Performance of sorption-based portable air cleaners in formaldehyde removal: Laboratory tests and field verification

Accepted Manuscript Performance of sorption-based portable air cleaners in formaldehyde removal: Laboratory tests and field verification Xiaoyue Zhu, Mengqiang Lv, Xudong Yang PII:

S0360-1323(18)30165-3

DOI:

10.1016/j.buildenv.2018.03.030

Reference:

BAE 5364

To appear in:

Building and Environment

Received Date: 14 November 2017 Revised Date:

19 March 2018

Accepted Date: 20 March 2018

Please cite this article as: Zhu X, Lv M, Yang X, Performance of sorption-based portable air cleaners in formaldehyde removal: Laboratory tests and field verification, Building and Environment (2018), doi: 10.1016/j.buildenv.2018.03.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT 1

Performance of sorption-based portable air cleaners in

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formaldehyde

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verification

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Xiaoyue Zhu, Mengqiang Lv, Xudong Yang*

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Beijing Key Laboratory of Indoor Air Quality Evaluation and Control, Department of

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Building Science, Tsinghua University, Beijing 100084, P. R. China

tests

and

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*

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Dr. Xudong Yang

Corresponding author:

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Department of Building Science, Tsinghua University, Beijing 100084, China

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Tel: +86 (10) 62788845

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Fax: +86(10) 62773461

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Email: [email protected]

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field

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laboratory

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removal:

ACCEPTED MANUSCRIPT Abstract

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Formaldehyde is one of the primary indoor air contaminants that widely exists in

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construction materials and household consumable products. Acute exposure to

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formaldehyde causes irritation and dermal allergies, while chronic exposure can result

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in DNA and chromosomal damage. However, only a handful studies have evaluated

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formaldehyde removal capacities of portable air cleaners (PACs) both in the field and

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in the laboratory. The laboratory performance of PACs has not been statistically

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compared with their field performance. This study evaluated the initial formaldehyde

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removal capabilities of several relatively popular commercial sorption-based PACs in

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Chinese market by measuring their clean air delivery rates (CADRs) in an 8 m3

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environmental chamber. The modified ‘pull-down’ method was applied in this study,

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and the total operation time of the tested PACs was 1.5 h. The laboratory results

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showed wide variations in the CADRs (13.8 m3/h to 75.6 m3/h), which was in

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agreement with the CADRs reported in previous studies. A single-zone field test under

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natural ventilation was also conducted in a bedroom with an area of 25 m2 and a

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volume of 67.5 m3 using the best performing PAC. The results were statistically

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analyzed for any significant difference between the laboratory and field data. The

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difference between the laboratory and field performance of the tested PAC was

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insignificant at a confidence level of 95%.

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Key words: Portable air cleaner; Formaldehyde; Clean air delivery rate; Field test

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ACCEPTED MANUSCRIPT 1. Introduction

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With the rising living standards and growing health awareness, indoor air quality has

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become increasingly important. Formaldehyde is one of the primary indoor air

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contaminants that widely exists in construction materials and household consumable

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products. Acute exposure to formaldehyde can result in mucus membrane irritation,

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dermal allergies, and allergic asthma. Chronic exposure to high concentrations of

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formaldehyde has severe adverse impacts on health such as causing neurotoxicity

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syndromes, pulmonary function damage, DNA and chromosomal damage[1].

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Guideline value of indoor formaldehyde concentration stipulated in the Chinese

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national standard is 0.10 mg/m3[2]. However, in poorly ventilated new and renovated

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homes, formaldehyde concentration can surge up to two to four times of the upper

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limit[3]. Effective measures should be taken to remove indoor formaldehyde.

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Indoor air contaminants removal is mainly achieved by three means: 1) source control;

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2) ventilation; 3) air purification. However, occupants today are faced with an

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intractable dilemma. On the one hand, elimination of indoor source is difficult

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because it calls for joint effort of manufacturing, enforcement, and legislation. On the

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other hand, intensified airtightness of modern buildings and frequent haze limits

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natural ventilation in China. Consequently, air purification is indispensable for

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removing air pollutants.

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Portable air cleaners (PACs) have been readily received by the market. According to a

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survey carried out in 2005, three out of every ten households own at least one type of

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air cleaning device in the United States[4]. However, some researchers reviewed the

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literature on air cleaners and noted that studies focusing on the removal of volatile

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organic compounds (VOCs) were inadequate[5], not to mention studies about

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formaldehyde.

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With regard to laboratory tests, a large portion of the studies focused on in-duct air

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ACCEPTED MANUSCRIPT cleaners (or filters) [6-11]. Filters in the air channels were examined and evaluated

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according to the Korean or Japanese standards. These standards designate the

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single-pass efficiency, removal efficiency, and effectiveness as primary indicators of

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PAC performance. However, the same filter behaves differently when installed as

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in-duct air cleaners or stand-alone air cleaners. Furthermore, due to their limited

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single-pass removal efficiency, in-duct cleaning devices for VOCs are rare, hence

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investigating filters alone is not sufficient. Studies on PACs are rare and most of them

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were reported several years ago. In 1989, Daisey and Hodgson compared four air

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cleaners based on their initial clean air delivery rates (CADRs) for nitrogen dioxide,

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dichloromethane,

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hexanal[12]. Later in the 1990s, several researchers evaluated the effectiveness of air

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cleaners in the removal of diverse pollutants by measuring the CADRs. Tests were

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conducted in varisized environmental chambers. Nelson et al. developed a method

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that utilizes the real-time concentration to calculate the CADRs for different analytes

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in environmental tobacco smoke and tested three air cleaners (chamber volume: 18

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m3)[13]. Shaughnessy et al. tested the effectiveness of a dozen commercial air

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cleaners for dust particulates, environmental tobacco smoke, bio-contaminants, and

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formaldehyde in a 24.8 m3 chamber[14]. Niu et al. extended the Association of Home

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Appliance Manufacturers (AHAM) method to estimate the initial effective cleaning

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rate of nineteen air cleaners for toluene removal in a 6.4 m3 chamber [15]. According

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to these studies, sorption-based air cleaners were only marginally effective in

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removing gaseous contaminants. In 2005, Chen et al. used a mixture of sixteen VOCs

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to evaluate the VOC removal ability of fifteen air cleaners with different technologies

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in a 54.4 m3 chamber[16]. The results of this study suggested that sorption-type filter

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was the most effective off-the-shelf commercial technology for VOC removal,

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however, the removal efficiencies varied significantly with filter design. Cleaning

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technologies advance quickly. It is necessary to examine if the performance of today’s

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air cleaners exceed the performance of their predecessors.

n-heptane,

toluene,

tetrachloroethylene,

and

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Laboratory tests are usually performed without field verification. It remains

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ambiguous whether laboratory data can predict the behavior of air cleaners in real

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situations. If this is feasible, significant efforts could be saved because field test is

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rather expensive and time-consuming. Most of the previous field tests employed a

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‘steady-state’ method[13,

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correct partitioning of the different stages of pollutant removal. However, field test

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should avoid elevating concentration to laboratorial level. Laboratory tests aim at

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evaluating PACs quickly. Therefore, devices are usually challenged with 10 times or

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even higher concentrations of contaminants compared to typical indoor environments.

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However, field tests aim to predict the performance of PACs close to real

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concentration levels. Unfortunately, portable devices that can accurately measure

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formaldehyde at low concentration are usually expensive and not very popularized.

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Moreover, the ‘steady-state’ method requires constant release intensity to calculate the

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CADR. Thus, it uses standard gases of the targeted contaminants and precise gas

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generator, which makes field tests even more laborious.

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Although diverse technologies for formaldehyde removal are available, many of them

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are for industrial use. In terms of technologies used in residential environment, where

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concentrations are low, the most commonly used technologies are 1) adsorption; 2)

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ozone/ion oxidation; 3) ultraviolet photo-catalytic oxidation. Previous studies noted

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that the technologies that require energy input often have problems related to the

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generation of hazardous byproducts and ozone exposure[19]. This has hindered wide

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acceptance of such technologies. Adsorption is still frequently used by many cleaning

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devices. Therefore, our study focused on sorption-based PACs.

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This study evaluated the initial formaldehyde removal capabilities of several

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commercial sorption-based PACs by measuring their CADRs in an 8 m3

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environmental chamber. The modified ‘pull-down’ method was applied, and the total

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operation time of the tested PACs was 1.5 h. A single-zone field test under natural

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17, 18]. Real-time concentration data is required for the

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67.5 m3 using the best performing PAC.

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The Mann Whitney U test was used to compare results from laboratory and field

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tests. The U test is a widely-used nonparametric test which is sufficient in comparing

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two independent samples from populations of skewed distributions and different

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variances[20]. Moreover, this method is robust even when sample sizes are small and

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different [21].

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2. Experimental methods

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2.1 Tested air cleaners

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The PAC market in China has experienced rapid growth over the past several years.

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Nowadays, customers can choose from hundreds of brands. To ensure the selection of

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representative PAC specimens, market research was conducted prior to the formal

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experiments. Consumer-oriented questionnaires were randomly circulated on

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professional online platforms. Respondents were asked about their age, gender, and

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whether they possessed a PAC. For people who had PACs, they were asked about the

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brand names used. Then respondents had to choose the most effective technology in

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their opinions from the options provided (shown in Fig.1). The online platform

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recorded IP locations and time used for finishing the survey to exclude repeated and

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irresponsive answers. Totally 360 valid responses were collected. Fig.1 summarizes

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the favorability and usage rate (derived from a statistical analysis of the 85 most

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salable PACs) of the most common technologies in China. The favorability of a

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technology is the ratio of the number of respondents that considered the technology

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attractive to the total number of respondents. Usage rate of a technology is the ratio of

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the number of PACs utilizing the technology to the total number of investigated PACs.

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For instance, the usage rate of ‘High Efficiency Particulate Air filter (HEPA)+

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Activated Carbon (AC)’ technology was 97.6%. Results indicated that well developed

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ACCEPTED MANUSCRIPT and handy technologies such as ‘HEPA+AC’ were more likely to be adopted as a

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standard configuration, while advanced technologies played a secondary role to

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enhance cleaning effect. However, even though all the PACs tested used activated

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carbon, the adsorption effect varied widely with the specific filter design. We carried

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out further studies on these PACs with the objective to select some best-selling brands

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according to sales record in 2016 from the major online shopping websites of China,

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major manufacturing areas, and common filter forms. Table 1 lists the general

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information about the PACs used in this study.

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Fig. 1 Usage rate and favorability of different cleaning technologies in China: HEPA:

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High efficiency particulate air filter; AC: Activated carbon; PCO: Photo-catalytic

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oxidation; ESP: Electrostatic precipitator. Usage rate: the ratio of the number of PACs

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utilizing the technology to the total number of investigated PACs; Favorability: the

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number of respondents that considered the technology attractive to the total number of

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respondents.

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Table. 1 Information of tested portable air cleaners 7

ACCEPTED MANUSCRIPT PAC Code

C

>1500

306.5

Similar to A, but with double-face organization (350x350x40 mm, 0.85 kg)

455

90

Np

320

200

>1500

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4 filters: Pre-filter + Coarse granular carbon filter (433x235x10 mm, 0.85 kg, deodorization) + Chemical-impregnated filter paper (433x235x10 mm, 0.80 kg, for formaldehyde) + HEPA (433x235x34 mm, 1.00 kg)

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2 filters: Pre-filter + Composite filter (363x278x45 mm, 0.80 kg, fine granular activated carbon is sandwiched between the HEPA filter pleats)

>1500

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(mg)

300

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CCM* (Claimed by Manufacturer)

Pre-filter + 'Smokestop' filter (456x365x45 mm, 1.00 kg, polyester fiber woven with coconut shell granular activated carbon) +HEPA

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Airflow Rate (m3/h)

Type of Filter

Nominal CADR for Formaldehyde (m3/h)

*CCM: cumulate clean mass. It is a performance index defined by China national standard which

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refers to the total mass of the cleaned contaminant when CADR drops to 50% of the initial value;

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Np: Not provided

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2.2 Materials

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Formaldehyde gas was generated by evaporating liquid formalin (37% solution;

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impurity content < 0.38%; Tianjin Yongda Chemical Reagent Company, China). The

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dosage was determined according to desired initial concentration and pilot

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experiments. The theoretical dosage of the solution Vs (ml) was calculated as:

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Vs =

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C0 (g/m3) is the initial concentration; V (m3) is the chamber volume; Cs (%) is the

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mass concentration of the solution (37%); ρs (g/ml) is the density of the solution (1.08

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g/ml). Subsequently, pilot experiments were performed to check whether the dosage

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was appropriate and adjustments were made accordingly until the difference between

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the tested initial concentration and the target value fell below 0.5 mg/m3.

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Formaldehyde solution was transferred into light-proof vials using a Dragon-lab top

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pipette (100-1000 µl, ± 2.5%). The vials were then placed on an electrical heater, near

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the oscillating fan so that the formaldehyde evaporated could swiftly enter the mixing

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zone.

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This study used a spectrophotometric method with 3-methyl-2-benzothiazolinone

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hydrazine (MBTH) reagent to measure the concentration of formaldehyde in air. This

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method is recommended by the Chinese Standard GB/T 18204.26-2000 and rivals the

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high-performance liquid chromatography (HPLC)[22]. Fig. 2 is the comparison curve

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of the MBTH and HPLC methods. The concentration range was 0.1-6.0 mg/m3. A

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1 mol/L solution of chromogenic reagent was prepared from ammonium ferric sulfate

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dodecahydrate (purity > 99%), and a spectrophotometer (WF J 7200, 325-1000 nm, ±

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2.0 nm) was used to measure the absorbance.

C0 ⋅ V Cs ⋅ ρs

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Fig. 2 Comparison curve of the MBTH and HPLC methods

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2.3 Facility setup

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The air-tight environmental chamber that was used for the laboratory tests was made

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of stainless-steel to minimize the adsorption effect. The natural decay rate of

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formaldehyde inside the chamber was below 0.05 h-1, and the infiltration rate

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measured with CO2 was 0.0002 h-1. The dimensions of this chamber were 2.0 m

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(length) x 2.0 m (width) x 2.0 m (height), with a total volume of 8.0 m3. This volume

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was considered because it was big enough to house the needed testing device while

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still small enough to achieve a uniform pollutant distribution with the air from the

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small fans installed in the environmental chamber. Before each test, the inner surfaces

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of the chamber were scrubbed with deionized water and the chamber was flushed with

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clean air for at least 12 h. The clean air was directly drawn from outdoor atmosphere.

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We monitored the concentration of formaldehyde in the cleansing air, which was 1%

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to 5% of the concentration inside the environmental chamber during a test.

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Additionally, we took empty samples to measure the background concentration before

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the targeted contaminant was released. The background concentration was subtracted

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ACCEPTED MANUSCRIPT when calculating the CADR, thus the remaining formaldehyde was not a major

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problem either. To monitor the properties of air, an automatic temperature and

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humidity recording device (WSZY-1, Tian Jian Hua Yi Co., China) was fixed on the

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wall of the chamber. The use of a portable electrothermal oven and desktop humidifier

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were sufficient to regulate the temperature and relative humidity inside the chamber

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because of its small volume. The power of the equipment was closely regulated until

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the air properties reached the set point (temperature: 23 ± 0.5 ˚C, relative humidity: 50

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± 10 %) and readings taken by the automatic temperature and humidity recording

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device were constant for at least 10 min. Thermal insulation materials and high levels

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of airtightness helped maintain stable temperature and relative humidity in the

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chamber. In a typical test, fluctuations in temperature and relative humidity were less

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than 1% and 5%, respectively.

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Besides the pre-mounted ceiling fan, which constantly stirred the upper layer of air, a

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stand-alone oscillating fan was also installed to homogenize the lower layer of air

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throughout the process. Although uniformity verification test was not conducted

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because of sampling condition restrictions, the fans used in this study were as

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powerful as those used in the 30 m3 chamber, where uniform field could be created.

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Therefore, it was safe to assume a well-mixed field in the 8 m3 chamber used in this

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study.

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Considering that the orifice of the prewired sampling tube was near the center of

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volume of the chamber, the PAC should be placed away from the middle of the

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chamber, otherwise the purified air stream would directly flush the orifice. Meanwhile,

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an adequate distance (0.5 m) between the PAC and the wall was maintained to avoid

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hindering airflow as illustrated in Fig. 3.

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Fig. 3 Schematic of the environmental chamber used for the laboratory test

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A single-zone field test was performed in the research bedroom of a flat located on the

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third floor of a laboratory building in Changzhou, Jiangsu Province. Fig. 4 shows the

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layout of the flat. The bedroom was about 5.0 m in length and width and 2.7 m in

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height (Volume: 67.5 m3). It had an outer door and two external windows, which

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separated the room from a corridor that was exposed to the outside. The internal door,

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connecting the bedroom and the adjoining living room, was tightly sealed to prevent

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cross-impact. As demonstrated in Fig. 5, the room was only primitively painted and

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had no furniture. In fact, the bedroom was painted more than a year before the

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research started. The background concentration of formaldehyde in the bedroom was

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lower than 0.05 mg/m3 (0.035 ppm). Three oscillating stand-alone fans were arranged

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to overcome the heterogeneous distribution of air pollutants in the room. The air

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cleaner to be tested was placed in the middle of the room.

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Fig. 4 Layout of the flat where field test of the air cleaners was conducted

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Fig. 5 Photograph of the research bedroom in the flat

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To simulate a real situation, the room temperature and relative humidity were not

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manually controlled, but were continuously monitored by at least three automatic

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recording devices. Like in the laboratory test, formalin solution was heated to generate

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formaldehyde gas. The location of the electric heater is marked with a triangle in Fig. 3.

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Considering the different using habits of people, the PAC was tested under three natural

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ventilation modes: 1) infiltration: only one of the external windows was slightly opened

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(2.0 cm); 2) moderate ventilation: an external window was opened (one-third of its

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area); 3) full ventilation: both the external windows were fully opened. The PAC was

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tested under each mode, which was repeated at least twice.

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2.4 Calculation and measurement of the CADR

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The clean air delivery rate (CADR) is the most commonly used index when evaluating

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the initial performance of PACs. The mass balance model from which the CADR was

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derived is established below (Fig. 6).

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Fig. 6 Schematic of the mass balance model of indoor air pollutants. Q: fresh air

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volume (m3/h); ks: loss ratio due to chamber characteristics (h-1); E: total source

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intensity (mg/m3∙h); Cout: outdoor formaldehyde concentration (mg/m3);

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(2)

(3)

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dC = Q (Cout − C ) + E − ( k s + CADR )C dt

QCout + E Q + ks = B, = Kn Q + k s + CADR V

Let

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Q stands for fresh air volume (m3/h); E refers to total source intensity (mg/m3∙h); ks is

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the loss ratio due to chamber characteristics; Kn is the decline rate of the pollutant

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when there is no PAC in the room. Obviously, CADR can be calculated from the

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difference of the decay rate of the contaminant between experiments with and without

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the PAC. For this purpose, the ‘pull-down’ method was used. It is recommended in

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studies[12-16].

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Firstly, certain amount of the targeted contaminant was injected into the experimental

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space. After the concentration reached the peak and stabilized for 10 min, the PAC

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was turned on. The concentration was measured at a certain interval during the decay

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period, and the time when the PAC started to operate was defined as time zero. Since

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the concentration established in the chamber was much higher than that in the outdoor

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environment, Cout can be approximated to zero. Source intensity E also approximated

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zero during the decay period. Whereupon, Equation (3) can be simplified to Equation

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(4). Thus, by employing a linear regression analysis of ln(

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obtained.

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C  CADR   ln   = −  K n + t V    C0 

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Although many researchers used chambers that had volumes bigger than 30 m3 in

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their experiments[15, 16], chambers smaller than 10 m3 (this study used an 8 m3

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chamber) were also efficient as long as test conditions were properly modified. First

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of all, the test duration and the interval between samplings were shortened. Because

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with the same PAC operating, concentration of the contaminant decays faster in the

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smaller chamber. Shortened test duration insured that the concentration did not drop

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below the detection limit of the gas inspection method. On the other hand, Chen et al.

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noticed[16] that CADR was not constant when the test duration became excessively

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long. In other words, the ‘pull-down’ method which assumes a constant CADR is

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inappropriate for long-term performance evaluation[16]. The CADR was nearly

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constant at the expense of limiting the evaluation to only an initial performance

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evaluation. Additionally, we increased the amount of the injected contaminant to

C ) , the CADR can be C0

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ACCEPTED MANUSCRIPT obtain reasonable initial concentrations. This insured the amount of adsorbed

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formaldehyde in each sample after the sampling time was shortened.

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Consequently, questions came down to finding out the relationship between C0 and

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the maximum measurable CADR (CADRmax ) . If the minimum detectable

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formaldehyde content of the MBTH method and the sample volume are known, the

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minimum allowable final concentration (Cmin) can be calculated. Obviously, the

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maximum measurable CADR reduces the concentration to its inferior limit within the

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test time. Based on the mathematic model mentioned above, the following inequations

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can be established.

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Ct ≥ C min

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CADRmax  −  kn + V 

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(6)

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Here, t refers to the test duration and Ct refers to the concentration at the end of the

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test. We suppose that the chamber volume is 8 m3 and C0 varies from 1 mg/m3 to 15

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mg/m3. Fig. 7 shows the correlation between the CADRmax and test duration at

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different C0 levels. As illustrated by the curve, when C0 reaches the value stipulated

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by the Chinese national standard test protocol for PACs, which is 1.0 mg/m3, the test

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duration should be reduced to 10 min if devices with a CADR greater than 100 m3/h

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are to be examined. However, this is impractical because sufficient data points are

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required to improve the quality of the regression model. On the other hand, if samples

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are drawn every 10 min to obtain 7 data points, the CADRmax will be restricted to 50

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m3/h even when the C0 is increased to 15 mg/m3. In addition, although higher C0

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allows higher CADRmax, this benefit decreases with increasing C0. To sum up, we

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conducted some preliminary tests to obtain a general estimation of the capacity of

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PACs, according to which the final test conditions were determined (see Table. 2).

(5)

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Our results showed that the data quality improved significantly when the modified

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method was applied.

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Fig. 7 Correlation between the CADRmax and test duration at different C0 levels: a

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higher CADRmax can be measured within the same period of time with a higher C0, but

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this boosting effect decreases with increase in C0

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Table 2 Comparison of standard and modified test method

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Volume

Duration

(m3)

(min)

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Sampling

Sampling

interval

duration

(min)

(min)

Flow rate (L/min)

Initial Concentration (mg/m3)

Standard [23]

30

60

10

5

0.5

1.0-1.2

Modified

8

30

5

2

0.5

2.5-3.5

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2.5 Sampling and analyzing the samples

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Samples were drawn by an air pump (Beijing Municipal Institute of Labor Protection, 17

ACCEPTED MANUSCRIPT QC-2 atmospheric sampler, China) through a stainless-steel tube located at the center

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of the chamber and a section of Teflon pipe. The flow rate was set at 0.50 L/min by a

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float flowmeter in order to provide intensive contact between the air sample and the

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absorbent i.e., MBTH (0.05 mg/ml, 5 ml). The sample volume was calibrated for

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temperature and pressure differences between the test condition and the standard

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condition (equation 7):

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Qc = Q ⋅

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Qc is the calibrated sample volume; Q is the original sample volume; Ts and Ps are the

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standard temperature (273.15 K) and the standard atmospheric pressure (101.35 kPa),

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respectively; T and P are the real temperature and atmospheric pressure, respectively.

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The first sample, based on which the initial concentration of formaldehyde was

346

calculated, should be drawn about 10 min after the release of formaldehyde gas. The

347

air cleaner to be tested was turned on after the first sample was drawn and the rest of

348

the samples were drawn at an interval of 5 min for a period of 30 min. The entire

349

procedure is illustrated in Fig. 8. Before releasing the contaminant, a sample, which

350

served as the empty sample, was taken to remove the impact of background

351

formaldehyde concentration.

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TS P ⋅ T PS

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Fig. 8 Sampling process of the PAC test in the environmental chamber

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The basic procedures used in the field test were similar to those used in the laboratory. 18

ACCEPTED MANUSCRIPT However, a subtle distinction should be noted; real rooms usually have a much higher

356

permeability than the environmental chamber used in the laboratory, which means that

357

the air exchange rate in real rooms is strongly affected by complex factors such as

358

temperature and pressure difference. Therefore, each air cleaner test should be

359

coupled with a blank test to determine the natural decay rate (kn). The interval

360

between these two tests should be as short as possible. Meanwhile, CO2 and SF6 were

361

released as tracer gas to measure the ventilation rate. This was done to check if the

362

saltation in fresh air volume radically changes the natural decay rate. CO2 was

363

monitored by TES-1370 portable CO2 detectors, and SF6 was sampled by

364

INNOVA-1303 multipoint sampler and analyzed by INNOVA-1412i photoacoustic

365

gas monitor. These devices did not release or adsorb formaldehyde. Meanwhile, the

366

reagent used for formaldehyde measurement did not adsorb CO2 or SF6.

367

The samples were promptly taken to the laboratory after the test and analyzed using a

368

spectrophotometer at a wavelength of 630 nm. Before spectrophotometric analysis,

369

400 µL of ferric sulfate solution (0.10 mol/L) was added to the samples and allowed

370

to react for at least 15 min to develop color. Hence, absorbance of the sample was

371

measured 20 min after the chromogenic reagent was added. Every absorbance value

372

corresponded to a particular value of formaldehyde content on the standard curve

373

plotted beforehand; the formaldehyde concentration was calculated by dividing the

374

formaldehyde content by the sample volume, which was calibrated according to

375

temperature. By taking the natural logarithm of formaldehyde concentrations and

376

performing linear regression, the slope of the fitting line that indicates the total decay

377

rate was determined. The product of the slope and space volume is numerically equal

378

to the CADR. Only the lines with an R-squared value greater than 0.90 were

379

considered.

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ACCEPTED MANUSCRIPT 3. Results and Discussion

381

3.1 Measured CADRs of PACs

382

Fig. 9 provides a visual comparison of the average CADR (of repetitive tests) of the

383

four PACs. The number inside each column refers to the average CADR, and the

384

highest point of the error bar is the maximum while the lowest point is the minimum

385

CADR of repetitive tests.

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Fig. 9 Histogram with error bars of the averaged CADR of the four PACs

388

Filters were changed after they had been used for three times. In fact, the adsorbed

389

mass of formaldehyde in each test was around 15 mg, and the cumulate cleaned mass

390

(CCM) of the PACs, which refers to the total mass of the cleaned contaminant when

391

CADR drops to 50% of the initial value, was at least 1500 mg. The CADRs of PAC-A

392

and PAC-C appear to be very small for the real rooms. Considering a regular bedroom

393

with an area of 20 m2 and a volume of about 60 m3, PAC-A can only provide an

394

equivalent ventilation rate of 0.23 h-1 for formaldehyde while PAC-C can provide an

395

equivalent ventilation rate of 0.41 h-1 for formaldehyde. For instance, if the initial

396

formaldehyde concentration is 0.6 mg/m3, which is usually the case in newly

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ACCEPTED MANUSCRIPT furnished residential rooms in Beijing [24], the use of PAC-A can only lower the

398

formaldehyde concentration to 0.12 mg/m3 (approximately the upper limit specified in

399

the Chinese national Standard) after 7 h.

400

PAC-D outperformed all the other PACs with an average CADR of 75.6 m3/h. This

401

suggests its ability to provide an equivalent ventilation rate of 1.26 h-1 in a regular

402

bedroom, which rivals mechanical methods of ventilation. Analyzing reasons for

403

PAC-D’s outstanding performance was beyond the scope of this paper. Nevertheless, a

404

reasonable hypothesis could be made by inspecting the mass transfer process. The

405

adsorption of VOCs involves three main stages: 1) mass transfer from bulk to sorbent

406

surface; 2) diffusion inside the solid; 3) adsorption and desorption onto the pore

407

surfaces [25]. Firstly, in order to be compacted into interlayers of the HEPA filter,

408

activated carbon should be processed into super fine particles; the activated carbon

409

granules incorporated into PAC-D were the smallest in size. It has been proved that

410

finer particle size is advantageous for the mass transfer process[26] because it

411

promotes sufficient contact. Meanwhile, the density of the HEPA filter was increased

412

and therefore the volume of air that could pass through them was reduced.

413

Performance of the filter in the subsequent stages largely depends on the

414

characteristics of the activated carbon incorporated into the filter.

415

Among the PACs tested, PAC-B was the only one to use chemisorption as a dominant

416

mechanism for formaldehyde removal. As mentioned earlier, PAC-B was mounted

417

with a cardboard-like filter shown in Fig. 10, and claimed to be a specialized

418

formaldehyde scrubber. Chemisorption is rarely used alone; porous materials that

419

promote physical adsorption are often used as media to provide reaction sites. The

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most common porous material used is modified activated carbon; a number of studies

421

have proved that it enhances chemisorption[27, 28]. Oddly, it was observed that the

422

media of the ‘formaldehyde removal filter’ of PAC-B could not develop a porous

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ACCEPTED MANUSCRIPT structure. Compared to activated carbon with a specific surface area that usually

424

exceeds 1000 m2/g, the pleats designed to increase contact surface in the

425

cardboard-like filter was rather insufficient. Since physical adsorption on this filter is

426

constrained, it needs to form abundant functional groups on its surface to let

427

chemisorption play the dominant role. Previous studies have reported that the

428

potential key parameters that affect chemisorption include relative humidity,

429

significant change in concentration of the targeted contaminant, and the media

430

incorporated into the filter[29]. In our tests, the concentration only had minor changes,

431

hence it did not have a strong effect on the CADR of PAC-B.

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Fig. 10 Structure and size information of the ‘formaldehyde removal filter’ used in

435

PAC-B.

436

Unfortunately, all the PACs that were tested had a CADR that was lower than their

437

nominal values. Manufacturers and certifying authorities have a somewhat casual

438

attitude regarding the VOC removal ability of PACs because the market demand for

439

PACs with particulate matter removal ability is higher. Consequently, the metrics

440

related to particulate matter are cautiously inspected and has a more reliable nominal

441

value. We tested the CADR of PAC-B for removal of particulate matter as well. An

442

average CADR of 282 m3/h was observed, which is close to its nominal value (307

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ACCEPTED MANUSCRIPT m3/h). Customers are readily satisfied by the efficient removal of particulate matter by

444

PACs, and usually do not care about the comparatively insufficient VOC removal

445

capacities. To make explicit, the applicable area is calculated with reference to the

446

CADR for particulate matter so that it is likely to be oversized for VOC removal.

447

Additionally, occupants usually close windows when a PAC is being operated, thus

448

creating a condition for VOCs to accumulate.

449

However, there was some relevance between the actual CADR and the nominal value;

450

the performance of PACs with a higher nominal CADR was better than the

451

performance of PACs with a lower nominal CADR. Another comforting news is that

452

in terms of formaldehyde removal, the overall performance of the PACs tested in this

453

study was remarkably better than the performance of products that were reported in

454

previous studies: the highest CADR of the 18 household air cleaners tested by Kim et

455

al. did not exceed 24 m3/h[30]; the CADR of the best performing PAC tested by Chen

456

et al. was merely 10.9 m3/h[16].

457

One limitation was that the ‘pull-down’ method can only test the initial performance

458

of PACs. Evaluation based on initial performance cannot be arbitrarily extended to

459

long-term performance. Because in addition to the rate of mass transfer process and

460

adsorption, adsorption capacity also has significant influence on long-term

461

performance. A PAC showing satisfactory initial performance does not necessarily

462

perform well in long term especially when the filter saturates easily. Therefore, the

463

initial performance is not enough to estimate a PAC. Long-term performance deserves

464

further study as well.

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3.2 Comparison between laboratory and field tests

467

Proper attention was paid to the comparability between the laboratory and field test 23

ACCEPTED MANUSCRIPT before analyzing the results. The influences of test conditions including temperature,

469

relative humidity, and initial concentration need to be specified because these

470

parameters were stably maintained in the environmental chamber but fluctuated in

471

real situations. Therefore, PAC-D was further examined under different temperatures,

472

relative humidity (RH), and initial concentrations (C0) to cover their variation range in

473

the field test. Table 3 lists all the test conditions and Fig. 11 provides the visual

474

comparison of the CADR of PAC-D under different test conditions. Although from the

475

perspective of adsorption mechanism, temperature is associated with several physical

476

parameters and water molecules compete with formaldehyde for active adsorption

477

sites. The values of both the parameters are far from extreme in normal rooms and

478

fluctuate within a relatively narrow range; PAC-D seemed robust enough to resist

479

these fluctuations. No significant change in the results was observed when the initial

480

concentration rose from 1.1 mg/m3 to 2.8 mg/m3. However, it should be noted that this

481

was only a minor change in initial concentration; different results could be obtained

482

when the gap becomes wider. Additionally, sensitivity to these test conditions varied

483

with different adsorbents. The influence may be significant for other adsorbents[31].

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ACCEPTED MANUSCRIPT 486

Fig. 11 CADR of PAC-D under varying (within a limited range) temperatures, relative

487

humidity, and initial formaldehyde concentrations. Parameter specifications of “low”

488

and “high” can be seen in Table 3.

489

Table 3. Test conditions for the studies on the influence of variations (within limited

491

ranges) in relative humidity, temperature, and initial concentration on PAC

492

performance

PAC-D

High RH (%)

Temperature (˚C)

C0 (mg/m3)

47.7

75.1

23.0

2.5

Low Temperature (˚C)

High Temperature (˚C)

RH (%)

C0 (mg/m3)

18.6 Low C0 (mg/m3) 1.1

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Low RH (%)

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29.4

51.5

2.7

High C0 (mg/m3)

Temperature (˚C)

RH (%)

2.8

25.0

50.3

Field test is challenging mainly because of the following reasons. First, the sink and

495

source effect in real situations is complicated and sometimes cannot be overlooked

496

even in unfurnished rooms. However, the universal ‘pull-down’ method is based on a

497

model that does not consider this effect (emission rate = 0), hence it presumably

498

brings about poor regression quality. Second, real rooms have much higher air

499

exchange rates than environmental chambers. Subtracting such high air exchange

500

rates from the total decay rate to obtain the CADR causes significant errors. The air

501

exchange rate depends on multiple factors such as temperature and air velocity, and is

502

likely to fluctuate during a test. This fluctuation can alter the sink and source

503

characteristics indoors. Lastly, real rooms are not mounted with necessary equipment

504

and do not have any specialized sampling apertures. Since formaldehyde usually

505

cannot be accurately measured by online equipment, frequent sampling, which

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ACCEPTED MANUSCRIPT unavoidably interferes with the conditions in the room, is required. Therefore, perfect

507

conformity between the conditions in the field and laboratory test is unrealistic; a

508

more sensible method is conducting a divergence analysis based on descriptive

509

statistical data.

510

Only part of the laboratory data was selected for comparison. Tests conducted under

511

similar conditions, i.e. temperature, relative humidity, and initial concentrations, were

512

selected for comparison. Table 4 summarizes all the test results and the corresponding

513

test conditions. Category ‘Field-A’, ‘Field-B’, and ‘Field-C’ refers to the field test

514

with infiltration, moderate ventilation, and full ventilation respectively. The average

515

values of the CADR (field test: 72.1 m3/h; laboratory: 74.5 m3/h) were very close, but

516

discrepancy between their maximum and minimum values was observed. This was

517

likely because of the scattered characteristics of the field test data; the standard

518

deviation of field test data was 12.1. By contrast, data from the laboratory tests had a

519

standard deviation of 2.3, which suggests better stability.

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Table 4. Conditions and results of the laboratory and field tests. T is the temperature,

522

RH is the relative humidity, C0 is the initial concentration, ACH is the air exchange

523

rate

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RH (%)

C0 (mg/m3)

ACH (h-1)

CADR (m3/h)

26.7

62.6

1.04

0.59

91.2

Field-A

26.7

48.9

1.08

0.58

69.7

Field-A

31.4

50.6

1.04

0.56

77.0

Field-B

26.9

75.0

1.05

0.58

74.5

Field-B

31.3

49.0

1.13

0.65

62.8

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Category

26

ACCEPTED MANUSCRIPT 23.2

33.0

1.23

0.61

56.2

Field-B

24.2

56.4

1.04

0.72

84.6

Field-C

27.1

69.6

1.08

0.96

60.9

Laboratory

18.6

50.5

2.37

<0.01

77.1

Laboratory

26.8

50.1

2.80

Laboratory

29.6

50.3

2.95

Laboratory

23.2

50.6

2.38

<0.01

76.2

Laboratory

22.9

47.7

2.49

<0.01

72.3

Laboratory

23.3

Laboratory

22.7

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73.1

<0.01

76.3

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<0.01

65.2

1.06

<0.01

71.0

75.1

1.32

<0.01

75.3

Field-A: field test with infiltration only (windows closed); Field-B: field test with moderate

525

natural ventilation (windows partially open); Field-C: field test with full natural ventilation

526

(windows fully open)

527

We used IBM SPSS Statistics V22.0 to check whether there is a significant difference

528

between the field and laboratory data. The output is presented in Table 5. The

529

significance of the null hypothesis that assumes equal variances is 0.006, which is

530

much smaller than 0.05, therefore unequal variances should be accepted. The bottom

531

section of the table shows the indicator of difference analysis i.e., sig, which is 0.613

532

for this case. This value is significantly higher than 0.05, suggesting that the

533

difference is insignificant at a confidence level of 95%.

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Table 5. Independent samples U test output

27

ACCEPTED MANUSCRIPT Laboratory

Field

N

7

8

Mean

74.5

72.1

Standard Deviation

2.32

12.08

F

10.648

Levene's Test for Equality of Variances Sig.

Sig.

0.006

0.613

SC

Independent Samples-Mann-Whitney U Test

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CADR

536

Our results indicate that the behavior of PAC-D in the field was similar to that in the

538

environmental chamber. However, it is important to keep in mind that the effect will

539

be completely different for the field and laboratory because it depends on the relative

540

magnitude of the CADR and volume of the space. The performance of PAC-D may be

541

satisfactory for a 60 m3 room but it can hardly meet the demands of a 120 m3 room.

542

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4. Conclusion

544

This study estimated the formaldehyde removal abilities of four sorption-based

545

portable air cleaners (PACs). Tests were conducted both in the laboratory and field in

546

order to compare the performance of PACs in the well-controlled laboratory

547

environment to that in real situation. The following conclusions could be drawn from

548

this study:

549

1) The CADRs of the four PACs tested in the laboratory had great discrepancies.

550

Values ranged from 13.8 m3/h to 75.6 m3/h. The best performing PAC used a

551

composite filter with ultra-fine activated carbon sandwiched between the

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ACCEPTED MANUSCRIPT 552

interlayers of the HEPA filter. This cut down airflow bypass and promoted

553

sufficient contact between the air to be treated and the composite filter.

554 555

2) Difference between laboratory and field results was insignificant at a confidence level of 95%.

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Acknowledgements s

558

This study was supported by the National Key Basic Research and Development

559

Program of China (Grant No. 2016YFC0700500) and the Innovative Research Groups

560

of the National Natural Science Foundation of China (Grant No. 51521005).

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Research Highlights Formaldehyde removal by portable room air cleaners were tested in the laboratory and the field Clean air delivery rates of tested air cleaners varied from 13.8 m3/h to 75.6 m3/h

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The laboratory and field performances of the tested air cleaner matched

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reasonably well