Why atmospheric nanoparticles grow at similar rates everywhere

The effects of temperature on the chemical composition, volatility and diffusivity of condensable organic vapors largely cancel each other out. This effectively buffers the rate at which new aerosol particles grow in the atmosphere.

Authors: Thomas Berkemeier, Ulrich Pöschl

Aerosols have a profound influence on air quality, human health, clouds, and climate, but new particle formation in the atmosphere has remained a scientific conundrum. Large fractions of airborne fine particulate matter consist of secondary organic aerosols (SOA) formed by gas-to-particle conversion of organic precursor molecules in the atmosphere. The growth rates of these atmospheric nanoparticles, however, have remained enigmatic: they are often smaller and less dependent on condensable vapor concentration than expected. Growth rates observed in field measurements are fairly uniform around 1–10 nanometers per hour. This growth is only weakly dependent on vapor concentrations that can vary by multiple orders of magnitude.

New kinetic model reconciles field observations and laboratory experiments
Scientists at the Max Planck Institute for Chemistry (MPIC) in Mainz have now taken a new integrative approach to analyze observational data from field measurements in the boreal forest (Hyytiälä, Finland) and from sophisticated laboratory experiments at the CERN CLOUD chamber. These data previously appeared inconsistent with theory and model predictions. Using a new kinetic multilayer model of multiphase chemistry (KM3C), the researchers show that the observed growth rates can be predicted when the temperature dependence and multiphase kinetics of gas-particle partitioning are resolved.

“Our model considers the reactivity, diffusivity, and concentration gradients of different chemical species across the gas phase, the condensed phase, and the interface between them”, says Zhiqiang Zhang, first author of the study and scientist at MPIC. “Slow surface-to-bulk transport limits the rate of vapor uptake by solid or semi-solid particles with low diffusivity, whereas low temperatures reduce vapor volatility and thereby enhance growth rates.”

In summer at the boreal forest site, the model reproduces the observed growth rates assuming a diffusivity characteristic of highly viscous, nearly glassy substances, whereas the springtime observations are consistent with liquid, well-mixed particles. The low diffusivity leads to an enrichment of relatively more volatile compounds at the particle surface, decelerates their uptake into the particle bulk, and delays the equilibration of gas-particle partitioning.

Buffer effects keep nanoparticle growth within a narrow range
The study answers the long-standing scientific question of why atmospheric nanoparticle growth rates are fairly uniform under widely different ambient conditions and exhibit a low dependence on temperature and organic vapor concentration. “Whether measured above the boreal forest in Finland or in a megacity like Beijing, newly formed particles grow at remarkably similar rates, even though the concentrations of condensable vapors differ by orders of magnitude”, explains Thomas Berkemeier, research group leader at MPIC and lead author of the study. “We found that temperature-related changes in the production, volatility, and diffusivity of condensing organic vapors largely offset each other and effectively buffer the rate of nanoparticle growth.”

Implications for air quality, ecosystem, and climate research
“When newly formed nanoparticles grow rapidly, they can become large enough to form clouds, thereby influencing the water cycle and energy balance of the Earth. Whether that happens  depends on the growth rates”, says Ulrich Pöschl, co-author and director at the Max Planck Institute for Chemistry. “Our findings improve the scientific understanding and predictability of how aerosols affect the interplay of air quality, ecosystems, and climate over the course of Earth’s history as well as in the Anthropocene, which is the current epoch characterized by rapidly increasing human interfluence.”

The insights gained in this study will be incorporated into regional and global investigations of chemistry–climate interactions in different environments, and the newly developed KM3C model has been made available online at https://multiphasekinetics.org/km3c/nano.