The World Health Organization calls air pollution the invisible killer, as it can be difficult to trace yet is responsible for 36% of lung cancer, 35% of pulmonary disease, and 27% of heart disease fatalities each year.
To better track air pollution, scientists use a variety of measurements to quantify the problem. One of these, aerosol optical depth, is the measure of how much sunlight is blocked from reaching Earth’s surface by particles in the atmosphere, such as dust and smoke, which absorb and scatter light.
With the advent of satellites, aerosol optical depth can be determined over both land and sea with sufficient accuracy for use in a broad range of scientific applications, from climate studies to air quality, among others. Modern satellite sensors fly hundreds of miles above Earth’s surface, scanning vast regions as they pass by, viewing the whole world every day or so. These frequent updates contribute to a growing record of data from multiple sensors that have been in orbit for years. The first advanced very high resolution radiometer (AVHRR) was launched by the National Oceanic and Atmospheric Administration in 1978, for example, and versions of the instrument have flown on 16 satellites in total and continue to the present day.
Scientists use computer algorithms to process aerosol optical depth data: For example, the Deep Blue algorithm has been used to process data collected over land, and the Satellite Ocean Aerosol Retrieval (SOAR) algorithm has been used for over-sea data. Recently, Hsu et al. and Sayer et al. applied a combined version of Deep Blue and SOAR to the nearly 40-year data record from AVHRR. Many past studies have used AVHRR data, but those focused on the ocean, whereas these two studies cover both land and sea.
In addition to creating this new, two-pronged algorithm, the researchers cross-checked it by comparing the results to data collected on board ships, via satellite, and from ground-based instrumentation, such as AERONET, NASA’s network of Sun-sky scanning sensors. Their data and user guide are both available online.
Not only does this study extend existing Deep Blue and SOAR data sets, but it lays the groundwork for a long-term, continuous data record of aerosol optical depth. Such a record would be invaluable for future climate studies and to help improve environmental and public health. (Journal of Geophysical Research: Atmospheres, https://doi.org/10.1002/2017JD026932, 2017 and https://doi.org/10.1002/2017JD026934, 2017)
—Sarah Witman, Freelance Writer