Effects of roughness, dielectric constant and electrical resistivity of wall on deposition of submicron particles driven by ionic air purifier

Highlights

• Particle diameter significantly affect the deposition of particles.

• Wall roughness is key factor beneficial to particle deposition driven by ionizer.

• Roughness changes air flow pattern and reduces particle-transport resistance.

• Back corona arises easily on walls with high resistivity and dielectric constant.

Abstract

The surface characteristics of wall play an important role in the deposition rate of submicron aerosol particles (SAPs) in a room. However, the effects of wall characteristics, including surface roughness (SR), dielectric constant (DC) and electrical resistivity (ER), on the deposition of SAPs driven by the ionic air purifier (IAP) is still unclear, and this study aims to investigate this important issue.

The experiments were conducted in a stainless steel chamber equipped with an IAP. The surfaces of chamber wall were covered by six different wall materials for testing. In the experiments, the NaCl_(s) monodisperse SAPs in the range of 50 to 300 nm (50, 80, 100, 200 and 300 nm) were introduced into the chamber, and then the decay of particle number concentration was monitored continuously. The effects of wall surface characteristics on the performance of IAP is analyzed by mixed model to recognize the effectiveness of each parameter.

SR of the wall is the key parameter response for the IAP performance on the deposition of SAPs. SR may diminish the thickness of particle-concentration boundary layer and change the air flow pattern near the walls and is hence beneficial to the IAP performance. Besides, SRof the test wall materials is negatively correlated to the DC, and the effect of SR may overwhelm the effect of DC. Hence, these two effects are highly correlated. Additionally, when the IAP is operating, the “back corona” develops more likely on the materials with higher ER. The back corona significantly reduces the IAP performance.

Introduction

Submicron aerosol particles are considered to be hazardous due to their small size, high number concentration and ability to penetrate deeply into the alveoli [1], [2], [3], [4], and the amounts of ultrafine particles deposited on alveolar surface area can be as high as 89.2 μm²/cm³ during typical weekday in urban area [5]. Many recent studies demonstrated that the toxicological effects of the inhaled particles mainly depend on the particle size, and ultrafine aerosol particles may cause more severe health effect than fine particles [1]. Furthermore, numerous studies have revealed that the appropriate dosimetry for ultrafine aerosol particle exposure is particle number rather than particle mass [6], [7], [8]. Inhaled ultrafine particles may cause harmful effects on the respiratory tract, such as pulmonary inflammation and fibrosis, pleural effusion, granuloma, and lung cancer [9], [10], [11]. Besides, people nowadays spend approximately 87% of their time indoors [12], and some previous researchers evaluating combined residential exposure to ultrafine aerosol particles demonstrated that the indoor sources contributed as much as 76% [13]. Common indoor sources of particles include frying, grilling, candle burning, ironing, ovening, etc. [14], [15], [16], [17], [18]. Moreover, it was estimated that around 10 to 30% of the total burden of disease caused by the aerosol particle exposure was attributable to indoor-produced ones, and hence the exposure to indoor submicron aerosol particles cause concern [13].

Ionic air purifier (IAP) is one of the most common air purifiers applied for removing indoor submicron aerosol particles. And its performance on air cleaning is influenced by many factors, including turbulence intensity, relative humidity, room size, particle size and composition, and physical characteristic of wall surface materials [19], [20], [21], [22], [23], [24]. It has been demonstrated that the strong air movement (turbulence intensity) can considerably enhance the particle deposition rate, but may decrease the effectiveness of an IAP on aerosol particle deposition [22], [24], [25]. Besides, the negative air ions have better stability under higher relative humidity [26], and thus an IAP performs better when the relative humidity is higher [20]. The effectiveness of IAP on the aerosol particle deposition is positively proportional to the ion concentration, which reduces significantly with the distance away from the IAP [20], [22], [23], [27]. Consequently, the IAP functions better in a smaller room [21], [24]. Also, the charging efficiency of an IAP on aerosol particles is associated with the particle size, and resistivity and dielectric constants [27], [28], [29], and hence the efficiency of an IAP on the deposition rate of the aerosol particles is also relevant to these influential factors. Finally, the physical characteristics of the wall surface in a room plays an important role in the deposition rate of submicron aerosol particles [30], [31], [32] and the performance of the IAP. Our previous study preliminary investigated the relationship between the effectiveness of the IAP and surface electrical resistivity and found that these two parameters were roughly positively correlated [19]. In the present study, we systematically investigated the correlation between the effectiveness of the IAP and the surface roughness, dielectric constants and electrical resistivity of the wall materials as well as the particle sizes. The results of this study can provide a better understanding of the effects of surface characteristics on the deposition of submicron aerosol particles driven by the IAP.