Donghyun Rim

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

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 high-concentration 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 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. Copyright 2012 American Association for Aerosol Research

The influence of chemical interactions at the human surface on breathing zone levels of reactants and products

UNLABELLED Using computational fluid dynamics simulations of an occupant in a ventilated room, we find that breathing zone ozone levels can be substantially lower and ozone reaction products associated with human surfaces (ORPHS) levels considerably higher than room levels. For air exchange rates <3/h, the ratio of the breathing zone to the ozone concentration 1 m from the body (bulk air), r(ozone), ranges from 0.59 to 0.75 for floor or ceiling air supply. ORPHS are enriched in the breathing zone, with concentrations for these conditions ranging from 1.2 to 2.5 greater than bulk air concentrations. At high air exchange rates (>8/h), the breathing zone concentrations approach bulk air concentrations (r(ozone) > 0.9) with a floor supply, whereas large concentration gradients occur between breathing zone and bulk air with a ceiling supply. At these high air exchange rates, ORPHS levels are 1.6-2.0 and 2.9-6.0 times the bulk air concentrations for floor and ceiling supply, respectively. The extent of depletion of ozone or enrichment of ORPHS is large enough that reliance on micro-environmental measurements alone, to assess the intake of ozone or ORPHS, is undesirable. PRACTICAL IMPLICATIONS Chemical reactions between ozone and human and clothing surfaces are predicted to significantly reduce ozone concentrations, and increase ozone reaction products associated with human surfaces (ORPHS) concentrations, in the breathing zone, relative to those concentrations in the larger microenvironment of a room. Existing measurements may overestimate ozone exposure and intake in typical indoor environments.

Ultrafine particle removal and ozone generation by in-duct electrostatic precipitators.

Human exposure to airborne ultrafine particles (UFP, < 100 nm) has been shown to have adverse health effects and can be elevated in buildings. In-duct electrostatic precipitator filters (ESP) have been shown to be an effective particulate control device for reducing UFP concentrations (20-100 nm) in buildings, although they have the potential to increase indoor ozone concentrations. This study investigated residential ESP filters to reduce ultrafine particles between 4 to 15 nm and quantified the resulting ozone generation. In-duct ESPs were operated in the central air handling unit of a test house. Results for the two tested ESP brands indicate that removal efficiency of 8 to 14 nm particles was near zero and always less than 10% (± 15%), possibly due to particle generation or low charging efficiency. Adding a media filter downstream of the ESP increased the decay rate for particles in the same size range. Continuous operation of one brand of ESP raised indoor ozone concentrations to 77 ppbv and 20 ppbv for a second brand. Using commercial filters containing activated carbon downstream of the installed ESP reduced the indoor steady-state ozone concentrations between 6% and 39%.

Occupational Exposure to Hazardous Airborne Pollutants: Effects of Air Mixing and Source Location

The presence of airborne pollutants in indoor environments has been associated with occupants’ discomfort and/or adverse health effects. This study investigates occupational exposure in relation to indoor air mixing and source location relative to a human body. Experimental and computational methods were used to provide information about the pollutant distribution in the vicinity of the human body for different levels of room air mixing. Study results show that the often used assumption of uniform pollutant distribution in an occupied space is not always appropriate for estimation of inhalation exposure. Results also indicate that an occupant may experience very high acute exposure to airborne pollutants when little air mixing exists in a space and the pollutant source is in the vicinity of the occupant. The buoyancy-driven flow induced by the convective heat transfer from an occupants body can transport pollutants in the occupants vicinity to the breathing zone. Specific study results reveal that a source located in the occupants front chest region makes a relatively large contribution to the breathing zone concentration compared with the other sources in the vicinity of the human body. With the source position in this region, exposure can be nine times greater than that calculated with the uniform mixing assumption. The buoyancy-driven convective plume around a body seems to have a significant influence on pollutant transport and human exposure, especially in the absence of room air mixing.

Indoor ultrafine particles of outdoor origin: importance of window opening area and fan operation condition.

Inhalation exposure to ambient ultrafine particles (UFP) has been shown to induce adverse health effects such as respiratory and cardiovascular mortality. Human exposure to particles of outdoor origin often occurs indoors due to entry of UFP into buildings. The objective of the present study is to investigate entry of UFP into a building considering building operational characteristics and their size-dependent effects on UFP concentrations. Indoor and outdoor UFP concentrations along with air change rates were continuously measured in a full-scale test building. Estimates of infiltration factor, penetration coefficient, and deposition rate have been made for a range of particle sizes from 4 to 100 nm. The results show that UFP infiltration factor varies with particle diameter, window position, air change rate, and central fan operation. When the central fan was on continuously, the average infiltration factor ranged from 0.26 (particles <10 nm) to 0.82 (particles >90 nm) for two large window openings, and from 0.07 to 0.60 for two small window openings. Under the central fan-off condition, the average infiltration factor ranged from 0.25 (particles <10 nm) to 0.72 (particles >90 nm) for two small window openings, while it ranged from 0.01 to 0.48 with all windows closed. Larger window openings led to higher infiltration factors due to the larger extent of particle penetration into the building. The fan operation mode (on vs off) also has a strong impact, as the infiltration factor was consistently lower (up to 40%) when the fan was on due to additional particle deposition loss to the furnace filter and duct surfaces.

Low-cost coarse airborne particulate matter sensing for indoor occupancy detection

In the energy-efficient smart building, occupancy detection and localization is an area of increasing interest, as services, such as lighting and ventilation, could be targeted towards individual occupants instead of an entire room or floor. Also, an increasing quantity and diversity of environmental sensors are being added to smart buildings to ensure the quality of services provided by the building. The need for particulate matter (PM) sensors in consumer devices such as air purifiers, is an example where manufacturing advances have made the sensors much less expensive than laboratory equipment. Beyond their original intended use, air quality, they can also be used for occupancy monitoring. The work presented in this article proposes to use a low-cost (<; 8 USD) PM sensor to infer the local movement of occupants in a corridor by sensing the resuspension of coarse (≥ 2.5 μm) particles. To obtain meaningful values from the inexpensive sensors, we have calibrated them against a laboratory-grade instrument. After calibration, we conducted a 7.8 hour experiment measuring coarse PM within a pedestrian corridor of a heavily-used office area. Comparing against ground truth data obtained by a camera, we show that the PM sensor readings are correlated with human activity, thus enabling statistical methods to infer one from the other.

Energy and Cost Associated with Ventilating Office Buildings in a Tropical Climate

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This study evaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the following factors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor air temperature and humidity, air conditioning system coefficient of performance (COP), and cost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore’s tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment. Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore’s. This result implies that the benefits of increasing outdoor air ventilation rate up to 25 L/s per person — which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave — can be much larger than the incremental cost of ventilation.

Size-Resolved Source Emission Rates of Indoor Ultrafine Particles Considering Coagulation

Indoor ultrafine particles (UFP, <100 nm) released from combustion and consumer products lead to elevated human exposure to UFP. UFP emitted from the sources undergo aerosol transformation processes such as coagulation and deposition. The coagulation effect can be significant during the source emission due to high concentration and high mobility of nanosize particles. However, few studies have estimated size-resolved UFP source emission strengths while considering coagulation in their theoretical and experimental research work. The primary objective of this study is to characterize UFP source strength by considering coagulation in addition to other indoor processes (i.e., deposition and ventilation) in a realistic setting. A secondary objective is to test a hypothesis that size-resolved UFP source emission rates are unimodal and log-normally distributed for three common indoor UFP sources: an electric stove, a natural gas burner, and a paraffin wax candle. Experimental investigations were performed in a full-scale test building. Size- and time-resolved concentrations of UFP ranging from 2 to 100 nm were monitored using a scanning mobility particle sizer (SMPS). Based on the temporal evolution of the particle size distribution during the source emission period, the size-dependent source emission rate was determined using a material-balance modeling approach. The results indicate that, for a given UFP source, the source strength varies with particle size and source type. The analytical model assuming a log-normally distributed source emission rate could predict the temporal evolution of the particle size distribution with reasonable accuracy for the gas stove and the candle. Including the effect of coagulation was found to increase the estimates of source strengths by up to a factor of 8. This result implies that previous studies on indoor UFP source strengths considering only deposition and ventilation might have largely underestimated the true values of UFP source strengths, especially for combustion due to the natural gas stove and the candle.

Resuspension of biological particles from indoor surfaces: Effects of humidity and air swirl

Human exposure to airborne particles can lead to adverse health outcomes such as respiratory and allergic symptoms. Understanding the transport mechanism of respirable particles in occupied spaces is a first step towards assessing inhalation exposure. Several studies have contributed to the current knowledge of particle resuspension from indoor surfaces; however, few published studies are available on resuspension of biological particles from indoor surfaces. The objective of this study is to investigate the impacts of humidity and air swirl on resuspension of biological particles from floor and duct surfaces. Controlled laboratory experiments were conducted under varying degrees of humidity and airflow conditions. Resuspension rates of five types of particles (quartz, dust mite, cat fur, dog fur, and bacterial spore-Bacillus thuringiensis as an anthrax simulant) were determined for two types of floor surface (carpet and linoleum) and a duct surface (galvanized sheet metal). The results show that the particle property of being hydrophilic or hydrophobic plays an important role in particle resuspension rate. Resuspension rates of hydrophilic dust mite particles increase up to two orders of magnitude as relative humidity (RH) decreased from 80% to 10% at 25°C. However, resuspension rates of cat fur and dog fur particles that are hydrophobic are within the measurement error range (±15%) over 10-80% RH. With regard to resuspension of bacterial spores (Bacillus thuringiensis) from a duct surface, the resuspension rates are substantially affected by air swirl velocity and particle size. However, no discernible increase in particle resuspension was observed with duct vibration.

Energy and Cost Associated with Ventilating Office Buildings in a Tropical Climate (vol 10, e0122310, 2015)

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This study evaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the following factors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor air temperature and humidity, air conditioning system coefficient of performance (COP), and cost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore’s tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment. Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore’s. This result implies that the benefits of increasing outdoor air ventilation rate up to 25 L/s per person— which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave— can be much larger than the incremental cost of ventilation.