Air flow and hence dispersion of toxic air pollutants in urban areas are influenced by the presence of buildings and other obstacles, with effects becoming more complex as building density and height increases; complicated road infrastructure involving tunnels and road elevation also impact on dispersion processes. For some pollutants, urban air quality is dominated by the cumulative effects of emissions from urban, industrial, agricultural and rural areas across a wider region through long-range transport and chemical transformations. This regional pollution influences local air quality via urban background concentrations and the alteration of conditions for local chemistry. Urban populations are increasingly aware of local air quality issues, inspiring the development of high-resolution air quality forecasting systems which enable individuals to choose lower-exposure activities or transport routes during times of high concentration. CERC have worked with HKUST on modelling air quality in Hong Kong’s very complex urban environment since 2014, using the world-leading ADMS-Urban street-scale dispersion model developed to account for dispersion from complex street canyons, road tunnels and elevated roads, alongside urban canopy flow effects. The ADMS-Urban Regional Model Link, coupling ADMS-Urban with regional modelling, is operational within the PRAISE-HK public air quality forecasting system for Hong Kong.

1. Routes to Clean Air 2021
Routes to Clean Air
12th October 2021
Christina Hood

2. Routes to Clean Air 2021
Outline
 Introduction to complex urban environments
 Modelling approaches
 Street canyon dispersion
 Urban canopy flow
 Tunnels
 Elevated roads
 Coupled regional and local modelling
 Hong Kong model application and evaluation
 Further modelling challenges and developments

3. Routes to Clean Air 2021
Introduction to complex urban environments
Sparse buildings
Regular buildings
forming wide canyons
Irregular buildings
forming tall canyons
Tall canyons and
multiple road levels
Increasing complexity

4. Routes to Clean Air 2021
Introduction to complex urban environments
Common features
 Dense building layouts change air flow and alter dispersion
 Street canyons and urban canopy flow
 Traffic infrastructure for managing high traffic flows can have
complex geometry
 Tunnels and elevated roads
 Urban air pollution can be influenced by regional air quality
 Coupled regional and local modelling
 Dense population exposed to high levels of air pollution
 Air quality forecasting

5. Routes to Clean Air 2021
Modelling approach: Street canyon dispersion
 Original ADMS-Urban canyon model based on OSPM
 Symmetric street canyons, height/width ≤ 1, solid buildings
 Emissions spread throughout canyon width
 ADMS-Urban Advanced street canyon module initially
developed for Hong Kong
 Needed to allow for:
 Separation of road carriageway and pedestrian areas
 Tall canyons (height/width > 1)
 Smooth variations between open road and solid canyon
 Asymmetry of canyon properties (height, width, building
density)
 Road elevation within canyon
NO2
Observed
Advanced canyon
No canyon Basic canyon

6. Routes to Clean Air 2021
Modelling approach: Street canyon dispersion
 Street canyon effects modelled using
component sources
1. Along-canyon channeling
2. Across-canyon direct dispersion
3. Recirculation
4. Non-canyon through gaps between buildings
5. Canyon top
6. Canyon end
 Weighting between components depends on
canyon properties and wind direction
 Evaluated using UK, European and Hong Kong
measurement data
Hood et al. 2021 JA&WMA doi: 10.1080/10962247.2020.1803158

7. Routes to Clean Air 2021
Modelling approach: Urban canopy flow
 Urban building effects on air flow
 Reduced wind speed
 Increased turbulent intensity
 Boundary layer height increased
 Gridded building properties used to calculate spatial
variation of flow
Particularly below
average building height
Wind
U
z
Wind speeds are higher
in more open areas
Wind speeds are lower in
heavily built up areas
https://www.harmo.org/Conferences/Proceedings/_Varna/publishedSections/H16-067-Hood-EA.pdf

8. Routes to Clean Air 2021
Modelling approach: Tunnels
 Vehicle emissions in tunnels are displaced to
tunnel end and/or vents
 Emissions can be entrained along outflow road in
vehicle wakes
 ADMS-Urban portal modelling approach based on
Ginzburg and Schattanek
 Evaluation carried out for tunnels in Austria and
UK (Bell Common, M25)
0
50
100
150
200
0 50 100 150 200
Modelled
(ug/m3)
Measured (ug/m3)
Outflow
Inflow
1:1
1:2
Road tunnel
specification
P35/01A/17
Receptor network
Evaluation (NO2 diffusion
tubes, 4 week averages)

9. Routes to Clean Air 2021
Modelling approach: Elevated roads
 Original ADMS-Urban modelling of elevated
roads allowed emissions to disperse down
through the road surface
 Recent development funded by Highways
England has modified downward dispersion
for elevated road sources
 Evaluated at London receptors near
elevated M4 and ground-level A4
HS5
Image © 2019 Google
NOx µg/m³
HS5
HS010
M4
A4
Project report: 2020-2021 publication from Highways England
/https://highwaysengland.co.uk/industry/innovation/academic-research/research-publications/
Evaluation
(annual)
Sites

10. Routes to Clean Air 2021
Modelling approach: Coupled regional to local modelling
 Combine regional model output (CMAQ, CAMx, EMEP4UK, CHIMERE or WRF-Chem)
and local ADMS-Urban modelling for street-scale concentration output with large
scale effects
 Avoids double-counting emissions by separating regional and local modelling using
‘mixing time’
Concentration within
nested domain
=
Regional
modelling of
emissions
-
Gridded locally
modelled
emissions (ΔT)
+
Explicit locally
modelled
emissions (ΔT)
Regional model
data: WRF, CAMx,
CMAQ, EMEP4UK,
CHIMERE,
WRF-Chem
User inputs via
text files
adms-rml and
subsidiary control
scripts
ADMS-Urban
RML Outputs
Utilities
Local model:
ADMS-Urban
Hood et al. 2018 https://acp.copernicus.org/articles/18/11221/2018/
NO2
µg/m3
Hong Kong

11. Routes to Clean Air 2021
Hong Kong model application and evaluation
Model application
 Regional model data: WRF (meteorology) and CMAQ
(concentration)
 Local model data: ADMS-Urban with over 32000 road sources
 >1 million model calculation output locations
 Daily street-scale forecast running on HKUST HPC, feeds mobile
app
Evaluation
 14-15 permanent monitors, 3 near-road all at complex junctions
 Ongoing evaluation and use of measurements for ‘Artificial
Intelligence’ forecast bias correction
 Short campaign measurements in individual canyons to
investigate canyon modelling

12. Routes to Clean Air 2021
Hong Kong model application and evaluation
 Ongoing work on improving explicit road layout and emissions
 Rapid changes to road layout – new bridges, tunnels, noise barriers
 Adding data relating to road elevation – GPS survey
 Different approaches to road emissions tested – both bottom-up and top-down
 Urban canopy and advanced canyon properties calculated from explicit building
footprint and height data
 32000 road sources, of which:
 Over 24000 street canyons
 60 road tunnel sources
 24 tunnel vents, grouped in 3 stacks
 Maximum road elevation ~80 m
(m)

13. Routes to Clean Air 2021
PRAISE-HK mobile app
Hourly exposure
tracking between
different micro-
environment
Exposure levels
between routes
Phase 1
Phase 2
(proposed)

14. Routes to Clean Air 2021
Further model challenges and developments
 Better emissions data
 Further investigation of tall street canyons
 More detailed local modelling of other
source types
 Aircraft
 Shipping
 Faster run times!
 Wind shear modelling for HK airport
0
2
4
6
8
10
12
14
16
0.0 0.5 1.0 1.5
Floor
number
Mean concentration ratio to near-ground
concentration
Summer mod Winter mod
Summer obs Winter obs

15. Routes to Clean Air 2021
Any questions?
Links to more information
 www.cerc.co.uk/UserGuides
 www.cerc.co.uk/TechSpec
 www.cerc.co.uk/publications
 help@cerc.co.uk
 Praise.ust.hk
Thanks to:
 Hong Kong University of Science and Technology:
Prof Jimmy Fung and team
 Hong Kong Environmental Protection Department
 PRAISE-HK team, HSBC funding
 Highways England (elevated roads)
 TRL (M25 tunnel concentration dataset)