Evolution of Ultrafine Particle Size Distributions Following Indoor Episodic Releases: Relative Importance of Coagulation, Deposition and Ventilation

Abstract

Indoor ultrafine particles (UFP, <100 nm) undergo aerosol processes such as coagulation and deposition, which alter UFP size distribution and accordingly the level of exposure to UFP of different sizes. This study investigates the decay of indoor UFP originated from five different sources: a gas stove and an electric stove, a candle, a hair dryer, and power tools in a residential test building. An indoor aerosol model was developed to investigate differential effects of coagulation, deposition, and ventilation. The coagulation model includes Brownian, van der Waals, and viscosity forces, and also fractal geometry for particles of >24 nm. The model was parameterized using different values of the Hamaker constant for predicting the coagulation rate. Deposition was determined for two different conditions: central fan on versus central fan off. For the case of a central fan running, deposition rates were measured by using a nonlinear solution to the mass balance equation for the whole building. For the central fan off case, an empirical model was used to estimate deposition rates. Ventilation was measured continuously using an automated tracer gas injection and sampling system. The study results show that coagulation is a significant aerosol process for UFP dynamics and the primary cause for the shift of particle size distribution following an episodic highconcentration UFP release with no fans operating. However, with the central mechanical fan on, UFP deposition loss is substantial and comparable to the coagulation loss. These results suggest that Received 1 July 2011; accepted 25 October 2011. Donghyun Rim's participation in this project was funded by the National Institute of Standards and Technology (NIST) through a US Intergovernmental Personal Act with the University of Texas at Austin. Michal Green participated in this study as a foreign guest researcher at the NIST while on sabbatical from the Israel Atomic Energy Commission; she contributed by collecting ultrafine particle data from consumer product tests. Jung-Il Choi is supported by Basic Science Research program (2011-0014558) and World Class University (WCU) program (R31-10049) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. However, the manuscript does not necessarily reflect the views of these agencies and no official endorsement should be inferred.Address correspondence to Jung-Il Choi, Department of Computational Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, South Korea. E-mail: jic@yonsei.ac.kr coagulation should be considered during high concentration periods (>20,000 cm −3 ), while particle deposition should be treated as a major loss mechanism when air recirculates through ductwork or mechanical systems.

Size Dependence of the Ratio of Aerosol Coagulation to Deposition Rates for Indoor Aerosols

“…Hussein et al were the first to propose a criterion for the negligence of coagulation using the specific total particle number concentration of ∼1.0 × 10 4 cm −3 (Hussein et al 2009), which has become a dominant criterion in the estimation of the deposition rate coefficient in following studies (Koivisto et al 2010(Koivisto et al , 2012. In support of this criterion Rim et al (2012) found that the coagulation mechanism should not be ignored when particle number concentration is more than 2.0 × 10 4 cm −3 for ultrafine particles. Although these criteria are all dependent on their specific experimental setting, they have a common feature in that only the total particle number concentration was used in the definition.…”

“…However, there are very few studies available in the literature addressing this issue. Due to the basic difference between the coagulation and deposition mechanisms (Friedlander 2000;Chan et al 2006;Lin et al 2007;Schripp et al 2008;Rim et al 2012), the relative ratio of coagulation to deposition rapidly decreases with the decrease of the total particle number concentration. In experimental studies, however, the particle number concentration cannot be decreased without limitation since the statistical error of the measurement instrumentation becomes too large to measure reliable concentrations.…”

Lung Deposition Analyses of Inhaled Toxic Aerosols in Conventional and Less Harmful Cigarette Smoke: A Review

“…It has been found that for ultrafine particles, the coagulation should not be ignored when particle number concentration is more than 2.0e+4 cm −3 [133]. Simply, the particle size change due to coagulation can be calculated with empirical (or analytical) coagulation rates [128]: …”

Ultrafine Particles in Residential Indoors and Doses Deposited in the Human Respiratory System

“…On this point, Rim et al [42] developed a particle dynamics model that predicts the temporal changes of the aerosol size distribution following an episodic release of high concentration UFPs. The authors focused on the decay period after particle emission from different sources.…”