Prof. Alexis Kai-Hon Lau Profile

Prof. Alexis Kai-Hon Lau

Associate Director at HKUST

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Publications (4)

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Proceedings Article | 15 September 2004 Paper

Validation of MODIS AOD products with 1-km resolution and their application in the study of urban air pollution in Hong Kong

Chengcai Li, Alexis Lau, Jietai Mao

Proceedings Volume 5547, (2004) https://doi.org/10.1117/12.561404

KEYWORDS: Aerosols, Satellites, MODIS, Air contamination, Clouds, Reflectivity, Atmospheric modeling, Remote sensing, Atmospheric particles, Pollution

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Remote sensing products of aerosol from MODIS have been released by NASA for a long time, and the Level 2 products showed to be very useful in monitoring regional aerosol pollution pattern and tracing pollutant transports. However, for a city, with an area of several hundreds square-kilometers and complicated terrain, the products with 10 km resolution are not enough especially in depicting the detail particulate matter (PM) distribution in the urban area. Recently, the aerosol optical depth (AOD) retrieval method proposed by NASA has been used to obtain the 1km resolution products in Hong Kong. Aerosol model was improved by sun-photometer observations and the calculation of a radiation transfer model, and finally looking up tables were created for real time aerosol products retrieval. The AOD products were validated by the sun-photometer observations at the HKUST. It was found the relative bias between the satellite products and the ground observations was within the range of about 20%, which was mostly equal to the estimation of NASA for their 10km level 2 products in most AERONET sites. The good results are mainly because of the most regions of Hong Kong are covered by dense dark vegetations (DDV) with very low surface reflectance in visual and near-infrared satellite channels. This high-resolution product was used in the study of air pollution in Hong Kong, and it was found the 1km products were more useful to describe the local urban PM pollutant distribution than the 10 km Level 2 products.

Proceedings Article | 15 September 2004 Paper

Study on particulate matter air pollution in Beijing with MODIS aerosol level 2 products

Jietai Mao, Chengcai Li, Alexis Lau

Proceedings Volume 5547, (2004) https://doi.org/10.1117/12.561426

KEYWORDS: Aerosols, MODIS, Air contamination, Satellites, Pollution, Clouds, Atmospheric particles, Remote sensing, Roads, Reflectivity

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In the run-up to the 2008 Olympic Games in Beijing, Chinese government officials at both the central and municipal levels are keenly aware that they must transform Beijing into a world-class city. According to the Beijing Municipal Environmental Protection Bureau (BJEPB) to improve its air quality some actions are adopting, including taking steps to increase the forested area surrounding the city preventing dust storms, reducing the automotive vehicles, moving polluting factories now inside the fourth ring road ringing the inner city to locations outside of the fourth ring road, and switching the fuel of public buses and taxis from diesel to natural gas, etc. Will they eliminate most serious environmental problems in Beijing? MODIS aerosol products are helping us to answer this kind of questions. A long-term validation has been finished by sun-photometer observations, and the results proved the relative error of MODIS level 2 products was slightly larger than the estimation of Chu et al. (2002) from the results in most AERONET sites. However, the comparison between the products and moisture-corrected air pollution index (API) data, which were daily released to public by EPB, showed a high correlation coefficient. An air pollution episode in 2003 was investigated by the usage of satellite products. Our conclusion for the air pollution control strategy in Beijing is that only reducing the pollution sources from inner city can't fully solve the pollution problems in Beijing and the regional transports from the nearby southern provinces are contributing a lot to the pollution situation in Beijing.

Proceedings Article | 9 April 2003 Paper

Research on air pollution in Beijing and its surroundings with MODIS aerosol products

Chengcai Li, Jietai Mao, Kai-Hon Lau

Proceedings Volume 4891, (2003) https://doi.org/10.1117/12.466349

KEYWORDS: Aerosols, MODIS, Air contamination, Visibility, Remote sensing, Atmospheric particles, Visualization, Meteorology, Satellites, Combustion

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In this paper the MODIS Level2 aerosol products over Beijing are validated by comparison with the observations from sun-photometer at Peking University (PKU) at first. The MODIS aerosol optical depths (AOD) also correlates well with the averaged mass concentration of respirable suspended particulate (RSP, PM10) calculated from released API data. The relations between the visibilities and the AOD values with distinct aerosol scale heights in different seasons are estimated. The cases described by the AOD distributions over Beijing and its surroundings show some dramatic processes. The analysis assisted by the relevant changes of meteorological variables can help us find the corresponding answer to each air pollution episodes. The seasonal AOD variations in Beijing show the mean AOD value is highest in summer, and has a decreasing trend from summer to autumn, and then to winter. It has a rapid increasing from winter to spring due to spring dust in North China. Finally we gain the seasonal mean visibility distributions of Beijing and its surroundings. The mean air quality gets worst in winter due to increased pollutant source in winter and bad dispersion conditions, and becomes the best in spring because strong wind provides fast dispersion in spite of occasional dust weathers. The results indicate the air pollution in Beijing urban area is both contributed by local urban emission and regional transport. Remote sensing from space has provided us a new view to study the air pollution.

Proceedings Article | 9 April 2003 Paper

A new way of using MODIS data to study air pollution over Hong Kong and the Pearl River Delta

Kai-Hon Lau, Chengcai Li, Jietai Mao, Jay-Chung Chen

Proceedings Volume 4891, (2003) https://doi.org/10.1117/12.466766

KEYWORDS: Aerosols, MODIS, Air contamination, Satellites, Pollution, Remote sensing, Spatial resolution, Atmospheric particles, Polonium, Quality measurement

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Aerosols are the main air pollutant in Asia. In this paper, the MODIS level 2 aerosol optical depth (AOD) products derived by NASA were validated with in situ sun-photometer observations over Hong Kong (HK). The MODIS AOD values were correlated with mass concentrations of respirable suspended particulates (RSP) measured at air quality monitoring stations over HK and Macau. Correlation between RSP and AOD were found to be statistically significant, suggesting that the satellite data is very useful for aerosol-related air pollution studies. Compared with concentrations measured from ground-based air quality monitoring networks, the AOD data cover a much larger area and have much better spatial resolution. Combining with meteorological information, the AOD data also proved to be very useful for the understanding of RSP variations at air quality monitoring stations. An example of using AOD data to help understand a pollution event over the PRD will be presented. Finally, monthly-mean distributions of AOD over Eastern China showed a distinct local maximum over the PRD, separated from high AOD areas to the north, suggesting that the aerosol problem over the PRD are mostly regional. Remote-sensing from space has provided a new and powerful way to study air pollution. To fully utilize this technique for air quality studies, the combination of a lidar and an X-band satellite receiver (for the MODIS data) is recommended. The AOD fields are vertically integrated products, together with the vertical profiles of extinction coefficients provided by a lidar, the surface distribution of aerosol could be derived.

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