Size hardly features in discussions about suspended particulate matter in the air—and rarely do we enquire about the size of the particle. But this is one place where size does matter—as the most prominent cause of premature deaths due to air pollution in developing countries is PM 2.5. Most data however pertains to PM10 as the absence of surface-based air pollution sensors in many developing countries, makes it difficult and in some cases impossible, to get even a rough estimate of the abundance of a subcategory of airborne particles that epidemiologists suspect contributes to millions of premature deaths each year. The problematic particles, called fine particulate matter (PM2.5), are 2.5 micrometers or less in diameter, about a tenth the fraction of human hair. These small particles can get past the body’s normal defenses and penetrate deep into the lungs. To fill in these gaps in surface-based PM 2.5 measurements, experts look toward satellites to provide a global perspective. Yet, satellite instruments have generally struggled to achieve accurate measurements of the particles in near-surface air. The problem is that most satellite instruments can’t distinguish between particles close to the ground and those high in the atmosphere. In addition, clouds tend to obscure the view. And bright land surfaces, such as snow, desert sand and those found in certain urban areas can mar measurements.
NASA’s Socioeconomic Data and Applications Center (SEDAC) Global Annual PM2.5 Grids from Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-Angle Imaging Spectroradiometer – Aerosol Optical Depth (MISR- AOD) data sets represent a series of annual average grids (2001-2010) of fine particulate matter (solid particles and liquid droplets) that were derived from MODIS and MISR -AOD satellite data (Fig 1). Together the grids provide a continuous surface of concentrations in micrograms per cubic meter of particulate matter of 2.5 micrometers or smaller (PM2.5) for health and environmental research. The satellite AOD retrievals were converted to ground-level concentrations based on a conversion factor developed by researchers at Dalhousie University, Canada, that accounts for spatial and temporal variations in aerosol properties and vertical structure as derived from a global 3-D chemical transport model (GEOS-Chem).This is the most comprehensive view of the health-sapping particles to date.
Though the new blending technique has not necessarily produced more accurate pollution measurements over developed regions that have well-established surface-based monitoring networks, it has provided the first PM2.5 satellite estimates in a number of developing countries that have had no estimates of air pollution levels until now.
The map shows Global Annual PM2.5 Grids which provide annual ‘snap shots’ of particulate matter 2.5 micrometers or smaller in diameter from 2001-2010. Exposure to fine particles is associated with premature deaths as well as increased morbidity from respiratory and cardiovascular disease, especially in the elderly, young children. The grids were derived from Moderate-Resolution Imaging Spectro Radiometer (MODIS) and Multi-Angle Imaging SpectroRadiometer (MISR) Aerosol Optical Depth( AOD) data. The raster grid cell size is approximately 50 sq km at the equator and the extent is from 70oN TO 60oS latitude. When compared with maps of population density, it suggests more than 80 per cent of the population breathe polluted air that exceeds the World Health Organization’s recommended level of 10 micrograms per cubic meter.