- Email: [email protected]
Calibration of the Ogawa passive ozone sampler for aircraft cabins
Atmospheric Environment 65 (2013) 21e24
Contents lists available at SciVerse ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Technical note
Calibration of the Ogawa passive ozone sampler for aircraft cabins Seema Bhangar a, *, Brett C. Singer b, William W. Nazaroff a, b a b
Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720-1710, USA Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Passive samplers can facilitate ozone monitoring in aircraft cabins. < We report the first field calibration of the Ogawa sampler in aircraft cabins. < Ozone was actively and passively measured on 11 flights in Spring 2007. < We estimate an in-flight effective collection rate of 14.3 0.9 atm cm3 min1. < The estimated sampling rate is similar to others from environments with low airflow.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 June 2012 Received in revised form 4 October 2012 Accepted 8 October 2012
Elevated ozone levels in aircraft cabins would pose a health hazard to exposed passengers and crew. The Ogawa passive sampler is a potentially useful tool for measuring in-cabin ozone levels. Accurate interpretation of measured values requires knowing the effective collection rate of the sampler. To calibrate the passive sampler for the aircraft-cabin environment, ozone was measured simultaneously with an Ogawa sampler and an active ozone analyzer that served as a transfer standard, on 11 commercial passenger flights, during FebeApr 2007. An empirical pressure-independent effective collection rate that can be used to convert nitrate mass to ozone mixing ratio was determined to be 14.3 0.9 atm cm3 min1 (mean standard error). This value is similar to estimates from other applications where airflow rates are low, such as in personal monitoring and in chamber studies. This study represents the first field calibration of any passive sampler for the aircraft cabin environment. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Passive sampling Transportation Ozone Effective collection rate
1. Introduction Along their flight paths, airplanes can encounter elevated levels of ozone that originated in the stratosphere. Measurements made during flights in the 1960s and 1970s revealed that in-cabin ozone was commonly above 100 ppb (Brabets et al., 1967; Bischof, 1973; Nastrom et al., 1980). In response to these data and to associated
* Corresponding author. 609 Davis Hall, University of California, Berkeley, CA 94720, USA. Tel.: þ1 925 980 5501. E-mail address: [email protected] (S. Bhangar). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.012
health concerns for flight attendants, the Federal Aviation Administration established standards (FAR 25.832 and FAR 121.578) designed to limit levels of ozone in airplane cabins (National Research Council, 2002). Since the standards were introduced, the ventilation systems of many aircraft have been equipped with ozone converters to limit the exposure of passengers and crew. However, in part owing to the challenges of sampling in the aircraft cabin environment, published data characterizing current levels of cabin ozone are scant (Spengler et al., 2004; Bhangar et al., 2008). Epidemiological studies indicate that environmental ozone exposure is associated with an increased risk of mortality, down to low levels and with no apparent threshold (Bell et al., 2006). There is
22
S. Bhangar et al. / Atmospheric Environment 65 (2013) 21e24
a public health interest, therefore, in being able to more readily measure ozone in aircraft cabin environments, particularly in aircraft lacking converters. Passive sampling for the measurement of a gaseous contaminant involves assessing the quantity of a gas transferred to a collector by means of molecular diffusion. Samplers based on this principle provide time-integrated measurements. Typically they are easy to use, inexpensive, small, lightweight, unbreakable, and create no noise. Furthermore, as they operate passively, they do not require electromagnetic interference (EMI) certification for use on board aircraft. These features make them good candidates for the application considered here. The Ogawa sampler, developed specifically for ozone monitoring (Koutrakis et al., 1993), contains two chambers that each hold a collection filter protected by a screen and an end cap that regulates the rate of diffusive mass transfer. These filters are coated with a nitrite-based solution; ozone oxidizes the nitrite to nitrate. After sampling, an analysis of the mass of nitrate ion on the filter indicates the amount of ozone collected. From this measurement, the ozone exposure (e.g. in ppb-h units) can be determined if the effective collection rate (ECR) is known. The theoretical collection rate (TCR) is computed from the ozone diffusion coefficient (D) and the dimensions of the sampler e the cross section area of diffusion (A) and the length (L) of the diffusion pathway e as shown in Equation (1a). In Equation (1b), the dependence of the diffusion coefficient on temperature (T) and pressure (P) is made explicit.
TCR ¼
TCR ¼
DA L D0 T 1:5 P
(1a) !
A L
(1b)
The ECR, defined as the collection rate under real sampling conditions, may be higher or lower than the TCR, depending on factors such as airflow conditions near the sampler and homogenous or heterogeneous reactions that can deplete the ozone concentration adjacent to the collector. In application, the mass of nitrate ion measured on the filter ðMNO3 Þ is converted to a molar concentration of ozone ðMO3 Þ by dividing by sampling duration t, the molecular mass of nitrate ðMWNO3 Þ, and the ECR as shown in Equation (2) (Koutrakis et al., 1993).
MO3 ¼
MNO3 t MWNO3 ECR
(2)
The ideal gas law is applied to convert the molar concentration of ozone to a mixing ratio ðYO3 Þ, as shown in Equation (3)
YO3 ¼
MNO3 RT t MWNO3 ECR P
(3)
where R is the gas constant. Equation (3) may be rewritten by defining a new parameter, ECRP, defined as the ECR multiplied by pressure as shown in Equation (4).
YO3 ¼
MNO3 RT t MWNO3 ECRP
(4)
The parameter ECRP provides a convenient basis for converting the mass of nitrate collected to an ozone reading when a mixing ratio (rather than mass concentration) of ozone is sought. Relative to ECR, the ECRP has the advantage of enabling the mass of nitrate ion to be converted to an ozone mixing ratio without requiring knowledge of pressure. Hence, in the context of assessing ozone
mixing ratios, ECRP is denoted as a “pressure adjusted” effective collection rate. For the Ogawa sampler, the TCR at standard temperature and pressure conditions is 24.5 cm3 min1 (Koutrakis et al., 1993). The ECR has been empirically determined under several conditions, including in a laboratory chamber, outdoors, and during use as a personal monitor (Koutrakis et al., 1993; Liu et al., 1994; Black, 2000). Values of the ECR so determined vary over a moderate range. The interior of an aircraft cabin is a distinctive environment with a complex configuration of space, and with conditions that include a high air-exchange rate, low pressure, and a large surfaceto-volume ratio. One cannot know, a priori, whether any of the previously studied environments provides a good value of the ECR for the Ogawa sampler deployed in aircraft cabins. Consequently, the aim of this study was to calibrate the Ogawa sampler for the aircabin environment by assessing an effective collection rate for the device for commercial passenger aircraft during flight. 2. Methods 2.1. Experimental protocols Ozone was measured on eleven flights (see Table 1) simultaneously with the Ogawa passive sampler and a UV-photometric real-time ozone analyzer (2B Technologies, Inc. Model 202). The active monitor had been recently calibrated and was treated as a transfer standard for this effort. Details concerning calibration of the transfer standard are described by Bhangar et al. (2008). As we aimed to obtain a comparison of the Ogawa sampler and active ozone measurements for a range of ozone levels, we monitored during the northern hemisphere low-tropopause season when ambient ozone levels in the air-traffic corridor are most variable. To avoid intentionally including flights with very low mean ozone, domestic flights with control devices were excluded from the sample, as these were found to have flight-mean ozone levels of only a few ppb (Bhangar et al., 2008, 2012). Active ozone sampling in the aircraft cabin was conducted as previously described (Bhangar et al., 2008). In brief, a researcher placed the active monitor under the seat in front of her or him, in the economy cabin of the airplane. The open end of the monitor’s sampling line was clipped to the seat back in front of the researcher to sample air close to the passenger’s breathing zone. The Ogawa monitor was affixed to the seat in front of the passenger, close to the sampling tube inlet of the active monitor. To avoid encountering an ozone-depleted boundary layer, the passive sampler was not placed adjacent to a fleeced or cloth surface. Rather, the seat Table 1 Monitored segments. Batch
Aircraft
Ca
Sample dur. (h)
Mean O3 (ppb)
Nitrate (ng)b
ECRP (atm cm3 min1)c
2 1 2 2 2 2 2 1 1 1 1
B737-300 B757 B737-300 B757 B757 B757 B757 B777 B777 B747-400 B747-400
N N N N N N N Y Y Y Y
4.0 2.6 3.7 4.1 3.6 5.1 4.0 6.3 10 7.8 8.2
6.1 11 13 22 17 24 32 0.2 0.1 18 30
56 70 120 220 94 250 360 26 7.9 300 480
14.6 e 16.8 15.7 9.7 13.0 17.8 e e 14.1 12.5
a Y ¼ converter present (transoceanic flight); N ¼ converter absent (domestic flight). b Measured mass minus the mean “blank” mass associated with the batch. c ECRP not assessed when nitrate mass was below the detection limit, which was 95 ng for samples in Batch 1, and 36 ng for samples in Batch 2.
S. Bhangar et al. / Atmospheric Environment 65 (2013) 21e24
back surface was covered with a paperboard backing before the sampler was pinned to it, or the sampler was attached such that there was at least 2.5 cm of air space separating it from the surface. The sampler was handled as specified by the manufacturer. Coated pads were stored in a refrigerator prior to use. A “batch” was designated as all samples exposed during the month following their assembly. Prior to analysis, assembled samplers were stored in individual dark, sealed vials at room temperature. Three blanks were assigned to each batch and remained with their corresponding samples but were not exposed. On board, samplers were exposed when the transfer standard was turned on and its warm-up period (w20 min) was concluded, and re-sealed when the active monitor was turned off. As a consequence, samples were collected during the major portion of the flight above 3000 m (10,000 ft). Samples and blanks were analyzed via ion chromatography. The analysis included duplicate measurements for quality control. The limit of detection was computed for a batch as three times the standard deviation of field blanks. A background correction factor was computed as the mean of the three blanks in each batch (Koutrakis et al., 1993; Spengler et al., 2004). 2.2. Data analysis ECRP is assessed for flights where the measured nitrate mass was above the passive sampler detection limit, using paired Ogawa readings (in ng nitrate) and ozone exposures ðYO3 t; in ppb-hÞ, as shown in Equation (5), which is derived from Equation (4), and assuming an in-cabin temperature of T ¼ 293 K.
ECRP ¼
MNO3 RT YO3 t MWNO3
(5)
3. Results Blank correction factors of 460 ng nitrate for batch 1 and 360 ng for batch 2 were subtracted from each reading. Limits of detection were 95 ng and 36 ng nitrate for batch 1 and batch 2, respectively. As presented in Table 1, measurements (blank-corrected) ranged from 7.9 to 480 ng nitrate, corresponding to a range of ozone exposures, computed based on measurements made by the active monitor, of 1.3 to 250 ppb-h. Fig. 1 presents a comparison between background-corrected Ogawa passive sampler raw readings and paired ozone exposure values based on readings from the active ozone monitor. The mean ECRP for paired active and passive sampler ozone exposures is 14.3 0.9 atm cm3 min1 (mean standard error).
Fig. 1. A comparison between background-corrected Ogawa passive sampler raw readings and ozone exposure values based on readings from an active ozone monitor when the two were simultaneously deployed. The single-parameter regression slope is 2.1 ng nitrate per ppb-h ozone, which corresponds to ECRP ¼ 14 atm cm3 min1.
23
4. Discussion The empirical ECRP determined in this study can be converted to an ECR estimate by dividing by pressure. During cruise, the pressure in the flight cabin is maintained between approximately 0.75 atm and 0.83 atm. The equivalent ECR for P between 0.75 atm and 0.83 atm is in the range 17e19 cm3 min1. The ozone exposure data that these estimates are based on span a moderate range: 0e 246 ppb-h. Care should be exercised in extrapolating above this range. Results from this calibration effort reinforce the importance of accurate blank correction for unbiased interpretation of Ogawa passive sampler readings. The nitrate levels measured on blank samples in the present study were high (360 and 460 ng nitrate) compared to levels obtained for exposed samples (450e940 ng nitrate). Spengler et al. (2004) reported even higher blank nitrate levels, 700e2600 ng. For low to moderate ozone exposures, a relatively small fractional error in the blank measurement produces a substantial error in the determination of ozone level. The conversion from nitrate mass to ozone mixing ratio requires information on temperature. It is adequate for the flight-cabin environment to assume that normal indoor temperatures prevail (Palmes and Gunnison, 1973). Fractional changes in absolute temperature (in Kelvin) that are commonly encountered in the airplane cabin are small. The Ogawa passive sampler was previously used to measure aircraft-cabin ozone in one study, conducted on board 106 commercial passenger flight segments (Spengler et al., 2004). That study used an ECR of 21.6 cm3 min1 (P ¼ 0.75 atm), w20% higher than the ECR determined here. In that study, samplers were suspended from the ceiling above an unoccupied seat in the first-class cabin. We reiterate that field monitoring in our study was conducted in the more densely packed economy cabin with monitors clipped to the seat back in front of the passenger. As deployed in the study by Spengler et al., the Ogawa sampler is likely to have a higher ECR than would prevail for the use reported here. The influence of variables such as sampler placement, location within the cabin, aircraft occupant density, flight altitude, and aircraft type on ECR was not systematically investigated in this study and has not been the subject of previous research. Hence, we cannot report on the spatial and temporal variability of the ECR within an airplane or its variability among flights. However, an approximate assessment of the anticipated variability and the factors that influence it is gauged by considering previous research on the influence of airflow on the ECR for passive samplers. Koutrakis et al. (1993) attribute the differences among estimates mainly to face velocity effects. Brown (2000) noted that in low wind conditions, the analyte may not be replenished efficiently at the sampler surface, resulting in an increased effective diffusion path length. In high wind conditions, the effective diffusion path length may be decreased by disturbance of the stagnant air layer within the sampler. The range of ECR estimates obtained across monitoring applications indicates the degree to which airflow conditions influence sampler performance. Outdoors, where air speeds are commonly higher than indoors, and where airflow is relatively unimpeded, the ECR has been estimated to be 29.0 2.7 cm3 min1 (N ¼ 37) (Koutrakis et al., 1993). The ECR assessed in a laboratory chamber study was 18.1 1.9 cm3 min1 (N ¼ 20) (Koutrakis et al., 1993). ECR estimates associated with personal monitoring were found to be 9e 38% lower. In personal sampling applications, the reduction in ECR has been attributed to ozone depletion caused by reaction with clothing, dilution associated with human expiratory flow, and human activities blocking the airflow around the sampler (Liu et al., 1994). The equivalent ECR estimate determined in the present
24
S. Bhangar et al. / Atmospheric Environment 65 (2013) 21e24
study of aircraft cabin environments e 17e19 cm3 min1 e is similar to estimates from personal monitoring and from the laboratory chamber. Acknowledgments Sampling containers, vials, and pads were provided courtesy of Ogawa & Co., Inc. The authors thank Donald Schaeffer (Ogawa & Co.) for his advice on sampling protocols. Dr. Eva Hardison (RTI, NC, USA) conducted the ion chromatography. We thank Shannon Cowlin who was instrumental in initiating the active monitoring campaign. The US Federal Aviation Administration (FAA) Office of Aerospace Medicine funded this work through the Air Transportation Center of Excellence for Airliner Cabin Environment Research (ACER), Cooperative Agreement 04-C-ACE-UCB. Although the FAA sponsored this project, it neither endorses nor rejects the findings of this research. References Bell, M.L., Peng, R.D., Dominici, F., 2006. The exposureeresponse curve for ozone and risk of mortality and the adequacy of current ozone regulations. Environmental Health Perspectives 114, 532e536.
Bhangar, S., Cowlin, S.C., Singer, B.C., Sextro, R.G., Nazaroff, W.W., 2008. Ozone levels in passenger cabins of commercial aircraft on North American and transoceanic routes. Environmental Science and Technology 42, 3938e3943. Bhangar, S., Cowlin, S.C., Singer, B.C., Sextro, R.G., Nazaroff, W.W., 2012. Correction to Ozone levels in passenger cabins of commercial aircraft on North American and transoceanic routes. Environmental Science and Technology 46, 1952. Bischof, W., 1973. Ozone measurements in jet airliner cabin air. Water, Air, and Soil Pollution 2, 3e14. Black, D.R., 2000. Development and Application of a Sensor for Real-time Microenvironmental and Personal Ozone Measurements. Ph.D. thesis, UC Berkeley. Brabets, R.I., Hersh, C.K., Klein, M.J., 1967. Ozone measurement survey in commercial jet aircraft. Journal of Aircraft 4, 59e64. Brown, R.H., 2000. Monitoring the ambient environment with diffusive samplers: theory and practical considerations. Journal of Environmental Monitoring 2, 1e9. Koutrakis, P., Wolfson, J.M., Bunyaviroch, A., Froehlich, S.E., Hirano, K., Mulik, J.D., 1993. Measurement of ambient ozone using a nitrite-coated filter. Analytical Chemistry 65, 209e214. Liu, L.J.S., Olson, M.P., Allen, G.A., Koutrakis, P., McDonnell, W.F., Gerrity, T.R., 1994. Evaluation of the Harvard ozone passive sampler on human subjects indoors. Environmental Science and Technology 28, 915e923. Nastrom, G.D., Holdeman, J.D., Perkins, P.J., 1980. Measurements of cabin and ambient ozone on B747 airplanes. Journal of Aircraft 17, 246e249. National Research Council, Committee on Air Quality in Passenger Cabins of Commercial Aircraft, 2002. The Airliner Cabin Environment and the Health of Passengers and Crew. National Academy Press, Washington, DC. Palmes, E.D., Gunnison, A.F., 1973. Personal monitoring device for gaseous contaminants. American Industrial Hygiene Association Journal 34, 78e81. Spengler, J.D., Ludwig, S., Weker, R.A., 2004. Ozone exposures during transcontinental and trans-pacific flights. Indoor Air 14 (Supp. 7), 67e73.
Contents lists available at SciVerse ScienceDirect
Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv
Technical note
Calibration of the Ogawa passive ozone sampler for aircraft cabins Seema Bhangar a, *, Brett C. Singer b, William W. Nazaroff a, b a b
Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720-1710, USA Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Passive samplers can facilitate ozone monitoring in aircraft cabins. < We report the first field calibration of the Ogawa sampler in aircraft cabins. < Ozone was actively and passively measured on 11 flights in Spring 2007. < We estimate an in-flight effective collection rate of 14.3 0.9 atm cm3 min1. < The estimated sampling rate is similar to others from environments with low airflow.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 June 2012 Received in revised form 4 October 2012 Accepted 8 October 2012
Elevated ozone levels in aircraft cabins would pose a health hazard to exposed passengers and crew. The Ogawa passive sampler is a potentially useful tool for measuring in-cabin ozone levels. Accurate interpretation of measured values requires knowing the effective collection rate of the sampler. To calibrate the passive sampler for the aircraft-cabin environment, ozone was measured simultaneously with an Ogawa sampler and an active ozone analyzer that served as a transfer standard, on 11 commercial passenger flights, during FebeApr 2007. An empirical pressure-independent effective collection rate that can be used to convert nitrate mass to ozone mixing ratio was determined to be 14.3 0.9 atm cm3 min1 (mean standard error). This value is similar to estimates from other applications where airflow rates are low, such as in personal monitoring and in chamber studies. This study represents the first field calibration of any passive sampler for the aircraft cabin environment. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Passive sampling Transportation Ozone Effective collection rate
1. Introduction Along their flight paths, airplanes can encounter elevated levels of ozone that originated in the stratosphere. Measurements made during flights in the 1960s and 1970s revealed that in-cabin ozone was commonly above 100 ppb (Brabets et al., 1967; Bischof, 1973; Nastrom et al., 1980). In response to these data and to associated
* Corresponding author. 609 Davis Hall, University of California, Berkeley, CA 94720, USA. Tel.: þ1 925 980 5501. E-mail address: [email protected] (S. Bhangar). 1352-2310/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.atmosenv.2012.10.012
health concerns for flight attendants, the Federal Aviation Administration established standards (FAR 25.832 and FAR 121.578) designed to limit levels of ozone in airplane cabins (National Research Council, 2002). Since the standards were introduced, the ventilation systems of many aircraft have been equipped with ozone converters to limit the exposure of passengers and crew. However, in part owing to the challenges of sampling in the aircraft cabin environment, published data characterizing current levels of cabin ozone are scant (Spengler et al., 2004; Bhangar et al., 2008). Epidemiological studies indicate that environmental ozone exposure is associated with an increased risk of mortality, down to low levels and with no apparent threshold (Bell et al., 2006). There is
22
S. Bhangar et al. / Atmospheric Environment 65 (2013) 21e24
a public health interest, therefore, in being able to more readily measure ozone in aircraft cabin environments, particularly in aircraft lacking converters. Passive sampling for the measurement of a gaseous contaminant involves assessing the quantity of a gas transferred to a collector by means of molecular diffusion. Samplers based on this principle provide time-integrated measurements. Typically they are easy to use, inexpensive, small, lightweight, unbreakable, and create no noise. Furthermore, as they operate passively, they do not require electromagnetic interference (EMI) certification for use on board aircraft. These features make them good candidates for the application considered here. The Ogawa sampler, developed specifically for ozone monitoring (Koutrakis et al., 1993), contains two chambers that each hold a collection filter protected by a screen and an end cap that regulates the rate of diffusive mass transfer. These filters are coated with a nitrite-based solution; ozone oxidizes the nitrite to nitrate. After sampling, an analysis of the mass of nitrate ion on the filter indicates the amount of ozone collected. From this measurement, the ozone exposure (e.g. in ppb-h units) can be determined if the effective collection rate (ECR) is known. The theoretical collection rate (TCR) is computed from the ozone diffusion coefficient (D) and the dimensions of the sampler e the cross section area of diffusion (A) and the length (L) of the diffusion pathway e as shown in Equation (1a). In Equation (1b), the dependence of the diffusion coefficient on temperature (T) and pressure (P) is made explicit.
TCR ¼
TCR ¼
DA L D0 T 1:5 P
(1a) !
A L
(1b)
The ECR, defined as the collection rate under real sampling conditions, may be higher or lower than the TCR, depending on factors such as airflow conditions near the sampler and homogenous or heterogeneous reactions that can deplete the ozone concentration adjacent to the collector. In application, the mass of nitrate ion measured on the filter ðMNO3 Þ is converted to a molar concentration of ozone ðMO3 Þ by dividing by sampling duration t, the molecular mass of nitrate ðMWNO3 Þ, and the ECR as shown in Equation (2) (Koutrakis et al., 1993).
MO3 ¼
MNO3 t MWNO3 ECR
(2)
The ideal gas law is applied to convert the molar concentration of ozone to a mixing ratio ðYO3 Þ, as shown in Equation (3)
YO3 ¼
MNO3 RT t MWNO3 ECR P
(3)
where R is the gas constant. Equation (3) may be rewritten by defining a new parameter, ECRP, defined as the ECR multiplied by pressure as shown in Equation (4).
YO3 ¼
MNO3 RT t MWNO3 ECRP
(4)
The parameter ECRP provides a convenient basis for converting the mass of nitrate collected to an ozone reading when a mixing ratio (rather than mass concentration) of ozone is sought. Relative to ECR, the ECRP has the advantage of enabling the mass of nitrate ion to be converted to an ozone mixing ratio without requiring knowledge of pressure. Hence, in the context of assessing ozone
mixing ratios, ECRP is denoted as a “pressure adjusted” effective collection rate. For the Ogawa sampler, the TCR at standard temperature and pressure conditions is 24.5 cm3 min1 (Koutrakis et al., 1993). The ECR has been empirically determined under several conditions, including in a laboratory chamber, outdoors, and during use as a personal monitor (Koutrakis et al., 1993; Liu et al., 1994; Black, 2000). Values of the ECR so determined vary over a moderate range. The interior of an aircraft cabin is a distinctive environment with a complex configuration of space, and with conditions that include a high air-exchange rate, low pressure, and a large surfaceto-volume ratio. One cannot know, a priori, whether any of the previously studied environments provides a good value of the ECR for the Ogawa sampler deployed in aircraft cabins. Consequently, the aim of this study was to calibrate the Ogawa sampler for the aircabin environment by assessing an effective collection rate for the device for commercial passenger aircraft during flight. 2. Methods 2.1. Experimental protocols Ozone was measured on eleven flights (see Table 1) simultaneously with the Ogawa passive sampler and a UV-photometric real-time ozone analyzer (2B Technologies, Inc. Model 202). The active monitor had been recently calibrated and was treated as a transfer standard for this effort. Details concerning calibration of the transfer standard are described by Bhangar et al. (2008). As we aimed to obtain a comparison of the Ogawa sampler and active ozone measurements for a range of ozone levels, we monitored during the northern hemisphere low-tropopause season when ambient ozone levels in the air-traffic corridor are most variable. To avoid intentionally including flights with very low mean ozone, domestic flights with control devices were excluded from the sample, as these were found to have flight-mean ozone levels of only a few ppb (Bhangar et al., 2008, 2012). Active ozone sampling in the aircraft cabin was conducted as previously described (Bhangar et al., 2008). In brief, a researcher placed the active monitor under the seat in front of her or him, in the economy cabin of the airplane. The open end of the monitor’s sampling line was clipped to the seat back in front of the researcher to sample air close to the passenger’s breathing zone. The Ogawa monitor was affixed to the seat in front of the passenger, close to the sampling tube inlet of the active monitor. To avoid encountering an ozone-depleted boundary layer, the passive sampler was not placed adjacent to a fleeced or cloth surface. Rather, the seat Table 1 Monitored segments. Batch
Aircraft
Ca
Sample dur. (h)
Mean O3 (ppb)
Nitrate (ng)b
ECRP (atm cm3 min1)c
2 1 2 2 2 2 2 1 1 1 1
B737-300 B757 B737-300 B757 B757 B757 B757 B777 B777 B747-400 B747-400
N N N N N N N Y Y Y Y
4.0 2.6 3.7 4.1 3.6 5.1 4.0 6.3 10 7.8 8.2
6.1 11 13 22 17 24 32 0.2 0.1 18 30
56 70 120 220 94 250 360 26 7.9 300 480
14.6 e 16.8 15.7 9.7 13.0 17.8 e e 14.1 12.5
a Y ¼ converter present (transoceanic flight); N ¼ converter absent (domestic flight). b Measured mass minus the mean “blank” mass associated with the batch. c ECRP not assessed when nitrate mass was below the detection limit, which was 95 ng for samples in Batch 1, and 36 ng for samples in Batch 2.
S. Bhangar et al. / Atmospheric Environment 65 (2013) 21e24
back surface was covered with a paperboard backing before the sampler was pinned to it, or the sampler was attached such that there was at least 2.5 cm of air space separating it from the surface. The sampler was handled as specified by the manufacturer. Coated pads were stored in a refrigerator prior to use. A “batch” was designated as all samples exposed during the month following their assembly. Prior to analysis, assembled samplers were stored in individual dark, sealed vials at room temperature. Three blanks were assigned to each batch and remained with their corresponding samples but were not exposed. On board, samplers were exposed when the transfer standard was turned on and its warm-up period (w20 min) was concluded, and re-sealed when the active monitor was turned off. As a consequence, samples were collected during the major portion of the flight above 3000 m (10,000 ft). Samples and blanks were analyzed via ion chromatography. The analysis included duplicate measurements for quality control. The limit of detection was computed for a batch as three times the standard deviation of field blanks. A background correction factor was computed as the mean of the three blanks in each batch (Koutrakis et al., 1993; Spengler et al., 2004). 2.2. Data analysis ECRP is assessed for flights where the measured nitrate mass was above the passive sampler detection limit, using paired Ogawa readings (in ng nitrate) and ozone exposures ðYO3 t; in ppb-hÞ, as shown in Equation (5), which is derived from Equation (4), and assuming an in-cabin temperature of T ¼ 293 K.
ECRP ¼
MNO3 RT YO3 t MWNO3
(5)
3. Results Blank correction factors of 460 ng nitrate for batch 1 and 360 ng for batch 2 were subtracted from each reading. Limits of detection were 95 ng and 36 ng nitrate for batch 1 and batch 2, respectively. As presented in Table 1, measurements (blank-corrected) ranged from 7.9 to 480 ng nitrate, corresponding to a range of ozone exposures, computed based on measurements made by the active monitor, of 1.3 to 250 ppb-h. Fig. 1 presents a comparison between background-corrected Ogawa passive sampler raw readings and paired ozone exposure values based on readings from the active ozone monitor. The mean ECRP for paired active and passive sampler ozone exposures is 14.3 0.9 atm cm3 min1 (mean standard error).
Fig. 1. A comparison between background-corrected Ogawa passive sampler raw readings and ozone exposure values based on readings from an active ozone monitor when the two were simultaneously deployed. The single-parameter regression slope is 2.1 ng nitrate per ppb-h ozone, which corresponds to ECRP ¼ 14 atm cm3 min1.
23
4. Discussion The empirical ECRP determined in this study can be converted to an ECR estimate by dividing by pressure. During cruise, the pressure in the flight cabin is maintained between approximately 0.75 atm and 0.83 atm. The equivalent ECR for P between 0.75 atm and 0.83 atm is in the range 17e19 cm3 min1. The ozone exposure data that these estimates are based on span a moderate range: 0e 246 ppb-h. Care should be exercised in extrapolating above this range. Results from this calibration effort reinforce the importance of accurate blank correction for unbiased interpretation of Ogawa passive sampler readings. The nitrate levels measured on blank samples in the present study were high (360 and 460 ng nitrate) compared to levels obtained for exposed samples (450e940 ng nitrate). Spengler et al. (2004) reported even higher blank nitrate levels, 700e2600 ng. For low to moderate ozone exposures, a relatively small fractional error in the blank measurement produces a substantial error in the determination of ozone level. The conversion from nitrate mass to ozone mixing ratio requires information on temperature. It is adequate for the flight-cabin environment to assume that normal indoor temperatures prevail (Palmes and Gunnison, 1973). Fractional changes in absolute temperature (in Kelvin) that are commonly encountered in the airplane cabin are small. The Ogawa passive sampler was previously used to measure aircraft-cabin ozone in one study, conducted on board 106 commercial passenger flight segments (Spengler et al., 2004). That study used an ECR of 21.6 cm3 min1 (P ¼ 0.75 atm), w20% higher than the ECR determined here. In that study, samplers were suspended from the ceiling above an unoccupied seat in the first-class cabin. We reiterate that field monitoring in our study was conducted in the more densely packed economy cabin with monitors clipped to the seat back in front of the passenger. As deployed in the study by Spengler et al., the Ogawa sampler is likely to have a higher ECR than would prevail for the use reported here. The influence of variables such as sampler placement, location within the cabin, aircraft occupant density, flight altitude, and aircraft type on ECR was not systematically investigated in this study and has not been the subject of previous research. Hence, we cannot report on the spatial and temporal variability of the ECR within an airplane or its variability among flights. However, an approximate assessment of the anticipated variability and the factors that influence it is gauged by considering previous research on the influence of airflow on the ECR for passive samplers. Koutrakis et al. (1993) attribute the differences among estimates mainly to face velocity effects. Brown (2000) noted that in low wind conditions, the analyte may not be replenished efficiently at the sampler surface, resulting in an increased effective diffusion path length. In high wind conditions, the effective diffusion path length may be decreased by disturbance of the stagnant air layer within the sampler. The range of ECR estimates obtained across monitoring applications indicates the degree to which airflow conditions influence sampler performance. Outdoors, where air speeds are commonly higher than indoors, and where airflow is relatively unimpeded, the ECR has been estimated to be 29.0 2.7 cm3 min1 (N ¼ 37) (Koutrakis et al., 1993). The ECR assessed in a laboratory chamber study was 18.1 1.9 cm3 min1 (N ¼ 20) (Koutrakis et al., 1993). ECR estimates associated with personal monitoring were found to be 9e 38% lower. In personal sampling applications, the reduction in ECR has been attributed to ozone depletion caused by reaction with clothing, dilution associated with human expiratory flow, and human activities blocking the airflow around the sampler (Liu et al., 1994). The equivalent ECR estimate determined in the present
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study of aircraft cabin environments e 17e19 cm3 min1 e is similar to estimates from personal monitoring and from the laboratory chamber. Acknowledgments Sampling containers, vials, and pads were provided courtesy of Ogawa & Co., Inc. The authors thank Donald Schaeffer (Ogawa & Co.) for his advice on sampling protocols. Dr. Eva Hardison (RTI, NC, USA) conducted the ion chromatography. We thank Shannon Cowlin who was instrumental in initiating the active monitoring campaign. The US Federal Aviation Administration (FAA) Office of Aerospace Medicine funded this work through the Air Transportation Center of Excellence for Airliner Cabin Environment Research (ACER), Cooperative Agreement 04-C-ACE-UCB. Although the FAA sponsored this project, it neither endorses nor rejects the findings of this research. References Bell, M.L., Peng, R.D., Dominici, F., 2006. The exposureeresponse curve for ozone and risk of mortality and the adequacy of current ozone regulations. Environmental Health Perspectives 114, 532e536.
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