Man Sing Wong

Derivation of nighttime urban air temperatures using a satellite thermal image

Abstract The aim of this study is to characterize the urban heat island (UHI) intensity in Hong Kong. The first objective is to explore the UHI intensity in Hong Kong by using the mobile transverse and remote sensing techniques. The second objective is to produce a satellite-derived air temperature image by integrating satellite remote sensing with a mobile survey, the methodology involved in making simultaneous ground measurements when the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite made an overpass. The average UHI intensity of Hong Kong was about 2°–3.5°C, although a very high value of 12°C UHI was observed on a calm winter night by ASTER. The satellite-derived surface temperature was compared with the in situ measurements. The bias was found to be only about 1.1°C. A good correlation was also found between the in situ surface and air temperature pair of readings at nighttime on 31 January 2007. The linear regression lines of temperatures in urban and suburban areas...

Spatial variability of frontal area index and its relationship with urban heat island intensity

In densely urbanized regions, the local climate is greatly influenced by the urban morphology, including interactions between buildings, space, and human activities. The Kowloon Peninsula in Hong Kong, with some of the greatest urban population densities in the world, represents an extreme case of the influence of the built environment on climate, with high-rise buildings, narrow street canyons, and little green space. In this study, the building frontal area index (FAI), a parameter for estimating aerodynamic resistance of the urban surface as a predictor of wind ventilation, was derived from a three-dimensional building database. The relationship between FAI and urban heat island intensity (UHII) from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite image was calculated at different scales. The highest correlation (r = 0.574, n = 4900) was obtained at 100 m resolution, which suggests that the optimum operational scale of FAI is 100 m resolution for the study area, i.e. the scale and size at which FAI impacts on the urban climate. The presence/trend of any higher correlation at different resolutions was tested using the nonparametric Mann–Kendall test, and the results show that the statistic Z-value generated from the test is smaller than the hypothesis significance levels of 90%, 95%, and 99%; thus, the hypothesis of having a higher correlation at any scale other than 100 m resolution is rejected. Planning authorities may use the FAI generated at 100 m resolution for designing wind ventilation corridors across Hong Kong at this scale, especially when temperature and air quality in the inner city are of major concern. However, for applications to other cities with different standard morphologies, the FAI–UHII relationship should be re-evaluated.

Retrieval of Aerosol Optical Thickness Using MODIS

Aerosol detection and monitoring by satellite observations has been substantially developed over the past decades. While several state-of-the-art aerosol retrieval techniques provide aerosol properties at global scale, high spatial detail that is suitable for urbanized regions is unavailable because most of the satellite-based products are at coarse resolution. A refined aerosol retrieval algorithm using the MODerate Resolution Imaging Spectroradiometer (MODIS) to retrieve aerosol properties at 500-m resolution over land is described here. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the top-of-atmosphere reflectance from surface reflectance and Rayleigh path reflectance. For the determination of surface reflectances, a modified minimum reflectance technique (MRT) is used, and MRT images are computed for different seasons. A good agreement is obtained between the surface reflectances of MRT images and MODIS land surface reflectance products (MOD09), with a correlation of 0.9. For conversion of aerosol reflectance to aerosol optical thickness (AOT), comprehensive lookup tables are constructed which consider aerosol properties and sun-viewing geometry in the radiative transfer calculations. The resulting 500-m AOT images are highly correlated (r = 0.937) with AErosol RObotic NETwork sunphotometer observations in Hong Kong for most of the year corresponding to the long dry season. This study demonstrates a method for aerosol retrieval at fine resolution over urbanized regions, which can assist the study of aerosol spatial distribution. In addition, the MODIS 500-m AOT images can also be used to pinpoint source areas of cross-boundary aerosols from the Pearl River Delta region.

\hbox{500} \times \hbox{500}\ \hbox{m}^{2}

Urban environmental quality (UEQ) is a complex and spatially variable parameter of increasing concern, especially in densely populated cities of the tropics and subtropics where climate, air quality, and the urban infrastructure may interact to produce uncomfortable and hazardous effects. The study investigates the application of multispectral remote sensing from the Landsat ETM+ and IKONOS satellite sensors for the mapping of UEQ in urban Hong Kong at a detailed level, using the measurable, image-based parameters of temperature and biomass, and examines the relationship between these and air quality in selected study districts. Multiple-criteria queries on these two parameters show that spatial variations in UEQ are closely related to natural topographic factors and urban morphology. The amount of biomass, as opposed to total area of vegetation, is also shown to be an important factor in the spatial variation of UEQ. The data permit visualisation of the relationship between the human and natural factors involved in UEQ and generate recommendations for future planning and urban-renewal projects.

, a Study in Hong Kong and the Pearl River Delta Region

The use of Geographic Information Systems (GIS) and Remote Sensing (RS) by climatologists, environmentalists and urban planners for three dimensional modeling and visualization of the landscape is well established. However no previous study has implemented these techniques for 3D modeling of atmospheric aerosols because air quality data is traditionally measured at ground points, or from satellite images, with no vertical dimension. This study presents a prototype for modeling and visualizing aerosol vertical profiles over a 3D urban landscape in Hong Kong. The method uses a newly developed technique for the derivation of aerosol vertical profiles from AERONET sunphotometer measurements and surface visibility data, and links these to a 3D urban model. This permits automated modeling and visualization of aerosol concentrations at different atmospheric levels over the urban landscape in near-real time. Since the GIS platform permits presentation of the aerosol vertical distribution in 3D, it can be related to the built environment of the city. Examples are given of the applications of the model, including diagnosis of the relative contribution of vehicle emissions to pollution levels in the city, based on increased near-surface concentrations around weekday rush-hour times. The ability to model changes in air quality and visibility from ground level to the top of tall buildings is also demonstrated, and this has implications for energy use and environmental policies for the tall mega-cities of the future.

Assessment of Urban Environmental Quality in a Subtropical City Using Multispectral Satellite Images

Aerosol detection and monitoring by satellite observations has been substantially developed over past decades. While several state-of-art aerosol retrieval techniques provide aerosol properties in global scale, the more detail characteristics remain unknown because most of the satellite sensors are limited to 1 km resolution observations. However, a new aerosol retrieval algorithm for the Moderate Resolution Imaging Spectroradiometer (MODIS) 500 m resolution data is developed to retrieve aerosol properties over land, which helps address the aerosol climatic issues on the local/urban scale. The rationale of our technique is to first estimate the aerosol reflectances by decomposing the top-of-atmosphere (TOA) reflectance from surface reflectance and Rayleigh path radiance. The modified Minimum Reflectance Technique (MRT) is adopted for the determination of the seasonal surface reflectances. A good agreement is revealed between the surface reflectances of MRT images and MODIS land surface reflectance products (MOD09), with a strong correlation of 0.9. Moreover, comprehensive look up tables (LUT) are constructed with the considerations of various aerosol optical properties and sun-viewing geometry in the radiative transfer calculations. The resulting 500 m aerosol optical thickness (AOT) data are highly correlated (r=0.94) with the AERONET sunphotometer observations in Hong Kong. This study demonstrates the applicability of aerosol retrieval at fine resolution in urban areas, which can assist the study of aerosol loading distribution and the impact of transient pollution on urban air quality. In addition, the MODIS 500 m AOT images can also be used to study the cross-boundary aerosols and feasible on locating the pollutant sources in the Pearl River Delta (PRD) region.

Modeling of Aerosol Vertical Profiles Using GIS and Remote Sensing.

Anthropogenic heat is the heat flux generated by human activities and is a major contributor to the formation of an urban heat island. In a city such as Hong Kong, obtaining pure pixels from medium- or coarse-resolution remote sensing images is challenging. Considering the completely different thermal properties of vegetation and impervious surfaces, this letter developed a novel algorithm to estimate anthropogenic heat fluxes by decomposing image pixels into fractions of impervious surfaces and vegetation, and by estimating the total heat flux for the mixed pixel. The Chinese small satellite HJ-1B images with a spatial resolution of 30 and 300 m for visible and thermal wavebands, respectively, and the temporal resolution of four days were used for the heat flux modeling. Results show that anthropogenic heat fluxes in Hong Kong are correlated to the building density and the building height, with r2 = 0.92 and 0.58 on October 11, 2012 and r2 = 0.94 and 0.62 on January 13, 2013, respectively. The average anthropogenic heat fluxes in urban areas are 289.16 and 283.17 W/m2 on October 11, 2012 and on January 13, 2013, respectively, and the commercial areas emit the largest anthropogenic heat fluxes around 500-600 W/m2 compared with other land-use types. The derived anthropogenic heat fluxes can help in planning and environmental authorities to pinpoint “hot-spot” areas, and they can be used for compliance monitoring.

Retrieval of aerosol optical thickness using MODIS 500 × 500m 2 , a study in Hong Kong and Pearl River delta region

Possible source regions of the PM2.5 mass concentrations in Beijing, China have been identified using the Potential Source Contribution Function (PSCF) analysis based on the backward trajectory modeling. Five–day backward trajectories for the arrival of air masses to the ground observation station at Chaoyang Olympic Sports Center were calculated, using the Hybrid Single–Particle Lagrangian Integrated Trajectory (HYSPLIT) model in 2013. The PSCF results demonstrated that regional sources in different seasons could be one of the crucial contributors to PM2.5 mass concentrations in Beijing, especially in the winter season. Compared PSCF results with the MODIS fine–mode aerosol optical thickness (AOTfine), the regions showing high AOTfine especially the Southern Hebei, Northern Henan and Southwest Shandong are the significant potential PM2.5 contributors to Beijing. Environmental authorities may use the derived models and results for pinpointing the source areas, for improving air quality in Beijing City.

Modeling of Anthropogenic Heat Flux Using HJ-1B Chinese Small Satellite Image: A Study of Heterogeneous Urbanized Areas in Hong Kong

Hong Kong, located adjacent to the rapidly growing urban-industrial region of south China, provides a case of mixed aerosol types (urban, industrial, marine, and long-distance, including dust) from diverse activities and has suffered many serious air pollution episodes over the last decade. However, the sources and transport pathways of aerosols measured and recorded in Hong Kong have not been well researched due to the lack of air quality monitoring stations in east Asia. Here, an integrated method combining Aerosol Robotic Network (AERONET) data, backward trajectories, and Potential Source Contribution Function (PSCF) modelling is used to identify probable transport pathways and magnitudes of source contributions for four characteristic aerosol types. These types, which are dominant in Hong Kong during defined climatic and environmental conditions, are urban fine aerosols, urban mixed aerosols, dust, and heavy pollution. They were defined by clustering a total of 730 AERONET data sets between 2005 and 2008. Results show that aerosol types 1 and 2 (urban fine and urban mixed) are associated with regional fine particulate urban emissions and predominantly local urban emissions, respectively, suggesting that mitigating measures taken within Hong Kong itself would be partially effective. Heavy pollution and dust (types 4 and 3) are more associated with short- and long-distance sources, notably heavy industries in nearby southern Guangdong and the Pearl River Delta region, and desert dust from arid regions in north China. The PSCF map representing dust aerosol type shows a wide range of eastward and southeastward trajectories from northwest China to Hong Kong. Although the contribution of dust sources is small compared to anthropogenic aerosols, a serious recent dust outbreak observed in Hong Kong was associated with an elevation of the air pollution index to 500, compared with 50–100 on normal days. The combined use of clustered AERONET, backward trajectories, and PSCF model can help to resolve long-standing issues about source regions and characteristics of pollutants carried to Hong Kong.

Estimation of potential source regions of PM2.5 in Beijing using backward trajectories

Aerosol observations are essential for understanding the Earths radiation budget and the complexities of climate change, as they are involved in the backscattering of solar radiation and the formation of cloud condensation nuclei. In Hong Kong, the most direct effect is on air quality. Atmospheric haze caused by the emission of aerosols from industrial and vehicular sources creates visibility lower than 8 km for approximately 20% of the time, having risen at 6% per decade since 1980, but regional emissions are at least as influential as local ones. The 179,000 km2 covered by Hong Kong and neighbouring Guangdong Province cannot be adequately covered by the 76 monitoring stations set up by the two governments, and satellite images offer the only potential source of regional air quality data. However, the current satellite-based aerosol optical thickness (AOT) products are intended for global air quality monitoring, and may contain errors over a humid coastal city such as Hong Kong and its surrounding industrialized regions. This research compares the AOT retrieved from several AOT operational products, namely the Moderate Resolution Imaging Spectroradiometer (MODIS) MOD04 product, the MODIS 500 m product, the Multiangle Imaging Spectroradiometer (MISR) product, the Ozone Monitoring Instrument (OMI) multiwavelength aerosol product, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) product, with ground-based AOT from sunphotometers in Hong Kong. These sunphotometers include two AERONET stations, and are deployed in Hong Kong over urban, suburban, and coastal areas. The rigorous correlations, root mean square errors, and mean absolute differences available from the multilocational field data within one city region provide a strong base for validating the AOT products from different sensors and at different spatial scales over different land surface types. The results suggest that the AOT products, especially those from MODIS 10 km, provide reliable and accurate observations for daily air quality monitoring over a variety of land-cover types, as well as for identifying emission sources for coordinated actions by the governments of Hong Kong and the Chinese mainland.