Mixing state of printer generated ultrafine particles: Implications for the complexity of indoor aerosols

Highlight

• Mixing state of printer-generated particles was measured for the first time.

• Three different types of particles were identified in the printer emissions.

• Physical models were developed for particle formation and ageing under different printing scenarios.

Abstract

The operation of laser printers can lead to the emission of high numbers of ultrafine particles. Evidence on the toxicology and adverse health effects of these particles has been mounting, however few studies have investigated the complexity of these particles in terms of their volatility, hygroscopicity and mixing state. This study utilized a Volatility Hygroscopic Tandem Differential Mobility Analyzer (VH-TDMA) to explore the internal and external mixing states of printer-generated particles. Up to 6.0 × 10⁵ particles. cm⁻³ were observed during the operation of the laser printer, and these ultrafine particles could be classified into three groups, each with its own particular volatility (i.e., gradually shrinkable, suddenly shrinkable and expandable), owing to the different internal mixing states. In particular, we propose shell-core structures to explain the special volatility of these particles. In addition, whilst the majority of the generated particles were initially hydrophobic at 90% relative humidity (RH), it was observed that there were a very small number of volatilized particles (less than 5%) that shrank (i.e., decreased in size) after humidification. This study can be used as a model for investigating the complex particle formation processes in relation to secondary organic aerosols from other sources. Furthermore, our results shed light on the complexity of indoor aerosols, which should be investigated further in future indoor air quality studies.

Introduction

Ultrafine particles (<100 nm) have been linked to significant health effects due to their large surface area and high alveolar deposition, as well as their complex chemical composition. Their toxicological effects have been acknowledged by the World Health Organization (2005).

Laser printers, a well-recognized indoor source of volatile organic compounds (VOCs) and ozone (O₃), have been found to emit high numbers of ultrafine particles (Pirela et al., 2015). The particles generated by laser printers can easily elevate the particle number concentration in a large office area to the levels encountered, for example, near a busy road (He et al., 2007), or over eleven times the background level (Koivisto et al., 2010). In addition, growing evidence shows that these particles may possess genotoxicity (Tang et al., 2012a) and are very likely to induce upper airway inflammation and oxidative stress in healthy people (Khatri et al., 2013; Pirela et al., 2017; Sisler et al., 2015). Both the scientific and medical communities have raised increasing concerns on this issue (He et al., 2010; Khatri et al., 2013; McGarry et al., 2011; Morawska et al., 2009; Pirela et al., 2017; Scungio et al., 2017; Sisler et al., 2015; Tang et al., 2012b). In recent years, the Pirela group developed a laboratory-based Printer Exposure Generation System (PEGS) to further demonstrate that engineered nanoparticles emitted from laser printers may be harmful to lung cells, and also that exposure to these emitted particles increases the risk of cardiovascular disease through rat exposure experiments (Carll et al., 2020; Pirela et al., 2016).

Printer-generated particles have a complex chemical composition. They mainly contain semi-volatile organic compounds (SVOCs), such as flame-retardant tri-xylyl phosphate, naphthalene and siloxane (Wensing et al., 2008), and some inorganic elements (e.g. Si, S, Ca, Ti, Fe and Zn) (Barthel et al., 2011; Bello et al., 2013; Castellano et al., 2012; Wang et al., 2011). In relation to their physical properties, they are volatile (Barthel et al., 2011; Morawska et al., 2009; Wang et al., 2011) and have an apparent unimodal size distribution, with a peak at less than 100 nm (He et al., 2007; Kagi et al., 2007; Morawska et al., 2008; Schripp et al., 2008). Also, they are typically charged (Jayaratne et al., 2012; Jiang and Lu, 2010; Wang et al., 2011), which is likely to increase their deposition rates in the human lung (Cohen et al., 1998).

A few studies have been conducted to investigate the particle formation mechanisms in relation to laser printers. It was found that these particles are not the result of fugitive toner particles and paper fibres (Kagi et al., 2007; Smola et al., 2002). In contrast, they are of a secondary nature and formed in the air via ion-induced nucleation (Wang et al., 2011) or the homogeneous and/or heterogeneous nucleation of SVOCs (Morawska et al., 2009). Multiple printer components, such as paper, toner, fuser roller and lubricant oil, contribute to particle formation processes by supplying VOC precursors when heated (Morawska et al., 2009). The ozone emitted during printing also plays an important role in particle formation through the oxidation of unsaturated VOCs (Wang et al., 2012). In addition, the temperature of printer fuser rollers (He et al., 2010), along with printing speed (Byeon and Kim, 2012) have been identified as important factors influencing particle emissions from laser printers. However, our understanding of the formation mechanisms of printer-generated particles is still incomplete.

It has been reported that mixtures of different nanoparticles, such as carbon nanotubes and fullerenes, may have increased or decreased toxicity due to synergistic or antagonistic interactions with pollutants, resulting in mixture health effects that differ from those of the individual particles (Naasz et al., 2018). Modifications on the surface of nanoparticles may or may not negatively affect the toxicity of the particles. For example, aged combustion particles can form surface-modified core-shell black carbon structures with altered toxicity characteristics (Krapf et al., 2017; Park et al., 2018). The mixing states of particles can be used to explain their formation mechanisms, because particle composition is affected by mixing history and interactions with condensing species (Loza et al., 2013). A good example of this was provided by Sullivan and Prather (2007) who proposed a mechanism to explain the formation of individual particles in Asian aerosol outflow by investigating the mixing state of oxalic acid.

This study aimed to gain an insight into mixing states of the printer-generated particles in order to better understand their particle formation and aging mechanisms. We first comprehensively characterized the volatility and hygroscopicity of the printer-generated particles using a Volatility Hygroscopic Tandem Differential Mobility Analyzer (VH-TDMA). We then discuss the internal and external mixing states of these particles in the context of possible formation and aging mechanisms. Asides from its specific importance in identifying the mixing states of printer-generated particles, this study has a more general benefit of improving the scientific knowledge of complex processes of secondary aerosol formation and ageing, especially in indoor environments. Finally, the results improve our understanding of the surprising complexity of indoor aerosols.