Numerical study on the ozone formation inside street canyons using a chemistry box model

doi:10.1016/S1001-0742(08)62134-8

Journal of Environmental Sciences

Volume 20, Issue 7, 2008, Pages 832-837

https://doi.org/10.1016/S1001-0742(08)62134-8 Get rights and content

Abstract

Tropospheric ozone is a secondary air pollutant produced in the presence of nitrogen oxides (NO_x), volatile organic compounds (VOCs), and solar radiation. In an urban environment, ground-level vehicular exhaust is the major anthropogenic source of ozone precursors. In the cases of street canyons, pollutant dilution is weakened by the surrounding buildings that creates localized high concentration of NO_x and VOCs, and thus leads to high potential of ozone formation. By considering the major physical and chemical processes, a chemistry box model is employed to investigate the characteristics of ozone formation due to vehicular exhaust inside street canyons under the worst case scenario, i.e. the calm wind condition. It is found that a high level of ozone concentration, of the order of 100 ppbv and higher, would occur inside the street canyons, in particular, when the emission rate (concentration) ratio of VOCs to NO_x is greater than 10. This elevated ozone concentration appears at the transition from VOCs to NO_x sensitivity and may extend to a few hundreds.

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Citation Excerpt :
For a practical application, the turbulent exchange mechanism is a major research challenge as this plays the key role in controlling the pollutant abundance in the street canyon (Barlow et al., 2004). This phenomenon can be represented by a simplified parameter called ‘transfer velocity’ (Salizzoni et al., 2009) or ‘air ventilation rate’ (Liu and Leung, 2008), herein referred to as ‘exchange velocity’ (Bright et al., 2013), which is responsible for quantifying the exchange of mass between the street canyon and the overlying atmospheric boundary layer. However, many emissions from vehicles are reactive, evolving chemically as the air parcel is circulated inside the street canyon and exchanged with the air above the rooftop.
Air pollution associated with road transport is a major environmental issue in urban areas. Buildings in urban areas are the artificial obstacles to atmospheric flow and cause reduced ventilation for street canyons. For a deep street canyon, there is evidence of the formation of multiple segregated vortices, which generate flow regimes such that pollutants exhibit a significant contrast between these vortices. This results in poor air ventilation conditions at pedestrian level, thereby leading to elevated pollutant levels and potential breaches of air quality limits. The hypothesis of a well-mixed deep street canyon in the practical one-box model approach is shown to be inappropriate. This study implements a simplified simulation of the canyon volume: a coupled two-box model with a reduced chemical scheme to represent the key photochemical processes with timescales similar to and smaller than the turbulent mixing timescale. The two-box model captures the significant pollutant contrast between the lower and upper parts of a deep street canyon, particularly for NO₂. Core important parameters (i.e. heterogeneity coefficient, exchange velocity and box height ratio) in the two-box model approach were investigated through sensitivity tests. The two-box model results identify the emission regimes and the meteorological conditions under which NO₂ in the lower canyon (i.e. the region of interest for the assessment of human health effects) is in breach of air quality standards. Higher NO₂ levels were observed for the cases with higher heterogeneity coefficients (the two boxes are more segregated), with lower exchange velocities (worse ventilation conditions), or with smaller box height ratios (reduced dilution possibly due to secondary smaller eddies in the lower canyon). The performance of a one-box model using the same chemical scheme is also evaluated against the two-box model. The one-box model was found to systematically underestimate NO₂ levels compared with those in the lower box of the two-box model for all test scenarios. This underestimation generally tends to worsen for higher heterogeneity coefficients, lower exchange velocities or smaller box height ratios. This study highlights the limitation of the assumption of homogeneity in single box models for street canyon simulation, and the inherent uncertainties that must be borne in mind to appropriately interpret such model output (in particular, that a single-box treatment will systematically underestimate NO₂ as experienced at street level).

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Numerical study on the ozone formation inside street canyons using a chemistry box model

Abstract

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Atmospheric Environment

Scientific and Technical Issues Related to the Application of Incremental Reactivity