Ultrahigh Passive Cooling Power in Hydrogel with Rationally Designed Optofluidic Properties.

The cooling power of a radiative cooler is more than halved in the tropics, e.g., Singapore, because of its harsh weather conditions including high humidity (84% on average), strong downward atmospheric radiation (∼40% higher than elsewhere), abundant rainfall, and intense solar radiation (up to 1200 W/m2 with ∼58% higher UV irradiation). So far, there has been no report of daytime radiative cooling that well achieves effective subambient cooling. Herein, through integrated passive cooling strategies in a hydrogel with desirable optofluidic properties, we demonstrate stable subambient (4-8 °C) cooling even under the strongest solar radiation in Singapore. The integrated passive cooler achieves an ultrahigh cooling power of ∼350 W/m2, 6-10 times higher than a radiative cooler in a tropical climate. An in situ study of radiative cooling with various hydration levels and ambient humidity is conducted to understand the interaction between radiation and evaporative cooling. This work provides insights for the design of an integrated cooler for various climates.

Experimental study on the impact of indoor air quality on creativity by Serious Brick Play method.

Companies are increasingly asking their employees to find creative solutions to their problems. However, the office environment may reduce an employee's creative potential. In this study, the role of indoor air quality parameters (PM2.5, TVOC, and CO2) in maintaining a creative environment (involving lateral thinking ability) was evaluated by Serious Brick Play (SBP), an adaptation of the LEGO Serious Play (LSP) framework. This study was conducted in a simulated office space with 92 participants over a period of 6 weeks. The SBP required participants to address a challenge by building using Lego bricks, and then describe the solution within a given timeframe. The creations and descriptions were then graded in terms of originality, fluency, and build. The results indicated that higher TVOC levels were significantly associated with lower-rated creative solutions. A 71.9% reduction in TVOC (from 1000 ppb), improves an individual's full creative potential by 11.5%. Thus, maintaining a low TVOC level will critically enhance creativity in offices.

Sub-hourly measurement datasets from 6 real buildings: Energy use and indoor climate.

The data presented here were collected independently for 6 real buildings by researchers of different institutions and gathered in the context of the IEA EBC Annex 81 Data-driven Smart Buildings, as a joint effort to compile a diverse range of datasets suitable for advanced control applications of indoor climate and energy use in buildings. The data were acquired by energy meters, both consumption and PV generation, and sensors of technical installation and indoor climate variables, such as temperature, flow rate, relative humidity, CO2 level, illuminance. Weather variables were either acquired by local sensors or obtained from a close by meteorological station. The data were collected either during normal operation of the building, with observation periods between 2 weeks and 2 months, or during experiments designed to excite the thermal mass of the building, with observation periods of approximately one week. The data have a time resolution varying between 1 min and 15 min; in some case the highest resolution data are also averaged at larger intervals, up to 30 min.

Diurnal trends of indoor and outdoor fluorescent biological aerosol particles in a tropical urban area.

We evaluated diurnal trends of size-resolved indoor and outdoor fluorescent biological airborne particles (FBAPs) and their contributions to particulate matter (PM) within 0.5-20 µm. After a ten-week continuous sampling via two identical wideband integrated bioaerosol sensors, we found that both indoor and outdoor diurnal trends of PM were driven by its bioaerosol component. Outdoors, the median [interquartile range] FBAP mass concentration peaked at 8.2 [5.8-9.9] µg/m3 around sunrise and showed a downtrend from 6:00 to 18:00 during the daytime and an uptrend during the night. The nighttime FBAP level was 1.8 [1.4-2.2] times higher than that during the daytime, and FBAPs accounted for 45 % and 56 % of PM during daytime and nighttime, respectively. Indoors, the rise in concentrations of FBAPs smaller than 1 µm coincided with the starting operation of the heating, ventilation, and air conditioning (HVAC) system at 6:00, and the concentration peaked at 8:00 and dropped to the daily average by noontime. This indicated that the starting operation of the HVAC system dislodged the overnight settled and accumulated fine bioaerosols into the indoor environment. For particles larger than 1 µm, the variation of mass concentration was driven by occupancy. Based on regression modeling, the contributions of indoor PM, non-FBAP, and FBAP sources to indoor mass concentrations were estimated to be 93 %, 67 %, and 97 % during the occupied period.

Aerosols from speaking can linger in the air for up to nine hours.

Airborne transmission of respiratory diseases has been under intense spotlight in the context of coronavirus disease 2019 (COVID-19) where continued resurgence is linked to the relaxation of social interaction measures. To understand the role of speech aerosols in the spread of COVID-19 globally, the lifetime and size distribution of the aerosols are studied through a combination of light scattering observation and aerosol sampling. It was found that aerosols from speaking suspended in stagnant air for up to 9 h with a half-life of 87.2 min. The half-life of the aerosols declined with the increase in air change per hour from 28 to 40 min (1 h-1), 10-14 min (4 h-1), to 4-6 min (9 h-1). The speech aerosols in the size range of about 0.3-2 µm (after dehydration) witnessed the longest lifetime compared to larger aerosols (2-10 µm). These results suggest that speech aerosols have the potential to transmit respiratory viruses across long duration (hours), and long-distance (over social distance) through the airborne route. These findings are important for researchers and engineers to simulate the airborne dispersion of viruses in indoor environments and to design new ventilation systems in the future.

Dynamic Analysis of Particle Emissions from FDM 3D Printers through a Comparative Study of Chamber and Flow Tunnel Measurements.

Ultrafine particle emissions originating from fused deposition modeling (FDM) three-dimensional (3D) printers have received widespread attention recently. However, the obvious inconsistency and uncertainty in particle emission rates (PERs, #/min) measured by chamber systems still remain, owing to different measurement conditions and calculation models used. Here, a dynamic analysis of the size-resolved PER is conducted through a comparative study of chamber and flow tunnel measurements. Two models to resolve PER from the chamber and a model for flow tunnel measurements were examined. It was found that chamber measurements for different materials underestimated PER by up to an order of magnitude and overestimated particle diameters by up to 2.3 times, while the flow tunnel measurements provided more accurate results. Field measurements of the time-resolved particle size distribution (PSD) in a typical room environment could be predicted well by the flow tunnel measurements, while the chamber measurements could not represent the main PSD characteristics (e.g., particle diameter mode). Secondary aerosols (>30 nm) formed in chambers were not observed in field measurements. Flow tunnel measurements were adopted for the first time as a possible alternative for the study of 3D printer emissions to overcome the disadvantages in chamber methods and as a means to predict exposure levels.

Size-resolved dynamics of indoor and outdoor fluorescent biological aerosol particles in a bedroom: A one-month case study in Singapore.

This study evaluated the interrelations between indoor and outdoor bioaerosols in a bedroom under a living condition. Two wideband integrated bioaerosol sensors were utilized to measure indoor and outdoor particulate matter (PM) and fluorescent biological airborne particles (FBAPs), which were within a size range of 0.5-20 µm. Throughout this one-month case study, the median proportion of FBAPs in PM by number was 19% (5%; the interquartile range, hereafter) and 17% (3%) for indoors and outdoors, respectively, and those by mass were 78% (12%) and 55% (9%). According to the size-resolved data, FBAPs dominated above 2 and 3.5 µm indoors and outdoors, respectively. Comparing indoor upon outdoor ratios among occupancy and window conditions, the indoor FBAPs larger than 3.16 µm were dominated by indoor sources, while non-FBAPs were mainly from outdoors. The occupant dominated the indoor source of both FBAPs and non-FBAPs. Under awake and asleep, count- and mass-based mean emission rates were 45.9 and 18.7 × 106 #/h and 5.02 and 2.83 mg/h, respectively. Based on indoor activities and local outdoor air quality in Singapore, this study recommended opening the window when awake and closing it during sleep to lower indoor bioaerosol exposure.

The characteristics and formation mechanisms of emissions from thermal decomposition of 3D printer polymer filaments.

Ultrafine particles (UFP) and volatile organic compounds (VOC) emitted from fused deposition modelling (FDM) 3D printing have received widespread attention. Here, we characterize the formation mechanisms of emissions from polymer filaments commonly used in FDM 3D printing. The temporal relationship between the amount and species of total VOC (TVOC) at any desired operating thermal condition is obtained through a combination of evolved gas analysis (EGA) and thermogravimetric analysis (TGA) to capture physicochemical reactions, in which the furnace of EGA or TGA closely resembles the heating process of the nozzle in the FDM 3D printer. It is generally observed that emissions initiate at the start of the glass transition process and peak during liquefaction for filaments. Initial increment in emissions during liquefaction and the relatively constant decomposition of products in the liquid phase are two main TVOC formation mechanisms. More importantly, low heating rate has the potential to restrain the formation of carcinogenic monomer, styrene, from ABS. A TVOC measurement method based on weight loss is further proposed and found that TVOC mass yield was 0.03%, 0.21% and 2.14% for PLA, ABS, and PVA, respectively, at 220 °C. Among TVOC, UFP mass accounts for 1% to 5% of TVOC mass depending on the type of filaments used. Also, for the first time, emission of UFP from the nozzle is directly observed through laser imaging.

Application of acoustic agglomeration to enhance air filtration efficiency in air-conditioning and mechanical ventilation (ACMV) systems.

The recent episodes of haze in Southeast Asia have caused some of the worst regional atmospheric pollution ever recorded in history. In order to control the levels of airborne fine particulate matters (PM) indoors, filtration systems providing high PM capturing efficiency are often sought, which inadvertently also results in high airflow resistance (or pressure drop) that increases the energy consumption for air distribution. A pre-conditioning mechanism promoting the formation of particle clusters to enhance PM capturing efficiency without adding flow resistance in the air distribution ductwork could provide an energy-efficient solution. This pre-conditioning mechanism can be fulfilled by acoustic agglomeration, which is a phenomenon that promotes the coagulation of suspended particles by acoustic waves propagating in the fluid medium. This paper discusses the basic mechanisms of acoustic agglomeration along with influencing factors that could affect the agglomeration efficiency. The feasibility to apply acoustic agglomeration to improve filtration in air-conditioning and mechanical ventilation (ACMV) systems is investigated experimentally in a small-scale wind tunnel. Experimental results indicate that this novel application of acoustic pre-conditioning improves the PM2.5 filtration efficiency of the test filters by up to 10% without introducing additional pressure drop. The fan energy savings from not having to switch to a high capturing efficiency filter largely outstrip the additional energy consumed by the acoustics system. This, as a whole, demonstrates potential energy savings from the combined acoustic-enhanced filtration system without compromising on PM capturing efficiency.

Removal of haloacetic acids from swimming pool water by reverse osmosis and nanofiltration.

Recent studies report high concentrations of haloacetic acids (HAAs), a prevalent class of toxic disinfection by-products, in swimming pool water (SPW). We investigated the removal of 9 HAAs by four commercial reverse osmosis (RO) and nanofiltration (NF) membranes. Under typical SPW conditions (pH 7.5 and 50 mM ionic strength), HAA rejections were >60% for NF270 with molecular weight cut-off (MWCO) equal to 266 Da and equal or higher than 90% for XLE, NF90 and SB50 with MWCOs of 96, 118 and 152 Da, respectively, as a result of the combined effects of size exclusion and charge repulsion. We further included 7 neutral hydrophilic surrogates as molecular probes to resolve the rejection mechanisms. In the absence of strong electrostatic interaction (e.g., pH 3.5), the rejection data of HAAs and surrogates by various membranes fall onto an identical size-exclusion (SE) curve when plotted against the relative-size parameter, i.e., the ratio of molecular radius over membrane pore radius. The independence of this SE curve on molecular structures and membrane properties reveals that the relative-size parameter is a more fundamental SE descriptor compared to molecular weight. An effective molecular size with the Stokes radius accounting for size exclusion and the Debye length accounting for electrostatic interaction was further used to evaluate the rejection. The current study provides valuable insights on the rejection of trace contaminants by RO/NF membranes.

Role of calcium ions on the removal of haloacetic acids from swimming pool water by nanofiltration: mechanisms and implications.

We investigated the removal of haloacetic acids (HAAs) from swimming pool waters (SPWs) by two nanofiltration membranes NF270 and NF90. The strong matrix effect (particularly by Ca2+) on membrane rejection prompts us to systematically investigate the mechanistic role of Ca2+ in HAA rejection. At typical SPW pH of 7.5, NF90 maintained consistently high rejection of HAAs (>95%) with little influence by Ca2+, thanks to the dominance of size exclusion effect for this tight membrane (pore radius âˆ¼ 0.31 nm). In contrast, the rejections of both inorganic ions (e.g., Na+ and Cl-) and HAA anions were decreased at higher Ca2+ concentration for NF270 (pore radius âˆ¼ 0.40 nm). Further tests show that the rejection of neutral hydrophilic molecular probes and the membrane pore size were not affected by Ca2+. Although Ca2+ is unable to form strong complex with HAAs, we observed the binding of Ca2+ to NF270 together with a reduction in its surface charge. Therefore, the formation of membrane-Ca2+ complex, which weakens charge interaction effect, was responsible for the reduced HAA rejection. The current study reveals important mechanistic insights of the matrix effect on trace contaminant rejection, which is critical for a better understanding of their fate and removal in membrane-based treatment.

An insight of disinfection by-product (DBP) formation by alternative disinfectants for swimming pool disinfection under tropical conditions.

Sodium hypochlorite (NaClO) is the most commonly used disinfectant in pool treatment system. Outdoor pools usually suffer from the strong sunlight irradiation which degrades the free chlorine rapidly. In addition, more pools start to adopt the recirculation of swimming pool water, which intensifies the disinfection by-product (DBP) accumulation issue. Given these potential drawbacks of using NaClO in the tropical environment, two alternative organic-based disinfectants, trichloroisocyanuric acid (TCCA, C3Cl3N3O3) and bromochlorodimethylhydantoin (BCDMH, C5H6BrClN2O2), were investigated and compared to NaClO in terms of their self-degradation and the formation of DBPs, including trihalomethanes (THMs) and haloacetic acids (HAAs), under simulated tropical climate conditions. The result reveals that halogen stabilizer, TCCA, had the advantages of slower free chlorine degradation and lower DBP concentration compared to NaClO, which makes it a good alternative disinfectant. BCDMH was not recommended mainly due to the highly reactive disinfecting ingredient, hypobromous acid (HBrO), which fails to sustain the continuous disinfection requirement. Total disinfectant dosage was the main factor that affects residual chlorine/bromine and THM/HAA formation regardless of different disinfectant dosing methods, e.g. shock dosing (one-time spiking) in the beginning, and continuous dosing during the whole experimental period. Two-stage second-order-kinetic-based models demonstrate a good correlation between the measured and predicted data for chlorine decay (R(2) ≥ 0.95), THM (R(2) ≥ 0.99) and HAA (R(2) ≥ 0.83) formation. Higher temperature was found to enhance the DBP formation due to the temperature dependence of reaction rates. Thus, temperature control of pools, especially for those preferring higher temperatures (e.g. hydrotherapy and spa), should take both bather comfort and DBP formation potential into consideration. It is also observed that chlorine competition existed between different precursors from natural organic matters (NOM) in filling water and body fluid analogue (BFA). Among the composition of BFA, uric acid, citric acid and hippuric acid were found to be the main precursors for HAA formation.

A Risk Assessment Scheme of Infection Transmission Indoors Incorporating the Impact of Resuspension.

A new risk assessment scheme was developed to quantify the impact of resuspension to infection transmission indoors. Airborne and surface pathogenic particle concentration models including the effect of two major resuspension scenarios (airflow-induced particle resuspension [AIPR] and walking-induced particle resuspension [WIPR]) were derived based on two-compartment mass balance models and validated against experimental data found in the literature. The inhalation exposure to pathogenic particles was estimated using the derived airborne concentration model, and subsequently incorporated into a dose-response model to assess the infection risk. Using the proposed risk assessment scheme, the influences of resuspension towards indoor infection transmission were examined by two hypothetical case studies. In the case of AIPR, the infection risk increased from 0 to 0.54 during 0-0.5 hours and from 0.54 to 0.57 during 0.5-4 hours. In the case of WIPR, the infection risk increased from 0 to 0.87 during 0-0.5 hours and from 0.87 to 1 during 0.5-4 hours. Sensitivity analysis was conducted based on the design-of-experiments method and showed that the factors that are related to the inspiratory rate of viable pathogens and pathogen virulence have the most significant effect on the infection probability under the occurrence of AIPR and WIPR. The risk assessment scheme could serve as an effective tool for the risk assessment of infection transmission indoors.

Modeling and experiments of the adhesion force distribution between particles and a surface.

Due to the existence of surface roughness in real surfaces, the adhesion force between particles and the surface where the particles are deposited exhibits certain statistical distributions. Despite the importance of adhesion force distribution in a variety of applications, the current understanding of modeling adhesion force distribution is still limited. In this work, an adhesion force distribution model based on integrating the root-mean-square (RMS) roughness distribution (i.e., the variation of RMS roughness on the surface in terms of location) into recently proposed mean adhesion force models was proposed. The integration was accomplished by statistical analysis and Monte Carlo simulation. A series of centrifuge experiments were conducted to measure the adhesion force distributions between polystyrene particles (146.1 ± 1.99 µm) and various substrates (stainless steel, aluminum and plastic, respectively). The proposed model was validated against the measured adhesion force distributions from this work and another previous study. Based on the proposed model, the effect of RMS roughness distribution on the adhesion force distribution of particles on a rough surface was explored, showing that both the median and standard deviation of adhesion force distribution could be affected by the RMS roughness distribution. The proposed model could predict both van der Waals force and capillary force distributions and consider the multiscale roughness feature, greatly extending the current capability of adhesion force distribution prediction.

Particle concentration dynamics in the ventilation duct after an artificial release: for countering potential bioterrorist attack.

Ventilation duct serves as a potential target for bioterrorist attack. Understanding the dynamics of aerosolized harmful agents in the ventilation ducts provides the fundamentals for effective control and management, e.g., risk assessment. In this work, new models for predicting the concentration dynamics in the ventilation duct after a particle resuspension (representing the case that harmful agents are dosed when the ventilation is off and subsequently being turned on) or puff injection (representing the case that harmful agents are dosed when the ventilation is running) event were derived based on the mass balance model. The models were validated by a series of wind tunnel experiments. Indoor airborne particle concentration models were derived by incorporating the proposed ventilation duct models for resuspension and injection cases. The effects of resuspension and injection in the duct on indoor airborne particle concentration were examined by two hypothetical cases of Bacillus anthracis dosage using the derived models. For the same amount of BW agent dosage in the ventilation duct, the resuspension type release prolongs the exposure of harmful agents whereas the injection type release produces a higher peak concentration.

Mathematical models for the van der Waals force and capillary force between a rough particle and surface.

The capability of predicting the adhesion forces between a rough particle and surface including the van der Waals force and capillary force is important for modeling various processes involving particle surface retention and resuspension. On the basis of the fractal theory describing the behavior of multiple roughness scales and the Gaussian roughness distribution, a set of mathematical models for the van der Waals force and capillary force is proposed. The proposed models provide the adhesion force predictions in good agreement with the existing experimental data and converge to the previous classical solutions of the adhesion forces between a smooth particle and surface as the roughness goes to zero. The influences of roughness for the combination of particle and surface, relative humidity (RH), contact angle, and Hurst exponent toward the adhesion forces are examined using the proposed models. The decline mode of the adhesion force with surface roughness and contact angle, as well as the increase mode with RH and the Hurst exponent are reasonably predicted by the proposed models. The comparison between the proposed models and those from the existing studies is also performed, which shows the similarities and differences between the proposed models and the existing models.

Removal and leakage of environmental tobacco smoke from a model smoking room.

Experimental studies on the removal of accumulated environmental tobacco smoke (ETS) and the effectiveness of ETS leakage control were carried out in a model smoking room using carbon monoxide, nicotine, 3-ethenylpyridine, respirable suspended particulates, and ultrafine particles (UFP) as the ETS tracers. The study investigated the effectiveness of the designated smoking room, equipped with a displacement ventilation system under different ventilation rates (10-58 L/sec per person,) in removing the ETS tracers. The extent of ETS leakage through different door operating scenarios under various ventilation rates was intensively studied. In particular, a manikin installed on a motorized rail was used to study the effect of human movement on the leakage of the ETS tracers. A double-door anteroom design was incorporated into the smoking room to study its effectiveness in ETS leakage prevention. It shows that at least 5 Pa of negative pressure, a fresh air supply rate 3-5 times higher than a typical office, direct air exhaust without air recirculation, and keeping the door closed are important for reducing ETS leakage. However, with the smokers moving in and out and the opening of the door, noticeable leakage of ETS can occur. The double-door anteroom design can improve leakage prevention. Among the five tracers, nicotine required the longest purging time to remove, after the smoking activity was stopped in the smoking room, due to its highly sorptive property. At least 4.4-6 hr of purging is needed for minimizing ETS exposure by non-smokers entering the smoking room. The peak size of particulate matter inside the smoking room is about 80-100 nm, suggesting the importance of including UFP as an indicator for monitoring the exposure and leakage of ETS. The impact of manikin movement on contaminant transport was studied, providing useful information on the effects of human activities on indoor air quality multicompartmental modeling.