Spectro-Microscopic Investigation of the Aerosol Impact on Hydrological Processes

Abstract

This project will investigate combined physical and chemical properties of atmospheric particles collected during field campaigns, organized and managed by the Atmospheric Radiation Measurement (ARM) program of the Department of Energy, and how these airborne particles are involved in the water cycle. We will focus our efforts on comprehensive chemical imaging and molecular characterization of particles collected during the Surface-Atmosphere Integrated Field Laboratory (SAIL) and the Tracking Aerosol Convection Interactions ExpeRiment (TRACER) campaigns.  We will examine elemental and molecular composition, mixing state, size, and internal structures of individual particles, along with molecular-level characterization of complex light-absorbing organic constituents in bulk aerosol and snow samples (from SAIL campaign). By combining our analyses with concurrent trace gases, meteorological, cloud, and in-situ aerosol measurements, we will investigate relevant aerosol regimes, aerosol-precipitation interactions, and changes in surface energy balance induced by snowpack deposits. Our measurements will be used to evaluate particle propensity to serve as cloud condensation nuclei (CCN) and ice nucleating particles (INP), assess aerosol radiative properties, and the link between snowpack albedo and atmospheric deposition of particulate matter. Our specific goals for the SAIL campaign are:  (1) Characterize aerosol regimes and sources, identify particle-type populations, mixing states, and atmospheric transformations based on the statistical analysis of our detailed measurements and the real-time records of the ARM operated Mobile Aerosol Observing System deployed in the field and additional measurements available from other research groups participating in the SAIL experiment; (2) Quantify the optical and chemical properties of snowpack deposits to investigate how aerosol deposition influence snowpack lifetime; (3) Link the chemical and optical properties of aerosols and snowpack deposits to determine impacts on the radiative forcing of climate from its atmospheric and surface components. Successful completion of these objectives will advance scientific understanding of aerosols regimes, sources, aerosol-precipitation interactions, and atmosphere-land interactions.

In the TRACER study, we will provide chemical imaging insights into the particle-type composition and particle mixing states informing interpretation of hygroscopicity parameters inferred from the real-time measurements of size-resolved CCN spectra and from the growth factors determined from relative humidity (RH)-controlled particle size distributions. We will provide comprehensive microscopic characterization of individual particles necessary to understand and quantify variations and differences in the values of hygroscopicity parameters for aerosols of representative compositions and sources. In turn, application of the better constrained values of those hygroscopicity parameters in atmospheric models will advance predictions of aerosol effects on formation of clouds and their life cycles. We will also investigate the effects of particle composition and mixing states on the integrated optical characteristics of aerosol measured by filter-based techniques. Currently employed filter-based measurements of aerosol light absorption are strongly influenced by many factors (e.g., filter type, location of particles on the filter, particle composition, mixing state and morphology, etc.) that typically result in absorption enhancement due to multiple scattering from the filter medium and deposited particles. Existing corrections for retrieval of atmospheric aerosol optical properties from filter-based measurements were derived from simplified reference materials which are not universally applicable to real-world aerosol mixtures. Our chemical imaging measurements of particles from the TRACER campaign will inform new observation-based corrections for improved retrieval of aerosol optical properties from filter-based measurement techniques, validating their results and associated errors.

In addition, a quantitative description of particles’ mixing state using will allow for a meaningful comparison with the particle-resolved process models capable of simulation of gas-particle partitioning, atmospheric aging of aerosols on local and regional scales, and corresponding changes in their optical and cloud-forming properties. In turn, calculations by the process models will serve as an input for the Community Earth System Model and by extension to DOE’s Energy Exascale Earth System Model (E3SM) to evaluate the mass and number concentrations of different particle types at regional-to-global scales, and their impact on direct and indirect radiative forcing of climate.