In 2021, a large number of fires occurred in Siberia. Smoke plumes were frequently transported over large distances from the fire sources, because of high injection in the atmosphere.
These are particularly favorable conditions to detect HONO with the IASI instruments. The image shows IASI/ MetOp-A, -B, and -C observations of HONO vertical columns (up to 2.5 x 1016 molec. cm-2) over the Siberian fires on August 2, 2021, overlaid on the visible image from the MODIS/Aqua sensor. The white-to-red dots indicate the fire radiative power values detected by MODIS/Aqua. These IASI measurements are taken in the evening. The absence of sunlight significantly increases the lifetime of HONO. This explains why HONO is detecting at several 100s of kilometres from the fires, likely combined with secondary formation along the transport.