Could outdoor tree coverage help an auditorium meet an indoor air-quality target?

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Researchers used computational fluid dynamics (CFD) models to investigate how outdoor trees affect the dispersion of PM1.0, PM2.5 and PM10 inside a naturally ventilated auditorium. The models coupled a standard k-ε model based on the Reynolds-averaged Navier-Stokes (RANS) approach with a revised generalized drift flux model.

They introduced crown volume coverage (CVC) to quantify outdoor trees and simulated different CVCs, oncoming wind speeds and window opening sizes. Half-open and fully open windows corresponded to wall porosities of 3.5% and 7.0%, respectively.

In the baseline scenario, a vortex formed inside the auditorium. Recirculating air created a well-mixed zone with little variation in particle concentrations. Indoor PM10 decreased noticeably farther from the inlet boundary because of turbulent diffusion.

Assuming pollution sources were diluted through the inlet, average indoor particle concentrations rose exponentially as oncoming wind speed increased. PM10 changed most significantly, attributed to turbulent diffusion and reduced surface deposition intensified by the higher wind speed. Larger window openings improved indoor cross-ventilation and reduced indoor particle concentrations.

At a CVC of 2.87–4.73 m³/m², indoor PM2.5 could meet the World Health Organization air quality guidelines’ interim target 3 (IT-3) for 24-hour mean concentrations. Average indoor particle concentrations were positively correlated with natural ventilation rates. At 3.5% wall porosity, R² was 0.9085 for PM1.0, 0.961 for PM2.5 and 0.9683 for PM10; at 7.0% porosity, the respective values were 0.9158, 0.9734 and 0.976.