Enhanced "urban breathability" leads to deterioration in ground-level air-quality

Barnes, Matthew J. and Brade, Thomas K. and MacKenzie, A. Robert and Whyatt, J. Duncan and Carruthers, David J. and Stocker, Jenny and Cai, Xioaming and Hewitt, Nicolas (2013) Enhanced "urban breathability" leads to deterioration in ground-level air-quality. In: 15th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, HARMO 2013, 2013-05-06 - 2013-05-09.

Abstract

Generally, there are four strategies to reduce exposure to poor urban air quality and improve the health of the inhabitants of a city: reduce overall emissions; increase the depositional sink for pollutants; relocate people and/or polluting industries or improve the ventilation of city neighbourhoods and streets. The ventilation of a city is intricately linked with urban form because urban form controls the overall aerodynamic roughness of the urban area, produces specific quasi-stationary modifications to the impinging flow and interacts with the radiative and turbulent energy transfer between the surface and the atmosphere. Surface aerodynamic roughness is a function of the spatial density, orientation and height of obstacles to the wind and plays a significant role in how air flow interacts with the urban landscape. This paper reports on changes in model performance resulting from the introduction of variable surface roughness values in ADMS-Urban (v3.1) before going on to assess whether significant reductions in pollutant concentrations can be achieved simply through local reductions in these values. ADMS-Urban was initially used to model NOx and NO2 concentrations across Birmingham City Centre, UK, using a file derived from airborne LiDAR data in which roughness values ranged from 0.4 to 3.1m. The model was then re-run with a modified surface roughness file in which selected values near the city centre were reduced to 0.1m to represent a change in landuse to urban parkland. Our results show that reducing surface roughness in the city centre would increase ground-level pollutant concentrations, both locally in the area of reduced roughness and downwind of that area. We discuss our results in terms of vertical stirring and horizontal ventilation effects and in this instance conclude that the vertical stirring effect dominates. Since the model predicts that reducing surface roughness to enhance 'urban breathability' has the unexpected effect of increasing ground-level pollutant concentrations, we caution against using this type of modelling for urban planning and design studies in which the concept of breathability is important. We expect the results from this study to be relevant for all atmospheric dispersion models with urban-surface parameterisations based on roughness.