Science2 publishers2 min readPublished
Volatile organic compounds supply about 100 of the 460 teragrams of carbon that rain delivers each year
Ana Yanez-Serrano and colleagues estimate in a review that water-soluble VOCs account for 21.7 percent of the annual wet-deposition carbon flux, a term most atmospheric carbon budgets leave out. The figure is a synthesis assembled in a review.
The Scientist · Science desk
What happened
- A review by Ana Yanez-Serrano and colleagues estimates that about 100 of the roughly 460 teragrams of carbon delivered to the ground each year by wet deposition arrives as volatile organic compounds.
- For decades the field had no way to detect VOCs in rainwater, so it assumed rainwater-based cycling of them was negligible and concentrated on nitrate and sulfate instead.
- Improved analytical techniques then found VOCs in rainwater at higher concentrations than scientists had expected. Those measurements are what make the new estimate possible.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint Adding this flux to a budget takes more than inserting a number: the review says modelers have to build VOC transport and chemistry into Earth system models, and detection must improve before the term can be constrained.
- exposure Nutrient-poor ecosystems are where a small deposition dose produces the fastest response, so the places most sensitive to this carbon are the ones a single global flux figure describes least well.
- decision Anyone recompiling a wet-deposition carbon budget now has to choose between carrying a review-level estimate that comes without a published uncertainty range and leaving the term where it has been.
One hundred teragrams out of 460 is 21.7 percent, so "about a fifth" is a fair reading of what Ana Yanez-Serrano and her co-authors put in the Journal of Geophysical Research: Biogeosciences [1][1][2]. The estimate is a synthesis assembled in a review. The two accounts of it published this month are one text: a research spotlight written by Rebecca Dzombak for Eos, which phys.org republished [3]. Neither version publishes an uncertainty range for the 100 teragrams or a breakdown by emission source [2].
The reason this term sat near zero is instrumental. Wet deposition chemistry grew up around nitrate and sulfate, the compounds that drive acid rain [4]. VOCs in rainwater could not be detected, and the flux was assumed negligible [5]. Better analytical methods then found them at higher concentrations than expected [6]. The review's authors call dissolved atmospheric VOCs "largely invisible" to the biogeosciences and a missing link in carbon cycle modeling [7].
The 100 teragrams is a delivery flux. It describes carbon arriving at the ground in rain [8]. It does not establish that land gained carbon it had not already released, and a roughly 22 percent correction to a delivery term is not a 22 percent correction to any net uptake number [1]. Some of the compounds involved turn up in petroleum fuels, paint thinners and pharmaceuticals [9].
The more testable claim in the review holds regardless of the global total. VOCs are highly reactive and biologically labile, so even small concentrations can affect water chemistry, plants and microorganisms at local to regional scales [10]. In carbon-limited ecosystems, a small amount of VOC deposition can trigger a rapid biological response [11]. A spiking or exclusion experiment could address that hypothesis in a single field season, and the authors list such observations and experiments as work still to be done [12].
Most atmospheric carbon budgets currently omit or underestimate water-soluble VOCs and their oxidized products [13]. Closing that gap means putting VOC transport and chemistry into Earth system models [14], and detection has to improve first, because these compounds are reactive and chemically diverse [15]. Climate change and human activity are pushing VOC emissions up while rainfall patterns change [16]. I would treat 100 teragrams as an order-of-magnitude flag. It justifies a monitoring effort, and closing a budget will take more than it.
What to watch
- Whether an uncertainty range or a regional breakdown is published alongside the 100 teragram estimate as monitoring data accumulate.
- Whether any Earth system model group adds water-soluble VOC deposition and its oxidized products as a tracked carbon term.
- Field experiments in carbon-limited soils that test whether VOC deposition changes carbon storage and not only water chemistry.