Chemical characteristics and source apportionment of free amino acids in the Qinling forest atmosphere of central China

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Free amino acids (FAAs) in ambient PM 2.5 samples were investigated at a forest site in the Qinling Mountains region of central China to explore their chemical characteristics, potential atmospheric processes, and sources across four seasons of 2021–2022. A total of 14 FAAs were quantitatively characterized using ultrahigh performance liquid chromatograph coupled with a high-resolution Orbitrap mass spectrometer. The mass concentrations of total FAAs varied from 6.4 to 48.5 ng m −3 (mean 23 ± 11 ng m −3 ) over the study periods, with higher levels in spring and summer. The compositional characteristics of total FAAs varied in different seasons. Among them, valine was mostly abundant, accounting for 39 ± 5 % of total FAAs, followed by alanine (14 ± 5 %) and glycine (14 ± 6 %). The correlations of FAAs with oxidants O 3 and NO 2 suggest that atmospheric oxidation capacity promoted their production, but the effects on individual ones varied. The total FAAs correlated negatively with ambient relative humidity but positively with temperature, indicating that meteorology was important factors governing the FAAs formation. Seven sources for PM 2.5 and FAAs, including biomass burning, gasoline vehicles exhaust, biological emissions, coal combustion, secondary formation, dust, and agricultural activities, were resolved from positive matrix factorization analysis. Biological emissions and agricultural activities were found to be the dominant sources of FAAs in this forest atmosphere. The contributions of these sources to individual FAAs varied substantially across different seasons. This study highlights the seasonal variability in the compositional characteristics of FAAs affected by the environmental factors and sources.