Seasonal Variation in Molecular Composition of Organic Aerosol at the Southern Great Plains Observatory

GREGORY W. VANDERGRIFT, Sonja Moons, Nurun Nahar Lata, Zezhen Cheng, Darielle Dexheimer, Rahul Zaveri, Fan Mei, Swarup China, Pacific Northwest National Laboratory

     Abstract Number: 361
     Working Group: Aerosol Chemistry

Abstract
Agricultural land accounts for approximately 52% of the United States land base alone, demonstrating the dominance and importance of this land type. This study aims to better understand the organic molecular composition of atmospheric particles influenced by agricultural land, informing the landscape impact on weather and climate. Specifically, 80 samples were collected from the Atmospheric Radiation Measurement's (ARM) Southern Great Plains (SGP) Observatory (agricultural region of Oklahoma, USA) during periods of day and night across different seasons spanning a period of two years (May, July, October 2021 and February, April 2022). Bulk aerosol was sampled directly from filter substrates via nanospray desorption electrospray ionization coupled with high resolution mass spectrometry (nano-DESI HRMS). Resultant mass spectra were subjected to molecular formulae (MF) assignment, resulting in over 3300 unique MF containing carbon, hydrogen, oxygen, nitrogen, and/or sulfur atoms. A total of 15 distinct intersections containing ≥50 MF between the five different seasons were identified, informing the seasonality of secondary organic aerosol (SOA) molecular composition above agricultural land. Particularly populous intersections included MF unique to April 2022 (483 MF, informing wildfire related SOA), MF unique to October 2022 (360 MF, showing increased aromaticity and oligomeric MF), MF unique to May 2021 (231 MF, dominated by organosulfates and correlated with southerly winds), and MF detected in all periods except February 2022 (254 MF, detailing SOA that is not observed in cold weather conditions). Current work involves leveraging additional datasets collected at the SGP site to better understand various atmospheric processes such as new particle formation (NPF). Combined with modelling and HRMS datasets, a molecular level understanding NPF and subsequent aerosol growth at the SGP observatory is being investigated.