By Dr. David Carruthers, Technical Director of Cambridge Environmental Research Consultants (CERC); Project Consultant for PRAISE-HK.
The varying quality of the air you breathe as you move around a city is determined by a multitude of factors on many spatial scales, from industrial and agricultural pollution emissions over a wide region, through regional and local weather conditions to the traffic flows and building geometries of a particular street. The PRAISE-HK project takes account of all these complexities, giving public access to predictions of the street-scale variation of air quality and allowing you to make informed choices about routes and transport modes to minimise your exposure to air pollution.
Air quality is measured at a small number of specific locations in the city. In Hong Kong, there are only 15 permanent air quality monitoring stations, so computational models are needed to predict the spatial variation of air quality throughout the urban area. In general these models use information about the current weather conditions and the emissions of different pollutants from a wide range of sources, both local and regional. They then perform calculations about where the emissions are transported and spread out by the wind and how they are transformed by chemical reactions in order to predict the final concentrations.
New model system producing a high-resolution air quality forecast
In PRAISE-HK two models will be linked together to create a state-of-the-art air quality forecasting system for Hong Kong. CMAQ is a typical ‘regional model’ which calculates concentrations of air pollution across the whole of the Pearl River Delta with a horizontal resolution of 1 km. This model is an advanced tool for predicting the transport and chemistry processes which affect pollutant emissions over a large area, but it cannot calculate the variations of air quality within our neighborhood areas on scales much smaller than 1 km. In contrast ADMS-Urban, developed by CERC, is a ‘local model’ which will calculate air pollution concentrations at high resolution within the HKSAR. Linking the two models together allows the system to take into account both small-scale and large-scale transport and chemistry effects, giving a much more powerful prediction of the street-scale air quality.
How does ADMS-Urban operate?
ADMS-Urban is designed to predict the transport of pollutant emissions from roads including the effects of adjacent buildings forming street canyons. Emissions in street canyons may be channelled along the road, trapped in recirculating flow regions or transported out of the canyon through gaps between buildings, above the buildings or from the end of the canyon. These effects result in higher concentrations with more complex spatial patterns compared to roads without adjacent buildings, details which are not available from existing air pollution information platforms.
The local weather conditions, in particular the speed and variability of the wind, are important for the accurate prediction of street-scale air quality. ADMS-Urban calculates the effects of local building height and density on the variation of wind speed, leading to reduced wind speeds in urban areas compared to rural areas.
How will detailed air quality predictions make a difference in Hong Kong?
Predicting the street-scale air quality in Hong Kong is particularly challenging due to the diverse nature of pollutant emissions, the steep terrain and tall densely packed buildings creating large local variations in airflow and turbulence, and the complex road geometries including deep street canyons and elevated highways. The ADMS-Urban model has been developed to address these challenges and will be used in PRAISE-HK to provide detailed information about the quality of the air you breathe in the streets of Hong Kong. You will be able to use this information to help plan your daily activities and protect your health from exposure to toxic air pollutants.