The operational street canyon models are a good choice for routinely practical applications in modelling pollutant concentrations inside urban street canyons, as they are able to calculate the pollutant concentrations down to street scale and require relatively short computational times and less expensive hardware resources.
Most of existing operational street canyon models are 2-dimensional models, considering the pollutant dispersions across the street and within the recirculation zone. They are mostly designed for the nearly regular street canyons with the aspect ratio equal to 1 (H/W ≈ 1), assuming the number of vortices in the street canyon no larger than 1, with no considerations of the pollutant depositions.
However, in Hong Kong, most urban street canyons have the aspect ratio larger than 1.5 (H/W > 1.5), under which the number of vortices in the street canyon is usually lager than 1. Besides, the pollutant dispersions along the street need to be taken into account for practical applications, as the building morphology are not uniformly distributed in Hong Kong.
This study uses the ADMS-Urban Regional Model link to model pollutant concentrations within street canyons in urban areas of Hong Kong. This model system is specially designed for Hong Kong’s routinely practical applications; and it is an automated system coupling the high-resolution local model (the ADMS-Urban) with the meso-scale regional model (the CMAQ) and the meso-scale meteorological model (the WRF).
The ADMS-Urban is an operational model, including a module of the advanced street canyon model, a module of modelling the pollutant deposition and a module of modelling the pollutant chemistry. The advanced street canyon model is able to produce the high resolution spatial distributions of pollutant concentrations at any height levels, with the horizontal resolution down to street scale (i.e., several meters); it is designed based on the consideration of the unique urban morphology in Hong Kong, which can model the street canyons with different aspect ratios (H/W), assuming the number of vortices in the street canyon proportional to the aspect ratio (H/W); it is a 3-dimensional dispersion model, which separate the pollutant dispersions into five different components: across the street, within the recirculation zone, along the street, in the opening areas between buildings and from the top of street canyons. The pollutant deposition module can model the effects of both the dry and wet depositions on the pollutant concentrations. The pollutant chemistry module can model the chemical reactions for the sulphate and NOX chemistries.
In current study, the ADMS-Urban Regional Model link (RML) is applied to spatially calculate the hourly NOX, NO2, PM10, and PM2.5 concentrations at three locations – Mong Kok, Central and Causeway Bay – where three Roadside Air Quality Monitoring Stations (RAQMSs) are installed, in the whole year of 2010. The agreements between the simulations from the RML and the observations from the RAQMSs are good with the acceptable error. Besides, the RML also performs better than the CMAQ. However, the RML has the tendency to under-predict the observations in most of the modelling periods, mainly due to the lack of traffic information for some minor roads in the vicinity of the RAQMSs and the rough calculations of emissions.
In the future study, we are going to further refine the advanced street canyon model and investigate the impact of meteorological conditions and urban building morphology on the street-level pollutions through the parametric studies on the quantified meteorological and urban morphology parameters respectively.