Vertical Profile of the Chemical Composition and Mixing State of Summertime Ambient Aerosols in the Southern Great Plains
XENA MANSOURA, Zezhen Cheng, Gregory W. Vandergrift, Nurun Nahar Lata, Valentina Sola, Zhenli Lai, Ashfiqur Rahman, Jeffery Dhas, Zihua Zhu, Damao Zhang, Fan Mei, Swarup China, Pacific Northwest National Laboratory
Abstract Number: 219
Working Group: Aerosols, Clouds and Climate
Abstract
Ambient aerosols in the US Southern Great Plains (SGP) were collected on June 19th, 2023, utilizing the Atmospheric Radiation Measurement (ARM) U.S. Department of Energy (DOE) unmanned aerial system (UAS) Arctic Shark. The UAS-ArcticShark took flight at 14:41 UTC and sampled ambient aerosols at six altitudes starting with position two (p2) at 1829m (AMSL), p3 at 1676m, p4 at 1372m, p5 at 1067m, p6 at 762m, and p7 at 610m. Samples were also collected for all altitudes (600-2000m) at the beginning and end of the flight (position 1 and position 8). The Size and Time-resolved Aerosol Collector (STAC) impactor system was used to collect these aerosol samples in the lower troposphere.
In-situ measurements collected particle size and concentration data, as well as relevant meteorological information such as relative humidity and temperature. Size, time, and altitude resolved-ambient aerosols in the SGP were analyzed offline by Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM/EDX) and Scanning transmission X-ray microscopy coupled with near-edge X-ray absorption and fine structure (STXM-NEXAFS) spectroscopy to probe their size, morphology, mixing state, and chemical composition. Further, nanospray desorption electrospray ionization (nanoDESI-HRMS) high resolution mass spectrometry analysis techniques was used to probe organic molecular formula.
In addition, we leveraged ARM ground lidar measurements to understand the heterogeneity of air masses. The lidar measurements presented an elevated aerosol layer around 2000m AMSL and a well-mixed boundary layer below 1100m AMSL. Based on this information, we classified our samples into two cases: case study 1 as high-altitude cloud influenced samples (p2-p4), and case study 2 as boundary layer influenced (p5-p7). The case 1 samples were transported from higher altitudes based on the back trajectory simulations and their compositions show carbonaceous sulfate particles dominating in larger size ranges (>0.5 µm), suggesting cloud influence. Case study 2 samples show an increase in the size mode with decreasing altitude, suggesting that possible new particle formation events may be occurring at these layers. The HRMS data shows case study 1 with a higher number fraction of nitrates and sulfate groups, further supporting that these layers may have been cloud influenced. Case study 2 shows changes in volatility suggesting that these samples may have been influenced by the boundary layer. The two layers seem to have been influenced by distinct airmasses—case study 1 from the clouds and case study 2 from the boundary layer.