Using Satellite-based Isoprene and Formaldehyde Measurements to Assess Atmospheric Oxidation Over Biogenic Source Regions

Isoprene oxidation by OH is a major formaldehyde (HCHO) source in the continental boundary layer. The fact that both species are short-lived, with HCHO removed by photolysis as well as by OH, gives rise to a tight, near-linear correlation between the isoprene:HCHO column ratio and 1/[OH] (or the isoprene lifetime) over isoprene-dominant source regions. This ratio can thus be used to test understanding of atmospheric oxidation over such areas. In this work, we use satellite-based measurements from CrIS and OMPS onboard Suomi-NPP to characterize isoprene:HCHO column variability across key source regions and from 2012-present. With a satellite overpass at 13:30 local time, the daily CrIS and OMPS observations are obtained during peak isoprene emission and photochemistry. We use a chemical transport model (GEOS-Chem v13.3) to interpret the space-based results in terms of implications for atmospheric oxidation and its variability over isoprene-emitting ecosystems. Analyses over Amazonia and the southern USA show that the sensitivity of the isoprene:HCHO ratio to OH depends on the biome, season, and year. Additionally, the model predicts a decreasing OH trend from 2012-present over the southern USA, but the satellite-measured ratio shows no change in oxidation capacity over this time. Overall, the isoprene:HCHO column ratio provides a framework to assess how the biosphere is influencing and changing the atmosphere's oxidizing capacity over biogenic source regions across time and space.