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Environmental Processes
Time-Resolved Measurements of Nitric Oxide, Nitrogen Dioxide, and Nitrous Acid in an Occupied New York Home Shan Zhou, Cora J Young, Trevor Casey VandenBoer, Shawn Finley Kowal, and Tara F Kahan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01792 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018
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Time-Resolved Measurements of Nitric Oxide, Ni-
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trogen Dioxide, and Nitrous Acid in an Occupied
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New York Home
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Shan Zhou1, Cora J. Young 2, Trevor C. VandenBoer 2, Shawn F. Kowal1, Tara F. Kahan1,*
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1
Department of Chemistry, Syracuse University, Syracuse, NY, USA
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2
Department of Chemistry, York University, Toronto, ON, Canada
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*
Corresponding Author: Tara Kahan,
[email protected], (315) 443-3285, Department of Chemis-
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try, Syracuse University, Syracuse, NY 13244
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KEYWORDS
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Photochemistry, air exchange rate (AER), indoor air quality, oxidants, combustion, photon flux
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ABSTRACT
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Indoor oxidizing capacity in occupied residences is poorly understood. We made simultaneous
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continuous time-resolved measurements of ozone (O3), nitric oxide (NO), nitrogen dioxide
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(NO2), and nitrous acid (HONO) for two months in an occupied detached home with gas appli-
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ances in Syracuse, New York. Indoor NO and HONO mixing ratios were higher than those out-
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doors, whereas O3 was much lower (sub-ppbv) indoors. Cooking led to peak NO, NO2, and
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HONO levels 20 - 100 times greater than background levels; HONO mixing ratios of up to 50
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ppbv were measured. Our results suggest that many reported NO2 levels may have a large posi1 ACS Paragon Plus Environment
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tive bias due to HONO interference. Nitrous acid, NO2, and NO were removed from indoor air
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more rapidly than CO2, indicative of reactive removal processes or surface uptake. We measured
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spectral irradiance from sunlight entering the residence through glass doors; hydroxyl radical
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(OH) production rates of (0.8 – 10) × 107 molec cm-3 s-1 were calculated in sunlit areas due to
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HONO photolysis, in some cases exceeding rates expected from ozone-alkene reactions. Steady
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state nitrate radical (NO3) mixing ratios indoors were predicted to be lower than 1.65 × 104 molec
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cm-3. This work will help constrain the temporal nature of oxidant concentrations in occupied
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residences and will improve indoor chemistry models.
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TOC Art
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INTRODUCTION
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Considering that people typically spend 90% of their time indoors (and 80% of that time in
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residences), air quality in residences is of major concern for human health.1 In addition to direct
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indoor sources and infiltration from the ambient atmosphere, harmful pollutants can be formed
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indoors through in-situ secondary chemistry initiated by oxidants.2-5 Yet our understanding of
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indoor oxidation processes, especially in occupied residences, lags substantially behind that for
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outdoors.
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The hydroxyl radical (OH) is the most important oxidant in the atmosphere. In indoor envi-
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ronments, OH levels are expected to be much lower than those outdoors because the primary pro-
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duction mechanism (ozone photolysis) does not occur.6 The primary indoor OH source is thought
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to be non-photochemical reactions between ozone and alkenes.7-8 Recent studies suggest that pho-
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tolysis of nitrous acid (HONO), formaldehyde, and hypochlorous acid (HOCl, which is generated
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during cleaning events using commercial bleach solution) can also be a source of indoor OH.9-15
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Nitrate radicals (NO3) have also been suggested to be potentially important oxidants indoors.4, 16
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There are few indoor measurements of reactive species such as OH and NO3, and none in resi-
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dences.17-18 Even levels of more stable oxidants are not well constrained in residences because
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most measurements have been performed in non-residential buildings; lower air exchange rates
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(AER) in residences may result in very different oxidant mixing ratios, as indoor levels can de-
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pend strongly on AER. For example, ozone is thought to be the most important oxidant in indoor
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environments, but some studies suggest that ozone levels in residences may be very low, likely
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due to low AER, adsorption to surfaces, and titration by nitric oxide (NO).19-20
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Nitrous acid is an emerging household oxidant that may be an important indoor photochemi-
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cal OH source.10, 12, 14-15, 21 It can be emitted directly in indoor settings by combustion processes
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such as burning candles, open fireplaces, gas stoves, and cigarette smoking.14, 22-23 Heterogeneous
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reactions of nitrogen dioxide (NO2) also lead to the secondary formation of HONO.24 However,
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there have been limited HONO measurements in residences and its indoor mixing ratio is subject
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to high levels of uncertainty. Peak HONO mixing ratios (15 min time resolution) of up to 100
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ppbv and 24 h averages of 40 ppbv were measured in controlled house experiments.25 Measure-
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ments made in real homes yielded much lower mean (1 – 7 days) HONO mixing ratios ranging
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from 0.8 to 11.3 ppbv.23, 26-28 In addition, most existing measurements of the important HONO
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precursor NO2 have been made with passive samplers that are exposed to the indoor environment 3 ACS Paragon Plus Environment
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for extended periods of time (1 to 14 days). Such extended sampling periods are unlikely to cap-
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ture short-term peak levels and temporal changes in mixing ratios, which are important for under-
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standing chemistry that occurs indoors. Furthermore, it has been shown that HONO interferes
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with most NO2 measurement methods used indoors.29-30 This suggests that many reported indoor
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NO2 values may be positively biased, further confounding the relative importance of NO2 and
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HONO in residences.
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In this study, we conduct time-resolved measurements of a suite of oxidants and oxidant pre-
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cursors including O3, NO, NO2, and HONO in an occupied residence to better characterize their
72
mixing ratios, sources, and sinks. We also measure spectral irradiance from sunlight entering the
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residence, and calculate OH production rates from HONO photolysis as well as steady-state NO3
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concentrations under various conditions.
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EXPERIMENTAL METHODS
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Research House
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From September 27 to November 20, 2017, continuous ambient sampling of indoor air was
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made in a single-family dwelling in Syracuse, NY. Located in an urban residential area, it is a 4-
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bedroom 1.5 story house (250 m3 volume) with an unfinished basement and a detached garage.
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The house utilizes both natural ventilation (opening windows) and forced-air system (heating or
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air-conditioning) using 100% recirculated air. Most floors are hardwood; the family room has a
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mixture of carpet and bamboo flooring. Illumination is primarily provided by LED lights and
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sunlight filtered through windows. Combustion appliances include a natural gas stove with four
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burners and an oven. Water heater, heating, and air conditioning systems were also operated us-
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ing commercial natural gas supplied to the house. During the sampling period, the house was oc-
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cupied by two non-smoking adults, one child, and a dog. Daily activity was carried out normally
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and a human activity log book was kept at the residential site.
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Real-Time Measurements
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A custom-built mobile analytical laboratory (Mobile Indoor Laboratory for Oxidative Spe-
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cies; MILOS) was deployed for time-resolved measurements of O3, NO, NO2, HONO, carbon
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dioxide (CO2), relative humidity (RH), and temperature (T). Ozone mixing ratios were measured
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by an Ecotech Serinus 10 UV photometric analyser (accuracy of 0.5 ppbv or 2% of reading). Ox-
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ides of nitrogen (NOx: NO and NO2) were measured using an Ecotech Serinus 40 O3-based chem-
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iluminescence analyser (accuracy of 0.4 ppbv or 0.5% of reading). The NOx analyser was modi-
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fied to allow simultaneous quantitative measurements of HONO by a difference technique.30 A
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HONO inlet consisting of a gas denuder (GD) channel and a bypass (BP) channel was inserted
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upstream of the NOx analyser. The GD removes gaseous acids including HONO using a sodium
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carbonate-coated annular denuder (URG Corp.), but does not remove NO2. The denuder was pre-
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pared by coating the interior with a saturated aqueous Na2CO3 solution (Fisher Scientific), shak-
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ing gently, pouring out the excess solution, and drying with zero air for ~5 min. Air sampled
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through the BP contains NO2 and HONO; both molecules are converted to NO by the molyb-
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denum-catalyst.25, 30-32 A Teflon solenoid three-way valve enabled the sampling of GD (NO2 on-
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ly) and BP (sum of NO2 and HONO, Σ(NO2+HONO)) air alternating at 5 min intervals. Distinct
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measurements of NO2 and HONO (rather than the sum of the two species) were made after
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October 31. CO2, RH, and temperature indoors were monitored with a TSI IAQ7545. Data were
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logged at 30 s intervals for all measurements. For NO2 and HONO, during each 5-min interval,
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the first 1 minute of data was excluded to reject data influenced by the instrument response time.
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Data acquired over the remaining 4 minutes was averaged. MILOS was stationed in the living
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room while its Teflon sampling inlet was positioned in the kitchen or living room at head height 5 ACS Paragon Plus Environment
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(approximately 1.5 m above the floor). Outdoor mixing ratios were measured intermittently
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throughout the sampling period by moving the inlet to the patio or front stoop, generally during
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the day. Outdoor temperature (Tout) and relative humidity (RHout) at Syracuse Hancock Interna-
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tional Airport, ~7.3 km north to the sampling site, were retrieved from the NOAA National Cen-
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ters for Environmental Information’s Integrated Surface Hourly Database.33
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Calibration and Quality Control
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Given low nitric acid (HNO3) levels previously reported in residences,25, 34 its interference to
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NO2 and HONO measurements is considered negligible in this study. The conversion efficiency
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of HONO by the molybdenum catalyst was determined by introducing zero air containing
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HONO, generated using a custom-built apparatus,35 into the NOx analyser and comparing the
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level reported by the NOx analyser to the mixing ratio measured offline using ion chromatog-
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raphy with conductivity detection. The conversion efficiency was determined to be to be (83.5 ±
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3)%. Previous studies have shown that small amounts of NO2 can adsorb to annular denuders.36-37
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We flowed NO2 ranging from 13 to 270 ppbv through the GD line and NO2 losses were found to
125
be (7.0 ± 0.45)%. Reported values were corrected to account for the conversion efficiency and
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loss.
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Calibrations and linearity checks were performed at the beginning, middle, and end of the
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sampling period using a dilution calibrator and ozone generator (Ecotech GasCal 1100) and NO
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cylinder (19.8 ppmv in N2, analytical uncertainty of 5%). The 30 s limit of detection (LOD) of the
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analytes were determined as 3 times the standard deviations (3σ) of the corresponding signals in
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zero air. Limits of detection were 0.63, 1.1, 1.3, and 1.3 ppbv for O3, NO, NO2, and
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Σ(NO2+HONO), respectively. Higher LODs for NO, NO2, and HONO than for O3 are due to the
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use of different digital filters. The 5 min LOD for HONO was calculated as 0.7 ppbv, 3 times the
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standard deviation propagated from the subtraction of 5 min NO2 from Σ(NO2+HONO). The en6 ACS Paragon Plus Environment
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durance of the GD was tested and the annular denuder was recoated on average every day (6
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hours – 4 days depending on accessibility) to minimize the influence of denuder deactivation on
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NO2 and HONO mixing ratios. Time periods potentially influenced by denuder deactivation were
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identified based on decreasing HONO/NO2 ratios.
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Data Analysis
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The decay (removal) rate constants (k) of NO, NO2, HONO, and CO2 were estimated from
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indoor measurements during periods affected by a dominant indoor source (e.g. cooking). Only
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time periods when all species peaked at concentrations significantly higher than the background
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values, followed by a non-source period, were considered in the calculations. Assuming constant
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removal rates and background mixing ratios during the decay period, and assuming an even dis-
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tribution of indoor concentrations of these species,
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Ct = e-kt Co + Cb
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where Ct is the indoor mixing ratio after time t, Co is the initial (peak) mixing ratio, and Cb is the
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background mixing ratio of the analyte gas during the decay event. The decay rate constant was
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determined using an exponential regression of the observed mixing ratio versus time for each
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decay period. Decay rate constant error was determined as the standard error of k obtained from
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the regression. Time periods for CO2 decay calculations were identified based on stringent crite-
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ria to exclude potential impacts from occupant emissions (see the Supplementary Information for
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details).
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Light Measurements and Photolysis Rate Calculations
[1]
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Wavelength-resolved spectra of sunlight entering the residence through double-paned patio
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doors (4.8 m2) leading to a deck off of the south-facing family room were acquired using a cali-
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brated Ocean Optics USB4000 spectrometer coupled to a 1 m fiber optic cable and a cosine cor-
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rector. Full details of the measurement procedure and data analysis are described in Kowal et al.12 7 ACS Paragon Plus Environment
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In brief, 3 background-subtracted spectral irradiances of sunlight indoors were measured with 1.5
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ms integration. Average spectral irradiances were calculated, and photon fluxes (F) were estimat-
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ed. These measurements were acquired on September 21, 2017 at approximately noon; the sky
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was cloudless and the solar zenith angle was 41.4°.38
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HONO photolysis rate constants (JHONO) were calculated as described previously using meas-
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ured photon fluxes and reported absorption cross sections (σ) and photolysis quantum yields
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(ϕ).12
166 ఒ
ܬுைேை = න ߪுைேை ሺߣሻ߶ுைேை ሺߣሻܨఒ ݀ߣ
[2]
ఒ
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Rate constants were then multiplied by measured indoor HONO concentrations to calculate OH
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production rates.
ܴܽܬ = ݁ݐுைேை ሾܱܱܰܪሿ
[3]
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RESULTS and DISCUSSION
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Outdoor temperature (Tout) ranged from -8.3 to 32.2 ºC and RHout ranged from 29 to 100%,
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with means (± 1σ) of 9.8 ± 7.6 ºC and 78.4 ± 17.2%, respectively. Precipitation occurred 22% of
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the time throughout the study period as rain or snow. The indoor environment was generally
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warmer (average temperature of 20.5 ± 1.2 ºC) and drier (average RH of 44.5 ± 5.7%), with little
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temporal variation in T and RH. Figure 1 shows a sector of the real-time temperature, RH, and
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gas species mixing ratios measured indoors during the sampling period. The time series over the
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entire sampling period are shown in Fig. S1. Mixing ratios of all species varied dynamically with
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high amplitudes (Fig. S1). Ozone levels indoors were often below the detection limit (58% of the
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observation period), whereas outdoor levels of up to 34.2 ppbv were measured. 8 ACS Paragon Plus Environment
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Mixing Ratios under Background Conditions
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Indoor background oxidant mixing ratios during periods without emission sources (i.e., doors
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closed, no cooking or heating events) were examined to obtain a baseline of mixing ratios indoors.
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Nitric oxide mixing ratios were much higher indoors than outdoors, with an average (± 1σ) in-
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door background mixing ratio of 4.0 ± 2.5 ppbv. Most (70%) outdoor NO levels were below the
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LOD. Using the LOD as an upper limit for the NO mixing ratio under these conditions, we esti-
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mated a lower limit for the background indoor/outdoor (I/O) ratio of 2.1. Comparable average
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levels of the sum of NO2 and HONO (Σ(NO2+HONO)) were observed under indoor background
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conditions and outdoors (5.4 vs 5.2 ppbv). The average (± 1σ) indoor background NO2 and
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HONO mixing ratios (acquired after October 31, excluding periods with potential influence from
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denuder passivation) were 2.0 ± 0.68 and 4.3 ± 2.2 ppbv, respectively. Note that we had very lim-
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ited outdoor distinct measurements of NO2 and HONO; their outdoor levels are not reported. Alt-
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hough outdoor HONO mixing ratios can be variable, they are typically on the order of 1 – 2 ppbv
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at night and sub-ppbv at midday.39 This is suggestive of higher HONO levels indoors under
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background conditions than outdoors, in agreement with previous studies.23,
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above its detection limit, its average background mixing ratio was 1.0 ppbv, significantly lower
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than the outdoor levels measured (mean of 12.6 ppbv, Fig. S2).
28
When O3 was
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The observed complex relationship between the indoor background and outdoor oxidant lev-
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els reflect different source and sink strengths. Direct emission from traffic is the dominant source
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of NOx in ambient urban environments. As shown in Fig. S2, outdoor NO mixing ratios peaked at
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6 – 8 EST (12 ppbv) and 16 – 20 EST (6 ppbv). However, during the daytime with high ambient
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ozone levels (~20 ppbv, Fig. S2), which convert NO to NO2, ambient NO levels ranged from 2
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ppbv to below the LOD. A similar, yet less pronounced, diurnal profile was also observed for
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ambient Σ(NO2+HONO) (Fig. S2), likely corresponding to rush-hour traffic emissions of NOx. 9 ACS Paragon Plus Environment
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Nitrous acid on the other hand is photolyzed rapidly by sunlight, so its ambient outdoor concen-
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tration is quite low during the day, and measured outdoor Σ(NO2+HONO) levels during the day
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are likely almost entirely due to NO2.
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Significantly higher NO and HONO mixing ratios indoors compared to outdoors suggest that
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transport of outdoor NO and HONO by air infiltration were likely not the primary indoor sources
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of these species. This is supported by the absence of rush-hour peaks for indoor levels. Combus-
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tion processes indoors, e.g. biomass, incense, and candle burning, gas cooking, and smoking,
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have been shown as major direct sources of NOx and HONO in indoor environments.14, 40-42 Gas
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cooking is likely the main indoor HONO source in the current study. Elevated concentrations of
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NO and HONO throughout the study period (including in the absence of combustion) are likely
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due to weaker sinks for these species indoors; low O3 in the case of NO and reduced photolysis
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indoors in the case of HONO. Since surface area to volume ratios are generally greater indoors
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than outdoors, heterogeneous reactions of NO2 may be more important indoors, and may have
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contributed to higher indoor background HONO levels.43
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In addition to indoor sources (e.g. combustion), infiltration of NO2 from outdoor air also con-
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tributes to indoor NO2 levels.40, 44 Transport of ambient O3 is the dominant source of indoor O3,
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which often is present at mixing ratios 10 – 80% of ambient outdoor values, depending on the
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AER.45 Our observations of lower background mixing ratios of NO2 and O3 indoors are consistent
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with outdoor-to-indoor transport of these species. Weak indoor production of NO2 from the reac-
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tion of NO and O3 due to low levels of indoor O3 may account for low background indoor NO2
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mixing ratios. Strong sinks – e.g. NO2 deposition to indoor surfaces and O3 titration by NO – may
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have also contributed to lower NO2 and O3 mixing ratios indoors compared to outdoors in the
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absence of direct sources.
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Mixing Ratios Influenced by Cooking 10 ACS Paragon Plus Environment
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Indoor mixing ratios of NO frequently exceeded 100 ppbv, and occasionally reached 340
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ppbv, 2 orders of magnitude higher than background levels (Fig. S1c). Dramatically higher in-
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door mixing ratios of NO2 and HONO than the background values were also observed, peaking at
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59 and 50 ppbv (Fig. S1c), which correspond to mixing ratios approximately 30 and 12 times
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higher than background levels, respectively. These peak episodes correspond to cooking events,
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as suggested in Fig. 1c. The average diurnal variations of indoor NO, NO2, and HONO during
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this study also showed three recurring spikes corresponding to meal times at ~ 8 – 9, 12 – 14, and
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16 – 19 EST with the highest levels occurring around supper time (Fig. S3). These observations
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together suggest that direct emission from gas cooking was the major source of NOx and HONO
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in this residence. Reactions involving NO2 likely also contribute to HONO formation during
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cooking events.22 Our measurements do not have the temporal resolution required to distinguish
239
between primary and secondary HONO formation.
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Statistics of maximum 5 min peak concentrations during cooking events are presented in
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Fig. 2 and Table 1; means (± 1σ) were 113.2 ± 72.7, 17.3 ± 12.8, and 19.5 ± 10.5 ppbv for NO,
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NO2, and HONO, respectively. These values are significantly higher than the mixing ratios meas-
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ured under background conditions as well as those measured outdoors. Peak NO mixing ratios are
244
in agreement with those observed in other residences (~40 – 300 ppbv) but significantly lower
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than those measured during controlled cooking experiments (100 – 2000 ppbv).22, 46-48 The peak
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NO2 levels in this study were generally at the low end of the concentration range previously re-
247
ported (~20 – 550 ppbv),22,
248
ppbv).22
46, 48
whereas peak HONO levels were on the high end (3 – 16
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As shown in Fig. 1c, when cooking appliances were in use, mixing ratios of combustion by-
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products (NO, NO2, and HONO) rapidly rose. The rise time of the signal corresponded to the
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time the appliance was on (i.e. cooking duration) which varied between 4 and 86 minutes in this 11 ACS Paragon Plus Environment
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field study. After the appliance was turned off, the levels rapidly reached peak levels and slowly
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decayed away. To investigate possible factors that affect the peak mixing ratios, 74 cooking
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events were analyzed. Positive correlations between the measured peak mixing ratios of these
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species and cooking duration were observed (r2 = 0.42 - 0.72; Fig. 3), suggesting that operating
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time of cooking appliances could explain 42% – 72% of the variability in the peak levels of NOx
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and HONO. In addition, mixing ratios measured when cooking with individual burners at differ-
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ent positions overlaps with oven operation, which suggests that appliance type overall exerted
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little effect on the emissions. Furthermore, cooking events using multiple appliances fall tightly
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along the same trend, indicative of negligible effect of additional appliances on the mixing ratios
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observed. In previous controlled experiments, peak NOx levels depended on the number of gas
262
elements used.47 The reason for the lack of influence of combustion elements in this study is un-
263
certain. There may be other variables that are not accounted for such as the power of the appli-
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ances and the setting during individual cooking events (e.g. oven temperature, gas mark on stove).
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A ventilated range hood was used during some cooking events (N = 15) during the study pe-
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riod. Recent assessments of range hood performance in the US indicate wide variations across
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hoods and airflow and burner positions. Several studies reported that using overhead kitchen ex-
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haust fans was associated with reduced NOx levels indoors.42,
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Σ(NO2+HONO) production rates during cooking events were lower when the fan was in opera-
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tion than when it wasn’t: 2.4 vs. 4.2 ppbv min-1 for NO, and 0.97 vs. 1.32 ppbv min-1 for
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Σ(NO2+HONO) (Fig. 3a,3b). These observations suggest that the vented exhaust fan in the kitch-
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en reduced NO and the sum of NO2 and HONO mixing ratios by approximately 43% and 27%,
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respectively.
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Indoor Decay/Removal Rate Constants
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In this study, NO and
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For many of the cooking events, there was a clear exponential decay of the combustion by-
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product mixing ratios after the appliance was shut off. The decay/removal rate constants of the
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combustion by-products were estimated for 55 uninterrupted episodes after cooking. The decay
278
rate constant of CO2 can potentially be referred to as the indoor-outdoor air exchange rate (AER).
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Estimated AER from 19 selected cooking events ranged from 0.1 – 1.6 hr-1 with a mean value of
280
0.62 (± 0.36) hr-1. This is consistent with reported AER for US residential homes (median/mean
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of 0.3 – 1.12 hr-1).51-54 Decay rate constants of NO, Σ(NO2+HONO), NO2, and HONO were di-
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rectly compared to AER; all were significantly higher (Fig. 4). This suggests that mixing ratios of
283
all these species decrease more rapidly than can be accounted for by air exchange alone and that
284
reactive processes (gas phase reactions and surface uptake) in the house make a significant con-
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tribution to their removal.
286
The NO decay rate constant, kNO, was on average 0.92 ± 0.37 hr-1. A plot of kNO vs. kCO2
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yielded an intercept of 0.35 h-1 (Fig. 4a), which can be considered the approximate reactive re-
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moval rate constant of NO. The reactive NO removal rate constant can also be estimated as the
289
difference between kNO and the AER; rate constants determined by this method ranged from 0.12
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to 0.79 hr-1 and averaged 0.33 hr-1. Although we are not certain of NO removal mechanisms other
291
than air exchange, its reaction with O3 infiltrated indoors is one possibility. As NO mixing ratios
292
indoors were high, it possibly quickly reacted with O3 and resulted in stable low levels of ozone
293
whereas high NO levels were sustained for hours. The lifetime of NO, i.e. the time it took for the
294
mixing ratio to decrease to e-1 of its initial value, varied between 30 – 228 min.
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Reactive decay rate constants for NO2 and HONO were also determined via the difference
296
method. We note that limited distinct measurements of NO2 and HONO resulted in considerable
297
scatter in the data, especially for kNO2 (Fig. 4c, 4d). The performance of the denuder may also
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have contributed to the scatter, as the denuder was sometimes deactivated quickly during cooking 13 ACS Paragon Plus Environment
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events when very high HONO concentrations were generated. This denuder passivation could
300
result in overestimations of kHONO and underestimations of kNO2 in this study. However, since the
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decay rate constants calculated during periods with potential influence from denuder deactivation
302
match the trend of those without (Fig. 4c, 4d), the uncertainty introduced by this potential artefact
303
is minimal, and we are confident that the reactive HONO loss observed in this residence is real.
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On average, kNO2 was 1.54 ± 0.52 hr-1. The reactive NO2 removal rate constant determined
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from the difference method was 0.74 (± 0.35) hr-1, larger than the mean AER. This suggests that
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reactive removal was more important to NO2 lifetimes indoors than air exchange. The mean reac-
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tive NO2 removal rate constant determined in this house is similar to the mean reactive rate con-
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stants reported in controlled experiments in residences.44, 55 Laboratory experiments have demon-
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strated that heterogeneous reactions play an important role in removing NO2 from the air in in-
310
door environments due to the elevated surface area to volume ratios.24, 56-59 The reactive NO2 re-
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moval rate constants determined in this study are comparable to those determined through cham-
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ber experiments in the presence of carpets under various conditions,43 further suggesting that NO2
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surface chemistry may be the dominant reactive NO2 removal mechanism indoors. Photolysis is a
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significant sink for NO2 outdoors, but photolysis rates indoors are much lower due to the lack of
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high energy photons. An NO2 photolysis rate constant of 8.49 × 10-4 s-1 was determined for sunlit
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indoor regions based on our spectral irradiance measurements (see discussion below). This results
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in a photochemical loss rate of 1.70 × 10-3 ppb s-1 (4.57 × 107 molec cm-3 s-1) at background NO2
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levels (2.0 ppb) and 5.01 × 10-2 ppb s-1 (1.35 × 109 molec cm-3 s-1) at peak NO2 concentrations (59
319
ppb). However, photolysis likely did not significantly affect our measured kNO2 because only
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small volumes of air in the residence were in direct sunlight at any given time, and the sampling
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inlet was in shaded regions of the house throughout the campaign. Indeed, NO2 removal rate con-
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stants did not show dependence on time of day (or availability of indoor sunlight) in this study
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(Fig. S4).
324
The average decay rate constant for HONO (kHONO) was 1.68 (± 0.55) hr-1, with a mean reac-
325
tive removal rate constant of 0.58 (0.16 – 1.03) hr-1. Previous experiments have reported negligi-
326
ble reactive HONO removal constants indoors.43-44 One possible HONO sink is uptake to surfac-
327
es.57 Although the specific mechanism is not clear, heterogeneous reaction of HONO with sur-
328
face-deposited nicotine from tobacco smoke highlights the possible reactive sinks on indoor inter-
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faces involving HONO and organics.2 Thermodynamic partitioning of HONO into indoor materi-
330
als that act as HONO reservoirs could be another possible sink.43 As with NO2, photolysis is the
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dominant sink for HONO outdoors, but this is likely not the case indoors, for the same reasons
332
outlined above with respect to NO2 (Fig. S4).
333
Photochemical OH Production
334
HONO photolysis has been predicted to be an important source of indoor OH; this has been
335
experimentally verified in sunlit classrooms.10 Indoor OH production rates from HONO in resi-
336
dences have not been reported. We measured spectral irradiance from sunlight entering the resi-
337
dence through glass patio doors and calculated OH production rates at measured HONO levels.
338
We do not present steady state OH mixing ratios because mixing ratios of OH sinks (especially
339
VOCs) are poorly constrained in residences, and were not measured in this study. Estimated
340
steady state OH mixing ratios would therefore be associated with very large uncertainty. Hydrox-
341
yl radical production rates, which are much better constrained in this study, can be incorporated
342
into indoor chemistry models to estimate steady state OH mixing ratios under a range of condi-
343
tions. Figure 5 shows the noon maximum wavelength-resolved photon fluxes inside this resi-
344
dence through the glass patio doors. A spectrum with the glass doors open is shown for compari-
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son. Light is completely attenuated by the glass at wavelengths shorter than ~340 nm, but a sig-
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nificant amount of light is transmitted in the actinic region (
