Seasonal cycles of water-soluble organic nitrogen aerosols in a deciduous broadleaf forest in northern Japan

摘要

The seasonal variations in aerosol water-soluble organic nitrogen (WSON) concentrations measured in a deciduous forest canopy over an approximately 30month period were investigated for possible sources in the forest. The WSON concentrations (average 157 ± 127 ngNm~(-3)) and WSON/water-soluble total nitrogen mass fractions (average 20 ± 11) in the total suspended particulate matter exhibited a clear seasonal cycle with maxima in early summer. TheWSONmasswas found to reside mostly in the fine-mode size range (D_p1.9 μm), which was similar to the size distributions of sugar compounds, indicating that the major WSON sources in autumn are associated with primary biological emissions. The vertical differences in WSON concentrations suggest that the water-soluble organic aerosol is enriched with nitrogen below the canopy level relative to the forest floor. The WSON concentration increased with enhanced hydrogen ion concentrations in aerosol in the early summer, indicating that aerosol acidity associated with anthropogenic sources outside the forest likely plays an important role in the formation of WSON in that season. The study suggests that multiple sources of WSON within the forest canopy may dominate over others in specific seasons, providing insights into WSON formation processes in forest environments.