Jietai Mao

Global monitoring of air pollution over land from the Earth Observing System‐Terra Moderate Resolution Imaging Spectroradiometer (MODIS)

[1] Moderate Resolution Imaging Spectroradiometer (MODIS) measurements (7 channels: 0.47-2.1 μm, 250-500 m resolutions) provide us with new insights into the characteristics of global aerosols. MODIS retrieves not only aerosol loading but also the fraction of fine mode particle. In this paper we demonstrate MODIS capability for use in monitoring global, regional, and local air pollution. Three case studies in northern Italy, Los Angeles, and Beijing showed the conclusive results of applying MODIS-derived aerosol optical depths (T a ) to regional and local air pollution in terms of accuracy (ΔT a = ±0.05 ± 0.2τ a ) and spatial sensitivity of the retrievals. Under stagnant condition, accumulated aerosol abundance can reach T a > 1 (at 0.55 μm) before being removed by wind or precipitation. The correlation found between Aerosol Robotic Network (AERONET) daily averaged T a and 24-hour PM 10 (particulate matter with diameter <10 μm) concentration (μg/m 3 ) in northern Italy is encouraging with correlation coefficient ∼0.82. The derivation of PM concentration from satellite measurements may be possible once we know the detailed aerosol vertical distribution. To compare aerosol loading in different regions of the globe, we choose the two most populated regions (eastern China and India) and the two most industrialized regions (the eastern United States/Canada and western Europe). The time series of MODIS monthly mean T a from July 2000 to May 2001 depicts a strong seasonal variation with maxima in the spring/ summer and minima in the winter. The clear separation between (1) the eastern United States/Canada and western Europe and (2) eastern China and India shows that the T a values in (2) are 50% to 2-3 times higher compared to those in (1). The enhancements of aerosol loading were due to smoke as originated from Montana/Idaho forest fires transported to the eastern United States in late August 2000 and dust outbreaks from Taklimakan and Gobi Deserts to eastern China as well as smoke from Southeast Asia to southern China in February-April 2001.

Retrieval, validation, and application of the 1-km aerosol optical depth from MODIS measurements over Hong Kong

The Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol retrieval algorithm was developed to derive aerosol properties at a global scale, suitable for climate studies. Under favorable conditions (clear sky and over dark surfaces), the standard 10/spl times/10 km MODIS aerosol products are also useful on regional scales to monitor aerosol distributions and transports. However, the 10-km resolution is insufficient to depict aerosol variation on local or urban scales, due to inherent aerosol variability as well as complex surface terrain. In this study, we have modified the MODIS algorithm to retrieve aerosol optical depth (AOD) at 1-km resolution over Hong Kong, a city of just over 1000 km/sup 2/ with very complex surface features. Accompanied by the increased spatial resolution are new aerosol models derived with single-scattering albedo (SSA) around 0.91-0.94 to accommodate higher aerosol absorption encountered in Hong Kong than that was presumed for MODIS standard products (SSA/spl sim/0.97) over the region. The derived AOD data are compared to handheld Microtops II sunphotometer observations at the Hong Kong University of Science and Technology and other locations across Hong Kong. Retrieval errors within 15% to 20% of sunphotometer measurements are found. Moreover, when compared with the standard 10-km AOD products, the 1-km AOD data are much better correlated with PM/sub 10/ measurements across Hong Kong, suggesting that the new 1-km AOD data can be used to better characterize the particulate matter distribution for cities like Hong Kong than the MODIS standard products.

Localized 3D-structural features of dynamic-chemical processes of urban air pollution in Beijing winter

The Beijing City Air Pollution Observation Field Experiment (BECAPEX) is described with emphases on the “point-surface” research approach and composite analysis. The analysis results of measurements from four observation sites across the Beijing urban area from January to March indicate that the overall impact of urban emission sources in the heating season is significant, and the staggered impact of urban emission sources has different features at observation sites over different parts of Beijing in both heating and non-heating seasons. The pollutants NOx, SO2 and CO in the urban boundary layer have the in-phase variation features over a large area. O3 concentrations at different sites have the same variation trend but its change is reversed phases with above pollutants. The pollutants over the urban area in heating and non-heating seasons also have the synchronous variation trend. The comprehensive sounding of BECAPEX indicates that pollutants and aerosol vertical profiles are closely correlated to the vertical structure of the large-scale inversion layer in the urban boundary layer over the urban area. The localized 3D-structural features of local urban polluting processes associated with the peripheral areas are discussed with a “point-surface” comprehensive sounding technique.

Study on variational aerosol fields over Beijing and its adjoining areas derived from Terra-MODIS and ground sunphotometer observation

This paper presents a comprehensive observation technique on derived aerosols data from mobile sunphotometer graph and Terra-Moderate Resolution Imaging Spectroradiometer (MODIS) observation. Research results suggest that after being treated by the variational technique with respect to sunphotometer observations, the Terra-MODIS remote sensing aerosol data are remarkably improved, thus for the first time revealing features of the influence of aerosols and pollution emissions of Beijing and its adjoining areas (Hebei, Shandong, etc. provinces). The regional impact features of aerosols are related with the peripheral U-shape topography of Beijing. Analyses with Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT-4) and meteorological data in the case studied confirm the pollutants diffusion process along the trajectory from the sources in the south-west region, and the regional aerosol impact features.

Remote sensing precipitable water with GPS

The principle and method of remote sensing precipitable water (PW) by the Global Positioning System (GPS) are introduced. A series of tropospheric delay have been obtained from the dual frequency receiver’s data of global tracking stations in East Asia and precise satelllte ephemeris of IGS from July 31 to August 20, 1997. The continuous PW at the interval of 30 min have been estimated at Shanghai and Wuhan GPS sites. The results are compared with PW from conventional radiosonde during the same period, and the RMS is about 0.50 cm.

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

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.

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

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.

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

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.

Remote sensing of atmospheric water vapor in the region of southwest China using GPS

GPS technology provides a new powerful approach for measuring atmospheric water vapor. In this paper the experiment of observing integrated atmospheric water vapor or the precipitable water (PW) using ground-based GPS receivers performed in southwestern region of China is presented. Zenith total delay (ZPD) data are collected in a GPS network consisting of 10 stations. Time series ofwater vapor in one hour bins for 10 months is retrieved. The results of 3 stations in southwestern region affected by many monsoons show that the water vapor has large variability in spatial and temporal scales. The water vapor derived from GPS is compared with retrievals from the radiosondes. The GPS PW agrees well with data from radiosondes which indicates GPS PW are correct at this locations. The difference of PW from them is larger when the humidity is higher and more variable in summer. The results of comparison with the NCEP reanalysis PW data show that they are consistent with each other. In the mountain areas where observations are few PW from GPS can be used to validate the PW from numerical model analyses. Finally long-term monitoring water vapor using ground-based GPS is discussing.

Remote sensing of atmospheric aerosol over China

For better understanding the distribution of atmospheric aerosol over China and its impact on climate, remote sensing aerosol optical depth (AOD) from satellite over 25 lakes has been done in China. The distribution of aerosol over the whole country is interpolated from these data. Four Sun-photometers also have been used to measure the data of AOD nearby four of these lakes for validating the remote sensing algorithm of satellite. The result shows that the southeast region of China has the biggest AOD over four seasons of the year and the value of AOD has slight variety between different seasons; the northwest area of China has a maximum in spring, but in other seasons, the value of AOD is low. This result fit to the fact that spring is the dry season in northwest of China and dust storm often occurs in this area, other season seldom has dust storm. The high value of AOD in the southeast China is coincident with the intensive industry, agriculture and other human activities in this region.