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Aerosol observations are essential for understanding the Earth's 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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]