No abstract

Size distributions of expiratory droplets expelled during coughing and speaking and the velocities of the expiration air jets of healthy volunteers were measured. Droplet size was measured using the interferometric Mie imaging (IMI) technique while the particle image velocimetry (PIV) technique was used for measuring air velocity. These techniques allowed measurements in close proximity to the mouth and avoided air sampling losses. The average expiration air velocity was 11.7 m/s for coughing and 3.9 m/s for speaking. Under the experimental setting, evaporation and condensation effects had negligible impact on the measured droplet size. The geometric mean diameter of droplets from coughing was 13.5 m and it was 16.0 m for speaking (counting 1-100). The estimated total number of droplets expelled ranged from 947 to 2085 per cough and 112-6720 for speaking. The estimated droplet concentrations for coughing ranged from 2.4 to 5.2 cm −3 per cough and 0.004-0.223 cm −3 for speaking.

The transport and deposition of polydispersed expiratory aerosols in an aircraft cabin were simulated using a Lagrangianbased model validated by experiments conducted in an aircraft cabin mockup. Infection risk by inhalation was estimated using the aerosol dispersion data and a model was developed to estimate the risk of infection by contact. The environmental control system (ECS) in a cabin creates air circulation mainly in the lateral direction, making lateral dispersions of aerosols much faster than longitudinal dispersions. Aerosols with initial sizes under 28 µm in diameter can stay airborne for comparatively long periods and are favorable for airborne transport. Using influenza data as an example, the estimated risk of infection by inhalation are at least two orders of magnitude higher than the risk of infection by contact. An increase in the supply airflow rate enhances ventilation removal and the dispersion of these aerosols. It reduces the risk of infection by inhalation for passengers seated within one row and one column from the index patient but it increases the risk for passengers seated further away. The deposition fraction increases with aerosol size. The ECS supply airflow rate has insignificant impact on the deposition behavior of these large aerosols, making the impact on the risk of infection by contact insignificant. Comparatively, the contact behavior of passengers is highly influential to the contact infection risk. Passengers seated within one row from the index patient are subject to contact risks that are one to two orders of magnitude higher than are passengers seated further away.

Dispersion characteristics of expiratory aerosols were investigated in an enclosure with two different idealized airflow patterns: the ceiling-return and the unidirectional downward. A multiphase numerical model, which was able to capture the polydispersity and evaporation features of the aerosols, was adopted. Experiments employing optical techniques were conducted in a chamber with downward airflow pattern to measure the dispersion of aerosols. Some of the numerical results were compared with the chamber measurement results. Reasonable agreement was found. Small aerosols (initial size

No abstract

White and "green" (vegetated) roofs have begun replacing conventional black (dark-colored) roofs to mitigate the adverse environmental effects of dark and impervious urban surfaces. This paper presents an economic perspective on roof color choice, in particular the direct comparison of white and green roofs, by conducting a 50-year life-cycle cost analysis (LCCA). Using data collected from 22 flat roof projects in the U.S., we find that relative to black roofs, white roofs provide a 50-year net savings (NS) of $25/m 2 ($2.40/ft 2 ) and green roofs have a negative NS of $71/m 2 ($6.60/ft 2 ). An important implication is that green roofs never quite "pay." Despite lasting at least twice as long as white or black roofs and therefore requiring fewer replacements, green roofs cannot compensate for their sizable installation cost premium of $151/m 2 ($14/ft 2 ).However, while the 50-year NS of white roofs compared to green roofs is $96/m 2 ($8.90/ft 2 ), the annualized cost premium is just $3.20/m 2 -year ($0.30/ft 2 -year). This annual sum should not deter owners from reaping the environmental and aesthetic benefits of green roofs (such as enhanced biodiversity, greater carbon sequestration, and increased property value) that are not captured in this LCCA. Choosing between a white and green roof should therefore be based on the preferences of the building owner. If global warming is a priority, then an owner should choose white roofs, which are three times more effective than green roofs in cooling the globe. If local environmental benefits are favored, then an owner should choose green roofs, which offer builtin stormwater management and a "natural" urban landscape aesthetic. Although black roofs are more cost-effective than green roofs and even white roofs in rare cases, we strongly recommend building code policies that phases out dark-colored roofs in warm climates to protect against their adverse public health externalities.

No abstract

The transport and removal characteristics of expiratory droplets at different supply airflow rates and "coughing" orientations were investigated both numerically and experimentally in a three-bed hospital ward setting. A Lagrangian-based particletracking model with near-wall correction functions for turbulence was employed to simulate the fate of the expiratory droplets. The model was tested against experimental droplet dispersion data obtained in an experimental hospital ward using Interferometric Mie Imaging and a light-scattering aerosol spectrometer. The change in airflow supply rate had insignificant effect on the transport and deposition of very large droplets (initial sizes ≥ 87.5 µm) due to the dominance of gravitational settling on these behaviors. Smaller droplets (initial sizes ≤ 45 µm) exhibited certain airborne behaviors. The effect of thermal plumes from heat sources was observed only when the supply airflow was low and when the droplet size was small, as observed in the vertical mixing patterns of the droplets of various sizes. Larger droplets tended to settle lower and lateral dispersion of the droplets became weak at the low supply airflow rate. The deposition characteristics for different surfaces in the room are described. The heat plumes seemed to obstruct small droplets from being deposited onto heated surfaces. More deposition was predicted in the lateral injection case compared with the vertical injection case. Adopting near-wall correction for turbulence in the model reduced the predicted deposition removal fraction by 25% for 1.5 µm droplets. This reduction became less significant for larger droplets due to the smaller dependence on turbulent diffusion in their deposition. NOMENCLATURECunningham correction factor