Ozone consumption and volatile byproduct formation from surface reactions with aircraft cabin materials and clothing fabrics

Abstract

We measured ozone consumption and byproduct formation on materials commonly found in aircraft cabins at flight-relevant conditions. Two series of small-chamber experiments were conducted, with most runs at low relative humidity (10%) and high air-exchange rate (∼20 h⁻¹). New and used cabin materials (seat fabric, carpet, and plastic) and laundered and worn clothing fabrics (cotton, polyester, and wool) were studied. We measured ozone deposition to many material samples, and we measured ozone uptake and primary and secondary emissions of volatile organic compounds (VOCs) from a subset of samples. Deposition velocities ranged from 0.06 to 0.54 cm s⁻¹. Emissions of VOCs were higher with ozone than without ozone in every case. The most commonly detected secondary emissions were C₁ through C₁₀ saturated aldehydes and the squalene oxidation products 6-methyl-5-hepten-2-one and acetone. For the compounds measured, summed VOC emission rates in the presence of 55–128 ppb (residual level) ozone ranged from 1.0 to 8.9 μmol h⁻¹ m⁻². Total byproduct yield ranged from 0.07 to 0.24 moles of product volatilized per mole of ozone consumed. Results were used to estimate the relative contribution of different materials to ozone deposition and byproduct emissions in a typical aircraft cabin. The dominant contributor to both was clothing fabrics, followed by seat fabric. Results indicate that ozone reactions with surfaces substantially reduce the ozone concentration in the cabin but also generate volatile byproducts of potential concern for the health and comfort of passengers and crew.

Introduction

Passenger flights typically cruise at an altitude of 9–12 km, which is in the upper troposphere or the lower stratosphere. At this height, the air is virtually free of most pollutants; however, the natural level of ozone may be elevated, ranging up to hundreds of ppb (Newchurch et al., 2003). When the ozone level is high outside the plane, the ozone level may also be elevated in the cabin since airplanes are continuously ventilated at high air-exchange rates using ambient air.

As summarized in a National Research Council report (NRC, 2002), several investigations were published in the 1960s and 1970s, documenting that ozone levels posing health concerns occurred in aircraft cabins on some flights, especially those flying at high altitudes, high latitudes, and during the late winter and spring months. The Federal Aviation Administration adopted cabin ozone concentration limits in 1980. The regulations state that ozone can be controlled by means of route planning or through the use of ozone converters. Currently, not all planes have ozone converters and, even when present, there is no consistent protocol in place to ensure their effective performance (NRC, 2002). There have been very few studies on ozone in the cabin environment since the early 1980s. One recently published study measured in-flight cabin ozone concentrations using passive samplers and reported an average level of 80 ppb, suggesting that elevated ozone is still an issue of potential concern in aircraft cabins (Spengler et al., 2004).

The cabin environment is characterized by low relative humidity (RH) (∼10–20%), high air-exchange rate (∼10–20 h⁻¹) and reduced cabin air pressure (∼0.8 atm) (NRC, 2002). Relative to most occupied microenvironments, the occupant density and surface-to-volume ratio in the cabin are high; on a full flight there may be only 1–2 m³ per occupant, including shared spaces.

As with other indoor spaces, the ozone level inside the plane is lower than the level in outside air because ozone is consumed by reactions, principally occurring on surfaces (Weschler, 2000). While reactions with surfaces reduce the level of ozone in cabin air, the byproducts of those reactions may be more irritating or toxic than ozone itself (Weschler, 2004). Studies conducted in a simulated cabin have confirmed that surfaces, including those associated with passengers, are the dominant contributors to ozone consumption and byproduct formation in airplane cabins (Wisthaler et al., 2005; Tamás et al., 2006; Weschler et al., 2007).

Ozone deposition has been characterized in indoor spaces such as homes and offices (as summarized by Weschler, 2000), and ozone deposition to common residential and commercial indoor materials have been studied in chamber experiments and modeled (e.g., Grøntoft and Raychaudhuri, 2004; Morrison and Nazaroff, 2002a; Reiss et al., 1995). Byproducts of ozone reactions with surfaces have been measured for some typical home furnishings. They include toxic air contaminants, such as formaldehyde and acetaldehyde, and compounds with low odor thresholds, such as hexanal, heptanal, nonanal, and various nonenal isomers (Weschler et al., 1992; Morrison and Nazaroff, 2002a; Wang and Morrison, 2006).

Our objective was to measure ozone-surface reactions for individual materials common to the cabin environment at flight-relevant conditions. We carried out experiments in a small chamber where cabin materials (seat fabric, carpet, and plastic) and clothing fabrics (polyester, wool, and cotton) were individually exposed to ozone at low RH and high air exchange rate. The experimental data were interpreted to quantify ozone deposition and uptake rate, to characterize formation of volatile organic byproducts of ozone-initiated chemistry, and to quantify byproduct emission rates and yields.