Investigate the Vertical Profile of Size-Resolved Aerosol Chemical Composition and Aerosol Mixing State during TRacking Aerosol Convection Interactions ExpeRiment (TRACER)
ZEZHEN CHENG, Nurun Nahar Lata, Darielle Dexheimer, Matthew A. Marcus, Gregory W. Vandergrift, Tania Gautam, Chongai Kuang, Allison Steiner, Swarup China,
Pacific Northwest National Laboratory Abstract Number: 394
Working Group: Aerosol Chemistry
AbstractThis study reports a case study of the vertical gradient of aerosol chemical composition and mixing state. Samples were collected using the automated size and time-resolved aerosol collector (STAC) platform deployed on the Atmospheric Radiation Measurement's (ARM) tethered balloon system (TBS) at the TRacking Aerosol Convection interactions ExpeRiment (TRACER, Houston, TX) ancillary site. TBS has a printed optical particle spectrometer (POPS) and a condensation particle counter (CPC) below STAC to determine particle size distribution. Four STAC samples were collected at different above-ground level altitudes (S1: 0-250 m, S2: 250-500 M, S3: 750-1000m, and S4: 1000-1250m). 72-hour Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) simulation shows that air masses came from the southeast U.S. and across the Gulf of Mexico. We utilized Computer-controlled Scanning Electron Microscopy with an Energy-Dispersive X-ray Spectrometer (CCSEM/EDX) combined with Scanning Transmission X-ray Microscopy/Near Edge Fine Structure spectroscopy (STXM/NEXAFS) to quantitatively probe size-resolved chemical composition and mixing state based on individual particles’ relative atomic percentage. The preliminary size-resolved chemical compositions of all samples have bi-modal size distribution (Aitken and accumulation modes), and the relative height of the accumulation mode decreases with increased altitudes. This result is consistent with the POPS size distribution data. Carbonaceous aerosols dominate (>50% by number) in all samples, and the number fraction of sodium-rich sulfate aerosol decreases from ~16% to ~5% with altitude increase. Moreover, particle mixing state were estimated based on the mixing state index (χ). The χ values for all four samples are not significantly different and vary between 35 and 48%, which suggests these particles are not extensively internally mixed and might be less processed in the atmosphere. Results from this study aim to improve our knowledge of the mixing state of the aerosol vertical profile, which can help reduce uncertainties in climate models.