Nitrous Acid (HONO) Retrievals from wildfire events by using Geostationary Environment Monitoring Spectrometer (GEMS) ultraviolet spectra

Nitrous acid (HONO) is known to be the significant source of hydroxyl radicals (OH), impacting air quality and climate as a major oxidant in the atmosphere. Many studies have highlighted that the photolysis of HONO can produce substantial amounts of OH throughout the day. Despite the crucial role of HONO in tropospheric chemistry, more research is needed to improve understanding of global HONO budgets. To address this, we developed a prototype HONO retrieval algorithm from the Geostationary Environment Monitoring Spectrometer (GEMS). The retrieval algorithm comprises two major processes, commencing with the spectral fitting of UV spectral range (343-371 nm) using the direct fitting method to obtain the slant columns. Subsequently, the conversion of slant columns into vertical columns is achieved by applying the air mass factor. The last step involves background correction, wherein the slant column amounts of HONO included in the radiance reference spectrum are added to the differential slant columns. Enhancements of HONO resulting from wildfire events in Asia were detected using GEMS. Refining the GEMS HONO retrieval algorithm is expected to enhance our understanding of the diurnal cycle of HONO, along with tropospheric chemistry in Asia.