Tracing Photochemical Aging in Biomass Burning Aerosols Using Stable Carbon Isotopes

Carbonaceous aerosols from biomass burning emissions are major contributors to atmospheric particulate matter and play a critical role in air quality, climate, and human health. Stable carbon isotopic composition (δ¹³C) provides a powerful tool for identifying emission sources and evaluating the influence of atmospheric processing on source signatures. This study applies δ¹³C analysis of three-step OC to assess the impact of photochemical aging on biomass burning aerosol isotopic characteristics.

Aerosol samples (PM₁) from the combustion of twenty different biomass fuels were collected during the biomass burning experiment. Of the twenty biomass burning samples, six were selected for photochemical aging experiments based on their organic carbon mass, which exceeded the minimum detection and precision requirements for δ¹³C analysis across all three thermal OC fractions. Samples with lower carbon mass were excluded from aging to avoid increased analytical uncertainty associated with low signal-to-noise ratios. The isotopic composition of total carbon and organic carbon before and after aging experiment will be presented.

The isotopic composition of aerosol particles produced during uncontrolled combustion exhibit a broad distribution across biomass species. The average δ¹³C_TC of PM₁ of hardwood emissions is –26.9 ± 1.3 ‰and PM₁ from softwood burning is –25.2 ± 0.1 ‰. The provided dataset also reveals distinct patterns in isotopic fractionation and carbon emissions across different biomass fuels under controlled combustion conditions. The observed fractionation factor ε (‰) varies significantly among different biomass burning species. The average fractionation factor for all biomass species is 0.0 ±1.0 ‰  The measured δ¹³Coc of aged samples indicates isotopic fractionation of organic carbon induced by photochemical aging. OH-aged samples showed no significant isotopic shifts relative to un-aged samples, although minor variations in total carbon mass were observed at higher temperature fractions, likely related to filter loading heterogeneity. In contrast, UV-aged samples exhibited systematic depletion in ¹³C across three temperature steps (200 °C, 350 °C, and 650 °C). The 350 °C fraction generally displayed the highest δ¹³C values among UV-aged samples, indicating distinct isotopic fractionation during photochemical processing. For example, δ¹³C_OC values for wood pellet emissions changed from −25.6 ‰, −25.7 ‰, and −25.4 ‰ in un-aged samples to −25.3 ‰, −25.1 ‰, and −25.0 ‰ after UV aging at 200 °C, 350 °C, and 650 °C, respectively.

Photochemical aging (particularly UV exposure) reveals systematic modifications to biomass burning isotopic signatures. These findings support the use of stable carbon isotopes for robust source apportionment of carbonaceous aerosols and for interpreting atmospheric observations influenced by photochemical aging.