PNNL

Abstract

Molecular composition of organic aerosol, especially for diel variations of organosulfates above agricultural fields, is not well understood despite profound impacts on regional climate, crop production, air quality, and human health. Here, nanospray desorption electrospray ionization with high resolution mass spectrometry (nanoDESI-HRMS) is used to interrogate the molecular composition of organic aerosol collected at the Southern Great Plains, located in an agricultural region of Oklahoma, USA. Identified molecular formulae featured carbon, hydrogen, oxygen (CHO), nitrogen (CHNO), and/or sulfur (CHOS, CHNOS), with higher organosulfate proportions during daytime (41%) compared to nighttime (30%). Nighttime aerosol featured increases in CHO, CHNO, and extremely low volatility organic carbon (ELVOC) species. However, due to high relative humidity, nighttime aerosol phase state was found to be more liquid-like than daytime aerosol by using parametrized glass transition temperatures. Aerosol molecular composition from an anthropogenically influenced plume (southerly winds) showed significant increases in CHOS and ELVOC species. By comparison with chamber studies, CHOS species are suspected to be of mixed biogenic and anthropogenic origin, whereas CHNOS species (not identified in the southerly winds) are of suggested biogenic origin. Overall, this study provides key insight into organosulfates above agricultural fields, demonstrating dependence upon diel cycles and episodic anthropogenic emissions.