Atmospheric aerosols are tiny airborne particles that play an important role in Earth’s climate system. In addition, aerosols are a major component of air quality. In a new study published in Scientific Reports and led by researchers at ACES, a new method is presented for obtaining new information related to atmospheric particle formation from vapors.
The information is needed for understanding atmospheric impacts of aerosols, and for designing climate change and air pollution mitigation strategies.
“This is an important step forward in understanding and predicting the number and effects of atmospheric aerosols”, says Tinja Olenius, lead author and research scientist at the Department of Environmental Science and Analytical Chemistry (ACES).
Understanding the details of aerosol formation is a key question for predicting the number and impacts of these small particles. The study builds on the theoretical tools that are needed to interpret experimental observations on particle formation and growth. The focus is on the growth dynamics of the very smallest nanoparticles, which are only a few nanometers in diameter.
“This early growth is a critical factor affecting the total aerosol number: the smallest nanoparticles are easily lost from the air by removal processes, and thus faster nanoparticle growth leads to more particles surviving to larger sizes where they can have climatic effects”, says Tinja Olenius.
Size-resolved measurements of nanoparticle concentrations can be used to assess the properties of the vapors and particles, which are the key parameters determining the numbers and growth rates of newly-formed particles. However, interpreting experimental data is challenging due to the complex growth dynamics, including condensation and evaporation of vapor molecules, and particles sticking onto each other or other surfaces. These processes must be correctly described in models that are used to extract information from experiments.
The growth of the very smallest nanoparticles – that are rather clusters of individual molecules – must be modeled molecule-by-molecule, while larger particles are described as macroscopic matter.
“A central question is where this transition happens, and the problem is that the transition size range depends on the vapor and particle properties. This is a type of chicken-and-egg problem: to assess these properties from experiments by modeling, the transition size range should be known, but on the other hand this size range depends on the properties”, says Tinja Olenius.
To date, there have been no generalizable methods for quantifying the particle size regime where the discrete effects become negligible and macroscopic condensation models can be applied.
The new study presents a novel, simple metric for locating the threshold between molecule-by-molecule and macroscopic particle growth. The metric is based on fundamental theory, and can be directly obtained from nanoparticle measurements with no need for prior knowledge of the vapor and particle properties.
“This enables improved, robust interpretation of measurements and assessment of the key parameters. Such analysis is required for understanding aerosol formation in atmospheric environments and its response to changes in, for instance, vapor emissions from traffic and industry.”
This is linked to climate change and air pollution control strategies.
“By providing the new tool to data analysts and modelers, the study can contribute to quantifying nanoparticles from different vapor emission sources, and to reducing uncertainties in predictions for future scenarios”, says Tinja Olenius.
The article “Robust metric for quantifying the importance of stochastic effects on nanoparticle growth” is published in Scientific Reports.
Text by Tinja Olenius and Annika Hallman