Possible high COVID-19 airborne infection risk in deep and poorly ventilated 2D street canyons

Despite the widespread assumption that outdoor environments provide sufficient ventilation and dilution capacity to mitigate the risk of COVID-19 infection,there is little understanding of airborne infection risk in outdoor urban areas with poor ventilation.To address this gap,we propose a modified Wells-Riley model based on the purging flow rate(QPFR),by using computational fluid dynamics(CFD)simulations.The model quantifies the outdoor risk in 2D street canyons with different approaching wind speeds,urban heating patterns and aspect ratios(building height to street width).We show that urban morphology plays a critical role in controlling airborne infectious disease transmission in outdoor environments,especially under calm winds;with deep street canyons(aspect ratio>3)having a similar infection risk as typical indoor environments.While ground and leeward wall heating could reduce the risk,windward heating(e.g.,windward wall~10 K warmer than the ambient air)can increase the infection risk by up to 75%.Our research highlights the importance of considering outdoor infection risk and the critical role of urban morphology in mitigating airborne infection risk.By identifying and addressing these risks,we can inform measures that may enhance public health and safety,particularly in densely populated urban environments.

Three-Dimensional Urban Thermal Effect across a Large City Cluster during an Extreme Heat Wave:Observational Analysis

Given extensive and rapid urbanization globally,assessing regional urban thermal effects(UTE)in both canopy and boundary layers under extreme weather/climate conditions is of significant interest.Rapid population and economic growth in the Yangtze River Delta(YRD)have made it one of the largest city clusters in China.Here,we explore the three-dimensional(3D)UTE in the YRD using multi-source observations from high-resolution automatic weather stations,radiosondes,and eddy covariance sensors during the record-setting heat wave(HW)of July-August 2013.It is found that the regional canopy layer UTE is up to 0.6-1.2℃,and the nocturnal UTE(0.7-1.6℃)is larger than daytime UTE(0.2-0.5℃)during the HW.The regional canopy layer UTE is enhanced and expanded northwards,with some rural sites contaminated by the urban influences,especially at night.In the boundary layer,the strengthened regional UTE extends vertically to at least 925 hPa(~750 m)during this HW.The strengthened 3D UTE in the YRD is associated with an enlarged Bowen ratio difference between urban and non-urban areas.These findings about the 3D UTE are beneficial for better understanding of the thermal environment of large city clusters under HW and for more appropriate adaption and mitigation strategies.