The same number in very different air
Newly formed particles in the atmosphere grow at roughly 1 to 10 nanometres an hour, and they do it in the Hyytiälä boreal forest in Finland, in Beijing, at San Pietro di Capofiume in Italy and inside the CLOUD chamber at CERN. The oxygenated organic molecules that feed that growth vary by three orders of magnitude across those settings. That the supply swings so far while the growth rate barely moves is the discrepancy Zhiqiang Zhang, Hyun Gu Kang, Ulrich Pöschl and Thomas Berkemeier at the Max Planck Institute for Chemistry set out to explain in Atmospheric Chemistry and Physics.[1]
Their answer runs through two temperature dependencies that pull in opposite directions. As temperature falls, the volatility of the condensing organic vapours falls with it, so the shift in the volatility distribution that follows the Clausius-Clapeyron equation speeds growth up at low temperature and low concentration. At the same time the particles themselves are viscous, semi-solid rather than liquid, and slow surface-to-bulk transport limits how fast a particle with low diffusivity can take a molecule in. The team resolves both with a kinetic multilayer model of multiphase chemistry, KM3C, that tracks concentration gradients inside the particle.[1]
In the chamber experiments the pattern shows up as a crossing. At minus 25 degrees Celsius the measured growth ran much higher than the kinetic limit; at 25 degrees Celsius it ran well below; at 5 degrees Celsius it sat near it. Reproducing the field data required a much lower diffusivity inside summer particles than inside spring ones, with an activation energy of 40 kilojoules per mole for that diffusivity. Once the two effects largely cancel, what is left is the narrow band of rates that converges across widely varying conditions.[1]
Easy for the model, blind for the measurement
A rate that keeps its value while temperature and chemistry change is an easy quantity to carry in a large-scale model, because being approximately right does not demand a well-resolved local vapour field. The same steadiness weakens the measurement as evidence: because the buffering leaves growth rate weakly dependent on measured organic vapour concentrations, a growth rate measured on its own constrains the driving vapours only loosely. A second reading stays open too; condensed-phase or surface reactions, which the authors did not invoke, may also influence the volatility distribution and particle diffusivity and produce part of the same convergence.[1]
The limitation the authors state is the one that decides how far this result travels. The diffusion properties of complex secondary organic aerosol mixtures have not yet been constrained with high precision, and differences in particle composition between field and chamber studies, with different precursors and different photochemistry, can plausibly lead to differences in particle-phase diffusivity. The model reproduces the observations; it does not measure the diffusivity it assumes.[1]
The measurement that comes next
The authors also name the work that would test the explanation: laboratory experiments and field observations in which nanoparticle composition and phase state are determined alongside condensable vapour concentrations. If such paired measurements are reported for a boreal forest by the end of 2027 and the retrieved diffusivity contrast between summer and spring particles holds, the buffering explanation passes its first real test; if the contrast is absent, the convergence needs another cause.[1]
Aerosols have a profound influence on climate and human health, and new particle formation in the atmosphere has remained a conundrum. A growth rate that holds still is useful arithmetic for anyone keeping that ledger. What it gives in stability, it takes back from what a single measurement can say about the air it came from.[1]