Thermal Condition and Air Quality Investigation in Commercial Airliner Cabins

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Article Thermal Condition and Air Quality Investigation in Commercial Airliner Cabins

Nu Yu †, Yao Zhang *,† , Mengya Zhang and Haifeng Li

Department of Air Trafﬁc, College of Civil Aviation, Nanjing University of Aeronautics and Astronautics, Nanjing 211106, China; [email protected] (N.Y.); [email protected] (M.Z.); [email protected] (H.L.) * Correspondence: [email protected]; Tel.: +86-159-5195-6600 † These two authors contributed equally to this work.

Abstract: Cabin air quality and thermal conditions have a direct impact on passenger and ﬂight crew’s health and comfort. In this study, in-cabin thermal environment and particulate matter (PM) exposures were investigated in four China domestic ﬂights. The mean and standard deviation of

the in-cabin carbon dioxide (CO2) concentrations in two tested ﬂights are 1440 ± 111 ppm. The measured maximum in-cabin carbon monoxide (CO) concentration is 1.2 ppm, which is under the US Occupational Safety and Health Administration (OSHA) permissible exposure limit of 10 ppm. The tested relative humidity ranges from 13.8% to 67.0% with an average of 31.7%. The cabin pressure change rates at the end of the climbing stages and the beginning of the descending stages are close to 10 hPa·min−1, which might induce the uncomfortable feeling of passengers and crew members. PM mass concentrations were measured on four ﬂights. The results show that PM concentrations decreased after the aircraft cabin door closed and were affected by severe turbulences. The highest in-cabin PM concentrations were observed in the oldest aircraft with an age of 13.2 years, and the waiting phase in this aircraft generated the highest exposures. Citation: Yu, N.; Zhang, Y.; Zhang, M.; Li, H. Thermal Condition and Air Keywords: aircraft cabin air quality; aircraft cabin thermal environment; environmental health; Quality Investigation in Commercial particulate matter; exposures Airliner Cabins. Sustainability 2021, 13, 7047. https://doi.org/10.3390/ su13137047 1. Introduction Academic Editor: David Hou Along with the rapid growth of the transportation industry, environmental health Chi Chow concerns constitute an essential impediment for people to choose their travel methods. Aircraft cabin is a unique micro-environment, which relies on the aircraft’s environmental Received: 8 May 2021 control system (ECS) to adjust its air supply, cabin pressure (P), temperature (T), and Accepted: 18 June 2021 Published: 23 June 2021 relative humidity (RH) [1,2]. The air supplied to the aircraft cabin is a mixture of outside bleed air and interior recirculated air [2,3]. Both bleed air and return air contribute to in-

Publisher’s Note: MDPI stays neutral cabin air pollution and affect the health of cabin crew and passengers [4,5]. The recirculated with regard to jurisdictional claims in air contains various pollutants, such as airborne particles, volatile organic compounds, published maps and institutional affil- pathogenic bacteria, and viruses. The high-efficiency particulate air (HEPA) filter, as an iations. air conditioning system component, intercepts both microscopic particles and larger ones, even microorganisms in return air, but the filters are rendered less effective if bypassed, improperly used, or clogged by particulate matter (PM) [6,7]. Ambient pollutants are also introduced to aircraft cabin by the ECS along with bleed air, especially when the aircraft is waiting or taxiing on the ground. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Research shows that, when the aircraft was taxiing on the ground and flying in the This article is an open access article stratosphere, its in-cabin PM mass concentrations were higher than those when the aircraft distributed under the terms and was cruising in the troposphere [3]. According to Ren et al., the ultrafine particle (UFP) conditions of the Creative Commons number concentrations in an MD-82 aircraft parked at Tianjin International Airport ranges 4 −3 4 −3 Attribution (CC BY) license (https:// from 1.0 × 10 particles·cm to 3.7 × 10 particles·cm , which means waiting at 100 m creativecommons.org/licenses/by/ downwind of the runway for 40 min, is equivalent to a 4-h exposure in typical urban 4.0/). conditions [8]. Commercial flights waiting on the ground will cause more fine particles to

Sustainability 2021, 13, 7047. https://doi.org/10.3390/su13137047 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 7047 2 of 15

be deposited in their ECSs, and the deposition is harmful not only to the health of crew and passengers but also to the performance of the aircrafts [9,10]. During cruising, the particle concentration spikes show responses to severe turbulences (ST) and cabin activities [11,12]. The research of health and safety and thermal comfort of cabin crew and passengers has become one of the hot research topics in the past decades. Many health issues of frequent air travelers and cabin crew are related to their in-cabin experiences. The health issues include mental stress, high altitude conditions, hormonal dysregulation, physical inactivity, fatigue, and biological infections, and the health risks during waiting and taxiing phases are more worthy of attention [13–16]. Pang, L. et al. indicated that the thermal comfort is not controlled well in some short-haul flights, which lead to overheating or undercooling, and an adaptive model was developed to help crew control the temperature and RH [17]. Some studies observed arid inside aircraft cabin during the cruise phase [18]. When assessing mental performance, the in-cabin temperature has a greater impact than the carbon dioxide (CO2) concentration does on a flight [19]. Tang et al. indicated that the heart rates increase significantly with the increase of exposure time in the aircraft cabin [20]. However, Zitter et al. found no difference between aircraft using 100% fresh air and 50% recirculated air in terms of the passenger likelihood of cold symptoms during the week after their flights [6]. This research investigates the realistic thermal environment and particulate matter (PM) exposures in four Chinese domestic flights, aiming to explore the in-cabin factors affecting airliner crew and passengers’ comfort and health, in order to provide preliminary data and directions for future studies on their safety and health.

2. Experiment and Methods The experiments were performed on four different airliners in April 2019. As Figure1 Sustainability 2021, 13, x FOR PEER REVIEWshows, the four airliners executed two China domestic round trips starting and ending 3 of 15 at Nanjing Lukou International Airport (NKG). The first round trip headed to and flew back from Guangzhou Baiyun International Airport (CAN), and the second round trip headed to and flew back from Haikou Meilan International Airport (HAK). The planned mercialflight duration aircraft of cabin these singleenvironment, way trips such is about as taxiing, two hours. climbing, A blank cruising, flight log descending, (FL) form taxi- ingwas segments prepared by [4]. the Park researchers et al. divided and filled the out flight by the into pilots nine after phases the aircraft to study landed. the The local and overallFL collected thermal information comfort about in an the aircraft aircraft cabin model, and age, their capacity, interrelations, flight duration, such cruising as processing, boarding,altitude, and taxiing, flight take distance. off, climb, Researchers cruise, in landing, the flights descending, recorded all post-processing cabin activities andphases [23]. severe turbulences.

Figure 1.1.The The tested tested round round trips trips from from Nanjing Nanjing Lukou Lukou International International Airport Airport (NKG). (NKG). The destinations The destinations are GuangzhouGuangzhou Baiyun Baiyun International International Airport Airport (CAN) and (CAN) Haikou and Meilan Haikou International Meilan AirportInternational (HAK). Airport (HAK).

Table 1. Information about the tested flights.

Flight Aircraft Aircraft Passengers Total Sampling Parameters ID Flight Date Duration Type Age (Year) Onboard Seats Location Collected (min) 18 April Airbus E1 NKG→CAN 13.2 135 128 128 19F PM 2019 319 20 April Airbus E2 CAN→NKG 0.7 135 195 195 31B PM 2019 321 27 April Airbus PM, CO2, CO, P, E3 NKG→HAK 5.9 175 174 174 24E 2019 320 T, RH 28 April Airbus PM, CO2, CO, P, E4 HAK→NKG 3.6 170 158 158 20E 2019 320 T, RH

Table 2 summarizes the instrument information. During the experiments, a Particu- late Matter Sensor (PMS) 3003 (Plantower Inc., Beijing, China) was deployed to monitor the PM mass concentrations throughout all tested flights. The measurements were recorded with one-second intervals and conveyed simultaneously to a laptop that the researcher carried onto the flights together with the instrument. The suspended particles scatter the laser beams in the PMS 3003, and the scattered light is collected at a certain angle to obtain a curve of the intensity. Then, the microprocessor uses an algorithm to obtain the equivalent particle sizes and the number concentrations per unit volume. Fi- nally, they were converted to total mass concentrations of different particle size ranges. The PMS 3003 has been calibrated against a TSI DustTrak 8532 for PM2.5 (R2 = 0.89). A 2.5 µm size-selective impactor was installed onto the DustTrak 8532 inlet before the calibration, and the calibration was done both indoor and outdoor to cover a 0–50 µg m−3 reading range. In E3 and E4, the CO2 and CO concentrations, along with the cabin pressure, temperature, and RH, were measured with a Q-Trak Indoor Air Quality Monitor 7575 (TSI Inc., Shoreview, MN, USA). These instruments were placed under the seat to avoid the Sustainability 2021, 13, 7047 3 of 15

PM concentrations were measured throughout all of the four tested flights (E1 through E4), however, due to the limitation of instrument availability, the CO2 and CO concentrations, cabin pressure, temperature, and RH were only monitored in the second-round trip (E3 and E4 only). Although a previous study shows that the sampling location inside the cabin did not well impact the measurements due to the strong ventilation, the in-cabin sampling locations in this study are all close to the rear-mounted jet engines in the four tested flights [21]. Table1 summarizes the aircraft and flight information of the four tested airliners, and the environmental parameters collected on them. The airliner ages range from 0.7 to 13.2 years, according to the information collected from the FL and the Umetrip website [22]. Each tested flight was divided into four phases in sequence starting from the cabin door closed to the cabin door reopened, based on the researcher’s observations. The four phases are waiting, taxiing-out, inflight, and taxiing-in phases. The inflight phase is the period from the aircraft leaving the ground till touching the ground again. The taxiing phase is the aircraft rolling on the airport ground. People may divide flight time into various phases based on their research needs. For example, Li et al. divided the flight phase into five segments to study the source apportionment of the airborne particles in the commercial aircraft cabin environment, such as taxiing, climbing, cruising, descending, taxiing segments [4]. Park et al. divided the flight into nine phases to study the local and overall thermal comfort in an aircraft cabin and their interrelations, such as processing, boarding, taxiing, take off, climb, cruise, landing, descending, post-processing phases [23].

Table 1. Information about the tested ﬂights.

Aircraft Aircraft Flight Duration Passengers Total Sampling Parameters ID Flight Date Type Age (Year) (min) Onboard Seats Location Collected E1 NKG→CAN 18 April 2019 Airbus 319 13.2 135 128 128 19F PM E2 CAN→NKG 20 April 2019 Airbus 321 0.7 135 195 195 31B PM PM, CO , E3 NKG→HAK 27 April 2019 Airbus 320 5.9 175 174 174 24E 2 CO, P, T, RH PM, CO , E4 HAK→NKG 28 April 2019 Airbus 320 3.6 170 158 158 20E 2 CO, P, T, RH

Table2 summarizes the instrument information. During the experiments, a Particulate Matter Sensor (PMS) 3003 (Plantower Inc., Beijing, China) was deployed to monitor the PM mass concentrations throughout all tested ﬂights. The measurements were recorded with one-second intervals and conveyed simultaneously to a laptop that the researcher carried onto the ﬂights together with the instrument. The suspended particles scatter the laser beams in the PMS 3003, and the scattered light is collected at a certain angle to obtain a curve of the intensity. Then, the microprocessor uses an algorithm to obtain the equivalent particle sizes and the number concentrations per unit volume. Finally, they were converted to total mass concentrations of different particle size ranges. The PMS 2 3003 has been calibrated against a TSI DustTrak 8532 for PM2.5 (R = 0.89). A 2.5 µm size-selective impactor was installed onto the DustTrak 8532 inlet before the calibration, and the calibration was done both indoor and outdoor to cover a 0–50 µg m−3 reading range. In E3 and E4, the CO2 and CO concentrations, along with the cabin pressure, temperature, and RH, were measured with a Q-Trak Indoor Air Quality Monitor 7575 (TSI Inc., Shoreview, MN, USA). These instruments were placed under the seat to avoid the panic among passengers. All data were recorded with a one-second interval automatically. One-minute averages were calculated for further analysis. The SPSS statistics 22 (IBM Inc, Endicott, NY, USA.) program was used for statistical analysis. Sustainability 2021, 13, 7047 4 of 15

Table 2. Instrument Information.

Parameters Instrument Unit Interval Range Accuracy Collected ±10% @ 100~500 µg m−3 PMS Model 3003 PM /PM /PM µg m−3 1 s 0~1000 µg m−3 1 2.5 10 ±10 µg m−3 @ 0~100 µg m−3

CO2 ppm 1 s 0~5000 ppm ±50 ppm CO ppm 1 s 0~500 ppm ±3 ppm Q-Trak indoor air quality P hPa 1 s 689.5~1241.1 hPa ±2% of Reading monitor 7575 RH % 1 s 5~95% RH ±3% RH T ◦C 1 s 0~60 ◦C ±0.5 ◦C

The air exchange rates (AER) inside the tested aircraft cabin can be calculated using CO2 as the tracer. The in-cabin CO2 mass balance model is shown in Equation (1) below,

dC V × in = E + Q(C − C ) (1) cabin dt out in

3 where Vcabin is the aircraft cabin volume (excludes cockpit), Vcabin = 139 m in this study for Airbus 320. Cin and Cout are the in-cabin and out-cabin CO2 concentrations in ppm. 405 ppm is used as the outside-cabin CO2 concentration in the stratosphere. t is time. −1 E is the in-cabin CO2 emission rate (0.4 L·min per person) [24]. Q is the ﬂow rate of outside-air being introduced to in-cabin. Equation (1) can be converted into the discrete form as shown in Equation (2),

Cin(t + ∆t) − Cin(t − ∆t) E = AER(t) × [Cout(t) − C (t)] + (2) 2∆t in V

where ∆t is the time change, AER(t) equals to Q/V in min−1 at time t.

3. Results and Discussion 3.1. Temperature, Relative Humidity, and Cabin Pressure Table3 summarizes the in-cabin temperature, pressure, and RH measurements in E3 and E4. Figure2 shows their time series. The temperature and RH levels in E3 and E4 fluctuated within similar narrow ranges. The means of the E3 and E4 in-cabin temperature are 27.5 and 27.2 ◦C, respectively. The RH in E3 ranges from 14.0 to 66.4% with a mean of 29.7%, and the RH in E4 ranges from 13.8 to 67.0% with a mean of 33.6%. The means and standard deviations (SD) of the measured cabin pressure in E3 and E4 are 910 ± 91 hPa and 938 ± 64 hPa, respectively (Table3). The mean ± SD of in-cabin T, RH, and P in E3 and E4 were applied to calculate the Predicted Mean Votes (PMV) and Predicted Percentage of Dissatisfied (PPD) based on ASHRAE Standard 55 [25], while assuming the mean passenger metabolic rate is 1.0 met (sit quietly in their seats), the in-cabin air speed is 0.1 m/s, and their clothing level is 0.61 clo (classic spring clothing: thin pants+long-sleeved shirts+socks+shoes, etc). The calculated means of PMV in E3 and E4 are 0.51 and 0.45, with ranges of (0.23, 0.80) and (−0.11, 1.09), respectively, which indicate that the E3 passengers feel neutral but the E4 passengers feel either neutral or slightly warm. The calculated means of PPD in E3 and E4 are 10% and 9%, with ranges of (6%, 18%) and (5%, 28%), respectively, which indicate the percentage of people who might feel discomfort with the thermal conditions in these flights. Martinez provides guidelines on aircraft thermal environment in his “Aircraft Envi- ronment Control” [26]. It recommends the ECS to provide comfort conditions with a temperature of 22.0 ± 2.0 ◦C, cabin pressure of 900~1000 hPa, and RH levels of 50.0~70.0% [26]. Table3 and Figure2 show that the means of the measured temperature in E3 and E4 are over 5.0 ◦C higher than the recommended, and the mean measured RH is only about half of the middle value of Martinez recommended range. The cabin pressure also fluctuated a Sustainability 2021, 13, x FOR PEER REVIEW 6 of 15

uses the landing elevation and corrected sea level pressure data from the FMGS to optimize the pressure schedule, and sends command signals to its respective motor, which directs the outflow value to the commanded position. The active CPC pressurizes the aircraft through 6 modes: ground, takeoff, cruise, descent, and abort. These modes are impacted by cabin altitude. Thus, the measured cabin pressure levels did not show smooth trends in E3 and E4 (Figure 2). In this study, the fluctuation ranges of cabin pressure in E3 and E4 are 225 hPa (796 to 1021 hPa) and 180 hPa (839 to 1019 hPa), respectively. Accord- ing to Martinez 2015, a sudden change of cabin pressure over 30 hPa causes pain and vertigo, and the change rate of cabin pressure should be less than 10 hPa·min−1 to decrease the risk of otitis media-related with air traveling [26]. Our measurements show that the cabin pressure change rates during E3 and E4 at the end of the climbing stage and the beginning of the descending stage are close to 1 kPa·min−1, and these might have induced the eardrum pains and transient hearing loss among passengers and crew members (Fig- ure 2). The researchers riding on the aircrafts felt ear pains during those occasions. In E4, the cabin pressure rises sharply from 20: 12 to 20: 22 (Figure 2b). This is because Sustainability 2021, 13, 7047 5 of 15 the E4 flight was scheduled to stop at Yichun Mingyueshan Airport (YIC) before landing on NKG (HAK→YIC→NKG). However, after the aircraft descended, it gave up approaching the YIC airport due to the unexpected meteorological conditions. The pressure change patternbit off the in recommendedE4 indicates that range. the aircraft However, did thenot meango back measured to the regular RH of cruising this study altitude is 1.7 (E3) for theand rest 1.9 (E4)of the times flight. higher than the average of Cui et al.’s measurements of 17.45% [18,26].

Table 3. Statistics of temperature, RH, and cabincabin pressurepressure inin E3E3 andand E4.E4.

T (℃) T( RH◦C) (%) RH (%) P (hPa) P (hPa) Sample ID Mean ± SD Mean ± SD Mean ± SD ID Size (N) Sample Size (N) Mean ± SD Mean ± SD Mean ± SD (min, max) (min,(min, max) max) (min, max)(min, max)(min, max) 27.5 ± 0.4 27.529.7± 0.4± 15.3 29.7 ± 15.3 910 ± 91 910 ± 91 E3 E3196 196 (26.2, 28.1) (26.2,(14.0, 28.1) 66.4) (14.0, 66.4)(796, 1021)(796, 1021) 27.2 ± 1.2 27.233.6± 1.2± 17.5 33.6 ± 17.5 938 ± 64 938 ± 64 E4 E4179 179 (21.0, 28.1) (21.0,(13.8, 28.1) 67.0) (13.8, 67.0)(839, 1019)(839, 1019)

Waiting Taxiing-Out Inflight Taxiing-In Waiting Taxiing-Out Inflight Taxiing-In (a) 30 (b) 30 1050 1050 T P RH 70 T P RH 70 29 29 1015 1015 60 60 28 980 28 980

50 50 945 945 27 27 T (℃) T (℃) T 40 910 40 910 P (hPa) P (hPa) P RH (%) 26 (%) RH 26 875 875 30 30 25 25 840 840

24 20 24 20 805 805

23 10 770 23 10 770 7:47 7:57 8:07 8:17 8:27 8:37 8:47 8:57 9:07 9:17 9:27 9:37 9:47 9:57 19:07 19:17 19:27 19:37 19:47 19:57 20:07 20:17 20:27 20:37 20:47 20:57 21:07 21:17 21:27 21:37 21:47 10:07 10:17 10:27 10:37 10:47 Time (hh:mm) Time (hh:mm) Figure 2. The RH, temperature, and cabin pressure time series in E3 (a) and E4 (b). Figure 2. The RH, temperature, and cabin pressure time series in E3 (a) and E4 (b). The in-cabin RH is usually controlled for two reasons: (1) lower the RH to reduce 3.2.the Carbon chance Dioxide of metal and corrosion Carbon Monoxide in the aircraft; (2) maintain comfort and low health risk environmentAs shown for in cabin Table workers 4, the measured and passengers. CO2 concentrations The previous range study from shows 1304 that to aluminum2135 ppm inalloy E3, corrodesand from much1069 to faster 1861 inppm the in environment E4. The mean where and standard the RHis deviation higher than of CO 70.0%2 concen- [27]. trationsBoth Martinez in E3 were and 1557 Cui et± 117 al. ppm, indicate and that 1323 the ± 104 aircraft ppm in-cabinin E4 (Table RH is4). often In this kept study, as lowthe aircraftas 10.0~20.0% type of toE3 avoid and E4 condensation is Airbus 320 problems with a passenger on cold walls compartment and equipment, volume which of 139 tend m3. However,to corrode the metals E3 aircraft and boost has 174 micro-organism seats, while the growth E4 has [ 18158,26 seats,]. However, and both low of them humidity were in aircraft cabin usually induce the feelings of dry skin, dry mucous membranes, and eye irritation, especially on long haul flights over 3 h [28]. For example, Uchiyama et al. found that increased dry eye symptoms were significantly associated with the low RH environmental conditions during air travel [29]. However, there is no conclusion of the correlation between biohazards and the cabin RH so far. Korves et al. indicated that low humidity suppresses the spread of bacteria [30], but Liu et al. found that the bacterial concentrations in the airliners decrease as the RH increases, and the number of colonies per cubic meter at 22.0% RH is 7.3 times of that at 30.0% RH [31]. Liu et al. recommended maintaining the aircraft cabin RH of 30.0% ± 3.0% considering both the aircraft safety and the thermal comfort and health [31]. The means of RH measurements in our study are close to this recommendation, but the ranges are much larger because we tested the cabin conditions from the cabin door closed to the door reopen. While the cabin doors were open to the ambient for passenger boarding and getting off, the departure or arrival airport weather conditions determine the temperature and RH at the start and endpoints of the tested flights. For example, at the departing time in E4, the instant RH in the cabin was over 65.0% when the aircraft cabin door was just closed, because it was raining at HAK at that Sustainability 2021, 13, 7047 6 of 15

moment (Figure2b). The RH levels went down during the inflight phase to their minimum values around 14% and then started to go up when the aircrafts descended (Figure2). Most commercial aircraft cabin air supply systems provide a mixture of 50% fresh air and 50% recirculated air to the cabin [32–35]. The cabin air gets exhausted through the grills on the cabin floor, and 50% of it is recirculated by the aircraft ventilation system. Dehumidifiers are installed in most aircrafts to remove the moisture in bleed air before the air enters the turbine and provide a relatively dry condition. However, no aircraft has a humidifier to increase the RH when the air enters the cabin. Besides, bleed air is processed by an ozone converter mostly installed behind the fresh air extraction port of the compressor of a jet engine to decompose the ozone in bleed air. Then, the dry air after ozone removal, mixed with recirculated air, is delivered into the cabin by the diffusers above the passenger seats. Because there is almost no moisture in the stratosphere, the cabin air keeps losing moisture when the aircraft is cruising, and the in-cabin RH keeps decreasing until the aircraft descends to the troposphere, where the ambient air contains moisture. The cabin pressure change depends on the combined effects of the amount of supply air, leak air, and exhaust air. The supply air is usually adequate and stable if the ECS functions well, and the leak air is not under control during the flight. Therefore, the cabin pressure adjustment is mainly controlled by the amount of exhaust air, that is, by the outflow valve. The automatic adjustment mode is most frequently used, while the cabin pressure is controlled by a cabin pressure controller (CPC) based on the signal from the flight management guidance computer (FMGC). There are two CPCs installed with paired motors, but only one CPC operates while the other one remains on standby. The active CPC uses the landing elevation and corrected sea level pressure data from the FMGS to optimize the pressure schedule, and sends command signals to its respective motor, which directs the outflow value to the commanded position. The active CPC pressurizes the aircraft through 6 modes: ground, takeoff, cruise, descent, and abort. These modes are impacted by cabin altitude. Thus, the measured cabin pressure levels did not show smooth trends in E3 and E4 (Figure2). In this study, the fluctuation ranges of cabin pressure in E3 and E4 are 225 hPa (796 to 1021 hPa) and 180 hPa (839 to 1019 hPa), respectively. According to Martinez 2015, a sudden change of cabin pressure over 30 hPa causes pain and vertigo, and the change rate of cabin pressure should be less than 10 hPa·min−1 to decrease the risk of otitis media-related with air traveling [26]. Our measurements show that the cabin pressure change rates during E3 and E4 at the end of the climbing stage and the beginning of the descending stage are close to 1 kPa·min−1, and these might have induced the eardrum pains and transient hearing loss among passengers and crew members (Figure2). The researchers riding on the aircrafts felt ear pains during those occasions. In E4, the cabin pressure rises sharply from 20: 12 to 20: 22 (Figure2b). This is because the E4 flight was scheduled to stop at Yichun Mingyueshan Airport (YIC) before landing on NKG (HAK→YIC→NKG). However, after the aircraft descended, it gave up approaching the YIC airport due to the unexpected meteorological conditions. The pressure change pattern in E4 indicates that the aircraft did not go back to the regular cruising altitude for the rest of the flight.

3.2. Carbon Dioxide and Carbon Monoxide

As shown in Table4, the measured CO 2 concentrations range from 1304 to 2135 ppm in E3, and from 1069 to 1861 ppm in E4. The mean and standard deviation of CO2 concentrations in E3 were 1557 ± 117 ppm, and 1323 ± 104 ppm in E4 (Table4). In this study, the aircraft type of E3 and E4 is Airbus 320 with a passenger compartment volume of 139 m3. However, the E3 aircraft has 174 seats, while the E4 has 158 seats, and both of them were fully loaded. The E3 aircraft is more occupied and older (5.9 yr vs. 3.6 yr) than E4, and these might explain the higher mean CO2 in E3. Sustainability 2021, 13, x FOR PEER REVIEW 7 of 15

fully loaded. The E3 aircraft is more occupied and older (5.9 yr vs. 3.6 yr) than E4, and these might explain the higher mean CO2 in E3. Sustainability 2021, 13, 7047 7 of 15 Table 4. Summary of in-cabin CO and CO2 concentrations in measured flights.

CO (ppm) CO2 (ppm) Sample Size ID Mean ± SD Mean ± SD Table 4. Summary of(N) in-cabin CO and CO2 concentrations in measured ﬂights. (min, max) (min, max) 0.12 ± 0.26 CO (ppm)1557 CO± 1172 (ppm) E3 196 Sample Size ID ± ± (N) (0.00, 1.18) Mean SD (1304,Mean 2135) SD 0.01 ± 0.02 (min, max) 1323 (min,± 104 max) E4 179 (0.00, 0.12) 0.12 ± 0.26 (1069,1557 1861)± 117 E3 196 (0.00, 1.18) (1304, 2135)

Figure 3 shows the CO2 concentration time series0.01 in± E30.02 and E4. There1323 is an± 104obvious E4 179 spike at the end of the waiting phase and the beginning(0.00, 0.12)of the taxiing-out(1069, phase 1861) in both E3 and E4. This is because the aircraft power supply system was switched from the auxil- iary power unit (APU) to the jet engine at the end of the waiting phase, so the air condi- Figure3 shows the CO 2 concentration time series in E3 and E4. There is an obvious tionerspike atwas the not end working of the waiting continuously phase at and that the moment. beginning Additionally, of the taxiing-out a huge phaseamount in of both CO E32 wasand emitted E4. This while is because the aviation the aircraft fuel powerstarted supply burning system and was was introduced switched from into thethe auxiliaryaircraft cabinpower with unit the (APU) jet engine to the bleed jet engine air while at the the end aircraft of the was waiting not moving. phase, soThe the Cao air et conditioner al. study also shows that the ground operation segments have higher in-cabin CO2 concentrations was not working continuously at that moment. Additionally, a huge amount of CO2 was thanemitted the whilecruising the segments, aviation fuel due started to theburning inadequate and ground was introduced ventilation into by the APU aircraft and cabin the higherwith the metabolic jet engine rates bleed of people air while during the boarding aircraft [36]. was In not this moving. study, the The CO Cao2 concentration et al. study decreases during the taxiing-out phase and has another rise during the inflight phase after also shows that the ground operation segments have higher in-cabin CO2 concentrations cabinthan thepressure cruising drop, segments, which is duesimilar to theto the inadequate study of Cao ground et al. ventilation [36]. by APU and the 2 higherAdditionally, metabolic rates when of the people cabin during pressure boarding was raised [36]. for In thisdescending, study, the the CO CO2 concentration concentra- tionsdecreases decrease during apparently the taxiing-out because phase more and ambi hasent another air was rise introduced during the into inﬂight the cabin, phase and after thecabin passenger pressure breathing drop, which load is of similar cabin CO to the2 was study diluted. of Cao et al. [36].

(a) Waiting Taxiing-Out Inflight Taxiing-In (b) Waiting Taxiing-Out Inflight Taxiing-In CO P 1050 1050 2200 2 2200 CO2 P 1015 1015 (ppm) 2000 (ppm) 2000 2 980 2 980 1800 945 1800 945

1600 910 1600 910 P (hPa) P P (hPa) 875 875 1400 1400 840 840 The concentration of CO of concentration The 1200 The concentration of CO 1200 805 805

1000 770 1000 770 7:47 7:57 8:07 8:17 8:27 8:37 8:47 8:57 9:07 9:17 9:27 9:37 9:47 9:57 10:07 10:17 10:27 10:37 10:47 19:07 19:17 19:27 19:37 19:47 19:57 20:07 20:17 20:27 20:37 20:47 20:57 21:07 21:17 21:27 21:37 21:47 Time (hh:mm) Time (hh:mm) FigureFigure 3. 3. TimeTime series series of of the the cabin cabin pressure pressure and and CO2 CO2 in in E3 E3 ( (aa)) and and E4 E4 ( (bb).).

TheAdditionally, US Occupational when the Safety cabin and pressure Health was Ad raisedministration for descending, (OSHA) has the established CO2 concentra- an tions decrease apparently because more ambient air was introduced into the cabin, and the 8-h occupational exposure limit (OEL) for a CO2 concentration of 5000 ppm [21]. The CO2 passenger breathing load of cabin CO was diluted. concentrations in E3 and E4 were all below2 this OEL. According to the US National Insti- The US Occupational Safety and Health Administration (OSHA) has established an tute for Occupational Safety and Health (NIOSH), chronic exposure to elevated indoor 8-h occupational exposure limit (OEL) for a CO2 concentration of 5000 ppm [21]. The CO2 levels is associated with the following symptoms: drowsiness (1000 to 2000 ppm); CO concentrations in E3 and E4 were all below this OEL. According to the US National headaches,2 sleepiness, stagnant, stale, stuffiness, loss of attention, increased heart rate, Institute for Occupational Safety and Health (NIOSH), chronic exposure to elevated indoor CO2 levels is associated with the following symptoms: drowsiness (1000 to 2000 ppm); headaches, sleepiness, stagnant, stale, stufﬁness, loss of attention, increased heart rate, and slight nausea (2000 to 5000 ppm) [37]. The results in E3 and E4 are both in the drowsiness range. The in-cabin mean CO2 concentration level of this study is similar to previous studies, and the measurements in the European Union Aviation Safety Agency (EASA) reports [21,36,38]. Sustainability 2021, 13, 7047 8 of 15

The mean CO2 concentration in E3 and E4 are 1.4 and 1.2 times higher than the mean level of 10 tested flights in the Cui et al. study (1096 ppm) [18]. Cui et al. tested Boeing 737 actual flights with a passenger compartment volume of 108 m3 configured with 120 seats (8-seat first class, 112 economy class). However, in that study, the average occupancy is 82.8%, although the average space of each seat is similar to our study. If we use the reference ambient CO2 concentration of 405 ppm [39,40], the mean level of CO2 concentration in E3 and E4 is about 3.5 times the reference ambient CO2 concentration. The calculated AERs of E3 and E4 are 0.32 min−1 and 0.43 min−1, respectively. That means on average, the E3 aircraft cabin introduces a whole cabin volume of fresh outside air into the cabin once every 3.10 min, and the E4 aircraft cabin introduces a whole cabin volume of fresh outside air into the cabin once every 2.45 min. These results are similar to the report of the Aviation Public Health Initiative (APHI), and satisfy design guide from Airbus and Boeing (the volume of cabin air exchanges once every 2–3 min) [41]. The CO concentrations in both E3 and E4 flights are below or equal to 1.18 ppm. The sensitivity and accuracy (±3 ppm) of the TSI CO sensor we used in this experiment are not sufficient to detect the low CO levels inside the realistic flight cabin, however, the collected data are within the instrument detection range and can be used as reference. The US OSHA has established an 8-h Permissible Exposure Limit for CO of 35.00 ppm. Because significant risks of cardiovascular disease mortality have been associated with CO exposure [42], the US Federal Aviation Administration (FAA) defines the limits for unhealthy CO exposure of 10.00 ppm time-weighted average. The sampling results in this study are all below these limits. Although the measured CO concentrations are far below the recommended permissible limits, and are not supposed to have any adverse health effects on flight passengers and cabin crew, the CO sampling results are still a very good index to show the aircraft self-pollution and airport effects. Figure4 indicates that the CO concentrations during the waiting phase and taxiing phases are higher than those in the inflight phase due to the incomplete combustion of fuel and the polluted cabin air intake. There was almost no CO during the inflight phase, but the concentrations returned to a relatively high level after aircraft landed on the destination airports in E4. When the aircraft waits for taxiing out after the cabin door closed, the idling engine generates CO due to the incomplete combustion of the aviation fuel. Besides, the ground support equipment (GSE) emits CO that enters the aircraft cabin under certain conditions [43]. However, CO is not usually present in the cabin under normal operating conditions during the inflight phase [43]. Thus, longer waiting and taxiing time in the airport might increase passengers’ CO exposures. The in-cabin CO concentrations can be influenced by the airport CO levels, ground wind direction, jet engine status, and the cabin ventilation system status. Figure4.b2 shows a magnified view of the CO concentrations in the aircraft cabin during E4. The average CO concentration in E3 is ten times of that in E4. The maximum CO readings occur during the taxiing-out stage of E3, however they are all below the allowable limits, such as the 9 ppm for outside air (USEPA) and 15 ppm for indoor (ASHREA). The aircraft age of E3 is also older than that of E4 (Table1), which may also contribute to higher CO concentrations in E3 flight. CO spikes coincidence with the PM spikes at waiting and taxiing phases in E3 and E4 indicate self-pollution and the airport effects on in-cabin air (Figure4). SustainabilitySustainability 20212021,, 1313,, x 7047 FOR PEER REVIEW 99 of of 15 15

(a) Waiting Taxiing-Out Inflight Taxiing-In (b1)Waiting Taxiing-Out Inflight Taxiing-In 1.20 1.20 CO CO 1.10 1.10 1.00 1.00 0.90 0.90 0.80 0.80 0.70 Take off 0.70 0.60 0.60 Landing 0.50 0.50

CO (ppm) Take off CO (ppm) 0.40 0.40 Landing 0.30 0.30 0.20 0.20 0.10 0.10 0.00 0.00 -0.10 -0.10 7:47 7:57 8:07 8:17 8:27 8:37 8:47 8:57 9:07 9:17 9:27 9:37 9:47 9:57 19:07 19:17 19:27 19:37 19:47 19:57 20:07 20:17 20:27 20:37 20:47 20:57 21:07 21:17 21:27 21:37 21:47 21:57 10:07 10:17 10:27 10:37 10:47 Time (hh:mm) Time (hh:mm) (b2) Waiting Taxiing-Out Inflight Taxiing-In

0.14 CO

0.12

0.10 Take off Landing 0.08

0.06 CO (ppm) 0.04

0.02

0.00 19:07 19:17 19:27 19:37 19:47 19:57 20:07 20:17 20:27 20:37 20:47 20:57 21:07 21:17 21:27 21:37 21:47 21:57 Time (hh:mm) FigureFigure 4. 4. COCO concentrations concentrations in in E3 E3 ( (aa)) and and E4 E4 ( (b1b1)) same same scale scale with with ( (aa);); ( (b2b2)) magnified magniﬁed scale. scale.

3.3. ParticleThe in-cabin Concentrations CO concentrations can be influenced by the airport CO levels, ground wind Thedirection, in-cabin jet engine particle status, measurements and the cabi aren summarizedventilation system in Table stat5us., and Figure their 4(b2) time shows series awith magnified manually view recorded of the CO turbulence concentrations shakes in are the shown aircraft in cabin Figure during5. The E4. mean The mass average concen- CO −3 concentrationtrations of PM in1, E3 PM is2.5 ten, and times PM of10 inthat E1 in are E4. 1.42, The 2.66,maximum and 3.19 COµ greadings·m , respectively, occur during which the taxiing-outare consistently stage theof E3, highest however among they the are four all below tested the flights. allowable The mean limits, mass such concentrations as the 9 ppm −3 forof PMoutside1, PM air2.5 (USEPA), and PM and10 in 15 E4 pp arem 0.04,for indoor 0.15, and(ASHREA). 0.26 µg· mThe ,aircraft respectively, age of whichE3 is also are olderconsistently than that the of lowestE4 (Table among 1), which the fourmay testedalso contribute flights. Theto higher mean CO PM concentrations1, PM2.5, and PMin E310 flight.mass CO concentrations spikes coincidence in E2 are with close the toPM but spikes consistently at waiting higher and taxiing than those phases in in E3 E3 (Table and E45 ). indicateTable5 also self-pollution indicates thatand thethe maximumairport effects PM on1, PM in-cabin2.5, and air PM (Figure10 concentrations 4). were all observed in E1, which are 9.31, 12.37, and 15.36 µg·m−3, respectively. Moreover, the standard 3.3.deviations Particle Concentrations of PM mass concentrations in E1 are also the highest among all flights, which indicateThe thatin-cabin the concentrationparticle measurements variations are are summa most prominentrized in Table in E1. 5, Itand is necessarytheir time to series note −3 −3 withthat themanually sensitivity recorded and accuracy turbulence (±10 shakesµg m are@ shown 0~100 inµ gFigure m ) 5. of The the PMSmean sensor mass concen- we used in this experiment are not sufficient to detect the low PM levels inside the realistic flight trations of PM1, PM2.5, and PM10 in E1 are 1.42, 2.66, and 3.19 µg·m−3, respectively, which arecabin, consistently however, the the highest collected among data the are four within tested the instrumentflights. The detectionmean mass range concentrations and can be used as reference. of PM1, PM2.5, and PM10 in E4 are 0.04, 0.15, and 0.26 µg·m−3, respectively, which are consistently the lowest among the four tested flights. The mean PM1, PM2.5, and PM10 mass Sustainability 2021, 13, 7047 10 of 15

Table 5. Summary of in-cabin PM mass concentrations in measured ﬂights.

−3 −3 −3 PM1 (µg m ) PM2.5 (µg m ) PM10 (µg m ) ID Sample Size (N) Mean ± SD Mean ± SD Mean ± SD (min, max) (min, max) (min, max) 1.42 ± 1.81 2.66 ± 2.90 3.19 ± 3.34 E1 213 (0.00, 9.31) (0.00, 12.37) (0.00, 15.36) 0.21 ± 0.62 0.44 ± 0.97 0.62 ± 1.18 E2 161 (0.00, 3.20) (0.00, 4.83) (0.00, 5.71) 0.19 ± 0.53 0.38 ± 0.92 0.48 ± 1.08 E3 196 (0.00, 2.89) (0.00, 4.89) (0.00, 5.29) Sustainability 2021, 13, x FOR PEER REVIEW 0.04 ± 0.16 0.15 ± 0.37 0.26 ± 0.5111 of 15 E4 179 (0.00, 1.17) (0.00, 1.83) (0.00, 2.43)

(a) Waiting Taxiing-Out Inflight Taxiing-In (b) Waiting Taxiing-Out Inflight Taxiing-In 18.0

PM1 E1 6.0 E2 PM1 16.0 PM2.5 PM2.5 ) ) 3 - 3 PM PM - 10 10

14.0 m 5.0 ⋅ m ⋅ g g μ μ 12.0 4.0 10.0 3.0 8.0

6.0 2.0

4.0 PM mass concentration ( concentration mass PM PM mass concentration ( 1.0 2.0

0.0 0.0 17:23 17:34 17:44 17:54 18:04 18:14 18:24 18:34 18:55 19:05 19:15 19:25 19:35 19:45 19:55 20:05 20:15 20:25 17:09 17:19 17:30 17:40 17:50 18:00 18:10 18:20 18:30 18:40 19:01 19:11 19:21 19:31 19:41 19:51 Time (hh:mm) Time (hh:mm) (c) Waiting Taxiing-Out Inflight Taxiing-In (d) Waiting Taxiing-Out Inflight Taxiing-In

6.0 E3 PM1 6.0 E4 PM1 PM2.5 PM2.5 ) ) 3 3 - PM10 - PM10 m

5.0 m ⋅ 5.0 ⋅ g g μ μ

4.0 4.0

3.0 3.0

2.0 2.0 PM mass concentration ( 1.0 PM mass concentration ( 1.0

0.0 0.0 7:47 7:57 8:07 8:17 8:27 8:37 8:47 8:57 9:07 9:17 9:27 9:37 9:47 9:57 10:07 10:17 10:27 10:37 10:47 19:07 19:17 19:27 19:37 19:47 19:57 20:07 20:17 20:27 20:37 20:47 20:57 21:07 21:17 21:27 21:37 21:47 Time (hh:mm) Time (hh:mm) Figure 5. The PM mass concentrationFigure 5. The time PM series mass in concentration aircraft cabins time (Grid series symbols in aircraft indicate cabins the (Grid time symbols of severe indicate turbulence) the time ((a): E1; (b): E2; (c): E3; (d):of E4). severe turbulence) ((a): E1; (b): E2; (c): E3; (d): E4).

One-way Analysis of Variance (ANOVA) and t-tests were used to compare the PM mass concentrations among different flights, and the results show that E1 and E4 are significantly different from those in E2 and E3, while the PM mass concentrations in E2 and E3 are not significantly different (α = 0.05). Therefore, E2 and E3 measurements are grouped together for further analysis, but E1 and E4 are analyzed separately. The boxplots of PM mass concentrations in E1 in different flight phases are shown in Figure 6a. The E2 and E3 PM data were combined and showed in Figure 6b. The results indicate that the average PM mass concentrations in the taxiing-out phase are about two times higher than those in the taxiing-in phase (Figure 6b). Figure 6c shows the PM10 mass concentrations rise after the E4 flight landing on the destination airport. After the E4 flight landing on NKG, ground particles entered the air conditioning system with the bleed air from the jet engine. The in-cabin CO concentrations show a rise at the same time (Figure 4). These indicate the departure and destination airports may affect the cabin interior environment during waiting and taxiing phases. Sustainability 2021, 13, 7047 11 of 15

Based on our record, the E1 is the oldest aircraft with a 13.2 yr age, which is about 2.24 times older than the second oldest one of 5.6 yr age (Table1). Deposition and re- suspension of particles on the interior of the air duct system and cabin can be much more prominent in E1, and its filtration in the ventilation system might not be working as well as other tested aircrafts due to its age [9–11,44]. In addition, severe turbulences during the inflight phases were recorded by researchers in this study. The coincidences of the PM spikes were observed from time series (Figure5). The PM re-suspension was apparent in all tested flights, however, E1 was affected most because the aircraft experienced more turbulences during the flight. These results implicate that frequent deep cleaning of the air ducts and interior surface of the aircraft cabin is essential in reducing in-cabin PM concentrations during the flight. The civil airport is usually highly polluted with fine and ultrafine particles due to its business, and the aircraft air conditioning systems collect a large amount of PM during their ground operation. The aircraft cabin is a unique environment that has the chance to get unstable experiences, and different levels of re-suspension may occur. Without deep cleaning, particles can be accumulated for years and re-suspend to cabin air and human breathing zone. Based on our knowledge, the civil aviation industry has grown rapidly in decades, and many aircrafts were scheduled for very fast cleaning and maintenance due to their tight schedule. Although most of the aircraft being used in China domestic flights are pretty new, there are still many aircraft older than ten years. More frequent thorough cleaning of the ECS and the interior surfaces of these aircraft cabins is recommended to lower cabin crew and passenger’s PM exposures. The self-cleaning functions of the ECS can be explored by the aircraft manufacturer for their future design. One-way Analysis of Variance (ANOVA) and t-tests were used to compare the PM mass concentrations among different flights, and the results show that E1 and E4 are significantly different from those in E2 and E3, while the PM mass concentrations in E2 and E3 are not significantly different (α = 0.05). Therefore, E2 and E3 measurements are grouped together for further analysis, but E1 and E4 are analyzed separately. The boxplots of PM mass concentrations in E1 in different flight phases are shown in Figure6a. The E2 and E3 PM data were combined and showed in Figure6b. The results indicate that the average PM mass concentrations in the taxiing-out phase are about two times higher than those in the taxiing-in phase (Figure6b). Figure6c shows the PM 10 mass concentrations rise after the E4 flight landing on the destination airport. After the E4 flight landing on NKG, ground particles entered the air conditioning system with the bleed air from the jet engine. The in-cabin CO concentrations show a rise at the same time (Figure4 ). These indicate the departure and destination airports may affect the cabin interior environment during waiting and taxiing phases. The taxiing-out and taxiing-in phases mean PM mass concentrations have no significant differences in all tested flight (α = 0.05). The inflight phase’s mean PM mass concentrations in E2 + E3 and E4 are significantly lower than other phases on the same flight, however, the inflight PM levels in E1 were not different from other phases (α = 0.05). The time series show E1 has experienced more frequent turbulences with longer durations than other tested flights. In addition, as mentioned, the E1 aircraft is 13.2 years old and the particle deposition could be severe in the ECS ducts and the interior surfaces of in the cabin. The turbulences made the particles re-suspended and the inflight phase PM concentrations increased. Beside the inflight phase, the waiting and taxiing phases PM in E1 were significantly higher than the comparable phases on other flights. Especially, −3 the mean PM10 mass concentration of the E1 waiting phase is about 2 µg·m , and the −3 maximum PM10 concentration reaches 12 µg·m . The waiting time in E1 was elongated to about an hour due to the air traffic control. The results show that reducing flight waiting time is helpful to lower cabin crew and passengers’ PM exposures. If the flight delays for a long time, waiting in the terminal buildings instead of staying inside the aircraft cabin will reduce passenger PM exposures. Sustainability 2021, 13, x 7047 FOR PEER REVIEW 1212 of of 15 15

(a) 12.0 (b) 6.0 Median E1 Median E2+E3 Mean Mean ) 10.0 ) 5.0 PM -3 PM1 1 -3 m ⋅ m PM PM2.5 ⋅ 2.5 μg μg ( PM PM 8.0 10 ( 4.0 10

6.0 3.0

4.0 2.0

2.0 1.0 PM mass concentration PM mass concentration concentration mass PM

0.0 0.0

Waiting Taxiing-Out Inflight Taxiing-In Waiting Taxiing-Out Inflight Taxiing-In N=54 N=16 N=110 N=7 N=37 N=47 N=251 N=22 (c) 3.0 Median E4 Mean

) 2.5 PM1 -3

m PM ⋅ 2.5

μg PM ( 2.0 10

1.5

1.0

0.5 PM mass concentration

0.0

Waiting Taxiing-Out Inflight Taxiing-In N=9 N=16 N=138 N=16 Figure 6. TheThe boxplot boxplot of of PM PM mass mass concentration concentration divided divided by by flight ﬂight phases phases (( ((aa):): E1; E1; ( (bb):): E2 E2 + + E3; E3; (c ():c): E4). E4). Boxes Boxes indicate indicate 25% 25% and 75%, whiskers indicate 10% 10% and and 90%. 90%.

TheTo findtaxiing-out out the and airport taxiing-in condition phases effects mean on PM the mass cabin concentrations air quality, Figure have7 nocompares signif- icantthe PM differences time series in all right tested before flight and (α = after 0.05). the The cabin inflight doors phase’s closed mean or openedPM mass at concen- NKG in trationsE1 and in E4, E2 respectively. + E3 and E4 The are masssignificantly concentrations lower than of the other PM phases1, PM2.5 on, PM the10 samereach flight, 37.67, −3 however,63.29, 77.88 theµ inflightg·m , respectively,PM levels in E1 10 minwere after not different the E4 cabin from door other opened. phases The(α = mean0.05). massThe −3 timeconcentrations series show of E1 the has PM experienced1, PM2.5, PM more10 were frequent 16.09, 21.92,turbulences 23.73 µwithg·m longer, respectively, durations at than10 min other before tested the flights. cabin door In addition, closed in as E1. mentioned, The difference the mayE1 aircraft be due is to 13.2 the airportyears old running and theand particle particle deposition accumulation could timebe severe at NKG in the is longerECS ducts in E4,and because the interior the surfaces E4 arrival of timein the is cabin.around The 10 turbulences p.m. Not like made the E1the data particles at NKG, re-suspended the mass concentrations and the inflight of PMphase1, PM PM2.5 con-, and centrationsPM10 were increased. quite different Beside during the inflight the E4 landingphase, the time. waiting This is and probably taxiing because phases thePM smaller in E1 wereparticles significantly grew up tohigher bigger than ones the at comparable the evening/night phases on time other comparing flights. withEspecially, the freshly the emitted smaller ones in the afternoon time (Figure7). mean PM10 mass concentration of the E1 waiting phase is about 2 µg·m−3, and the maxi- In this study, we first time measured cabin pressure changes along with thermal mum PM10 concentration reaches 12 µg·m−3. The waiting time in E1 was elongated to about anenvironment hour due to parameters the air traffic and control. particle The exposures results show in actual that reducing flight cabins. flight In-cabin waiting thermaltime is helpfulenvironment to lower and cabin PM concentrationscrew and passengers’ were investigated. PM exposures. However, If the flight it should delays be for noted a long that time,our study waiting has in several the terminal limitations. buildings For example,instead of due staying to the inside instrument the aircraft availability cabin will and reducerestrictions passenger for real PM flights, exposures. only four actual domestic flights were tested for PM mass concentrations,To find out andthe theairport thermal condition environment effects on parameters the cabin were air quality, only collected Figure in7 compares two flights. thePM PM number time series concentrations right before were and notafter measured the cabin indoors this closed study. or The opened time at lag NKG effects in E1 for thermal parameters and different size particles cannot be clearly identified from current and E4, respectively. The mass concentrations of the PM1, PM2.5, PM10 reach 37.67, 63.29, data. The airport emission sources and their effects on the in-cabin exposures cannot be Sustainability 2021, 13, x FOR PEER REVIEW 13 of 15

77.88 µg·m−3, respectively, 10 min after the E4 cabin door opened. The mean mass concentrations of the PM1, PM2.5, PM10 were 16.09, 21.92, 23.73 µg·m−3, respectively, at 10 min before the cabin door closed in E1. The difference may be due to the airport running and Sustainability 2021, 13, 7047 particle accumulation time at NKG is longer in E4, because the E4 arrival time13 is of around 15 10 p.m. Not like the E1 data at NKG, the mass concentrations of PM1, PM2.5, and PM10 were quite different during the E4 landing time. This is probably because the smaller particles grewassociated up to bigger with this ones study’s at the results. evening/night In addition, time the timecomparing of closing with the cabinthe freshly door, and emitted smallersevere ones turbulences in the afternoon were recorded time manually, (Figure 7). so human errors are unavoidable.

90 ) PM1

-3 80

m PM ⋅ 2.5 70 PM mg 10 ( 60 E4 E1 50

40 Cabin door opened Cabin door closed

10 PM mass concentration concentration PM mass 0

-10 20:05 20:15 20:25 20:35 20:45 20:55 21:05 21:15 21:25 21:35 21:45 21:55 22:05 15:35 15:45 15:55 16:05 16:15 16:25 16:35 16:45 16:55 17:05 17:15 Time (hh:mm) FigureFigure 7. The 7. PMThe PMmass mass concentration concentration instant instant changes changes at at NKG whenwhen the the cabin cabin doors doors open open or closeor close in E1 in and E1 E4.and E4.

4.In Conclusions this study, we first time measured cabin pressure changes along with thermal en- vironmentIn the parameters tested realistic and domestic particle flights,exposures the average in actual in-cabin flight temperature cabins. In-cabin is 27.4 ◦thermalC, RH en- vironmentis 31.7%, and cabinPM concentrations pressure is 924 hPa, were which investigated. are close to otherHowever, in-cabin it studies.should However,be noted that ◦ ourthe study measured has several mean temperature limitations. is For 5 C example, higher, and due the to RH the is 20%instrument lower than availability the Martinez and re- 2015 recommendations. The cabin pressure change rates at the end of the climbing stage strictions for real flights, only four actual domestic flights were tested for PM mass con- and the beginning of the descending stage are close to 10 hPa·min−1, and this rate may centrations,induce the and uncomfortable the thermal feeling environment of passengers parame and crewters were members. only collected in two flights. PM number concentrations were not measured in this study. The time lag effects for ther- The monitored CO2 concentrations vary from 1069 to 2135 ppm, with an average and malstandard parameters deviation and ofdifferent 1440 ± 111size ppm. particles The CO cannot concentrations be clearly are identified lower than from 1.20 current ppm all data. Thethe airport time, whichemission are undersources the and US OSHAtheir effects and FAA on exposurethe in-cabin limits. exposures cannot be associated withFrom this our study’s observations, results. the In meanaddition, PM massthe time concentrations of closing inthe the cabin oldest door, aircraft and are severe turbulencessignificantly were higher recorded than those manually, in the other so human aircrafts, errors and the are waiting unavoidable. phases mean PM mass concentration is significantly higher than other phases in the oldest aircraft. In addition, 4. Conclusionsthere are apparent concentration spikes when the aircraft encountered severe turbulence. These results suggest particle deposition and re-suspension inside aircraft, and assume frequentIn the thoroughtested realistic cleaning domestic in commercial flights, airliners the average and avoiding in-cabin severe temperature turbulence routesis 27.4 °C, RHcould is 31.7%, be of and help cabin to reduce pressure the passengers is 924 hPa, and which cabin are crew close exposures. to other However, in-cabin sincestudies. only How- ever,four the flights measured were tested mean in temperature this study, more is 5 research °C higher, needs and to bethe done RH to is make 20% thelower aircraft than the Martinezage effects 2015 on recommendations. PM more conclusive. The cabin pressure change rates at the end of the climbing stage and the beginning of the descending stage are close to 10 hPa·min−1, and this rate Author Contributions: Conceptualization, N.Y. and H.L.; methodology, N.Y.; software, Y.Z.; vali- may induce the uncomfortable feeling of passengers and crew members. dation, N.Y., Y.Z. and M.Z.; formal analysis, H.L.; investigation, Y.Z.; resources, Y.Z.; data curation, Y.Z.;The writing—original monitored CO draft2 concentrations preparation, Y.Z.; vary writing—review from 1069 and to 2135 editing, ppm, N.Y.; with visualization, an average Y.Z.; and standardsupervision, deviation N.Y.; project of 1440 administration, ± 111 ppm. N.Y.; The funding CO concentrations acquisition, N.Y. are and lower M.Z. All than authors 1.20 have ppm all theread time, and which agreed are to the under published the US version OSHA of the and manuscript. FAA exposure limits. Funding:From ourThis researchobservations, was funded the bymean the Nanjing PM mass University concentrations of Aeronautics in and the Astronautics oldest aircraft New are significantlyFaculty Start-up higher fund, than grant numberthose in 90YAH19018; the other This aircrafts, research wasand also the funded waiting by the phases Postgraduate mean PM massResearch concentration & Practice Innovationis significantly Program higher of Jiangsu than Province, other phases grant number in the SJCX20_0067. oldest aircraft. In addition,Institutional there are Reviewapparent Board concentration Statement: Not spikes applicable. when the aircraft encountered severe turbulence.Informed These Consent results Statement: suggest particleNot applicable. deposition and re-suspension inside aircraft, and assume frequent thorough cleaning in commercial airliners and avoiding severe turbulence Data Availability Statement: Not applicable. Sustainability 2021, 13, 7047 14 of 15

Acknowledgments: The authors sincerely thank Quan Ye for helping with the field measurement and preliminary data processing. Special thanks should also be given to Jie Li for providing the needed instruments during experiments. Conflicts of Interest: The authors declare no conflict of interest.

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