Global estimates of ambient fine particulate matter concentrations from satellite-based aerosol optical depth: development and application

Abstract

Background: Epidemiologic and health impact studies of fine particulate matter with diameter < 2.5 microm (PM2.5) are limited by the lack of monitoring data, especially in developing countries. Satellite observations offer valuable global information about PM2.5 concentrations.

Objective: In this study, we developed a technique for estimating surface PM2.5 concentrations from satellite observations.

Methods: We mapped global ground-level PM2.5 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) PM2.5 concentrations at approximately 10 km x 10 km resolution indicate a global population-weighted geometric mean PM2.5 concentration of 20 microg/m3. The World Health Organization Air Quality PM2.5 Interim Target-1 (35 microg/m3 annual average) is exceeded over central and eastern Asia for 38% and for 50% of the population, respectively. Annual mean PM2.5 concentrations exceed 80 microg/m3 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 PM2.5 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 microg/m3.

Conclusions: Satellite-derived total-column AOD, when combined with a chemical transport model, provides estimates of global long-term average PM2.5 concentrations.

Figure 1

Mean AOD for 2001–2006 from the MODIS (A) and MISR (B) satellite instruments. (C) Data from the combined product developed here. White space denotes water or < 50 measurements.

Figure 2

Annual mean η (ratio of PM_2.5 to AOD) for 35% relative humidity. White space indicates water.

Figure 3

Satellite-derived PM_2.5 and comparison with surface measurements. (A) Mean satellite-derived PM_2.5 between 2001 and 2006; white space denotes water or < 50 AOD measurements. (B) Average coincident values of both measured and satellite-estimated PM_2.5. The solid black line denotes unity; thin dotted lines show uncertainty of ± (1 μg/m³ + 15%); and the dashed line represents the best fit (Hirsh and Gilroy 1984). (C) Positions and mean values of coincidently measured surface sites.

Figure 4

Global satellite-derived PM_2.5 averaged over 2001–2006. White space indicates water or locations containing < 50 measurements. Circles correspond to values and locations of comparison sites outside Canada and the United States; the black box outlines European sites.

Figure 5

Regional satellite-derived PM_2.5 concentrations. Columns show mean satellite-derived PM_2.5 for 2001–2006 at locations that contain at least 50 measurements. Contours denote population density (left) and surface elevation (right). Crosses indicate city centers. Note the different color scales for each region. Altitude data are from the U.S. Geological Survey (1996).

Figure 6

Estimate of the satellite-derived PM_2.5 bias, defined as (satellite-derived PM_2.5 – truth) ÷ truth. Boxed areas outline the regions used in Figure 8; see Supplemental Material (doi:10.1289/ehp.0901623) for the two subregions.

Figure 7

Satellite-derived PM_2.5 sampling and its estimated induced uncertainty. (A) Total number of values used from satellite per 0.1° grid box. (B) Percentage change in average coincidently sampled simulated PM_2.5 concentrations relative to a full-year average.

Figure 8

Cumulative distribution of regional, annual mean PM_2.5 estimated from satellite-derived PM_2.5 at a resolution of 0.1° × 0.1° for 2001–2006. The top axis identifies WHO AQG and Interim Target (IT) associated with each concentration level. Regions are outlined in Figure 6.