Diurnal variations and vertical gradients of biogenic volatile and semi-volatile organic compounds at the Tomakomai larch forest station in Japan

Volatile and semi-volatile organic compounds (VOCs and SVOCs) in the atmosphere control oxidative capacity of the air and formation of organic aerosols. To investigate sources, variations and fluxes of VOCs and SVOCs in the forest atmosphere, samples were collected in a larch forest at different heights in Tomakomai, Japan, from 3 to 5 September 2003 and were analyzed for VOCs (isoprene and α-pinene) and SVOCs (n-nonanal, n-decanal, glycolaldehyde, hydroxyacetone, glyoxal and methylglyoxal) using GC-FID and GC-MS. Isoprene (8–851 pptv) showed a diurnal variation with a maximum in daytime, whereas diurnal pattern was indistinct for α-pinene (6–145 pptv). SVOCs showed diurnal variations similar to isoprene, suggesting their origin to be biogenic. Denuder samplings of SVOCs demonstrated that up to 60% of SVOCs were present in aerosol particles. n-Nonanal and n-decanal were more abundant at lower altitude, suggesting their primary emissions from vegetations. In contrast, other four SVOCs showed an opposite or no trend and are considered as oxidation products of biogenic precursors such as isoprene. The averaged upward fluxes of isoprene and α-pinene were obtained to be 34.8 and 9.8 ng m^-2 s^-1, respectively. Magnitudes of SVOC fluxes were comparable to those of α-pinene, indicating that SVOCs affect budgets of organic gases/aerosols over the forest.