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Article

Odor from Building Air Conditioners: Emission Characteristics, Odor Compounds and Influencing Factors

by
Jingjing Pei
* and
Luyao Sun
Tianjin Key Laboratory of Indoor Air Environment Quality Control, School of Environment Science and Engineering, Tianjin University, Tianjin 300072, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(2), 1495; https://doi.org/10.3390/su15021495
Submission received: 30 November 2022 / Revised: 2 January 2023 / Accepted: 5 January 2023 / Published: 12 January 2023
(This article belongs to the Special Issue Advanced Technologies on Indoor Environment Quality in Sustainable Buildings)
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Abstract

:
The odor generated by air conditioners is an important factor influencing the perceived air quality in buildings. In this study, different types of air conditioners and air filters were investigated to study the level of odor emission related to the operation state of the compressor, to identify the odor compounds and to analyze the cooling setpoint temperature on emitted odor intensity. Results show that the odor from constant frequency air conditioner use is periodic and stronger than that from variable frequency air conditioner use due to the different operation strategies of the compressor, which affect the evaporation of condensed water on the surface of the cooling coil. Ethyl acetate, acetic acid, 2-ethyl-1-hexanol, acetaldehyde, hexanal, nonanal, toluene and n-hexane are identified as odor compounds by Odor Active Value (OAV), Gas Chromatography/Olfactory/Mass Spectrometry (GC/O/MS) and Flavornet methods. The higher cooling setpoint temperature would lead to stronger odor, due to greater release of hydrophilic odorous compounds from condensed water. In our opinion, reducing the residual condensed water in air conditioners may be the key to control odor emission before purification.

1. Introduction

Indoor air pollution is now considered one of the top five environmental risks to public health, with the US Environmental Protection Agency (EPA) declaring that indoor air pollution is two to five times higher than outdoor air pollution [1]. With increasing sensitivity and demand for a clean and pleasant environment [2,3], indoor odor is more frequently becoming a cause for user complaints [4]. For energy saving purposes, buildings are constructed airtight, which can cause the persistent existence of objectionable odors. Studies have shown that long-term exposure to malodor will have a negative impact on people’s emotions [5] and cause headaches, concentration problems, allergies and other sick building syndromes [6,7,8]. Long-term exposure to moldy odor increases lower birth weights [9]. In addition, there is a strong correlation between human depression and olfactory function [10,11]. Therefore, it is necessary to maintain a healthy indoor olfactory environment. There are already some areas that focus on improving environmental odors, such as odors in exhaust emissions from livestock buildings [12,13,14], industrial activities [2,15,16] and odors in the microenvironment of refrigerators [17,18], but research on indoor odors is just beginning. There have been some standards such as ASHRAE Standard 62.1-2016 [19], which stipulates the acceptability of indoor odors should be greater than 80%. GB18883-2022 [20] requires no abnormal odor in indoor air quality. Regarding indoor odor sources, there has been some research on construction products [21,22,23] and occupants [24,25,26]. The odor of air conditioners has been rarely studied. As an air treatment system in the whole room, the importance of air conditioners to IAQ can be seen. In recent years, the odor from building air conditioners has been increasingly complained about by occupants [27,28]. On the one hand, 58% of occupants have reported unpleasant odor and 70% of occupants felt suffocated when using air conditioners with closed windows [28]. On the other hand, the study shows that an indoor ventilation system has the largest pollution contribution rate (TVOC: 39% [29], perceived air pollution quantified by olf: 42% [30]) compared with other pollution sources. Therefore, air conditioners cannot be ignored in the attempt to improve indoor odor environments.
The available investigations on odor from air conditioners are mostly about automotive air conditioner systems (ACS), and mainly focus on odor emission time profile, odor source, odor compound and factors affecting the odor intensity. First, about odor emission time profile, a market survey by the air conditioner company Gree of their consumers shows that odor emission from air conditioners mainly appears after a period of operation under “refrigeration” mode, which is usually the time of the air conditioner turning to “ventilation” mode without the compressor running [31]. Another study found that odors of ACS mainly occur at the time the device is switched from off to on or from on to off [32]. This indicates that the odor emission is related to the running status of air conditioner. However, there is a lack of systematic research on the characteristics of the odor emission time profile from building air conditioners, whose different operation may influence its odor emission.
Dirty filters, byproducts from microbial breeding, corroded metallic materials and non-metallic material are all the potential unpleasant air conditioner odor sources [33]. A study on ACS believes that microorganisms bred inside the air conditioner are the source of odor [34]. Simmons et al. [35,36] found that insulation materials in ACS which absorb moisture and VOCs appear to provide suitable substrates for fungal colonization. Human skin scales in the air entering air conditioner will intensify the odor because the scales are rich in keratin, which is a nutrient for microbes [7]. In addition, the hydrophilic coating on the surface of cooling coil, which is synthesized by odorous acrylic resin, can also be a potential source of odors [8]. The long-term use of the filter is not only conducive to the breeding of microorganisms, but its loose and porous structure caused by the accumulated indoor dust (such as skin scales) on the filter is also conducive to adsorption of various VOC. Therefore, the filter is a very important carrier of odor compounds, and the sensory pollution of the air conditioner system mainly comes from the filter [30]. Therefore, the specific odor compounds from the used air filters need to be identified in further detail.
For the compounds contributing to odor perception (called odor compounds or odorants hereafter), Kim et al. [32] identified some aldehydes, alcohols and organic acids for ACS. Kawakubo et al. [37] identified toluene and isovaleric acid. Kim et al. [38] extracted alcohol and lower fatty acid from the condensed water of a building air conditioner. However, most of these researches only show the categories of odor compounds, without specific compounds mentioned.
To identify the odor compounds, Gas Chromatography/Olfactory/Mass Spectrometry (GC/O/MS) and odor active value (OAV) methods are commonly used. In GC/O/MS, the chromatographic and odor analyses are performed simultaneously. By comparing the chromatogram and odor spectrum of VOCs, the odor compounds can be identified. This has been widely used in the research of various odorous products [39,40,41], like wood, food and perfume. While the VOCs are concentrated in the sampling tubes for sniffing, some odor compounds identified by GC/O/MS may not be perceived in reality because their real concentration may be too low to be perceived. However, the OAV method compares the indoor concentration and with the corresponding compound’s odor threshold to judge whether the odor compounds can be olfactorily perceived. This method has been used to identify odor compounds in a variety of environments, such as a newly renovated building [42], landfill and wastewater treatment plant [43].
There have been some studies about the factors affecting odor emission from air conditioner. Kim et al. [36] conducted an “odor reproduction tendency” experiment of ACS, and found that the shorter cooling duration and lower inlet air humidity would lead to stronger odor at the air conditioner outlet. For building air conditioners, consumers often complain that the odor becomes stronger when the cooling setpoint temperature is higher, especially during sleep time at night which the temperature is usually set above 28 °C. However, the reason has not been studied.
With the above knowledge gap in the field of building air conditioner odor, this study took several different types of building air conditioners and used air filters to investigate the correlation between odor emission characteristics and air conditioner running status, to identify specific odor compounds and to analyze the influence of cooling setpoint temperature on odor emission intensity.

2. Methods

2.1. Samples and Experimental Design

Four in-use air conditioners and three used air conditioner filters were tested in this study. The sample information is shown in Table 1.
AC #1–4 used in various environment were investigated to study the correlation between odor emission characteristics and operation state of the air conditioner, which refers to its compressor running status. The compressor of a constant frequency air conditioner (CFAC) is intermittently turned on and off according to indoor temperature, which may cause large temperature fluctuation of the supplied air in the room. The compressor of a variable frequency air conditioner (VFAC) runs continuously. It operates at a high frequency at the start, then turns to low frequency when it reaches the setpoint temperature to keep supply air and indoor temperature constant.
To identify the odor compounds from each air conditioner, the outlet air of CFAC #1–3 was collected on site, and filters #5–7 were tested for pollutant emission in laboratory.
CFAC #1 was investigated to study the influence of cooling setpoint temperature on odor intensity from the air conditioner outlet.

2.2. Test Procedure

2.2.1. Field Test of Odor Emission from Air Conditioner Outlet

The experimental schematic is shown in Figure 1. The background of the room was adjusted to 28 ± 1 °C, 55 ± 5% to ensure the normal operation of air conditioner and consistent inlet air humidity. The odor intensity (OI) level was less than 4 to avoid affecting personnel’s sniffing [44,45]. The measurement of temperature (T), relative humidity (RH), concentration of TVOCs and odor evaluation of air conditioner outlet were carried out on site. Time of testing for every parameter is shown in Figure 2. It refers to an operation cycle between two adjacent compressor starts. In an operation cycle, it is called “cool-on period” when the compressor is running and “cool-off period” when the compressor is stopped. T/RH were continuously measured by Onset HOBO (Onset Co. Limited, Massachusetts in USA. Model U10-003), while the concentration of TVOCs was measured by ppbRAE 3000 (RAE System Co. Limited, California in USA).
On-time on-site odor evaluation was carried out when the air conditioner was turned on. The odor evaluation panel included 4 persons with normal olfactory function. The training and evaluation methods were established according to ISO16000-30 [46], which is on sensory testing of indoor air. An odorless oil-free diaphragm pump (HLVP15, Kamoer, China) connected with PTFE pipe delivered same flow of air to the nose of the odor evaluators. They recorded the time, intensity and characteristics of odor during air conditioner running. At the same time, air samples were collected by gas sampling bags (Tedlar PVF, 10L, USA) for further VOC component analysis. The outlet VOC concentration presented a periodical pattern due to intermittent running of compressor for CFAC. A bag of air was collected every two minutes for a complete cycle. The flow rate of the air pump (HLVP6, Kamoer, China) was 5 L/min. The air in the sampling bags was then sampled by Tenax-TA sorbent tubes (Markes, UK) and analyzed by GC/MS. DNPH tubes (SEEQ-144102 DNPH-Silica column, ANPEL Laboratory Technologies (Shanghai) Inc.) connected with an ozone scrubber (SEEQ-144104, ANPEL Laboratory Technologies (Shanghai) Inc.) were used to collect outlet air and analyzed by HPLC for quantitative determination of small molecules of aldehydes and ketones. Background samples of the room air were also collected. Two tubes for each sample were used as parallel samples.
When studying the correlation between odor and operation state and identification of odor compounds, the air conditioner was set at 26 °C and medium wind speed. When studying the influence of the cooling setpoint temperature, it was set at 24 °C, 26 °C and 28 °C. Before each setpoint test, in order to avoid the influence of the previous test, the air conditioner was switched to run in ventilation mode for a while to ensure that no condensed water remained inside.

2.2.2. Laboratory Test of Odor Emission from Air Filter

The two identical filters of the same air conditioner were put into 10 L and 50 L Tedlar sampling bags for pollutant emission. The bags were filled with 8.5 L and 35 L of clean air, respectively, and then stored at 25 °C for 48 h. Two Tenax-TA tube samples were taken from the 10 L bag for GC/MS analysis, and three Supelco tube samples (Supelco, Germany) were taken from 50 L bag for GC/O/MS analysis.

2.2.3. Tenax-TA, DNPH and Supelco Tube Sampling and Analysis Method

The gas sampling pump (Laoying 2020, China) was employed to pump air into Tenax-TA, DNPH and Supelco sorbent tubes. Every Tenax-TA tube was sampled at 0.2 L/min for 20 min. Every DNPH tube was sampled at 0.4 L/min for 20 min. Every Supelco tube was sampled at 0.2 L/min for 50 min. These samples were immediately sent to laboratory for analysis. The analysis conditions are shown in Table 2.

2.3. Methods to Identify Odor Compounds

Compounds detected by GC/MS were identified for odorants by comparing with Flavornet https://www.flavornet.org/ (accessed on 17 August 2022) [47]. Flavornet summarized odorants that have been found in natural products or real environment at supra-threshold levels, that is, levels likely to stimulate olfactory receptor neurons (ORNs). Data were collected from articles published since 1984 using GC/O. They were collated by the team of Hannah E. Collins of Yale University who also checked the chemical data for odorants against Chemical Abstract Service databases. Therefore, the odorants library “Flavornet” was used for qualitative preliminary screening of compounds detected by GC/MS.
When GC/O/MS was used to identify odorants, each Supelco tube was analyzed by three odor evaluators using an alternative sniff method [48]. Each evaluator sniffed 1/3 of a tube sample analysis time for about 15 min, then the odor evaluators’ sequence was changed when switching parallel samples. This avoided olfactory fatigue and allowed each section of Supelco tube to be analyzed by three odor evaluators. Only those compounds identified by all three evaluators were considered as odorants. However, further quantitative measurements are needed to determine whether the concentration of these odorants identified by this method reached the degree of being perceived in the real environment.
Some small molecules aldehydes and ketones are more likely to be the odor compounds according to their odor characteristic and odor threshold. However, these compounds are usually not easily detected by GC-MS method. In this study, in addition to the GC/O/MS odor identification of the pollutants emitted from air filters, the concentration of some aldehydes and ketones was accurately quantified by High Performance Liquid Chromatography (HPLC) at the air outlet during the operation of air conditioner to identify the odor compounds using odor active value (OAV). Compounds with OAV value greater than 1 were identified as odor compounds. The odor threshold database summarized by Yoshio Nagata was used [49].

3. Results and Analysis

3.1. Correlation between Odor Emission Characteristics and Operation State of Air Conditioner

T/RH, concentration of TVOC and odor of CFACs all change periodically with the frequent start and stop of a compressor (Figure 3a–c). T and TVOC concentration both decrease during the cool-on period and increase during the cool-off period; RH decreases first during the cool-on period due to the dehumidification effect and increases later, especially during the cool-off period due to the evaporation of condensed water. The on-site odor evaluation, the air smells like metal/dust (OI: 2.5) when the compressor starts, which may be caused by corrosion of aluminum materials [37]. The smell changes to wet/fishy (OI: 2) when the compressor shuts down. After the humidity peaks, a strong acidic odor with OI in the range of 3~4 appears.
The outlet air parameter and odor emission characteristics of VFAC are very different with CFAC, as shown in Figure 3d. No periodical pattern was observed. RH and TVOC concentration reached the peak around 10 min after the air conditioner was turned on, then decayed continuously. During the later period of operation, T/RH fluctuated in a very small range. The odor of metal/dust or wet/fishy were not detected clearly. Only a slightly acidic odor with OI in the range of 2~3 was detected near the peak humidity time. No matter what type of air conditioner, the acidic odor with the largest OI always appeared after RH started to decrease rapidly from the peak, as shown by the orange dashed line in the figure. This is consistent with the study by Kim [36]. It seems that the emission of acidic odor is always accompanied by the evaporation of condensed water, and there has been a study to extract odor compounds from air conditioner condensed water [38].
The acidic odor intensity of CFAC is obviously greater than VFAC. For CFAC, its compressor frequently starts and stops, leading to large temperature fluctuation on the surface of cooling coil, so the condensed water is easy to evaporate during the cool-off period, while for VFAC, the condensed water does not evaporate so much because the surface of cooling coil keeps at low temperature level. As shown in Figure 3, the RH of CFAC is more than 70% in every cycle, while VFAC maintains about 40% for most of the operation time. Thus, more odor compounds were emitted from CFAC due to the increased evaporation of condensed water.

3.2. Identification of Odor Compounds

3.2.1. Odorants from Air Conditioner Outlet

Although the background VOC concentrations of CFAC #1–3 are low enough, their profiles are very different, as shown in Figure 4. They all contain many benzene series, while the office has more alkanes, the meeting room has more aldehydes and the kitchen has more acid esters. Nevertheless, the odor description of these air conditioners is very similar, as shown in Figure 3.
The VOCs from the air conditioner outlet measured by GC/MS (sampled by Tenax-TA tubes) are listed in Table 3. The average concentration of all the sampling time points was calculated as toluene-equivalent concentration for every compound. The odorants, which are identified by Flavornet and detected in all samples, are in bold. The last column only shows the odor description of those identified by Flavornet. Among these VOCs emitted by air conditioner, 17 species were identified as odorants, and 8 of these species were detected in all three samples. Similar odors in air conditioners should be caused by the same several odor compounds.
Table 4 lists the average concentration of the small molecule aldehydes and ketones detected by HPLC (sampled by DNPH tubes). The OAV is used a criterion to identify odorants from compounds measured. The OAV values of acetaldehyde and hexanal (put in bold) are much higher than 1 in all samples, therefore they are identified as the odorants in this group.

3.2.2. Odorants from Used Air Filter

The VOCs emitted from the air filters were analyzed by both GC/MS and GC/O/MS method. Their concentrations and OIs are listed in Table 5. The value is the average of parallel samples. The odorants detected in all filter samples (put in bold) are 2-ethyl-1-hexanol, nonanal, hexanal, toluene and n-hexane, in which the OI of nonanal and hexanal are bigger. 2-ethyl-1-Hexanol is one of the typical pollutants released from fungal metabolism [50], which can cause headaches, fatigue, intestinal problems and dizziness [51].
The odorants identified from the air conditioner outlet and used air filter are summarized in Table 6. Compounds identified at the air conditioner outlet by OAV method were regarded as type I odorants, because they can be definitely perceived. Compounds identified by GC/O/MS have the characteristics of odor but may not be perceived at the real concentration, and therefore were regarded as type II odorants. The other compounds detected from air conditioner outlet by GC/MS only passed the qualitative primary screening according to Flavornet are regarded as type III odorants in this study.
The eight odor compounds shown in Table 6 are partly consistent with the study of Kim et al. [32]. They concluded that alcohols, aldehydes and organic acids are mainly responsible for the acidic odor of the air conditioner by the GC/O/FID method. However, their research did not mention the specific compounds.

3.2.3. Variation of Odor Compounds with Cooling Status of CFAC

The odor perceived by personnel during the cool-on and cool-off periods are obviously different, as shown in Figure 3. Figure 5 compares the difference of VOC category composition between these two periods, with the data from air bags analyzed by GC/MS. The concentration of total VOCs during the cool-off period is higher than the cool-on period, where the main increase is oxygenated compounds (acid ester, alcohol, aldehyde and ketone). The concentration of alkanes decreases, and the benzene series does not change too much.
It is observed that those oxygenated compound VOCs which are higher during the cool-off period are generally hydrophilic [52], while the hydrophobic alkanes are more present during the cool-on period. This difference of VOC categories appearing during these two periods is consistent with the analysis of Kawakubo et al. [37]. As shown in Figure 7 of Kawakubo’s article [37], when the air conditioner is turned on, condensed water is generated and accumulated on the surface of evaporator, and then hydrophobic VOCs are released. The hydrophilic VOCs are dissolved in condensed water during the cooling-on period and then released when the condensed water starts to evaporate during the cooling-off period. Therefore, more hydrophilic compounds appeared in the cooling-off period.
The change in the eight odor compounds identified between the cool-on and cool-off periods are shown in Figure 6. The concentration of ethyl acetate, acetic acid, 2-ethyl-1-hexanol, acetaldehyde, hexanal and nonanal increased during the cool-off period, with acetic acid appearing only during the cool-off period. The concentration of toluene and n-hexane decreased in the cool-off period. On the other hand, according to their odor description, acetic acid and acetaldehyde with pungent smell are most likely responsible for the acidic odor of air conditioners. Ng et al. [7] and Kim et al. [53] also believe that acetic acid is one of the odor compounds leading to air conditioner odor. n-Hexane with metal smell is likely responsible for the dust/metal odor. This agrees with the perceived odor that dust/metal odor occurred during the cool-on period and the acidic odor occurred during the cool-off period.

3.3. Effect of Cooling Setpoint Temperature on Emitted Odor Intensity

For CFAC, cooling setpoint temperature influences the duration of the cool-off period and the surface temperature of the cooling coil, and therefore the evaporation of condensed water. The change of OI and TVOC concentration of one operation cycle under various cooling setpoints are shown in Figure 7. With the increase of temperature, the concentration of TVOC and the intensity of acidic odor increased, while the intensity of metal/dust odor and wet/fishy odor, which appeared in the cool-on period, did not change too much, and therefore are not shown in the figure. At 24 °C, the acidic odor was very weak (OI < 1) and almost imperceptible. When the temperature was raised to 26 °C, the OI increased abruptly to about 3.5. The OI at 28 °C was about 4, which was not much stronger than that at 26 °C.
The concentration of the eight odor compounds identified at various cooling setpoints are shown in Figure 8. The concentration of ethyl acetate, acetic acid, 2-ethyl-1-hexanol, acetaldehyde, hexanal and nonanal increased with the increase of temperature. Among them, 2-ethyl-1-hexanol, nonanal, acetic acid and acetaldehyde were almost absent at 24 °C. The concentrations of acetic acid and acetaldehyde with sour/pungent odor increased significantly when the cooling setpoint was adjusted to 26 °C. The concentration of n-hexane did not change too much. These results also agree with the odor evaluation results that the change of dust/metal odor intensity at three temperatures is not obvious and the acidic odor is stronger.
The temperature affects not only the evaporation of condensed water, but also Henry’s constant (air/water partition constant) of VOCs dissolved in water [54,55,56]. Studies have proved that Henry’s constant is higher at higher temperature [57]. The greater Henry’s constant indicated that the VOC is more hydrophobic. Therefore, the higher the setpoint temperature, the more easily the VOC can be separated from condensed water. In addition, the evaporation of condensed water and the emission of hydrophilic VOCs may not be synchronized, leading to the time delay between the RH peak and occurrence of acid odor, as shown in Figure 3 (indicated by the dash line). This means that the hydrophilic VOCs are not released by attaching to water vapor molecules as shown in Figure 8. More microscopic studies of molecular motion are needed.
In addition, the influence of cooling setpoint temperature on human air quality perception may not be only related with the pollutant emission from air conditioner as analyzed above. People may be less sensitive to odor at lower temperatures. People feel fresher when the air is colder and drier [58,59,60]. Pang et al. [61,62] also found that the perception of pollution is heavier than higher air enthalpy with the same pollutants.

4. Discussion

The hydrophilic compounds, like acetic acid and acetaldehyde, in combination with condensed water in air conditioners are major odor contributors, and Kim et al. [38] have extracted odor-active compounds from unevaporated condensed water. Therefore, controlling condensed water is the key to solve the odor problem of air conditioners. The independent control of temperature and humidity in an indoor environment, such as a cooling radiation air conditioner with separate fresh air system [63,64,65], or a dehumidification module at the inlet of air conditioner [66,67,68,69], could be a solution to reduce residual condensed water in the air conditioner. Less residue of condensed water would not only avoid the enrichment and emission of the odor compounds, but also avoids the breeding of microorganisms in a high humidity environment.
In this study, only 13 small molecules of aldehydes and ketones were detected by HPLC to identify the odor compounds using OAV method. However, the subjective method of GC/O/MS and reference of Flavornet provides a screening of the air conditioner odor compounds in this study. In addition, sulfide, ammonia and volatile fatty acids (VFAs) may also be air conditioner odor compounds due to their low odor thresholds, although almost no peaks were found in the GC/AED sulfur and nitrogen analysis when measuring pollutants from the air conditioners [32].

5. Conclusions

Different types of building air conditioners and used air filters were tested to study odor emission characteristics during operation. The following conclusions can be made:
Due to the different on/off strategies of compressor, which will influence the temperature and evaporation of condensed water of the cooling coil surface, the odor from a constant frequency air conditioner is periodic following the periodic running of the compressor, and its odor is stronger than that of a variable frequency air conditioner.
Ethyl acetate, acetic acid, 2-ethyl-1-hexanol, acetaldehyde, hexanal, nonanal, toluene and n-hexane are identified as the possible pollutants contributing to the unpleasant odor from air conditioners. Among them, acetic acid and acetaldehyde are most possibly responsible for the acidic odor during the cool-off period, while n-hexane is possibly responsible for the dust/metal odor during the cool-on period.
A higher cooling setpoint temperature would lead to a stronger acidic odor due to a greater release of odorants from the condensed water.

Author Contributions

L.S.: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing—Original Draft, Visualization. J.P.: Writing—Review and Editing, Supervision, Project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used in this study are available from the corresponding author on reasonable request, except for data that is subject to third party restrictions.

Acknowledgments

This research was conducted under the International Energy Agency’s Energy in Buildings and Communities Annex 68 project “Supplementing Ventilation with Gas phase Air Cleaning, Implementation and Energy Implications”. The authors would like to thank the collaborators from China Automotive Technology & Research Center Co. Ltd. for providing help during the GC/O/MS analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental schematic on-site.
Figure 1. Experimental schematic on-site.
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Figure 2. Time of testing for every parameter.
Figure 2. Time of testing for every parameter.
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Figure 3. Parameters in air conditioner outlet change with the operation state ((a–c) is the status of constant frequency air conditioner, (d) is the status of variable frequency air conditioner. Odor intensity: 0, no odor; 1, very weak; 2, weak; 3, distinct; 4, strong; 5, very strong; 6, extremely strong).
Figure 3. Parameters in air conditioner outlet change with the operation state ((a–c) is the status of constant frequency air conditioner, (d) is the status of variable frequency air conditioner. Odor intensity: 0, no odor; 1, very weak; 2, weak; 3, distinct; 4, strong; 5, very strong; 6, extremely strong).
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Figure 4. Background VOC profiles of CFAC (Data were collected by Tenax tubes and analyzed by GC/MS).
Figure 4. Background VOC profiles of CFAC (Data were collected by Tenax tubes and analyzed by GC/MS).
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Figure 5. VOCs profiles during cool-on and cool-off periods.
Figure 5. VOCs profiles during cool-on and cool-off periods.
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Figure 6. Concentration of odor compounds during cool-on and cool-off periods.
Figure 6. Concentration of odor compounds during cool-on and cool-off periods.
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Figure 7. TVOC concentration and intensity of acidic odor at various cooling setpoint.
Figure 7. TVOC concentration and intensity of acidic odor at various cooling setpoint.
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Figure 8. Influence of cooling setpoint on odor compounds concentration.
Figure 8. Influence of cooling setpoint on odor compounds concentration.
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Table
Table 1. Information about the samples tested.
Table 1. Information about the samples tested.
Sample No.TypeUsing EnvironmentRefrigeration Power (W)Stated Air Flow Rate (m3/h)Time after Last Clean (Years)
#1Constant frequency air conditionerMeeting room725012002–3
#2Office712011001–2
#3Kitchen720011002
#4Variable frequency air conditionerOffice7200
(1800–8500)		10002
#5Used Air filterBedroom--2–3
#6Study--2–3
#7Bedroom--1
Table
Table 2. Analytical conditions of GC/MS, GC/O/MS and HPLC.
Table 2. Analytical conditions of GC/MS, GC/O/MS and HPLC.
SamplesTenax-TA TubesSupelco TubesDNPH Tubes
Analysis methodTD-GC/MS (TD-100 a, Markes; Agilent 7890 B/5975 B, USA)GC/MS (Agilent 7890 B/5975B, USA); ODP (Gerstel C200, Germany)HPLC (Agilent 1260, USA) c
ColumnDB-624UI (60 m × 0.25 mm × 1.4 μm)UA-1 (60 m × 320 μm × 0.5 μm)Poroshell 120 EC-C18 (4.6 × 100 mm, 2.7 μm)
Column flow1.2 mL/min, HeMS: 3.0 mL/min, N2
ODP: 1.5 mL/min		1.5 mL/min, Acetonitrile
Oven temp.50 °C (2) b−9 °C/min-130 °C (1) b−10 °C/min-240 °C (10) b, 32.89 min analysis time40 °C (5) b−3℃/min−92 °C−5 °C/min−160 °C−10 °C/min−280 °C (15) b-300 °C65% (0–4.2), 55% (at 14.6 min), 30%(25–31 min), 65% (31.1–34 min)
Injector temp.200 °C--
DetectorMS-UV spectra (360 nm)
Quadrupole temp.230 °C--
Ion source temp.230 °C230 °C-
Trans line temp.250 °C250 °C-
Scan range40–300 m/z33–550 m/z-
Notes: a: The thermal desorption system was a two-stage desorption unit (1. Primary (tube) desorption (250 °C, 10 min); 2. Secondary (trap) desorption (300 °C, 3 min)). Cold Trap U-T15 ATA-2S, heating rate 100 °C/s, outlet splits; b: The numbers in parentheses indicate the hold time; c: The DNPH silica cartridges were eluted by 5 mL acetonitrile and 20 μL of sample solution was injected and analyzed by HPLC.
Table
Table 3. VOCs from the air conditioner outlet detected by GC/MS.
Table 3. VOCs from the air conditioner outlet detected by GC/MS.
CategoryCompoundCASCFAC #1
C(μg/m3)		CFAC #2
C(μg/m3)		CFAC #3
C(μg/m3)		Odor
Description
Acid esterEthyl Acetate141-78-61.992.8321.77pineapple
Ethyl iso-allocholate101230-69-71.211.14-/
Androstane-17-carboxylic acid, methyl ester15173-52-12.29--/
10,13-Eicosadienoic acid, methyl ester30223-50-81.81--/
1,3-benzene dicarboxylic acid112043-90-0-5.24-/
Acetic acid64-19-71.141.331.76sour
AlcoholEthanol64-17-5-5.0312.12sweet, alcohol
2-ethyl-1-Hexanol104-76-711.603.042.08rose, green
2-methyl-1-Propanol78-83-15.24-3.00wine, solvent, bitter
1-Butanol71-36-31.29-1.75medicine, fruit
AldehydeAcetaldehyde75-07-01.170.723.79pungent, ether
Hexanal66-25-11.250.751.21grass, tallow, fat
Nonanal124-19-65.675.550.96fat, citrus
Decanal112-31-24.603.63-soap, orange peel, tallow
Octanal124-13-0-1.45-fat, soap, lemon, green
KetoneAcetone67-64-18.856.2012.11/
Cyclohexanone108-94-14.25-1.00/
2-Oxetanone, 4-methyl-3068-88-0--6.22/
Benzene seriesToluene108-88-316.7312.2216.70aroma, paint
o-Xylene95-47-6-6.8813.80aroma, geranium
Ethylbenzene100-41-40.952.156.57/
Benzene71-43-20.570.732.72/
Styrene100-42-5-6.698.81balsamic, gasoline
Alkanen-Hexane110-54-33.654.3223.59alkane
Methylene chloride75-09-212.622.5516.33/
Methane, oxybis [dichloro-20524-86-11.561.780.79/
Heptane142-82-5-9.051.73alkane
Bicyclo [3.1.1] hept-2-ene23978-81-61.81-3.08/
Limonene138-86-3-0.961.28lemon, orange
Tetrachloroethylene127-18-43.55--/
Cyclobutane, methyl-598-61-8-11.58-/
OtherAcetonitrile75-05-87.294.674.63/
N, N, O-Triacetylhydroxylamine17720-63-72.03--/
Notes: “-” means not detected; “/” means the compound is not present in Flavornet; The odorants in bold are those identified by Flavornet and detected in all samples at the same time.
Table
Table 4. Aldehydes and ketones from the air conditioner outlet detected by HPLC.
Table 4. Aldehydes and ketones from the air conditioner outlet detected by HPLC.
CompoundOT a (μg/m3)CFAC #1CFAC #2CFAC #3
C(μg/m3)OAVC(μg/m3)OAVC(μg/m3)OAV
Formaldehyde670.3167.360.1052.530.0830.970.05
Acetaldehyde2.9515.215.1512.344.1815.205.15
Acetone10890027.950.0019.480.0021.010.00
Hexanal1.2512.6810.1210.518.398.516.80
Propanal2.592.631.01--1.230.47
Crotonaldehyde71.971.360.02----
2-Butanone1416.451.340.00----
Notes: a: Odor Threshold; “-” means not detected; The odorants in bold are those whose OAV values are higher than one.
Table
Table 5. VOCs emitted by the filters.
Table 5. VOCs emitted by the filters.
GroupCompoundFilter #5Filter #6Filter #7Odor
Description
C (μg/m3)OIC (μg/m3)OIC (μg/m3)OI
Acid esterOctadecanoic acid, 2-propenyl ester2.07/-/-//
Acetic acid ethenyl ester-/-/2.113yogurt, hawthorn
Ethyl Acetate-/-/2.423soap, freshener
Alcohol2-methyl-1-Propanol2.37/-/-//
2-ethyl-1-Hexanol1.400.55.05117.002slightly sour
AldehydeBenzaldehyde-/3.29/-//
Octanal-/1.3417.633.5freshener, soap
Nonanal6.3225.04211.493.5almond, sour
Hexanal1.060.757.042.59.143.5grass, alcohol
Butanal, 3-methyl--/-/2.763alcohol
KetoneAcetone2.18/1.43/1.63//
Acetylacetone-/12.71/-//
Benzene seriesBenzene1.99/1.99/0.93//
Toluene16.251.513.61124.552.5paint, leather, plastic
Ethylbenzene1.91/-/1.34//
o-Xylene2.88/1.57/3.00//
Styrene5.981.52.58/6.111.5garbage, charred
Benzene, 1,4-dichloro-195.22420.031.5-/hawthorn, irritation
Alkanen-Hexane2.5821.7811.060.75metal, grass
Heptadecane, 9-hexyl-1.08/2.35/-//
Limonene2.93/10.82/2.31//
Decane, 2,6,7-trimethyl--/3.39/-//
3-Ethyl-3-methyl heptane-/7.58/6.56//
Heptane-/-/9.393metal
Tetrachloroethylene-/-/2.64//
Octane, 4-methyl--/-/2.131.5cucumber
Decane, 2-methyl--/-/2.70//
Note: “-” means not detected; “/” means the compound was not identified by odor evaluators; The odorants in bold are those detected in all filter samples.
Table
Table 6. Summary of odorants from air conditioner.
Table 6. Summary of odorants from air conditioner.
CategoryOdorantsAir Conditioner OutletAir FilterOdor
Description
GC/MSHPLCGC/O/MS
Acid esterEthyl Acetate√ 3 pineapple, soap, freshener
Acetic acid√3 sour
Alcohol2-ethyl-1-Hexanol√ √ 2resin, flower, slight sour
AldehydeAcetaldehyde√√ 1 pungent, ether
Hexanal√√ 1√grass, tallow, fat, alcohol
Nonanal√ √2fat, citrus, almond, sour
Benzene seriesToluene√ √2aroma, paint, leather, plastic
Alkanen-Hexane√ √2alkane, metal, grass
Notes: “1”: type I odorants; “2”: type II odorants; “3”: type III odorants.
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Pei, J.; Sun, L. Odor from Building Air Conditioners: Emission Characteristics, Odor Compounds and Influencing Factors. Sustainability 2023, 15, 1495. https://doi.org/10.3390/su15021495

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Pei J, Sun L. Odor from Building Air Conditioners: Emission Characteristics, Odor Compounds and Influencing Factors. Sustainability. 2023; 15(2):1495. https://doi.org/10.3390/su15021495

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Pei, Jingjing, and Luyao Sun. 2023. "Odor from Building Air Conditioners: Emission Characteristics, Odor Compounds and Influencing Factors" Sustainability 15, no. 2: 1495. https://doi.org/10.3390/su15021495

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