40 ppb of NO2 is exposed to surfaces with differing amounts of iron. For all experiments, substrates were in equilibrium with 1 atm of air at an RH of 50%. In addition, the slurries used to coat the Pyrex inner tube were adjusted to pH 4.5 to control for pH effects, and the dry weight of the coatings was kept constant. Reaction kinetics are displayed in terms of flux from the substrate surface to facilitate comparison of the rate of NO2-to-HONO conversion.

Figure 1. Evidence for the role of Fe2+ in the conversion of NO2 into HONO on iron oxide and Fe-bearing clay surfaces. (A) Measured fluxes of HONO emitted from the surface of iron-containing substrates and alumina stemming from the reaction of 40 ppb of NO2 in air. (B) Fluxes of HONO formed from the reaction of 50 ppb of NO2 in air on iron oxide surfaces are correlated to the amount of Fe2+(aq) extracted from the iron oxide substrate. All substrates were adjusted to pH 4.5 ± 0.3 and experiments were carried out at a total pressure of 1 atm and 50% RH. Error bars represent the 95% confidence interval of the mean of at least three measurements.

As shown in Figure 1A, the largest HONO flux originates from the reaction of NO2 on nontronite (NAu-2), whereas γAl2O3 yielded the lowest flux of all substrates measured. Previous studies demonstrated that the formation rate of HONO from reaction 1 displays a dependence on the surface area of the reaction cell used, with higher formation rates observed with high surface areas.15 If reaction 1 were the sole source of the HONO in our experiments, we would have expected the same amount of HONO to be formed on the 8651


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Figure 2. (A) Conversion of NO2-to-HONO on Fe-bearing clay minerals and soils collected from Arizona and the Sahara desert depends on both the native pH (B, circles) and amount of Fe2+(aq) extracted from the substrate (B, triangles); see Table 1 for a key to substrate abbreviations. (C) Nitrous acid fluxes from substrates are shown following extraction of Fe by oxalic acid. Also shown are the pH values of substrates following oxalate extraction (D, circles) and the amount of Fe2+(aq) present in the oxalic acid extracts (D, triangles). In all cases, the NO2 concentration was 40 ppb in air at 50% RH, 1 atm, and 296 K. Error bars represent the 95% confidence interval of the mean of at least three measurements. The limit of detection for Fe2+ is 1 mg/L.

various substrates since by definition, the flux calculation accounts for any surface area differences. The significant variability in observed HONO fluxes indicates that a different mechanisms for NO2 → HONO conversion is operational that is related to the properties of the substrate. Reactivity appears to be related to the presence of Fe in the substrate. For example, NAu-2 and γ-Fe2O3 contain the most Fe of all the substrates (see Table 1), followed by Wyoming montmorillonite (SWy-2), and kaolinite (SA-K); synthetic γAl2O3 contains no iron. The high HONO flux observed with NAu-2 and γ-Fe2O3 prompted us to consider that HONO may be formed via redox chemistry involving reaction 3. To test this, γ-Fe2O3 powder was suspended in H2O2 overnight to oxidize Fe2+ to Fe3+ according to reaction 6, which is the basis of the Fenton reaction.73 Fe 2 + + H 2O2 + H+ → Fe3 + + OH + H 2O

extracted from aqueous slurries of these substrates (Figure 1B). The procedure used to quantify the Fe2+ concentrations is provided in the SI. For each measured flux, the initial concentration of NO2 (50 ppb in air at 50% RH), the pH of the slurry used to make the coatings (pH 4.3), and the mass of the surface coating were identical to facilitate comparison. The observed trend in HONO flux, Fe3O4/γ-Fe2O3 > α-FeOOH > γ-Fe2O3 > α-Fe2O3, is proportional to the amount of Fe2+ extracted from aqueous slurries (pH 4.3) of each substrate, as determined by the photometric 1,10-phenanthroline assay (see lower half of Figure 1B). This provides further evidence that the NO2-to-HONO conversion rates are dependent on the Fe2+ content of the substrate. The amount of Fe2+ measured in aqueous slurries of the studied Fe oxides reflects the abundance of structural Fe2+ present in the mineral and the rate of reductive dissolution of the various Fe oxides. Consistent with Figure 1B, previous studies have shown that the least stable Fe oxides (Fe3O4, followed by α-FeOOH and γ-Fe2O3) exhibit more extensive reductive dissolution than the more stable hematite (αFe2O3).46,75,76 The nonstoichiometric magnetite substrate (Fe3O4/γ-Fe2O3)77 used shows the highest reactivity, as it consists of 30 wt % Fe3O4, which dissolves more rapidly than Fe3+ oxides due to its Fe2+ content and the fact that its Fe3+ occupies both tetrahedral and octahedral positions. In the case of γ-Fe2O3, the amount of Fe2+ (aq) extracted is likely influenced by the 2% Fe3O4 impurity present.78 It should also be mentioned that minor Fe2+ fractions present in the Fe3+ oxides likely catalyze additional reductive dissolution via an interfacial Fe2+-Fe3+ electron-transfer mechanism.79 In addition, particle size may also play a role in facilitating reductive dissolution of the Fe-oxides. The amount of Fe2+(aq) extracted from the substrates used in Figure 1B is anticorrelated with the size of the respective particles as determined by TEM; this is consistent with previous studies showing that smaller particles dissolve more rapidly.46,80−83 All other factors being equal, the results here indicate that the reactivity of NO2 on Fe oxide surfaces depends on the amount of Fe2+ present in the substrate, which is a function of the reductive dissolution rate of the Fe oxide.

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As shown in Figure 1A, the flux of HONO formed from H2O2-treated γ-Fe2O3 coatings is a factor of 5 less than the amount of HONO formed on untreated γ-Fe2O3. Powder XRD analysis indicates that the origin of the Fe2+ in untreated γFe2O3 likely stems from a ∼2 wt % magnetite (Fe2+(Fe3+)2O4) impurity present in the sample (see SI Figure S1). This provides support for the hypothesis that surface sites comprised of Fe2+ are responsible for the NO2 → HONO conversion on the Fe-bearing substrates. Assuming that the HONO generated on redox-inactive γ-Al2O3 was due solely to reaction 1, the data in Figure 1 suggest that a mechanism involving Fe2+ in Febearing minerals is between 3−20 times more effective at converting NO2 to HONO than a hydrolysis mechanism, depending on the substrate. This observation is consistent with previous Knudsen cell measurements showing that the uptake coefficients (the fraction of surface collisions resulting in a reaction) for NO2 adsorption onto γ-Fe2O3 surfaces are 2 orders of magnitude greater than that measured for γ-Al2O3.74 Relationship between HONO Flux and Fe2+ Content in Fe-Oxides. The relationship between HONO formation and Fe2+ was further tested by comparing the reactivity of NO2 on well-defined (synthetic) iron oxides with the amount of Fe2+ 8652


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Figure 3. (A) Mineralogical composition of the magnetic fraction of soil samples as determined by powder X-ray diffraction; the “other” category consists of non Fe-bearing constituents (see SI). (B) Comparison of the Fe 2p3/2 binding energies (taken from main peak center of gravity) for Fe2+ and Fe3+ (hydr)oxides, oxides, and silicates. Green bars represent binding energies characteristic of Fe3+ bound to SiO44−, while orange bars are binding energies for Fe-oxides and Fe2+-containing minerals. Asterisks indicates values taken from references 85 and 87.

NO2-to-HONO Conversion on Fe-Bearing Soil and Clay Minerals. Further studies were conducted on Fe-bearing clay minerals and soil samples collected from Arizona and the Sahara desert to investigate the reaction of NO2 with Fe2+ on environmentally relevant substrates. Kinetic data from flowtube studies and information about Fe solubility and mineralogy were coupled to examine the relationship between Fe-bearing minerals and the observed NO 2 -to-HONO conversion rates. The soil samples were chosen because wind-blown mineral dust derived from these arid regions is known to impact atmospheric composition, including in polluted air masses.45,84 In addition, their low organic matter content ensures the results will be minimally impacted by organic redox chemistry. Figure 2A shows that the HONO flux stemming from the reaction of NO2 on Fe-bearing substrates at their native pH is highly variable, with the highest fluxes observed for samples collected from Arizona (Az-1) and the Sahara (Sah-1 and Sah2) Deserts. Two clay minerals (NAu-2 and PFl-1) and two soil

samples from other parts of Arizona (Az-2 and Az-3) were an order of magnitude less reactive by comparison. There is no relationship between the observed HONO flux and the total Fe content determined by ICP/MS (0.4−24 wt %) or XPS (0.3− 21 wt %) analysis of the samples (Table 1). These observations suggest that variations in HONO flux are not due to differences in total Fe content, but rather to the reactivity of Fe in each sample. We quantified Fe2+(aq) in aqueous slurries of the soils and clays (at their native pH) and found that Az-1 and Sah-1 were the only samples in which Fe2+(aq) concentrations were above the limit of detection (1 mg/L) of our spectroscopic method (defined as three times the standard deviation of the background signal of the spectrophotometer). That Az-1 and Sah-1 produced the most HONO and yielded the highest extractable Fe2+(aq) concentration present in the aqueous slurry further supports the role of Fe2+ in facilitating NO2 reduction. In addition to the presence of Fe2+, the amount of HONO formed on Az-1 appears to be influenced by the pH of 8653


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Environmental Science & Technology the aqueous slurry used to coat the flow tube wall (Figure 2A). This is expected based on equilibrium considerations: The amount of HONO formed will increase as the pH approaches the pKa of HONO. Consistent with this, the highest HONO flux was observed for Az-1, which has a pH that is ∼2 pH units lower than the other substrates studied. Interestingly, Sah-1 and Sah-2 soil produced the second and third highest HONO fluxes measured, although the pH values of these substrates in their aqueous suspensions were 8.65 and 8.74, respectively. The absence of detectable Fe2+ in the Sah-2 sample likely reflects the insolubility of the iron-bearing phase at near neutral pH. This implies that soil minerals play a dual role of a reductant and proton donor with respect to NO2-to-HONO conversion. The role of mineralogy and surface pH in this reaction is explored in more detail below. The Role of Poorly Crystalline Fe (Hydr)oxides. A significant fraction of iron in the soil samples may also be present as poorly crystalline Fe (hydr)oxides such as the ubiquitous ferrihydrite; these are derived from weathering of other Fe-bearing minerals in soil and would be difficult to quantify via XRD.49,53 In addition, poorly crystalline Fe (hydr)oxides may also be adsorbed to the surface of the clay minerals studied here as they were used directly from their source material, without further purification. A widely used method to selectively remove ferrihydrite from soil involves extraction by oxalic acid (OA) in the dark;50 the method is based on the fact that ferrihydrite dissolves up to three orders of magnitude faster than the other crystalline Fe (hydr)oxides. To assess the role of poorly crystalline Fe (hydr)oxides on soil reactivity toward NO2, we extracted the soil and clay minerals with OA and determined the HONO flux from the modified substrates with the coated-wall flow reactor. Figure 2C shows the HONO flux resulting when NO2 is reacted over soil samples and clay minerals that were subjected to oxalic acid extraction. Extraction with OA was accompanied by a significant decrease in the surface area of the soils (Table 1), indicating that poorly crystalline Fe phases contribute to 30−50% of the surface area of these substrates. Notably, the OA-extracted Az-1 and Sah-1 soils produced 2−3 times less HONO than they did in their native state, although the acidic nature (pH ∼ 4) of the OA-extracted substrates would favor release of HONO from the surface. The highest HONO flux was observed from Az-2 and Az-3 following extraction with OA, although these samples were among the most unreactive in their native state. Interestingly, the amount of Fe2+ extracted from Az-2 and Az-3 was around four times less than what was extracted from the other substrates even though the amount of total Fe present in these soils was relatively high (Table 1). In the case of Az-2 and Az-3, the high buffering capacity of these alkaline samples precluded the OA extract from reaching the acidic pH required for efficient extraction of Fe (ideally the extraction pH should be between 3−4).50 Thus, the higher reactivity of Az-2 and Az-3 relative to the other substrates likely reflects a higher abundance of Fe (hydr)oxides left over in the samples combined with the drop in pH (from ∼9 to 5) induced by the OA treatment. The Role of Soil Mineralogy. Powder XRD showed the presence of Fe-bearing minerals in the magnetic fraction of all soil samples (Figure 3A, SI Table S3). For example, Arizona samples contained magnetite and the mixed-valence silicate minerals augite and hornblende; Sahara desert soil contained magnetite, augite, hornblende, and ilmenite. However, there were no systematic trends between HONO flux and the

abundance of crystalline Fe2+-bearing in soil samples. X-ray photoelectron spectroscopy was used to assess the bonding environment of Fe present in soil and clay mineral samples. High resolution scans of the Fe 2p region were collected and the binding energies of the Fe 2p3/2 peak in soil and clay mineral samples are compared to those of various Fe oxides, (hydr)oxides, and silicates in Figure 3B. The binding energies of the Fe 2p3/2 peak for the soil and clay mineral samples fall within two categories. First, soil samples Az-1, Az-3, and the clay mineral palygorskite (PFl-1) in their native states have Fe 2p3/2 binding energies in the range 711.2−711.5 eV, which we attribute to Fe3+ associated with Fe oxides (e.g., hematite, magnetite) and (hydr)oxides (e.g., ferrihydrite) present in these samples.85,86 Second, samples labeled Sah-1, Sah-2, Az-2, and the clay minerals SA-K, NAu-2, and SWy-2 have Fe 2p3/2 peak binding energies that fall within a narrow range of 712.2−712.5 eV and are ∼1 eV higher than the Fe (hydr)oxides such as hematite or ferrihydrite. We attribute this to Fe3+ associated with silicates (i.e., Fe substituting for Al in clay minerals, or complexed to siloxane surfaces or OH-groups at edges as depicted in SI Figure S3). This assignment is based on the following observations: (i) the Fe-bearing mineral nontronite NAu-2 with its octahedral Fesilicate layers has an Fe 2p3/2 binding energy of 712.4 eV that shifts to 712.8 eV when ancillary Fe (hydr)oxides are removed via oxalate extraction (see NAu-2a); (ii) the Fe 2p3/2 binding energy of the ferric silicates such as andradite (Ca3(Fe3+) 2(SiO4) 3) occurs at 712.3 eV;87,88 and (iii) numerous studies have assigned Fe 2p3/2 peaks at ∼713 eV to Fe3+ complexed to silica surfaces.89 Indeed, adsorption of Fe3+ (from Fe2(SO4)3) on SA-K at pH 6 results in a binding energy for the Fe 2p3/2 peak of 712.9 eV (Figure 3A). The higher Fe 2p3/2 binding energy for Fe3+-silicates relative to the Fe3+-oxides likely reflects the greater difficulty in removing the Fe electrons from Fe bound to SiO44− than to O2−.87 Considering soil reactivity (e.g., Figure 2) in the context of surface Fe species and mineralogy, it is reasonable to conclude that Fe present in Az-1 is influenced mostly by the Fe (hydr)oxides (i.e., Fe2+ supplied from magnetite or via reductive dissolution of hematite or poorly crystalline Fe (hydr)oxides). For example, the Fe 2p3/2 binding energy for Az1 was indicative of Fe-oxides, and the ratio of Fe-bearing silicates to Fe-oxides (Σ(Fe-Silicates)/Σ(Fe-oxides); see SI Table S3) determined by XRD was 0.4. The Σ(Fe-Silicates)/ Σ(Fe-oxides) ratios for Sah-1, Sah-2, and Az-2 samples are 1.4, 1.6, and 2.0, respectively; this is consistent with XPS data suggesting that Fe-bearing silicates are dominant in these samples. An intermediate case is observed for Az-3, which has Σ(Fe-Silicates)/Σ(Fe-oxides) ratio of near unity and whose Fe 2p3/2 binding energy is at the higher limit of the Fe (hydr)oxides category. Taken as a whole, the data show that Fe2+ can be derived from a combination of Fe-bearing minerals present in a single sample, although the more soluble phases (e.g., ferrihydrite) may contribute disproportionately more Fe2+(aq). Influence of Substrate pH. It is clear that pH is an important variable controlling both Fe2+ availability and N(III) speciation (where N(III) ≡ NO2−, HONO, and H2ONO+). To examine the effect of pH on NO2-to-HONO conversion, we characterized the HONO flux-pH relationship for five different natural Fe-bearing minerals and mixtures (Fe3O4, 1% Fe3O4/ SA-K (w/w), 1% Fe2(SO4)3/SA-K (w/w), NAu-2, and SA-K) using the coated-wall flow reactor. Aqueous slurries of known 8654


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Figure 4. (A−E, J) pH profiles for HONO flux stemming from NO2 reduction on the indicated iron bearing surfaces. Initial NO2 concentration was 40 ppb in air at 50% relative humidity, 1 atm and 296 K. Gaussian peaks were used to fit the measured data to highlight HONO formation processes A (red) and B (green) described in the text. (F−I) Amount of Fe2+(aq) extracted from the indicated substrates increases with decreasing slurry pH. Error bars represent the 95% confidence interval of the mean of at least three measurements.

amounts of the minerals were pH-adjusted, coated onto the flow reactor wall, dried, and equilibrated at 50% RH prior to exposure to NO2. The steady-state fluxes of HONO generated from reaction of NO2 on the various substrates over the pH range 1−12 is shown in Figure 4; we refer to these plots as pHprof iles below. In addition, we measured the pH profile for Fe2+(aq) present in aqueous suspensions of each Fe-bearing substrate and plot them in Figure 4F−I. The pH profiles of six Fe-bearing substrates studied (Figure 4A−E, J) are characterized by two different processes that occur over two distinct pH ranges. To highlight the relative importance of the two processes, the pH profiles were fit to two overlapping Gaussian functions, also displayed in Figure 4. The first process (process A) is defined by a peak in HONO flux centered at pH 3.8. As pH drops below 3.8, an increasing fraction of the HONO formed will be protonated and exist as surface-adsorbed nitrous acidium cation (H2ONO+), the pKa1 of which is 1.7.64 As the peak HONO flux in the acidic pH range coincides with the high amounts of dissolved Fe2+ present (see Figure 4F−I), we attribute process A to the reduction of NO2 by Fe2+(aq)36 present in the thin film of water coating the substrates at 50% RH.90 It is well-known that reductive dissolution of Fe3+ to Fe2+ is favored under acidic conditions.46,47 This is especially evident when Fe3O4 is the substrate (Figure 4A), where structural Fe2+ dissolves at low pH.78 This also coincides with the pH range in which HONO is expected to be the dominant N(III) species based on wellknown aqueous equilibria.63,64

Remarkably, HONO also appears to be formed at near neutral pH in a second process (process B), with a peak that occurs between 5.5 and 6well above the aqueous pKa of HONO. For example, when Fe3O4 is diluted in a mixture with SA-K (1% w/w) (Figure 4B) process A is still operational, but a second peak in HONO flux is evident at pH 6 with HONO production tailing off to negligible amounts at pH 9. Similar pH profiles are observed when SA-K is treated with 1% Fe2(SO4)3 (Figure 4C) or when the smectite NAu-2 is used as the substrate (Figure 4D). The data in Figure 4 suggest that process B must be due to a mechanism other than reduction by aqueous Fe2+ as the pH profiles in Figure 4F−I show that Fe2+(aq) is not present at pH > 5. We hypothesize that process B stems from the reduction of NO2 by Fe2+ that is complexed to surface hydroxyl groups or exists as structural iron (i.e., present in an octahedral sheet as is the case for NAu-2, smectites and SA-K; see Figure 3B).56,91,92 To test this, we measured a pH profile for NAu-2 that had been extracted with OA and rinsed multiple times to remove the ancillary Fe oxides present as impurities in the native sample. Whereas NAu-2 in its native state showed HONO to be formed from process A and B, HONO was only formed via process B in the OA-extracted sample (NAu-2a). This indicates that process A was due to reaction of NO2 with Fe2+(aq) generated via reductive dissolution of ancillary Fe (hydr)oxides present in the native NAu-2 sample,93 whereas process B is due to surface complexed iron or structural Fe2+. This is further supported by XPS analysis of the NAu-2a sample, which shows the Fe 2p3/2 8655


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Considering this, it would seem the NO2 hydrolysis mechanism (reaction 1) may not always provide an accurate explanation for HONO production on ground-level surfaces. Exceptions to this include: (a) When high concentrations of NO2 are in contact with wet surfaces (e.g., engineered systems); (b) when NO2 encounters a surface where the Fe2+ concentration is too low; or (c) when NO2 is generated as a solvated reaction product in aqueous systems (e.g., during nitrate photolysis).101 Clearly, future efforts to measure and model HONO production need to consider the presence of redox active reactive sites on boundary layer surfaces when distinguishing between thermal sources of HONO. In addition to the arguments above, the environmental relevance of the Fe2+ + NO2 reaction in soil can be assessed in the context of recent field studies that observed nighttime NO2to-HONO conversion on ground-level surfaces.13,14 These field studies modeled the vertical profile of measured HONO concentrations to derive reactive uptake coefficients for NO2to-HONO conversion (γNO2). Here, the parameter γNO2 is defined as the fraction of collisions of NO2 with the surface that result in the formation of a HONO molecule.102 Wong et al. found that a γNO2 value of 10−5 was sufficient to model ambient levels of HONO in Houston, TX;13 Vandenboer et al. used a model of the vertical profile of HONO concentrations measured during the NACHTT-11 campaign to derive γNO2 values of (0.2−1.6) × 10−5.14 Using the methods described in the Supporting Information, we derived values of γNO2 for Febearing soil samples (Sah-1, Sah-2, Az-1, Az-2, and Az-3) that are between (0.1−2.9) × 10−5. The agreement between these value and those inferred from field data support that HONO formation on Fe-bearing soil surfaces is environmentally relevant. In addition to its importance in soil, it is reasonable to conclude that Fe present in airborne mineral dust can also impact atmospheric levels of NO2 and HONO. After all, the composition of airborne mineral dust reflects that of the crustal material from which it is derived.45 As mineral dust undergoes long-range atmospheric transport, it incorporates sulfuric acid and other strong acids.103 Over time, the amount of acid added will exceed the alkalinity of the deliquesced aerosol droplet; the corresponding drop in pH enhances dissolved Fe2+ concentrations.44,81,104−108 This acid dissolution mechanism has been postulated to be a significant source of soluble Fe to marine phytoplankton.107−111 Thus, there is significant evidence suggesting that Fe2+ may be sufficiently abundant in mineral dust aerosols to influence NO2 reactivity on these surfaces. Evidence for Fe-promoted reactive nitrogen chemistry on mineral dust comes from measurements of HONO and NO2 during dust storms. Wang et al. documented such events during a field study in Phoenix, AZ in 2001.84 In that study, differential optical absorption spectroscopy measurements revealed that the [HONO]/[NO2] ratio in air increased from a typical value of ≤0.03 in the absence to dust to near 0.19 during two different dust storm events; this observation provided evidence that mineral dust enhances NO2 → HONO conversion. Although Wang et al. did not measure the composition of mineral dust, it is possible that the high [HONO]/[NO2] ratios observed stemmed from Fe redox chemistry. This conclusion is based on the following line of reason: (a) The evidence presented above suggests the reactivity of NO2 with Az-1, Az-2, and Az-3 soil samples are likely driven by Fe2+ redox chemistry; (b) the dust storms encountered in the study by the Wang et al. study occurred in the region where we obtained our Arizona

peak binding energy at 712.8 eV, indicative of Fe complexed to SiO44−.87−89 We also measured pH profiles for SA-K to assess its contribution to HONO formation in the Fe-SA-K mixtures shown in Figure 4B and C. The pH profile shown in Figure 4J demonstrates that HONO is produced on SA-K by process B, albeit to a lesser degree than what is evident for NAu-2 and for mixtures of SA-K with Fe3O4 or Fe2(SO4)3. XPS analysis of SAK indicates the presence of 0.6 wt % Fe with a Fe 2p3/2 peak at 712.4 eV, consistent with Fe associated with silicate; no crystalline Fe phases were detected via XRD. It should also be mentioned that Fe2+(aq) from SA-K was below the detection limit of our assay over the pH range 1−12. Thus, we attribute the NO2-to-HONO conversion observed in Figure 4J to surface-complexed or structural Fe present in SA-K. The presence of structural Fe present in SA-K has been discussed previously with strong evidence for its existence coming from NMR studies.94 This conceptual model is also consistent with previous results showing that structural Fe2+ in clay minerals reduces pollutants over a range of redox conditions.55,92,95,96 With respect to surface acidity, relatively small amounts of complexed Fe and adsorbed hydrolysis products ( 5, rapid hydrolysis of Fe2+ at near-neutral pH yields Fe(O,OH)6 polymers that ultimately cover surfaces with an amorphous Fe (hydr)oxide coating.97,98 One might assume that these amorphous coatings are similar to ferrihydrite, which both theory and experiment show has a PZC of ∼8.53,58 At the PZC, the Fe (hydr)oxide surface is partially hydroxylated and the surface density of Fe− OH2+ and Fe−O− is equal.99 We showed previously that amorphous Al and Fe (hydr)oxides present in soil containing M−OH2+ surface groups are capable of protonating NO2− at a bulk pH that is 3 to 4 pH units above the aqueous pKa of HONO.60 In addition, previous flux chamber studies show that HONO emissions are possible from soils with pH values of up to 8.8.100 Thus, it appears that process B proceeds via reduction of NO2 to nitrite on Fe (hydr)oxide coatings, followed by protonation of NO2− by Fe−OH2+ surface groups that are present up to a pH of ∼8. This process is consistent with the data in Figure 2, which show that HONO is formed on the Arizona and Sahara Desert soil with pH values between 5 and 8. Above pH 8, NO2 can still be reduced by structural or surfacecomplexed Fe2+.91 However, the predominance of Fe−O− surface groups at higher pH means there is insufficient proton density to promote reaction 4, thereby leading to accumulation of adsorbed NO2− at pH > 8. Environmental Implications. As indicated by the data presented above, iron in soil plays three important roles in converting ambient NO2 to HONO. First, Fe-bearing minerals common to soil provide Fe2+(aq) or redox active surface sites on which to reduce NO2. Second, amphoteric Fe (hydr)oxides that coat soil surfaces are capable of protonating NO2− and driving release of HONO into the atmosphere over a wide range of soil pH; this includes soils that are considered to have near-neutral pH. Third, the high surface area contributed by Febearing clay minerals and poorly crystalline Fe (hydr)oxide coatings, which constitute a major fraction of soil reactive surface area, increases the probability that NO2 will adsorb to reactive Fe sites in soil. Primary Fe-bearing minerals and their pedogenic (hydr)oxides are ubiquitous in soil and mineral dust surfaces; thus, NO2 reactivity is likely not limited by the availability of iron reactive sites on ground-level surfaces. 8656


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Allegrini, I. Nitrous acid in the urban area of Rome. Atmos. Environ. 2006, 40, 3123−3133. (7) Sörgel, M.; Regelin, E.; Bozem, H.; Diesch, J. M.; Drewnick, F.; Fischer, H.; Harder, H.; Held, A.; Hosaynali-Beygi, Z.; Martinez, M.; Zetzsch, C. Quantification of the unknown HONO daytime source and its relation to NO2. Atmos. Chem. Phys. 2011, 11, 10433−10447. (8) Stutz, J.; Alicke, B.; Neftel, A. Nitrous acid formation in the urban atmosphere: Gradient measurements of NO2 and HONO over grass in Milan, Italy. J. Geophys. Res. 2002, 107, LOP 5−1−LOP 5−15. (9) Vogel, B.; Vogel, H.; Kleffmann, J.; Kurtenbach, R. Measured and simulated vertical profiles of nitrous acid-part II. Model simulations and indications for a photolytic source. Atmos. Environ. 2003, 37, 2957−2966. (10) Wong, K. W.; Oh, H.-J.; Lefer, B. L.; Rappenglück, B.; Stutz, J. Vertical profiles of nitrous acid formation in the nocturnal urban atmosphere of Houston, TX. Atmos. Chem. Phys. 2011, 11, 3595− 3609. (11) Villena, G.; Kleffmann, J.; Kurtenbach, R.; Wiesen, P.; Lissi, E.; Rubio, M. A.; Croxatto, G.; Rappenglück, B. Vertical gradients of HONO, NOx and O3 in Santiago de Chile. Atmos. Environ. 2011, 45, 3867−3873. (12) Wong, K. W.; Tsai, C.; Lefer, B.; Haman, C.; Grossberg, N.; Brune, W. H.; Ren, X.; Luke, W.; Stutz, J. Daytime HONO vertical gradients during SHARP 2009 in Houston, TX. Atmos. Chem. Phys. 2012, 12, 635−652. (13) Wong, K. W.; Tsai, C.; Lefer, B.; Grossberg, N.; Stutz, J. Modeling of daytime HONO vertical gradients during SHARP 2009. Atmos. Chem. Phys. 2013, 13, 3587−3601. (14) VandenBoer, T. C.; Brown, S. S.; Murphy, J. G.; Keene, W. C.; Young, C. J.; Pszenny, A. A. P.; Kim, S.; Warneke, C.; de Gouw, J. A.; Maben, J. R.; Wagner, N. L.; Riedel, T. P.; Thornton, J. A.; Wolfe, D. E.; Dubé, W. P.; Ö ztürk, F.; Brock, C. A.; Grossberg, N.; Lefer, B.; Lerner, B.; Middlebrook, A. M.; Roberts, J. M. Understanding the Role of the Ground Surface in HONO Vertical Structure: High Resolution Vertical Profiles During NACHTT-11. J. Geophys. Res. Atmos. 2013, 118, 1−17. (15) Finlayson-Pitts, B. J.; Wingen, L. M.; Sumner, A. L.; Syomin, D.; Ramazan, K. A. The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: an integrated mechanism. Phys. Chem. Chem. Phys. 2003, 5, 223−242. (16) Stutz, J.; Alicke, B.; Ackermann, R.; Geyer, A.; Wang, S. H.; White, A. B.; Williams, E. J.; Spicer, C. W.; Fast, J. D. Relative humidity dependence of HONO chemistry in urban areas. J. Geophys. Res. 2004, 109, D03307. (17) Pusede, S. E.; VandenBoer, T. C.; Murphy, J. G.; Markovic, M. Z.; Young, C. J.; Veres, P. R.; Roberts, J. M.; Washenfelder, R. A.; Brown, S. S.; Ren, X.; Tsai, C.; Stutz, J.; Brune, W. H.; Browne, E. C.; Wooldridge, P. J.; Graham, A. R.; Weber, R.; Goldstein, A. H.; Dusanter, S.; Griffith, S. M.; Stevens, P. S.; Lefer, B. L.; Cohen, R. C. An Atmospheric Constraint on the NO2 Dependence of Daytime Near-Surface Nitrous Acid (HONO). Environ. Sci. Technol. 2015, 49, 12774−12781. (18) Carter, W. P. L.; Atkinson, R.; Winer, A. M.; Pitts, J. N. J. Experimental investigation of chamber-dependent radical sources. Int. J. Chem. Kinet. 1982, 14, 1071−1103. (19) Sakamaki, F.; Hatakeyama, S.; Akimoto, H. Formation of nitrous acid and nitric oxide in the heterogeneous dark reaction of nitrogen dioxide and water vapor in a smog chamber. Int. J. Chem. Kinet. 1983, 15, 1013−1029. (20) Wang, J.; Koel, B. E. IRAS studies of NO2, N2O3, and N2O4 adsorbed on Au(111) surfaces and reactions with coadsorbed H2O. J. Phys. Chem. A 1998, 102, 8573−8579. (21) Goodman, A. L.; Miller, T. M.; Grassian, V. H. Heterogeneous reactions of NO2 on NaCl and Al2O3 particles. J. Vac. Sci. Technol., A 1998, 16, 2585−2590. (22) Goodman, A. L.; Underwood, G. M.; Grassian, V. H. Heterogeneous reaction of NO2: characterization of gas-phase and adsorbed products from the reaction, 2NO2(g)+H2O(a) → HONO-

soil samples; (c) it is likely that the dust from the storm and our soil samples have similar minerology. Finally, these results have important implications for designing and operating instruments that measure NO2 and HONO in the laboratory and field. For example, precautions are necessary to avoid redox active surfaces (e.g., steel)112 as a material for inlets and fittings. In addition, the accumulation of soil or mineral dust particles in tubing over time may decrease the transmission of NO2 through tubing sampling lines and lead to high background levels of HONO. This could lead to significant errors in ambient measurement data, especially when the aim is to measure NO2 or HONO at very low concentrations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b01915. Details of the Powder XRD, XPS, and CIMS measurements; description of procedures used to extract Fe from soil samples; additional details of the NO2 reactivity studies; description of how reactive uptake coefficients were derived (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +1-(812)-855-6525; e-mail: JDRaff@indiana.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from a National Science Foundation CAREER Award (AGS-1352375) and Indiana University is gratefully acknowledged. Some XPS measurements were performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory. We thank Dr. Maren Pink for powder X-ray diffraction analysis of iron oxide and kaolinite samples. We also thank Steve Cook, and Erin Cook for collecting soil samples; Dr. Barry Stein who helped with transmission electron microscopy.

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REFERENCES

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