Over the past several decades, the annual burned area in California's Sierra Nevada mountains has increased considerably, with significant social, economic, and ecosystem impacts that provide motivation for understanding how the history of forest management influences the composition of fuels and emissions in wildfires. Here, we measured the carbon concentration and radiocarbon abundance (∆¹⁴C) of fire-emitted particulate matter from the KNP Complex Fire, which burned through several groves of giant sequoia trees in the southern Sierra Nevada mountains during California’s 2021 wildfire season. Over a 26-hour sampling period, we measured the concentration of fine airborne particulate matter (PM_2.5) along with carbon monoxide (CO) and methane (CH₄) dry air mole fractions using a ground-based mobile laboratory. Filter samples of PM_2.5 were also collected and later analyzed for carbon concentration and ∆¹⁴C. Covariation of PM_2.5, CO, and CH₄ time series data confirmed that our PM_2.5 samples were representative of wildfire emissions. Using a Keeling plot approach, we estimated that the mean ∆¹⁴C of PM_2.5 was 111.5 ± 2.3‰ (n=12), which is considerably enriched relative to that of atmospheric carbon dioxide in the northern hemisphere in 2021 (-3.4 ± 1.4‰). By combining these ∆¹⁴C data with a steady-state one-box ecosystem model, we estimated that the mean age of fuels combusted in the KNP Complex Fire was 40 ± 6 years. This multi-decadal fuel age provides evidence for emissions from woody biomass, coarse woody debris, and larger-diameter fine fuels. The combustion of these larger-size fuel classes is consistent with independent field observations that indicate high fire intensity contributed to widespread giant sequoia mortality. With the expanded use of prescribed fires planned over the next decade in California to mitigate impacts of wildfires, our measurement approach has the potential to provide regionally-integrated estimates of the effectiveness of fuel treatment programs.

PM_2.5 concentrations were collected using a PurpleAir PA-II air quality sensor with laser particle counting (PurpleAir, Draper, UT, USA).

PM_2.5 filter samples were analyzed for total carbon (TC) concentration and ∆¹⁴C at the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry laboratory at the University of California, Irvine.

Carbon monoxide (CO) and methane (CH₄) dry air mole fractions were measured using a wavelength-scanned cavity ring-down spectrometer (G2401, Picarro, Santa Clara, CA, USA).