Global Estimates of Ambient Fine Particulate Matter Concentrations from Satellite-Based Aerosol Optical Depth: Development and Application

Article excerpt

BACKGROUND: Epidemiologic and health impact studies of fine particulate matter with diameter< 2.5 [micro]m [PM.sub.2.5] are limited by the lack of monitoring data, especially in developing countries. Satellite observations offer valuable global information about [PM.sub.2.5] concentrations.

OBJECTIVE: In this study, we developed a technique for estimating surface [PM.sub.2.5] concentrations from satellite observations.

METHODS: We mapped global ground-level [PM.sub.2.5] concentrations using total column aerosol optical depth (AOD) from the MODIS (Moderate Resolution Imaging Spectroradiometer) and MISR (Multiangle Imaging Spectroradiometer) satellite instruments and coincident aerosol vertical profiles from the GEOS-Chem global chemical transport model.

RESULTS: We determined that global estimates of long-term average (1 January 2001 to 31 December 2006) [PM.sub.2.5] concentrations at approximately 10 km X 10 km resolution indicate a global population-weighted geometric mean [PM.sub.2.5] concentration of 20 [micro]g/[m.sup.3] The World Health Organization Air Quality [PM.sub.2.5] Interim Target-1 (35 [micro]g/[m.sup.3] annual average) is exceeded over central and eastern Asia for 38% and for 50% of the population, respectively. Annual mean [PM.sub.2.5] concentrations exceed 80 [micro]g/[m.sup.3] over eastern China. Our evaluation of the satellite-derived estimate with ground-based in situ measurements indicates significant spatial agreement with North American measurements (r = 0.77; slope = 1.07; n= 1057) and with noncoincident measurements elsewhere (r = 0.83; slope = 0.86; n = 244). The 1 SD of uncertainty in the satellite-derived [PM.sub.2.5] is 25%, which is inferred from the AOD retrieval and from aerosol vertical profile errors and sampling. The global population-weighted mean uncertainty is 6.7 [micro]g/[m.sup.3].

CONCLUSIONS: Satellite-derived total-column AOD, when combined with a chemical transport model, provides estimates of global long-term average [PM.sub.2.5] concentrations.

KEY WORDS: aerosol, aerosol optical depth, AOD, particulate matter, [PM.sub.2.5]. Environ Health Perspect 118:847-855 (2010). doi:10.1289/ehp.0901623 [Online 16 March 2010]

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Chronic exposure to airborne fine particulate matter with diameter < 2.5 [micro]m [PM.sub.2.5] is associated with adverse human health impacts including morbidity and mortality (e.g., Dockery et al. 1993; McDonnell et al. 2000; Pope et al. 2009). Several national environmental agencies in North America and Europe monitor [PM.sub.2.5] concentrations at numerous sites throughout their jurisdictions, but even these relatively dense networks have limited geographic coverage. Few long-term measurement sites exist elsewhere in the world, particularly in rapidly developing countries where concentrations and estimated health impacts are greatest (Cohen et al. 2004). Point measurements collected at monitoring sites are not necessarily representative of regional concentration, and regional variability is difficult to assess from point measurements alone. In recent years, application of satellite observation to surface air quality has advanced considerably (Hoff and Christopher 2009; Martin 2008). In fact, global aerosol observations from satellite remote sensing could substantially improve estimates of population exposure to [PM.sub.2.5].

Since the mid 2000s, the MODIS (Moderate Resolution Imaging Spectroradiometer) and MISR (Multiangle Imaging Spectroradiometer) instruments onboard the National Aeronautics and Space Administration's (NASA) Terra satellite has provided global observations of aerosol optical depth (AOD), a measure of light extinction by aerosol in the atmospheric column above the earth's surface. Terra's sun-synchronous orbit encircles the earth approximately 15 times each day, with each pass crossing the equator at approximately 1030 hours local solar time. Observations of AOD from Terra provide daily insight into the global distribution of column-integrated aerosol. …