Quantifying the chemical mixing state index of atmospherically aged aerosols collected in the North Atlantic free troposphere

Abstract:

As atmospheric particles travel through the atmosphere, their initial chemical and physical properties transform through a variety of processes such as condensation, oxidation, and evaporation, changing the composition of the aerosol ensemble, as well as on an individual particle basis. These complex reactions result in large uncertainties in the radiative modules of climate models. Particularly uncertain is the role that these transformations have on particle-cloud interactions. Single particle characterizations of the chemical composition of well-aged and remote aerosols are sparse. To investigate these particle transformations, we performed micro-spectroscopic analysis on aerosol samples collected at the Pico Mountain Observatory. The Observatory is a remote station on top of Pico Mountain, on Pico Island in the Azores’ archipelago in the northern Atlantic, which typically receives air that has been transported over long distances in the free troposphere. We performed multi-modal micro-spectroscopic analysis such as computer controlled scanning electron microscopy with energy dispersive X-ray spectroscopy and scanning transmission X-ray microscopy with near-edge X-ray absorption fine structure spectroscopy on nine samples collected during June and July 2017. We classified the particles into different groups such as sea salt, dust, carbonaceous, and sulfates based on their elemental composition.

We developed Python routines that use the micro-spectroscopy data to determine the mixing state index (χ) of the individual particles; the mixing state index is a measure of how similar or different (internally or externally mixed) the elemental composition of particles are to or from each other. It was found that the analyzed samples were internally mixed (χ between 65% and 95%), and that larger particles had greater individual diversity. The chemical composition data were then related to average sample hygroscopicity, as well as cloud condensation nuclei concentration (CCNc) data collected in-situ at the site. The data, including the mixing state index, can be utilized in particle resolved models to improve our understanding of the role that atmospheric aging of aerosol plays in radiative forcing and in affecting cloud properties.