Up in Smoke: Experimental Constraints on Foliage- and Soil-Derived Fe in Biomass Burning Aerosols

Atmospheric deposition of iron (Fe) can limit primary productivity and carbon dioxide uptake in some marine ecosystems. Recent modeling studies suggest that biomass burning aerosols may contribute a significant amount of soluble Fe to the surface ocean. Existing studies of burn-induced trace element mobilization have often collected both entrained soil particles along with material from burning biomass, making it difficult to determine the actual source of aerosolized trace metals.

In order to better constrain the importance of biomass vs. entrained soil as a source of trace metals in burn aerosols, we conducted burn experiments using soil-free foliage representative of a variety of fire-impacted ecosystems. The resulting burn aerosols were collected in two stages (PM > 2.5 μm and PM < 2.5 μm) on cellulose filters using a high-volume air sampler equipped with an all-Teflon impactor. Unburned foliage and burn aerosols were analyzed for Fe and other trace metals using inductively coupled plasma mass spectrometry (ICP-MS). Our results show that about 1% of Fe in plant biomass is likely mobilized as atmospheric aerosols during biomass burning events.

We used these results and estimates of annual global wildfire area to estimate the impact of biomass burning aerosols on total atmospheric Fe flux to the ocean. We estimate that foliage-derived Fe contributes 114 Gg annually. Prior studies, which implicitly include both biomass and soil-derived Fe, concluded that biomass burning contributes approximately 690 Gg of Fe. Together, these studies suggest that fire-entrained soil particles contribute 83 % (576 Gg) of Fe in biomass burning emissions, while plant derived iron only accounts for 17%.