References:

Wang, J., and S.A. Christopher, Intercomparison between satellite-derived aerosol optical thickness and PM2.5 mass: Implication for air quality studies, Geophys. Res. Lett., 108 (21), doi:10.1029/2003GL018174, 2003. (pdf file)

Christopher, S. A. and J. Wang, 2003: Exploring the Potential of Satellite Data for Air Quality Applications, Eos Trans. AGU, 84(46), Fall Meet. Suppl., Abstract A11E-0033, 2003. (pdf file, download first and then open it)

1. Introduction

Particular matters, or aerosols, reduce visibility, affect human health, and also cause several ecological effects. As defined by Environment Protection Agency (EPA), the dry mass content of particular matter with aerodynamic diameter less than 2.5 µm (PM2.5) in the atmosphere is an important parameter for the evaluation of air quality. However, the large spatiotemporal variations of particular matter make it a challenge to judge the air quality and issue prompt health alert from the current ground-based measurement network, especially when the aerosol events come from sources outside the U.S. The launch of EOS TERRA and AQUA satellite provides an unprecedented opportunity to monitor the air pollution over the globe. The intent of this study is to explore the potential of satellite aerosol datasets for air quality applications.

2. Hypothesis and Methodology

Aerosols with diameters around 1 ~2µm are efficient in scattering the visible light. During MODIS passing time (locally, 10:30AM for TERRA and 1:30 for AQUA) in clear sky conditions, the atmospheric boundary layer is well mixed. Hence, the MODIS visible reflectance and its column aerosol optical thickness (AOT) retrievals can be used as indicators of the PM2.5 mass at the surface. In this study, we compared MODIS AOT with the ground-based PM2.5 hourly measurements.  For each comparison, MODIS AOT time is centered around the PM2.5 observation time period. The final goal of this comparison is to evaluate the quality of MODIS AOTs in the context of air quality applications before they are assimilated into the air quality models.  This is important because evaluation of data quality is a critical step in the data assimilation processes. 

3. Data and Study Area

  • MODIS AOT from TERRA and AQUA, 2002.
  • PM2.5 measured from Tapered-Element Oscillating Microbalance (TEOM) in Alabama and Texas.
  • Sunphotometer data in Stennis, MS.
  • EPA PM2.5 analysis and extinction analysis from IMRPOVE measurements.

4. A Case Demo. (more cases and analysis can be found in our paper and poster)

The following figures show a heavy haze event identified by the spatial distribution of MODIS AOT. Also shown is the linearly derived Air Quality Index (AQI) and the 700mb geopotential heights. Grey regions are areas where MODIS AOT is not available due to possible sun glint or cloud contamination.

5. Conclusions

  • Using one year of the MODIS AOT from the TERRA/AQUA satellites collocated with hourly particular matter mass measured at about 40 ground stations over Alabama and Texas, we show that the MODIS AOT has a good positive correlation with PM2.5 mass (linear coefficient around 0.7).
  • Through statistical analysis, the MODIS AOT product can be used to discern air quality categories such as good, moderate and unhealthy to a relatively high degree of confidence. However we would like to outline several factors that could affect the relationship between PM2.5 and satellite-derived AOT. These factors include vertical distribution, optical and hydroscopic properties of aerosols.
  • Aerosol extinction profile from ground based lidars or from satellite measurements such as CALIPSO are highly important for further enhancing the use of satellite data for air quality studies. This study implies that assimilation of MODIS AOT has the potential to improve the air quality forecasts.
     
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