Statistical Analysis of Individual Particle Mixing State and Morphology

Principal Investigator(s):
Ryan Moffet, University of the Pacific

Atmospheric aerosols – tiny particles suspended in the air - affect climate directly by absorbing and scattering the sun’s radiation and indirectly by impacting the properties of clouds [IPCC, 2013]. Most cloud formation in the atmosphere relies on aerosols to nucleate cloud droplets. The chemical composition of the aerosol particles impacts how quickly a cloud droplet forms, and how long it stays in in the air (lifetime). The chemical composition of aerosols is complicated by the fact that different chemical species may be spread out over all particles (internal mixture) or found only in certain particles (external mixture). The distribution of chemical species within the particles is termed the mixing state. It is found that the mixing state changes as an aerosol particle is chemically transformed in the atmosphere; this process is called “aging”. The aging and mixing state are deemed high priority research topics due to the potential sensitivity of both the direct and indirect effect on these variables. This proposal aims to fold simultaneous microscopic and chemical measurements of aerosol mixing state into a recently developed parameterization that can be interfaced with regional and global models. Aerosol processes, such as wet removal, responsible for the “rain out” of aerosol from the atmosphere, are poorly represented in climate models. There is evidence that wet removal processes are sensitive to the detailed composition of individual particles. In order to gain a better understanding of the effect of aerosol chemical transformation on these processes, detailed observations of particle populations at the single particle level are necessary.

Tropical locations are of specific interest due to the lack of such detailed climate-related measurements in these regions of the earth. The green ocean amazon (GOAmazon) field campaign was set up to study the interaction of biogenic emissions with an urban pollution plume in a tropical environment. Understanding the interaction of aerosol plumes with gaseous biogenic organic compounds emitted from plants and trees is of importance to determining the effects the aerosols have on climate. As an urban plume mixes with biogenic compounds, the chemical associations (mixing state) within the individual aerosol particles is expected to change the cloud forming properties of the particles. Specifically, as biogenic organic compounds become oxidized, they condense onto pre-existing aerosol. Both the chemical characteristics and amount of organic coatings have been shown to affect the ability of the particle to serve as a cloud particle nucleus. Here it is proposed to use state-of-the-art coupled microscopic and chemical measurements housed at the Department of Energy’s national user facilities to gain a detailed understanding of the effects the anthropogenic/biogenic interaction on the aging and mixing state of tens of thousands of individual particles sampled during the GOAmazon campaign.

Modeling nanoscale changes (mixing state transformation) that affect large scale processes (cloud formation) has been problematic in the modeling community. With the development of a new parameterization of aerosol mixing state that employs entropy and diversity metrics [Riemer and West, 2013], comparison with regional process models is more straightforward. Single particle analysis techniques such as those proposed here are the only ones that can be used with this parameterization to quantify the mixing state of the aerosol population. Never before have these detailed, single-particle measurements been so explicitly included in models.

With guidance from collocated measurements, samples collected in the Amazon basin during the GOAmazon field campaign will be analyzed for single particle chemical composition. Spatially resolved elemental and molecular mixing state for light elements will be obtained using scanning transmission X-ray microscopy coupled with near edge X-ray fine structure (STXM/NEXAFS) spectroscopy located at Lawrence Berkeley National Laboratory. Spatially resolved mixing state for heavy elements (Z>23) will be obtained using scanning electron microscopy coupled with energy dispersive X-ray (CCSEM/EDX) spectroscopy at Pacific Northwest National Laboratory’s Environmental and Molecular Sciences Laboratory. New methods will be developed in order to combine both data sets into a unified database. These methods will be general enough to be applicable to other data sets beyond the GOAmazon campaign.