PNNL

Abstract

Atmospheric particles play critical roles in climate. However, significant knowledge gaps remain regarding the vertically resolved organic molecular-level composition of atmospheric particles due to aloft sampling challenges. To address this, we use a tethered balloon system at the Southern Great Plains Observatory and high-resolution mass spectrometry (HRMS) to respectively collect and characterize organic molecular formulae (MF) in ground level and aloft (up to 750 m) samples. We show that organic MF uniquely detected aloft were dominated by organonitrates (139 MF; 54% of all uniquely detected aloft MF). Organonitrates that were uniquely detected aloft featured elevated O/C ratios (0.73 ± 0.23) compared to aloft organonitrates that were commonly observed at ground level (0.63 ± 0.22). Unique aloft organic molecular composition was positively associated with increased cloud coverage, increased aloft relative humidity (increase of ~40% compared to ground level), and decreased vertical wind variance. Furthermore, 29% of extremely low volatility organic compounds (ELVOCs) in the aloft sample were truly unique to the aloft sample compared to ground level, emphasizing potential oligomer formation at higher altitudes. To show relevance beyond the sampling location here, the Weather Research and Forecasting/chemistry (WRF-Chem) model predicted vertical dependencies for secondary organic aerosol concentrations that varied geospatially across the entire southeastern United States. Overall, this study highlights the importance of considering vertically-resolved organic molecular composition (particularly for organonitrates) and hypothesizes that aqueous phase transformations and vertical wind variance may be key variables affecting the molecular composition of aloft organic aerosol.