English: Aerosol particles of natural origin (such as windblown dust) tend to have a larger radius than human-produced aerosols such as particle pollution. These false-color maps show where there are natural aerosols, human pollution, or a mixture of both on a monthly basis. The maps are based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Green areas show aerosol plumes dominated by larger particles. Red areas show aerosol plumes dominated by small particles. Yellow areas show plumes in which large and small aerosol particles are intermingling. Gray shows where the sensor did not collect data. Among the most obvious patterns the time series illustrates is that in the planet’s most southerly latitudes, nearly all the aerosols are large, while in the high northern latitudes, smaller aerosols are very abundant. Most of the Southern Hemisphere is covered by ocean, which means the largest source of aerosols is natural sea salts. Because land is concentrated in the Northern Hemisphere, the amount of small aerosols from fires and human activities is greater there than in the Southern Hemisphere. Over land, patches of large-radius aerosols appear over deserts and arid regions, most prominently, the Sahara Desert in northern Africa and the Arabian Peninsula, where dust storms are common. Meanwhile, places where human-triggered or natural fire activity is common (land-clearing fires in the Amazon from August-October, for example, or lightning-triggered fires in the forests of northern Canada in Northern Hemisphere summer) are dominated by smaller aerosols. Human-produced (fossil fuel) pollution is largely responsible for the areas of small aerosols over developed areas such as the eastern United States and Europe, especially in their summer.