Understanding the chemical processes that affect growth rates of freshly nucleated particles

Description

The birth of new particles in the atmosphere is a significant process because both nucleation and growth rates are much higher than expected based on early naïve models. Our research aims to elucidate the mechanisms responsible for these high rates. This poster focuses on our recent work that contributes to our understanding of fast growth rates of freshly nucleated particles. This work includes the development of improved methods to measure size distributions of particles down to 1 nm and the use of these data to quantify size-dependent growth rates down to 1 nm. During the summer of 2010 chamber experiments were carried out at the University of Minnesota to study nucleation and growth following the photochemical formation of sulfuric acid from sulfur dioxide in the presence of amines. Size dependent growth rates of 1-3 nm particles were measured and found to increase with size and to be less than values that could be explained by collision-limited uptake of sulfuric acid. This contrasts with measurements in the atmosphere, where 1-3 nm particles grow faster than can be explained by sulfuric acid condensation, presumably because other compounds also contribute to growth. Although these observations do not explain why atmospheric growth rates are so fast, they do show one way in which chamber studies may not accurately reflect all of the processes that are important in the atmosphere. Another interesting observation in the chamber studies was the strong effect of amines on sub-10-nm size distributions. When amines were not intentionally added to the chamber, nucleation occurred in a short burst. When amines were added, however, high nucleation rates persisted for the duration of the experiment. Without amine addition, sub 10-nm number distributions decreased sharply with increasing size after the initial nucleation burst. In the presence of amines, number distributions decreased much more gradually with increasing size, due to the high rates at which stable nuclei were produced and grew. The chamber studies also showed that amines were always present at levels of 10-100 ppt, no matter how much effort was made to clean the chamber. The AmPMS, developed by David Hanson and coworkers, allowed measurements of amines for the first time in chamber studies. We think it is likely that amines have always been present in chamber studies.