Field New Insights into Bioaerosol Amino Acid Origin and Atmospheric Degradation: Significant Dust Contribution and Drying Effect

Proteinaceous matter (PrM), including combined and free amino acids (CAAs and FAAs), represents an abundant component of bioaerosols. Observations in nonarid areas suggested significant influences from aqueous-phase oxidation on FAA compositions and abundances. However, the origins and processing of aerosol PrM in arid and dusty areas remain poorly understood. The characterizations of CAAs and FAAs in PM_2.5 and dust from leaf surfaces, roads, and buildings were investigated throughout the cold seasons in arid Lanzhou (northwest China). Leucine dominated CAAs in both PM_2.5 and dust samples. This finding, combined with evidence from amino acid nitrogen isotope signal and Mantel test analysis, strongly suggests that soil (/dust) is the main contributor (up to 38%) to total aerosol amino acids in Lanzhou, followed by vegetation and biomass burning. Glycine dominated aerosol FAAs during winter (relative humidity = 50 ± 14%) and correlated significantly with atmospheric oxidation levels, while these patterns were absent during the drier spring with higher atmospheric oxidation capacity. We propose that low relative humidity (33 ± 15%), rather than weak aerosol acidity (pH = 8.1 ± 1.0), constrains PrM degradation during spring, leading to a significant reduction in free glycine mass fraction. This study presents the first observational evidence for constrained PrM degradation under dry conditions.