Formic acid (HCOOH) is a prevalent organic acid that influences rain acidity and aerosol properties. However, models largely underestimate its atmospheric abundance, particularly in biomass burning plumes. Here, we use the Infrared Atmospheric Sounding Interferometer (IASI) to track HCOOH evolution in ageing South American biomass burning plumes during the exceptionally active 2024 fire season. By integrating IASI retrievals of total column HCOOH (TCHCOOH) and carbon monoxide (TCCO) with atmospheric trajectory modelling, we analyse HCOOH changes over the rarely studied week-long period. We particularly focus on the free troposphere, consistent with satellite sensitivity and reported transport altitudes. We find that HCOOH formation is most pronounced between the day of emission (Day 0) and the following day (Day 1), with the TCHCOOH:TCCO ratio increasing by ∼5.7 ± 1.0 % day^−1. Between Day 2 and Day 6, the TCHCOOH:TCCO ratio decays at ∼4.8 ± 0.8 % day^−1 as HCOOH is lost, then stabilises by Day 8. Box modelling based on the UK Chemistry and Aerosol model, initialised in the free troposphere (850 hPa) using in-plume observations, captures the increase in HCOOH:CO on the first day (4.4 % day^−1) but slightly underestimates it. The model identifies hydroxymethyl-methyl-α-lactone (HMML, an isoprene oxidation product), ethene, and acetylene as key HCOOH precursors. HMML might be influenced by vegetation-derived emissions. After Day 1, the simulated HCOOH:CO ratio declines smoothly (2.7 % day^−1), dominated by OH oxidation. This work demonstrates a novel approach that combines different observations and methods to investigate atmospheric chemistry in biomass burning plumes over week-long timescales.