Evidence for GasâSurface Equilibrium Control of Indoor Nitrous Acid

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Environmental Processes

Evidence for Gas-Surface Equilibrium Control of Indoor Nitrous Acid Douglas B. Collins, Rachel F. Hems, Shouming Zhou, Chen Wang, Eloi Grignon, Masih Alavy, Jeffrey A Siegel, and Jonathan P.D. Abbatt Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04512 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

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Environmental Science & Technology

Evidence for Gas-Surface Equilibrium Control of Indoor Nitrous Acid Douglas B. Collins,a,1,* Rachel F. Hems,a Shouming Zhou,a Chen Wang,a Eloi Grignon,a Masih Alavy,b Jeffrey A. Siegel,b,c Jonathan P.D. Abbatta

aDepartment

of Chemistry, University of Toronto; 80 St. George Street, Toronto, ON, Canada, M5S 3H6

bDepartment

of Civil and Mineral Engineering, University of Toronto; 35 St. George Street, Toronto, ON, Canada, M5S 1A4

cDalla

Lana School of Public Health, University of Toronto; 223 College Street, Toronto, ON, Canada, M5T 1R4

*Corresponding Author: Douglas B. Collins, [email protected]

ACS Paragon Plus Environment

1


1Now

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at: Department of Chemistry, Bucknell University; 1 Dent Drive, Lewisburg, PA 17837

KEYWORDS: nitrous acid, nitrogen dioxide, indoor chemistry, multiphase chemistry, chemical ionization mass spectrometry

1

ABSTRACT

2

Nitrous acid (HONO) is an important component of indoor air as a photolabile precursor

3

to hydroxyl radicals and has direct health effects. HONO concentrations are typically

4

higher indoors than outdoors, although indoor concentrations have proved challenging to

5

predict using box models. In this study, time-resolved measurements of HONO and NO2

6

in a residence showed that [HONO] varied relatively weakly over contiguous periods of

7

hours, while [NO2] fluctuated in association with changes in outdoor [NO2]. Perturbation

8

experiments were performed in which indoor HONO was depleted or elevated and were


2

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9

interpreted using a two-compartment box model. To reproduce the measurements,

10

[HONO] had to be predicted using persistent source and sink processes that do not

11

directly involve NO2, suggesting that HONO was in equilibrium with indoor surfaces.

12

Production of gas phase HONO directly from conversion of NO2 on surfaces had a weak

13

influence on indoor [HONO] during the time of the perturbations. Highly similar temporal

14

responses of HONO and semi-volatile carboxylic acids to ventilation of the residence

15

along with the detection of nitrite on indoor surfaces support the concept that indoor

16

HONO mixing ratios are controlled strongly by gas-surface equilibrium.

17

TOC GRAPHIC

18


3


19

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Introduction

20

Oxides of nitrogen are common air pollutants and are key participants in the production

21

and cycling of atmospheric oxidants.1 Nitric oxide (NO) is a well-known primary emission

22

product of high temperature combustion. In the atmosphere, it can react with O3 or peroxy

23

radicals to form NO2, which can subsequently photolyze or react on surfaces.1 Within the

24

indoor environment, reactive uptake of NO2 to surfaces is an important removal pathway,2

25

given low levels of photons with sufficient energy for photolysis (λ < 400 nm) in regions

26

that are not directly sunlit.3,4 Uptake of NO2 to indoor surfaces can result in the

27

heterogeneous formation of nitrous acid (HONO):

28

𝑠𝑢𝑟𝑓𝑎𝑐𝑒

𝑁𝑂2

(R1)

𝐻𝑂𝑁𝑂

29

by various mechanisms including water-mediated disproportionation5 and one-electron

30

reduction reactions with electron donating organic compounds (e.g., phenols).6,7 NO2

31

reduction reactions with light-absorbing organic or semi-conducting inorganic substrates

32

can be photo-enhanced.7–13 Some NO2 surface uptake may also lead to adsorption,14

33

nitrate

34

generalized by R1 have been studied for decades5,16–19 on various types of

production,14

and/or

organo-nitrogen

formation.15

Altogether,

processes


4

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35

surfaces7,9,14,20–22 and have been incorporated into chemical models of the indoor

36

environment with a variety of parameterized mechanisms.23–25 HONO mixing ratios

37

indoors (see Section S1) have been mostly attributed to R1, although the direct impact of

38

the NO2 uptake process on indoor [HONO] has been questioned.17,24 Understanding the

39

processes that regulate indoor [HONO] is important due to its direct inhalation toxicity and

40

impact as a source of hydroxyl radicals via UV photolysis.26–28

41

The nocturnal planetary boundary layer is an interesting analogue for the indoor

42

environment; HONO has been shown to accumulate in the boundary layer at night,

43

although the accumulation rate slows as [HONO] increases.29,30 Consequently, Stutz et

44

al.29 proposed that HONO is in a steady state in the dark, resulting from the co-existence

45

of competing source and sink processes. It is known that HONO can be lost from the gas

46

phase due to surface uptake,2,20,31–33 which likely dominates the sink term in the dark.

47

HONO can be produced by reactions of NO2 on surfaces,2,5,6,18,34 from nitrite35 on mineral

48

and salt-containing surfaces depending on surface acidity,36 and/or from displacement

49

due to uptake of stronger acids (e.g., HCl, HNO3) taken up from the gas phase.33 Spicer

50

et al.2 note that while net gas-phase HONO production is observed at high [NO2] within


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51

an indoor environment, an equilibrium process is most likely active, wherein the authors

52

proposed that NO2- resides on surfaces as a precursor reservoir of gas-phase HONO.

53

Wainman et al.17 suggested an effective Henry’s Law equilibrium to explain the co-

54

dependence of RH and slow emissions of gaseous HONO, suggesting that HONO (or

55

NO2-) may be in aqueous solution on surfaces. The concept that gaseous HONO is in

56

equilibrium with surfaces via R2 and R3 indicates that there are active source and sink

57

processes occurring simultaneously and continuously within buildings, and that levels of

58

gaseous HONO are not necessarily sensitive to NO2 uptake.

59

𝐻𝑂𝑁𝑂(𝑔)→𝐻𝑂𝑁𝑂(𝑠𝑢𝑟𝑓)

(R2)

60

𝐻𝑂𝑁𝑂(𝑠𝑢𝑟𝑓)→𝐻𝑂𝑁𝑂(𝑔)

(R3)

61

The present study uses high frequency measurements of trace gases paired with a

62

chemical box model to explore the processes influencing HONO and NO2 in a residence.

63

Measurements from periods without deliberate experimentation are presented, showing

64

the near steady state behavior of HONO. Experimental perturbations to [HONO] and

65

[NO2] were conducted to explore the kinetics as the system relaxed from a perturbed state

66

back to steady state. Box modeling of both positive and negative perturbations to [HONO]


6

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67

illustrate the need for equilibrium-type processes to re-establish a steady state

68

concentration of gaseous HONO ([HONO]ss) within the residence. Similarity between

69

HONO and semivolatile organic acid behavior after residence ventilation, along with

70

detection of nitrite on indoor surfaces, provide support for the importance of gas-surface

71

equilibrium of HONO. We stress that the equilibrium processes R2 and R3 are distinct

72

from the initial sources of HONO indoors, which arise from R1 and primary combustion

73

emissions.

74

Materials and Methods

75

A brief overview of the study location, experimental procedures, measurements, and

76

model construction will be provided here with further details provided in the Supporting

77

Information (SI; Sections S2 to S7).

78

Location and Experiment Design

79

Measurements were conducted in an inhabited, semi-detached residence in Toronto

80

during November 2016. The residence was equipped with a forced air recirculation

81

system (no ventilation) in which the fan was set to operate continuously; heating and


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82

cooling functions were not operated during the study. Three types of conditions were

83

considered: non-experiment periods in which experimental activities were not being

84

performed (but the home was inhabited by two people), outdoor air ventilation

85

experiments that rapidly reduced [HONO] in the kitchen, and combustion experiments

86

that provided a primary source of HONO to the indoor space. All experimental

87

perturbations to indoor air composition focused on the kitchen; instrumentation was

88

located in a space adjacent to the kitchen (Figure S1). Ventilation experiments were

89

conducted by opening a door and windows in the kitchen to allow rapid indoor-outdoor air

90

exchange. A single research-grade cigarette (1R6F) was burned in the kitchen shortly

91

after closing the windows and doors; this activity will be largely neglected, as its effects

92

on nitrogen oxide chemistry were minor. Combustion experiments were performed using

93

an open flame stove burner with public utility natural gas fuel (Table S2). One large burner

94

was operated during each experiment at maximum output with no hardware in contact

95

with the flame. When noted, a cast iron pot support and stainless steel pot containing tap

96

water were placed over the flame.


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97

Trace gases were sampled from multiple locations during the study: kitchen, outside,

98

and (during some periods) at the air return vent on the first floor of the building (Figure

99

S1), although most of this study focused on kitchen and outdoor measurements. Sample

100

air was drawn through Teflon tubing (4.57 mm inner diameter; PFA) at each location to a

101

centralized set of 3-way Furon solenoid valves (Saint-Gobain). Valve states that

102

controlled the sampling location were automated and logged (1 Hz) using a custom

103

LabView control interface (National Instruments). Sample gas residence time in the tubing

104

was < 5 s. Zero air (Air grade zero 0.1; Linde [Canada], Inc.) was sampled by all

105

instruments for a period of 2.5 minutes every 60 minutes. After sampling zero air, all

106

instruments were allowed to stabilize for 2.5 minutes before resuming normal sampling.

107

Trace Gas Measurements

108

NO and NO2 were measured using a chemiluminescence trace gas monitor (Thermo

109

Scientific, Model 42i). The instrument was equipped with a blue light converter (Air Quality

110

Design, Inc) for selective observation of NO2. Uncertainty in NO and NO2 was

111

conservatively estimated to be ±10%. Interference from RO2• formed within the blue light

112

converter37 may have caused NO2 measurements to represent a lower limit when photo-


9


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113

labile organic compounds (e.g., glyoxal) were present. CO2 and CO measurements

114

(Thermo Scientific, Model 410i and Model 48i) were used to estimate the rates of air

115

exchange with outdoors and intra-zonal mixing within the building, respectively (Section

116

S5). Air introduced to an O3 instrument (Thermo Scientific, Model 49i) was passed

117

through a Teflon filter according to manufacturer specifications to limit interference from

118

aerosol particle scattering. Measurements of NO, NO2, O3, CO and CO2 were reported

119

as 60 second running average concentrations that were calculated on-board each

120

instrument. Temperature (T) and relative humidity (RH) were measured using data

121

logging sensors (HOBOware; Section S11).

122

HONO was measured using a high-resolution time-of-flight chemical ionization mass

123

spectrometer (HR-ToF-CIMS)38 with acetate (CH3COO-) reagent ion.39 The reagent ion,

124

generated from acetic anhydride using a 210Po radioactive source (NRD, Model P-2021),

125

was introduced into the ion-molecule reaction chamber of the HR-ToF-CIMS. Resulting

126

CH3COO- ions can then abstract a proton from compounds that have a gas phase acidity

127

greater than acetate. Negative ion signal detected by the HR-ToF mass analyzer was

128

saved with 0.5 Hz frequency. Since actuation of the switching valves induced a pressure


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129

fluctuation in the sample line, data collected within 20 s of a valve-switching event were

130

disregarded. Data processing was performed with Tofware (Version 2.5.7; Aerodyne

131

Research, Inc.) operating within Igor Pro (Version 6.37; Wavemetrics, Inc.). Ion signals

132

were first normalized to the CH3COO- reagent ion signal (m/z 59.01), and then were

133

background corrected by subtracting the linearly interpolated zero gas measurements

134

that were conducted approximately every 30 mins. HONO was quantified by applying a

135

calibration to normalized, background corrected NO2- ion signal (m/z 45.99). Potential

136

interferences to NO2- ion signal for HONO detection, sources of measurement

137

uncertainty, and the calibration method are discussed in Section S4. Uncertainty in

138

[HONO] was estimated to be ±30%.

139

Box Model

140

A time-dependent two-compartment box model was constructed to investigate the

141

processes that control the temporal behavior of NO2 and HONO following perturbations

142

to indoor air composition (see also Section S7). Figure 1 shows the two-compartment

143

scheme with processes labeled using corresponding rate constant symbols. One box was

144

attributed to the kitchen space, while the other box represents the remainder of the home.


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145

A two-compartment model was used in this study due to the need for a second air volume

146

(‘the house’) to replenish the kitchen (see ‘Ventilation Experiments’). Carbon monoxide

147

(CO) removal from the kitchen after cigarette combustion was used to estimate the inter-

148

zonal mixing rate (kmix). The indoor/outdoor air exchange rate (AER = kio) was estimated

149

using the decay of CO2 after perturbations. Mixing ratios were converted to absolute

150

concentration units using 2.461025 molecules m-3 for the total number density of

151

molecules in air. Both boxes were assigned a surface/volume ratio of 3 m-1.40 The model

152

was evaluated with a time step (t) of 0.01 s; model output was not sensitive to the time

153

step length. The number of processes affecting [HONO] and [NO2] was minimized and

154

the number of independent assessments to constrain rate constants was maximized

155

(Table S4).

156

157


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158

Figure 1. Two-compartment box model developed to study perturbation experiments.

159

Rate constant symbols represent different physical and chemical exchange processes.

160

Parameter descriptions are given in Table S4 and rate equations are provided in Table

161

S5.

162

Results and Discussion

163

Non-experiment Periods

164

During non-experiment periods, the inhabitants infrequently used the gas stove, which

165

produces NO2 and HONO. NO2 was mostly supplied to the indoor environment through

166

exchange with outdoor air. Similar to prior observations in buildings (Table S1), [HONO]

167

was 3 – 14.2 ppb (mean = 5.3 ppb; mode = 4.5 ppb; Figure 2a). The time-resolved

168

measurements reveal remarkable stability in [HONO] over periods of hours, suggesting

169

that it was in a near-steady state with balanced source and sink rates, when not perturbed

170

by occupant activities (e.g., ventilation or combustion).24,41 During these same non-

171

experiment periods, indoor [NO2] varied to a much larger degree (3 – 70 ppb), responding

172

mainly to changes in outdoor [NO2] (Figure 2b). O3 mixing ratios were generally less than


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173

10 ppb indoors, which was lower than or similar to outdoors. [NO] was highly variable and

174

tracked the outdoor concentration closely, indicating that it was unreactive indoors, which

175

is consistent with low indoor [O3].

176

Mixing ratios of HONO less than 3 ppb were rarely observed (see Section S4 for

177

discussion of possible instrumental interferences). [HONO] was largely independent of

178

[NO2], with the exception of 18 Nov (Figure 3). Measurements on 18 Nov indicate high

179

outdoor [HONO] and an indoor RH (36 – 40%) that was higher than the remainder of the

180

study period (RH = 28-35%). A linear least squares fit to the data prior to 18 Nov showed

181

that [HONO] and [NO2] were weakly correlated (r2 = 0.3, slope = 0.13). Data with [NO2] >

182

20 ppb and [HONO] > 7.5 ppb (Figure 3) were acquired during non-experiment use of the

183

gas stove by inhabitants. Neglecting inhabitant stove use resulted in even weaker linear

184

correlation between HONO and NO2 (r2 = 0.06, slope = 0.05).

185

Overall, the insensitivity of [HONO] to changes in [NO2] suggests that formation of

186

HONO is not controlled by incremental changes in the gas-phase concentration of NO2,

187

although a direct coupling is often included in models.23–25 In another study, [HONO] was

188

not proportional to [NO2], and temporal trends were modeled by invoking a saturation of


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189

reaction sites for NO2 on indoor surfaces.24 Such an adjustment to the parameterized

190

mechanism in this prior work dampened the sensitivity of HONO to NO2, but the predicted

191

[HONO] was still more sensitive to changes in [NO2] than the observations. Overall, our

192

observations of slow variations in HONO mixing ratios during non-experiment periods

193

suggests that a set of processes results in a HONO steady state within the building and

194

that changes in [HONO] were buffered against dynamic (minute-to-hour scale) changes

195

in [NO2].


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196 197

Figure 2. (a.) Histogram of [HONO] in the kitchen for all non-experiment periods. (b.)

198

Histogram of [NO2] in the kitchen for all non-experiment periods. (c.) Time series of trace


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199

gas measurements inside and outside the residence during all non-experiment periods.

200

Times are given in the Universal Time Coordinate (UTC = local time + 5 h).

201 202

Figure 3. Comparison of [HONO] and [NO2] in the kitchen during all non-experiment

203

periods in which simultaneous HONO and NO2 measurements were available. [HONO]

204

was averaged to 1-minute intervals and colorized based on the sampling date (in UTC).

205

Combustion Experiments

206

If HONO is in an equilibrium with a precursor reservoir, then external perturbations to

207

[HONO] and [NO2] can provide information on processes that establish the steady state.

208

Combustion is a known source of HONO and NO2, and has been implicated in elevated

209

indoor [HONO] in prior studies.2,26,42,43 In this study, [HONO] was deliberately elevated


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210

using a gas burner stove. Relaxation of HONO toward its steady state concentration was

211

monitored and modeled to investigate chemical processes. After operating the gas burner

212

for 12 – 25 minutes, [HONO] reached 20 – 50 ppb and [NO2] reached 40 – 50 ppb in the

213

kitchen. During experiments where concentrations of HONO and NO2 were measured in

214

the kitchen and at the air vent, both indoor locations in the home had near identical

215

concentrations within 10 minutes of initiating combustion (Figure S5), due to the rapidity

216

of inter-zonal mixing within the main floor of the building. Indeed, given that the

217

recirculating fan within the building ventilation system was operated continuously

218

throughout the study, it is likely that the measured levels of HONO and NO2 existed

219

throughout the residence.

220

Concentrations of both gases began to decrease immediately upon extinguishing the

221

open flame (Figure 4). [NO2] could be modeled well when including indoor/outdoor air

222

exchange, inter-zonal mixing, and deposition to surfaces (krem,NO2; the sole adjustable

223

parameter influencing modeled [NO2]). The probability that a collision between an NO2

224

molecule and an indoor surface will lead to an uptake event, known as the effective uptake

225

coefficient (γ), that corresponds to krem,NO2 can be evaluated using Equation 1.


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4𝑘𝑟𝑒𝑚,𝑁𝑂2𝑉

226

𝛾𝑁𝑂2 =

227

where V is the volume of air, S is the indoor surface area, and ωNO2 is the thermal velocity

228

of NO2 (at 25 °C). The rates of chemical removal of NO2 determined during combustion

229

experiments (Tables S4 and S5; krem,NO2 = (2.7 – 6.5)10-4 s-1; γNO2 = (1 – 2.3)10-6) were

230

similar to or slightly greater than prior studies.2,6,24,44,45

𝜔𝑁𝑂2𝑆

[Eq. 1]

231

In order to find the best measurement-model agreement for HONO, the aforementioned

232

air exchange processes must be accompanied by explicit inclusion of competing HONO

233

production (ksrc,HONO) and removal (krem,HONO) processes that represent an equilibrium with

234

a condensed phase reservoir (e.g. nitrite) and produce a steady state for HONO. The

235

competing processes are implemented in the box model as a first order removal of

236

gaseous HONO (γHONO = (6.2 – 6.9)10-6), and a zero-order production of gaseous

237

HONO. The use of a zero-order process is analogous to a first order production process

238

that has a large and practically invariant reservoir of precursor. As discussed below and

239

in the SI (Section S12), samples collected from surfaces in the residence indicate the

240

presence of HONO and/or nitrite, providing important support for an equilibrium source of


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241

gaseous HONO indoors. Model-measurement agreement was improved by a small

242

degree if we add some degree of chemical formation of HONO via NO2 uptake (kHONO) as

243

well.

244

The potential importance of equilibrium-type processes to indoor HONO concentrations

245

has been suggested previously by Spicer et al.2 and was discussed by Lee et al.46 but

246

has not been explored in detail. To illustrate the sensitivity of the model to each HONO-

247

related process, model output shown in Figure 4 was generated while eliminating some

248

or all of the chemical processes used to predict [HONO]. Omission of all chemical

249

processes such that only inter-zonal and indoor/outdoor air exchange processes remain

250

active (dashed gray line, ‘air exchange only’) clearly deviates from the observations.

251

Eliminating the direct production of gas phase HONO from NO2 uptake (kHONO = 0; black

252

dashed line) causes the model to slightly underestimate [HONO] and slightly overestimate

253

the overall rate of HONO removal from the kitchen. When the competing source and sink

254

(equilibrium) processes that lead to [HONO]ss were omitted (ksrc,HONO = 0 and krem,HONO =

255

0; solid gray line), an increase in [HONO] was predicted by the model after the flame was

256

extinguished but was not observed. Clearly, the pair of surface-based source and sink


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257

processes were key elements of the model, while direct chemical production from NO2

258

uptake provided a weaker contribution to measurement-model agreement.

259 260

Figure 4. Measurements (markers) and model calculations (lines) of trace gases during

261

a combustion experiment. Replicate experiments can be found in Figure S6.

262

Ventilation Experiments

263

Ventilation experiments were performed to complement the perturbations arising from

264

using the stove. Upon ventilation with outdoor air, [HONO] in the kitchen was reduced to

265

3 ppb, and immediately began increasing toward [HONO]ss of 6 ppb when ventilation


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266

ceased (Figure 5). Stable [HONO] was observed within 15-30 minutes ( ~ 300 s).

267

Meanwhile, [NO2] increased when outdoor air was introduced to the kitchen. Since both

268

[NO2] and [O3] were greater in outdoor air, NO2 was both introduced directly from outdoors

269

and produced by reaction of O3 with NO present indoors (allowing indoor [NO2] to briefly

270

exceed its outdoor concentration). [NO2] decreased upon cessation of ventilation, but did

271

so at a slower rate than the observed increase in HONO. Temporal profiles and mixing

272

ratios of HONO and NO2 in our ventilation experiments closely resemble observations

273

from previous studies in residences and a classroom.2,28,41,43 In addition, the different time

274

constants for NO2 and HONO relaxation clearly illustrate a lack of coupling between

275

processes that control these species’ gas phase abundances.

276

Relaxation back to a steady state was simulated to probe the processes that contribute

277

to this temporal behavior. Model output in Figure 5a represents calculated concentrations

278

in the kitchen. In the same manner as the combustion experiments, the model-

279

measurement agreement was found for NO2 by including air exchange processes (kio and

280

kmix) and uptake to surfaces (krem,NO2). Only krem,NO2 was adjusted to find agreement with

281

measurements. The modest deviation of the model from the data may result from a time-


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282

and/or concentration-varying krem,NO2 value, or may be due to uncertainties in physical

283

mixing processes (e.g., variability in AER, intra-zonal transport within the kitchen).

284

Analysis of the ventilation data with the box model indicates that γNO2 was 1×10-6.

285

[HONO] could be predicted well in the kitchen by including only physical exchange

286

processes (kio and kmix), as long as a constant value for [HONO]ss was applied to the

287

‘house’ compartment (kHONO, ksrc,HONO, and krem,HONO were set to zero in both boxes).

288

When modeling ventilation experiments, [HONO]ss in the ‘house’ compartment was held

289

constant at 6 ppb, which was the measured [HONO]ss in the kitchen at the end of the

290

ventilation experiment (see also Section S8). Setting a constant [HONO]ss for the ‘house’

291

represents an inferred source of HONO from a large reservoir of precursor that existed

292

consistently within the building in order to maintain mass balance. It was assumed that

293

ventilation using the windows and door near the kitchen did not necessarily ventilate the

294

rest of the house. Air exchange between compartments in the model (Figure 1, kmix)

295

allowed the concentration of HONO in the kitchen to achieve [HONO]ss in a short period

296

of time. Incorporation of an upper limit rate for HONO production directly from NO2 (kHONO)

297

only provided a small change to the model results (Figure 5a, dashed line).


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298

We note that although cigarette emissions were released into the kitchen from t = 960

299

– 1250 s, production of nitrogen oxide species during the cigarette burning episodes were

300

very small compared to the perturbation from ventilating the kitchen (Section S8).

301 302

Figure 5. Measurements (markers) and model calculations (lines) of trace gases in the

303

kitchen during ventilation experiments. The ‘HONO+chem’ model run (left panel)

304

simulates HONO production by including kHONO (Figure 1). The scale for HCOOH (right

305

panel) is presented in uncalibrated, arbitrary signal units. The time of day at t = 0 s for the

306

experiment shown is 14:45 (local time).


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307

The concept that HONO was in equilibrium with a precursor reservoir is supported by

308

the similarity in behavior between the signals for HONO and formic acid (HCOOH), which

309

has been long known to be a semi-volatile organic compound that maintains an

310

equilibrium with indoor surfaces.47 The temporal trend in HCOOH overlays with HONO

311

closely (Figure 5b), suggesting similar mechanistic drivers behind the replenishment of

312

both acids to the gas phase in the kitchen after the windows/door had been closed. Other

313

alkyl monocarboxylic acids (propionic, butyric, pentanoic) followed similar trends (Figure

314

S9).

315

Overall, the ventilation experiments illustrate a decoupling of the kinetic behavior

316

between NO2 and HONO, such that the dynamics of these gases in the indoor space

317

cannot be modeled using direct gas phase HONO production from NO2 uptake to

318

surfaces. Instead, in support of the combustion experiments, the results point to a large

319

reservoir of HONO precursor on the inner surfaces of the residence that readily releases

320

HONO to the gas phase, helping to re-establish steady state after a perturbation.

321

Mechanistic Considerations for HONO Production


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322

The ultimate source of HONO in the indoor environment, apart from combustion, is

323

presumably associated with the uptake of NO2 to surfaces (R1). Subsequent to formation

324

via R1, the equilibrium processes R2 and R3 can then dominate the control of gaseous

325

[HONO]. In particular, in both our ventilation and combustion experiments, a driving force

326

for [HONO]ss was required in order to find measurement-model agreement. In the

327

combustion case, when [HONO] was elevated in all zones of the residence, competing

328

source and removal processes that help to re-establish the steady state were

329

implemented explicitly in the model and were key components in achieving measurement-

330

model agreement for temporal trends in [HONO] (Figure 4). In the ventilation case, when

331

[HONO] was depleted in the kitchen, model-measurement agreement was achieved

332

mainly through inter-zonal mixing in which [HONO]ss in the ‘house’ box was taken to

333

persist throughout the experiment and act as a source for the kitchen. In order to maintain

334

mass balance of HONO, one must infer a responsive source of HONO to the ‘house’ box

335

as the kitchen is replenished and approaches [HONO]ss on a scale of minutes. A more

336

straightforward measurement-modeling comparison could be achieved in an indoor

337

space with fewer defined zones where both types of cases could be represented with


26

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338

identical model construction. Still, further strong support for equilibrium control of gaseous

339

HONO comes from the identical behavior of HCOOH, a well-known semi-volatile organic

340

compound, during the ventilation experiments (Figure 5). It may be possible that the

341

presence of the gas stove in the residence examined within the present study conditioned

342

the surfaces to have a greater surface reservoir of the precursor to gaseous HONO,

343

although a direct comparison study with homes not containing gas stoves was beyond

344

the scope of this work.

345

Direct chemical production of gaseous HONO from NO2 uptake was weak. When the

346

direct chemical production of gaseous HONO was included in the model for the ventilation

347

experiments, predicted HONO levels were higher than the measurements (Figure 5a). A

348

similarly small influence of direct gaseous HONO formation from NO2 uptake was

349

observed in the combustion experiments (Figure 4).

350

The set of processes driving [HONO] proposed in this study help explain similar

351

experiments reported in the literature where there is a rapid HONO increase after

352

ventilation.2,28,41,43 For example, data from ventilation experiments performed during the

353

SURFin campaign24,28 show temporal trends in [HONO] that approach a steady state and


27


Page 28 of 37

354

show similar HONO mixing ratios (~6 ppb). While investigators arrived at a model

355

construct that uses a limitation to NO2 uptake to dampen the HONO production rate, the

356

model retained more sensitivity to deliberate experimental additions of NO2 than the

357

measurements indicated. The authors concluded that indoor gaseous HONO formation

358

was generally not proportional to NO2,24 although some immediate HONO production did

359

result from NO2 uptake, as has been observed previously.2 Hence, mechanisms used to

360

simulate indoor [HONO] should be weakly sensitive to [NO2]; we propose one such

361

mechanism in this study.

362

The Proposed HONO Reservoir

363

Implementation of gas-surface equilibrium processes in the model implies that a

364

precursor reservoir to gaseous HONO persists on indoor surfaces. Overall, we suggest

365

that nitrite and/or HONO formed upon NO2 uptake may be dissolved in surface adsorbed

366

water or sorbed to building materials. Alkaline materials, in particular, such as grout or

367

concrete, could be of particular importance in the formation of nitrite.36,44 In addition to the

368

kinetic studies described above, this proposal is supported by new measurements of at


28

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369

least 1012 nitrite ions cm-2 detected on various types of hard indoor finishes with some

370

measurements larger than 1013 ions cm-2 (Section S12).

371

An early study of HONO formation from NO2 uptake illustrated that the yield of gas

372

phase HONO was reduced in environmental chambers that had not been cleaned via

373

photo-oxidation.18 Such soiled reactor surfaces may be somewhat representative of the

374

‘aged’ surfaces that exist in inhabited residences and could be conducive to the

375

accumulation of HONO or nitrite reservoirs. In laboratory studies, realistic building

376

materials released HONO for extended periods after NO2 exposure ceased; this behavior

377

was enhanced with increased RH,17 suggesting that nitrite dissolved and accumulated in

378

surface-bound water. Other studies of NO2 uptake to materials found indoors showed that

379

nitrite accumulated on alkaline materials (e.g., concrete, wallboard paper)44 and that

380

HONO deposited strongly to alkaline cleaning agents (likely as nitrite).9 Further, efficient

381

uptake of HONO from air to (alkaline) photocatalytic paint surfaces was suggested to

382

result from rapid oxidation of the nitrite surface reservoir by the photocatalyst, shifting the

383

balance of the multiphase equilibrium of HONO and nitrite.48 Overall, studies of HONO

384

and NO2 chemistry have been conducted on many types of realistic, chemically-complex


29


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385

surfaces7,9,17,22 and simpler proxy materials,16,20,49 from which much has been learned

386

about the mechanisms of NO2 uptake and HONO production. However, continued study

387

of the gas phase and the condensed phase in the same experiments will enhance

388

mechanistic insight and will clarify the degree to which multiphase equilibrium controls

389

gaseous HONO mixing ratios. Overall, surface composition clearly has an important

390

impact on HONO chemistry.26,27

391

Outdoors, soils have been suggested to act as a HONO source depending on surface

392

pH.11,33,35,36,50 Changes to surface pH via uptake of strong acids from the gas phase (e.g.,

393

HCl) may protonate nitrite on surfaces, resulting in a flux of HONO to the gas phase.16

394

Sources of strong acids in the gas-phase include primary emissions from various indoor

395

activities (e.g., cleaning, combustion)51,52 as well as secondary sources like

396

heterogeneous reactions of NO2 with chloride salts, Cl radical reactions with organics,53

397

and dark NOx chemistry.54 Recently, the tendency for ammonium nitrate aerosol to

398

evaporate and produce gaseous NH3 and HNO3 under warm and dry indoor conditions55

399

has been observed during winter upon outdoor-to-indoor transport.56 Further

400

measurements of gaseous strong acids, along with inorganic nitrogen speciation and


30

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401

water content on realistic indoor surfaces, are warranted to gain a better understanding

402

mechanistic aspects of this chemical system.

403

The findings of the present study suggest that comprehensive indoor air chemistry

404

models require inclusion of an equilibrium or dynamic exchange of HONO between the

405

gas phase and a condensed-phase reservoir. This behavior needs be examined in a wide

406

variety of indoor spaces and for other semi-volatile species.

407 408

ASSOCIATED CONTENT

409

Supporting Information. The following file is available free of charge at

410

http://pubs.acs.org.

411

Documentation of the building layout and conditions, HR-ToF-CIMS calibration and

412

uncertainty, natural gas fuel composition, air exchange rate determination, experimental

413

replicates, surface sampling of nitrite, and supplemental gas-phase organic acid

414

measurements (PDF).

415

AUTHOR INFORMATION


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416

Corresponding Author

417

*Douglas B. Collins, Department of Chemistry, Bucknell University, 1 Dent Drive,

418

Lewisburg, PA 17837; [email protected]

419

Present Address

420

†Department of Chemistry, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837

421

ACKNOWLEDGMENT

422

Support was provided by the Alfred P. Sloan Foundation (Chemistry of Indoor

423

Environments). The authors would like to thank Charles Weschler for useful

424

conversations and Jonathan Wang for help with trace gas analyzer calibrations.

425 426

REFERENCES

427

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