Impact of Morphology and Polymorph on Behavior of Succinic Acid Aerosols in Mixtures with Ammonium Sulfate

Particles on the scale of nanometers are incredibly important in atmospheric chemistry due to their complex role as aerosols in the atmosphere. Atmospheric aerosols and their morphologies directly impact cloud formation, which in turn affects global warming and climate change modeling. Many factors can affect the distribution and morphology of atmospheric aerosols, such as humidity, temperature, and the presence of other chemical species. In this project, a system of succinic acid, a dicarboxylic acid, and ammonium sulfate, an inorganic salt, was studied as both are commonly found in atmospheric aerosols.

There are a variety of ways to model and study atmospheric aerosols; this project utilized a syringe pump to feed aqueous solutions of varying concentrations and mixing ratios of succinic acid and ammonium sulfate into an atomizer to produce nanoscale particles. These particles were then dried and analyzed in a Scanning Mobility Particle Sizer (SMPS) system to obtain size distribution data. This method of analyzing particles typically results in a Gaussian particle size distribution. However, for specific mixtures of succinic acid and ammonium sulfate, a bimodal aerosol size distribution was observed. This phenomenon represents a gap in understanding of the atmospheric chemistry in the succinic acid/ammonium sulfate system.

It was initially hypothesized that the observed bimodal aerosol size distribution is due to the polymorphism of succinic acid, as a similar phenomenon is observed in aerosols of glutaric acid, another dicarboxylic acid. Succinic acid has two known polymorphs: the ꞵ polymorph which is stable at room temperature, and the ɑ polymorph which is stable above 137℃. To test for the presence of these polymorphs in different solutions of succinic acid and ammonium sulfate, particles were collected using a cascade impactor to then be analyzed with powder x-ray diffraction (PXRD). To further investigate the morphology of the particles collected, scanning electron microscopy (SEM) was used to take characteristic images of collected samples. This combination of SMPS, PXRD, and SEM data was collected for aerosols formed from aqueous solutions of succinic acid and ammonium sulfate to better understand the system and its relevant chemistry.