Towards the top of the image, smoke can be seen from the Little Giant Fire in the Okanogan-Wenatchee National Forest in Washington. Smoke plumes are also visible from other fires along the Cascade mountains. The impact of the fires on air quality can be seen in the aerosol forecast charts produced by the Copernicus Atmosphere Monitoring Service (CAMS). They can originate from desert dust, volcanic eruptions and wildfires. The aerosol optical depth (AOD) is a measure of the total amount of aerosol in a vertical column of the atmosphere.