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The Oxidative Capacity of Indoor Atmospheres Sasho Gligorovski*,† and Charles J. Weschler*,‡,§ †
Aix Marseille University, CNRS, Laboratoire de Chimie de l’Environnement (FRE 3416), (Case 29), 3 place Victor Hugo, F - 13331 Marseille Cedex 3, France ‡ Environmental and Occupational Health Sciences Institute, Rutgers University, 170 Frelinghuysen Road, Piscataway, New Jersey 08854, United States § International Centre for Indoor Environment and Energy, Dept. of Civil Engineering, Technical University of Denmark, Nils Koppels Allé 402, 2800, Lyngby, Denmark derived from the less volatile oxidized organics. In the troposphere, the main daytime sources of OH radicals are the photolysis of ozone, nitrous acid (HONO) and, to a lesser extent, formaldehyde (HCHO). Typical outdoor daytime OH concentrations are on the order of 106 cm−3. Indoors, the UV portion of solar light is largely attenuated, limiting photolysis as a source of OH. Consequently, the ozonolysis of alkenes has been hypothesized to be the major pathway for OH formation indoors.1 Historically, modeling and indirect measurements suggested typical indoor OH concentrations on the order of 105 cm−3. A detailed model of indoor atmospheric chemistry developed by Carslaw2 estimated that the ozonolysis of alkenes/monoterpenes contributes almost 90% to the total OH production rate, while HONO photolysis contributes slightly more than 10%. However, a recent study3 conducted in a school classroom in Marseille, France is forcing re-evaluation of OH as an indoor oxidant. This study measured OH radical concentrations directly using laser-induced fluorescence with a fluorescent assay by gas expansion. The measured concentrations peaked in late afternoon at 1.8 · 106 cm−3, an order of magnitude higher than that suggested by modeling and indirect studies. Analysis n the developed world humans typically spend more than of the results indicates that the photolysis of HONO makes a 85% of their time indoors. Yet our understanding of indoor greater contribution to indoor OH than previously assumed. oxidation processes lags substantially behind that for outdoor Indoor HONO concentrations are frequently larger than those processes. There is a need for real-time, continuous measuremeasured outdoors. Unvented or poorly vented gas-fired ments of hydroxyl radicals, nitrate radicals, and other shortappliances are a strong source of indoor NO2, which can lived, highly reactive oxidants in representative indoor environreact with water sorbed on indoor surfaces to generate HONO. ments. Such information is essential to properly evaluate the The Carslaw model2 indicates that the indoor OH production impact of indoor oxidation chemistry on human health. It is rate via HONO photolysis is very sensitive to the values used time to bring the sophisticated tools of outdoor atmospheric for the outdoor-to-indoor attenuation of light in the 300 nm ≤ chemistry indoors. λ ≤ 405 nm region. The Marseille field campaign found that The major indoor oxidants are thought to be ozone (O3), the wavelengths between 340 and 405 nm were readily available nitrate radical (NO3), and the hydroxyl radical (OH). Nitrogen within this indoor environment (Figure 1 in ref 3). It also dioxide (NO2), hydrogen peroxide (H2O2), hydroperoxy found OH levels as high as 1.5 × 106 cm−3 during periods of radicals (HO2), alkylperoxy radicals (RO2) and chlorine increased ventilation. This may be partially due to decreased atoms (Cl) may also be important indoor oxidants under scavenging of OH by VOCs emitted indoors (such VOCs have certain conditions. Such oxidants alter the mix of chemicals that lower mixing ratios during periods with increased ventilation). building occupants inhale, dermally absorb and incidentally It may also reflect greater outdoor-to-indoor transport of O3 ingest. However, oxidation processes that occur indoors are and NO, leading to increased O 3/alkene and NO/HO2 poorly characterized, and many of the anticipated reaction reactions, respectively. products are yet to be identified. Outdoors, the hydroxyl radical is considered to be the most important oxidant. It plays a central role in photo-oxidation Received: November 5, 2013 Revised: November 25, 2013 cycles, oxidizing primary volatile organic compounds (VOCs) Accepted: November 25, 2013 to form (1) ozone in the presence of NO2, (2) secondary Published: December 4, 2013 oxygenated VOCs, and (3) secondary organic aerosols (SOA)
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© 2013 American Chemical Society
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dx.doi.org/10.1021/es404928t | Environ. Sci. Technol. 2013, 47, 13905−13906
Environmental Science & Technology
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Figure 1. Simplified illustration highlighting some of the more important formation pathways of OH radicals indoors.
These results3 illustrate the value of real-time continuous OH measurements in indoor settings and indicate that OH radical chemistry may play a more important role in indoor oxidation processes than previously thought. However, there is an additional factor to considerhuman occupancy. It is only within the past decade that we have learned that human occupancy dramatically influences ozone levels, and hence the oxidative capacity of indoor atmospheres. For example, in experiments in a simulated office4 the ozone mixing ratio decreased to half its initial value when two people entered. At the same time the mixing ratios of various carbonyls, dicarbonyls and hydroxycarbonyls increased. These changes were primarily due to reactions of ozone with squalene and unsaturated fatty acids found in human skin surface lipids. It is anticipated that skin oil reactions and their products also impact the mixing ratios of other indoor oxidants, including OH radicals. Since the sources and sinks of indoor OH are coupled in a complex fashion, it is difficult to predict even the direction of this impact on indoor OH levels. Real-time, continuous measurements of OH should be conducted in the same environment, under otherwise identical conditions, when it is empty and occupied. Other measurements would further characterize the oxidative capacity of indoor environments and constrain current models of indoor atmospheric chemistry.5 These include better characterization of the light intensity at different wavelengths in various indoor settings. The solar light which penetrates indoors is highly variable, depending on the geographic location, the time of year, orientation of the room, size of the windows and type of window glass. Furthermore, the nature of indoor lighting is changing, with CFLs and LEDs replacing incandescent bulbs. The resultant impacts on indoor photochemistry have received almost no attention. Simultaneous measurements of other indoor oxidants, including NO3, HO2, Cl and nitryl chloride (ClNO2) are needed. We recognize that it is challenging to deploy the instruments used outdoors for reliable real time measurements of short-lived species indoors. However, the atmospheric community is known to be inventive and capable of addressing such challenges if they deem the issue of sufficient importance and if funding is available to pursue these goals. Measurements of indoor oxidants should be
coupled with continuous indoor and outdoor measurements of organic and inorganic compounds that play a role in the generation and/or removal of oxidants. Taken together such studies will better define the dominant oxidation processes that occur indoors, the oxidation products that result from this chemistry, the influence of occupants on the environments that they occupy and, ultimately, the impact of oxidative chemistry on human health.
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AUTHOR INFORMATION
Corresponding Authors
*(S.G.) E-mail:
[email protected]. *(C.J.W.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Weschler, C. J.; Shields, H. C. Production of the hydroxyl radical in indoor air. Environ. Sci. Technol. 1996, 30, 3250−3258. (2) Carslaw, N. A new detailed chemical model for indoor air pollution. Atmos. Environ. 2007, 41, 1164−1179. (3) Gómez Alvarez, E.; Amedro, D.; Afif, C.; Gligorovski, S.; Schoemacker, C.; Fittschen, C.; Doussin, J. F.; Wortham, H. Unexpectedly high indoor hydroxyl radical concentrations associated with nitrous acid. Proc. Natl. Acad. Sci. U.S.A. 2013, 110 (33), 13294− 13299. (4) Wisthaler, A.; Weschler, C. J. Reactions of ozone with human skin lipids: Sources of carbonyls, dicarbonyls, and hydroxycarbonyls in indoor air. Proc. Natl. Acad. Sci. U.S.A. 2010, 107 (15), 6568−6575. (5) Weschler, C. J. Chemistry in indoor environments: 20 years of research. Indoor Air 2011, 21, 205−218.
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