Building Energy & Environment Laboratory
Let the dream set sail

2019

Xia, T., Chen, C.* (2019). Differentiating between indoor exposure to PM2.5 of indoor and outdoor origin using time-resolved monitoring data. Building and Environment, 147, 528-539.


To effectively control indoor PM2.5 (particulate matter with diameter less than 2.5 μm) in residential buildings, it is essential to differentiate between the contributions of outdoor PM2.5 infiltration and indoor PM2.5 emissions to the total indoor exposure. This study developed a method for automatically differentiating between indoor exposure to PM2.5 of indoor and outdoor origin using only the time-resolved indoor and outdoor PM2.5 concentrations and information about window opening/closing behavior. This investigation focused on naturally ventilated buildings without the use of portable air cleaners. The proposed approach combines change point analysis, a statistical method; the mass balance for PM2.5, a physical model; and window behavior characteristics to analyze the data and identify the indoor PM2.5 emissions. A series of experiments in a small-scale laboratory setup were conducted to validate the proposed method. The results show that the proposed method can automatically and successfully identify the indoor PM2.5 emissions for all of the 17 cases. Also, the proposed method accurately estimated the indoor exposure to PM2.5 of indoor and outdoor origin as a percentage of the total indoor exposure for all 17 cases with an average absolute error of 0.32%. 


2018

Bian, Y., Wang, R., Ting S.H., Chen, C.*, Zhang, L.* (2018). Electrospun SF/PVA nanofiber filters for highly-efficient PM2.5 capture. IEEE Transactions on Nanotechnology, 17, 934-939.


Particulate matter (PM) air pollution is associated with various adverse health effects. Heavy outdoor air pollution, especially smog, has significantly impacted the living quality of residential environments. Here, we report the fabrication of silk fibroin/polyvinyl alcohol (SF/PVA) nanofiber air filters for the filtration of PM pollution using the electrospinning technique. Silk fibroin is a type of protein fibers which is widely available with excellent environmental sustainability, and PVA is commonly used for continuous nanofiber fabrication. Our results showed that, the PM2.5 removal efficiency of the SF/PVA filter was 99.11±0.24%, which was about 10% higher than that of the pure PVA filter under the same pressure drop of 50 Pa. In a long-term filtration test, the filtration efficiency of this composite SF/PVA filter remained at a high value of 98.97% after 24 hours. The results indicate that the superior filtration performance and long service life of the SF/PVA nanofiber air filer holds a great promise in PM pollution prevention. 


Bian, Y., Wang, R., Wang, S., Yao, C., Ren, W., Chen, C.*, Zhang, L.* (2018). Metal-organic frameworks-based nanofiber filters for effective indoor air quality control. Journal of Materials Chemistry A, 6, 15807-15814.


Indoor air quality is essential to public health as people spend most of their time indoors. Hence, effective indoor air filtering is under heavy demand to deal with this challenge. Metal-organic frameworks (MOFs) have been demonstrated as suitable candidates for air pollution control because of their unique properties, such as large surface area and rich functionalities. The integration of MOFs into an organic polymer matrix is considered to be an effective method to control air pollution; however, an efficient and low-cost method to fabricate such filters is still lacking. Herein, an immersion method was proposed for embedding the homogenous growth of MOFs into nanofibers with a superior capability of wind resistance without film failure. The prepared MOFs-filter shows effective PM2.5 and formaldehyde removal. The PM2.5 filtration efficiency increased from 74.5% to 87.2% after integrating ZIF-67 nanocrystals into the electrospun PAN nanofibers. Moreover, the PM2.5 filtration efficiency remain more than 99% during a long-term test over 30 days. The ZIF-67@PAN filter also achieves a formaldehyde removal efficiency of 84%. This work not only proposes a scalable, and low-cost method for fabricating flexible MOF-nanofiber filters but also holds great promise for improving indoor air quality and reducing the associated health risks. 


Bian, Y., Zhang, L.*, Chen, C.* (2018). Experimental and modeling study of pressure drop across electrospun nanofiber air filters. Building and Environment, 142,244-251.


Electrospun nanofiber air filters can achieve high PM2.5 removal efficiency with a relatively low pressure drop because of the slip effect. They may therefore be applied in buildings to reduce indoor exposure to PM2.5 with lower energy consumption. This study first fabricated 25 nylon nanofiber filters with different filter parameters of fiber diameter, filter thickness, and packing density. The pressure drop across each nanofiber filter was measured under five different face velocities. This study then developed a method for modeling the pressure drop across electrospun nanofiber air filters using the filter parameters. 125 sets of experimental data were obtained for the model development, and a semi-empirical model was developed to predict the pressure drop across nylon electrospun nanofiber filters. The results showed that the pressure drop was proportional to the face velocity and filter thickness. The product of drag coefficient and Reynolds number was a function of both packing density and Knudsen number. The semi-empirical model reasonably predicted the pressure drop across the nylon electrospun nanofiber filters with a median relative error of 4.3%. 


2017

Shi, S., Bian, Y., Zhang, L., Chen, C.* (2017). A method for assessing the performance of nanofiber films coated on window screens in reducing residential exposures to PM2.5 of outdoor origin in Beijing. Indoor Air, 27, 1190-1200.


Recently, many nanofiber films have been developed for air filtration applications. These films exhibit high PM2.5 (particles with aerodynamic diameters less than 2.5 μm) removal efficiency and relatively low air resistance. Thus, coating window screens with nanofiber films may be able to mitigate residential exposure to PM2.5 of outdoor origin. This study developed a method for assessing the performance of nanofiber window screens in reducing residential exposure to PM2.5 of outdoor origin in Beijing. The results show that the use of selected nanofiber window screens all the time throughout the year can reduce the mean value of the annual average indoor PM2.5 of outdoor origin by 64–66% for Beijing residences. However, the mean value of annual harmonic average air exchange rate when the windows are open was also reduced from 2.34 h-1 to 0.27–0.35 h-1, which is far below the national standard. If the nanofiber window screens were used only when the outdoor PM2.5 pollution was severe, the screens had less of an impact on residential natural ventilation, but the national standard still could not be met. Hence, more efforts are needed to further reduce the air resistance of nanofiber window screens in order to ensure proper residential ventilation. 


2012

Chen, C., Zhao, B., Weschler, C.J. (2012). Indoor exposure to outdoor PM10: assessing its influence on the relationship between PM10 and short-term mortality in U.S. cities. Epidemiology, 23, 870-878.


Background: Seasonal and regional differences have been reported for the increase in short-term mortality associated with a given increase in the concentration of outdoor particulate matter with an aerodynamic diameter smaller than 10 μm (PM10 mortality coefficient). Some of this difference may be due to seasonal and regional differences in indoor exposure to PM10 of outdoor origin. Methods: From a previous study we obtained PM10 mortality coefficients for each season in 7 U.S. regions. Hence we estimated the change in the sum of indoor and outdoor PM10 exposure per unit change in outdoor PM10 exposure (PM10 exposure coefficient) for each season in each region. This was originally accomplished by estimating PM10 exposure coefficients for 19 cities within the regions for which we had modeled building infiltration rates. We subsequently expanded the analysis to include 64 additional cities with less well characterized building infiltration rates. Results:  The correlation (r = 0.71 [ 0.46 to 0.86) between PM10 mortality and PM10 exposure (28 data pairs; 4 seasons in each of 7 regions) was strong using exposure coefficients derived from the originally targeted 19 NMMAPS cities within the regions. the correlation remained strong (r = 0.67 [0.40 to 0.84]) when PM10 exposure coefficients were derived using 83 cities within the regions (the original 19 plus the additional 64). Conclusions: Seasonal and regional differences in PM10 mortality appear to partially reflect seasonal and regional differences in total PM10 exposure per unit change in outdoor exposure.


Chen, C., Zhao, B., Weschler, C.J. (2012). Assessing the influence of indoor exposure to "outdoor ozone" on the relationship between ozone and short-term mortality in US communities. Environmental Health Perspectives, 120, 235-240.


Background: City-to-city differences have been reported for the increase in short-term mortality associated with a given increase in ozone concentration (ozone mortality coefficient). Although ozone concentrations are monitored at central outdoor locations, a large fraction of total ozone exposure occurs indoors. Objectives:  To clarify the influence of indoor exposure to ozone of outdoor origin on short term mortality, we conducted an analysis to determine whether variation in ozone mortality coefficients among U.S. cities might be partly explained by differences in total ozone exposure (from both outdoor and indoor exposures) resulting from the same outdoor ozone concentration. Methods:  We estimated average annual air change rates (the overall rate at which indoor air is replaced with outdoor air) and used these to estimate the change in total ozone exposure per unit change in outdoor ozone exposure (ozone exposure coefficient) for 18 cities that had been included in the National Morbidity and Mortality Air Pollution Study (NMMAPS). We then examined associations between both parameters and ozone mortality coefficients obtained from Smith et al. (2009). Results:  For the 18 targeted NMMAPS cities, the association between ozone mortality coefficients and ozone exposure coefficients is strong (1-h ozone metric: R2 = 0.58, p < 0.001; 8-h ozone: R2 = 0.56, p < 0.001; 24-h ozone: R2 = 0.48, p = 0.001). When extended to another 72 NMMAPS cities, the associations remain strong (R2 between 0.47 and 0.63; p < 0.001). Conclusions:  Differences in ozone mortality coefficients among cities appear to partially reflect differences in total ozone exposure resulting from differences in the amount of outdoor ozone that is transported indoors.


2011

Chen, C., Zhao, B., Yang, X. (2011). Preventing the entry of outdoor particles with the indoor positive pressure control method: Analysis of influencing factors and cost. Building and Environment, 46, 1167-1173.


Maintaining positive pressure indoors with a mechanical ventilation system is a popular control method to prevent the entry of outdoor airborne particles. This paper analyzes the influencing factors which affect the satisfied superfluous airflow rates of positive pressure control. Through modeling a large amount of cases with a validated model, the influencing factors, e.g. temperature difference, outdoor wind velocity, effective air leakage gaps in the envelopes, the area of the air leakage and the room, were analyzed. Based on the theoretical model, a correlating equation to calculate the satisfied superfluous airflow rate was established by multiple full quadratic regressions. The correlating equation is simple for engineers or designers to determine the satisfied superfluous airflow rate. This paper also aims to answer that which method, pressure control or indoor air cleaning, costs less to prevent the same amount of outdoor-originated particles from entering indoors. Generally speaking, except for a situation with a very low ratio of indoor/outdoor particle concentration (I/O ratio) requirement, indoor air cleaning control method requires less supply airflow rate than positive pressure control method for reducing the concentration of indoor particles with outdoor origin.


Chen, C., Zhao, B., Yang, X. (2011). Impact of two-way air flow due to temperature difference on preventing the entry of outdoor particles using indoor positive pressure control method. Journal of Hazardous Materials, 186, 1290-1299.


Maintaining positive pressure indoors using mechanical ventilation system is a popular control method for preventing the entry of outdoor airborne particles. The idea is, as long as the supply air flow rate is larger than return air flow rate, the pressure inside the ventilated room should be positive since the superfluous air flow must exfiltrate from air leakages or other openings of the room to the outdoors. Based on experimental and theoretical analysis this paper aims to show the impact of two-way air flow due to indoor/outdoor temperature difference on preventing the entry of outdoor particles using positive pressure control method. The indoor positive pressure control method is effective only when the size of the opening area is restricted to a certain level, opening degree less than 30° in this study, due to the two-way air flow effect induced by differential temperature. The theoretical model was validated using the experimental data. The impacts of two-way air flow due to temperature difference and the supply air flow rate were also analyzed using the theoretical model as well as experimental data. For real houses, it seems that the idea about the positive pressure control method for preventing the entry of outdoor particles has a blind side.


Chen, C., Zhao, B. (2011). Review of relationship between indoor and outdoor particles: I/O ratio, infiltration factor and penetration factor. Atmospheric Environment, 45, 275-288.


Epidemiologic evidence indicates a relationship between outdoor particle exposure and adverse health effects, while most people spend 85-90% of their time indoors, thus understanding the relationship between indoor and outdoor particles is quite important. This paper aims to provide an up-to-date revision for both experiment and modeling on relationship between indoor and outdoor particles. The use of three different parameters: Indoor/Outdoor (I/O) ratio, infiltration factor and penetration factor, to assess the relationship between indoor and outdoor particles were reviewed. The experimental data of the three parameters measured both in real houses and laboratories were summarized and analyzed. The I/O ratios vary considerably due to the difference in size-dependent indoor particle emission rates, the geometry of the cracks in building envelopes, and the air exchange rates. Thus, it is difficult to draw uniform conclusions as detailed information, which make I/O ratio hardly helpful for understanding the indoor/outdoor relationship. Infiltration factor represents the equilibrium fraction of ambient particles that penetrates indoors and remains suspended, which avoids the mixture with indoor particle sources. Penetration factor is the most relevant parameter for the particle penetration mechanism through cracks and leaks in the building envelope. We investigate the methods used in previously published studies to both measure and model the infiltration and penetration factors. We also discuss the application of the penetration factor models and provide recommendations for improvement.