Guidelines on Choice of Models and Model Parameters

2.1    General 
2.1.1    Coordinate system 
The Hong Kong 1980 Geodetic datum should be used for local modelling projects. A web-based transformation tool is available on Lands Department’s website for conversion of coordinates [LINK].

2.1.2    Executable version

The applicability or validity of the executable version of AERMET and AERMOD should be reviewed before the final submission of the modelling assessment. The corresponding bug fixes and enhancements implemented could be referred to USEPA Support Center for Regulatory Atmospheric Modeling (SCRAM) website. 

 

2.2    Meteorological Data
2.2.1    At least 1 year of recent meteorological data (including wind speed, wind direction, stability class, ambient temperature, cloud cover and mixing height) from PATH should be used to determine the highest short-term (hourly, 8-hour, daily) and long-term (annual) air quality impacts at identified air sensitive receivers in the assessment period. The range of mixing height should be adjusted according to the observation data from HKO at the reference year (i.e. 2019).

2.2.2    When using PATH's meteorological output to drive models for Tier 1 and Tier 2 assessment, the meteorological parameters from PATH, including wind speed and direction, should be taken as uniform over the whole relevant PATH grid cell(s) (i.e. the 3-dimensional volume) without further adjustment. In cases an anemometer height is required as input to models for Tier 1 and Tier 2 assessment, it should be set at the mid-layer height of the relevant PATH grid cell(s) (i.e. 8.5 m).

2.3    Geophysical Data
2.3.1    Land use and Surface Characteristics
The surface characteristics parameters should be determined according to the methods specified in AERMOD implementation guide published by USEPA. The current recommendation is excerpted below:

• The determination of the surface roughness length should be based on an inverse distance weighted geometric mean for a default upwind distance of 1 kilometer relative to the measurement site. Surface roughness length may be varied by sector to account for variations in land cover near the measurement site; however, the sector widths should be no smaller than 30 degrees.
   
• The determination of the Bowen ratio should be based on a simple unweighted geometric mean (i.e., no direction or distance dependency) for a representative domain, with a default domain defined by a 10km by 10km region centered on the measurement site.
   
• The determination of the albedo should be based on a simple unweighted arithmetic mean (i.e., no direction or distance dependency) for the same representative domain as defined for Bowen ratio, with a default domain defined by a 10km by 10km region centered on the measurement site.
   

The exact approach in determining these parameters should be fully justified and agreed by EPD. Consultants can generate the site specific AERMOD-ready AERMET output from the AERMET tool under the Air Modelling Platform (VIA).

For large scale infrastructural or strategic assessment projects, changes may be made to land use across an extensive area. As the meteorological conditions are influenced by the land use and cover, applicants working on these projects should update the land cover image based on the latest design so that the surface characteristics input in AERMET accurately represent the proposed future land use and cover.

2.3.2    Definition of Urban or Rural
AERMOD provides the “urban” option to account for the more convective boundary layer from the urban heat island effect by enhancing the turbulence for urban night-time conditions. According to the procedures for classifying whether a study area is urban or rural, consultants could refer to EPA’s 40 CFR Part 51 Appendix W (USEPA’s Guideline of Air Quality Model).  Based on these guidelines, most areas in Hong Kong would be classified as urban in AERMOD.

Consultants shall justify selecting the urban or rural option and the population assumed for the urban option in the assessment report. The population of the district where the study area is located should be inputted, referencing the latest census data available.

2.3.3    Terrain 
Considering the hilly topography in Hong Kong, the default ELEVATED mode in AERMOD should be applied unless there is a specific justification for the non-default FLAT terrain option. For the ELEVATED mode, all sources and receptors are required to have base elevation in meters above Principal Datum (mPD) in the model input file.

2.3.4    Building Downwash
When modelling industrial emission, additional turbulence may occur due to the presence of nearby buildings. The buildings within five times the lesser of the building height or maximum projected width should be considered in driving the building downwash when setting up the model. By default, the BPIPPRM building pre-processor should be used and the associated plots of the buildings shall be presented as figures in the report.

2.4 NO_X to NO₂ Conversion
2.4.1    The conversion of NO_X to NO₂ is a result of a series of complex photochemical reactions and determines the prediction of near field impacts of NO_X emissions. Two approaches are currently acceptable in the determination of NO₂:

1. Ozone Limiting Method (OLM); or
2. Empirical relationship for annual NO₂:NO_x ratio.

2.4.2    When applying OLM to determine motor vehicle impacts, a conservative 28% of tailpipe NO_X can be assumed to be NO₂ (on a mixing ratio basis) for all vehicle types. If further refinements are desirable, individual tailpipe NO₂ and NO_X output are available in EMFAC-HK model.

2.4.3    For other non-road sources, initial NO₂/NO_X ratios of 0.1 can be adopted (Ref 1). Otherwise, project specific ratios can be proposed with strong supporting evidence and should be fully agreed with EPD.

2.4.4 The hourly background ozone concentration for using OLM can be extracted from PATH.

2.4.5 2.4.5    As an alternative, the annual NO₂ concentrations can be estimated with project specific empirical relationship derived using the latest available and representative data from EPD’s air quality monitoring stations (AQMS). The empirical relationship should be described by a fitted curve of the selected annual NO₂ and NOx monitoring data (Ref 2,3 Jenkin 2004a, Environment Agency UK 2007). The annual NO₂ concentrations can be determined from the fitted curve using the total NO_x concentrations. The use of project specific empirical relationship and the calculation details should be approved by EPD on a case-by-case basis.  A tool is available on the Air Modelling Platform (VIA) for obtaining the constants of the Jenkin curve with user-defined selection of AQMS.

Ref.(1): USEPA (2015), Technical support document (TSD) for NO2-related AERMOD modifications, https://gaftp.epa.gov/Air/aqmg/SCRAM/models/preferred/aermod/AERMOD_NO2….

Ref.(2): Jenkin M E, 2004a. Analysis of sources and partitioning of oxidant in the UK – Part 1: The NOx-dependence of annual mean concentrations of nitrogen dioxide and ozone. Atmospheric Environment, 38, 5117-5129.

Ref.(3): Environment Agency UK 2007. Review of methods for NO to NO2 conversion in plumes at short ranges (https://www.gov.uk/government/publications/review-of-methods-for-no-to-…)

2.5 Odour Impact
In assessing odour impacts, a much shorter time-averaging period of 5 seconds is required due to the shorter exposure period tolerable by human receptors. Conversion of model-computed hourly average results to 5-second values is therefore necessary to enable comparison against the recommended standard. Depending on the source type and stability class, the following factors can be applied to convert the hourly results to 5-second averages (Ref 4). The stability class could be referred to the meteorological data extracted from PATH.
 

Source type	Stability Category	1-hour to 5-sec Conversion Factor
Area	A, B, C, D	2.5
	E & F	2.3
Line	A to F	6
Tall wake-free point	A, B, C	17
	D, E, F	35
Wake-affected point	A to F	2.3
Volume	A to F	2.3

Ref (4): Approved Methods for the Modelling and Assessment of Air Pollutants in New South Wales, https://www.epa.nsw.gov.au/your-environment/air/industrial-emissions/approved-methods-for-the-modelling-and-assessment-of-air-pollutants

2.6 Receptors
These include discrete receptors representing all the identified air sensitive receivers at their appropriate locations and elevations and any other discrete or grid receptors for supplementary information.  A receptor grid, whether Cartesian or Polar, may be used to generate results for contour outputs.  The receptors for plotting contours should be placed at sufficient resolution to allow for identification of zones with the highest impact or for determination of buffer zones.

2.7  Output
The highest short-term and long-term averages of pollutant concentrations at prescribed receptor locations are output by the models and appropriately summed up for comparison with the relevant air quality standards specified for the corresponding pollutant. Contours of pollutant concentration are also required for indicating the general impacts of emissions over a study area.

Copies of model files in electronic format should also be provided for EPD's reference. 
 

Air Quality Modelling & Forecasting Section, 
Air Science and Modelling Group
Environmental Protection Department
January 2024

Model – Type	Source	Application	Information source
1. AERMOD – Steady state dispersion	United States Environmental Protection Agency	For point, area and volume sources	1.    Available from USEPA website 2.   Technical note for Air Quality Modelling
2. PATH (latest version: PATH v3.0) – Grid-based comprehensive modelling system	Hong Kong Environmental Protection Department	Provide meteorological data for the model inputs and provide air pollutant concentration estimates for Tier 3 background contribution	Guidelines for Local-Scale Air Quality Assessment Using Models