RapidAir is based on the same modelling approach used in AERMOD, the USEPA’s accepted model for traffic emissions. It is a steady-state dispersion model, in which pollutants are assumed to disperse away from their source in a plume with a Gaussian concentration profile in both directions perpendicular to the plume centreline.
AERMOD was selected as the dispersion core of the default version of RapidAir due to the extensive research that underpins it, its recognised suitability for application and widespread acceptance for use internationally. AERMOD is an internationally recognised air dispersion modelling code and is a mandated USEPA model of choice for road traffic air quality assessments in the USA (USEPA, 2015). The USEPA imposes strict guidelines on use of dispersion models and the use of AERMOD is written into law in ‘Appendix W’ of the Guideline on Air Quality Models (USEPA, 2017). AERMOD also accounts for other factors that influence dispersion. For example, convective and stable boundary layer conditions, urban and rural land effects, terrain and the presence of buildings.
Traditional Gaussian models calculate concentrations at locations of interest individually, typically making calculations every hour of the input meteorological record, before averaging the results. This approach often results in long model run times, especially for large study areas. RapidAir makes use of a dispersion kernel modelling approach to greatly reduce computational effort for cases that can involve tens of thousands of discrete sources and millions of receptor locations. The application of this approach, combined with a highly optimised (and vectorised) computational framework, results in run times of several seconds for large model domains, delivering modelling speeds that are not possible when using traditional Gaussian models.
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RapidEMS
Traffic emissions module
The RapidEMS module enables the rapid calculation of emissions rates for a range of key pollutants – including oxides of nitrogen (NOx), particulate matter with a diameter of less than 10 microns or 2.5 microns (PM10 and PM2.5 respectively) and carbon dioxide (CO2) – for specific years, road types (e.g motorways, urban), vehicle speeds and fleet composition. The default version of RapidEMS uses COPERT coefficients. In the UK, RapidEMS closely aligns with the latest version of the Department for Environment, Food and Rural Affairs’ (Defra) Emissions Factors Toolkit.
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Met
Meteorogical module
Providing high-quality meteorological data is a key requirement for undertaking robust and defensible air quality modelling. RapidAir has been designed to enable users to quickly process and undertake quality assurance (gap filling using the United States Environmental Protection Agency (USEPA) methodology) of the met data driving their simulations.
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Canyons
Street canyon model and allocator
In built-up urban areas, air pollution can become trapped in street canyons – streets surrounded by tall buildings. As such, the impacts of canyons are important elements of any robust air quality modelling system. Street canyon models range from complex computational fluid dynamic (CFD) models to simpler empirical (such as the USEPA STREET box-model and semi-empirical models (such as the Danish Operational Street Pollution Model (OSPM)). RapidAir includes a canyon module based on the AEOLIUS system developed by the UK Met Office.
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