2011 年 23 巻 3 号 p. 81-88
Terpenoids such as isoprene and monoterpenes are major compounds emitted from plants, and contribute to the formation of photochemical oxidants and secondary organic aerosols in the troposphere. In this study, a portable terpenoid sampling devise was developed for a relaxed eddy accumulation (REA) method to measure the terpenoid flux above forests. The devise was light in weight, approximately 8 kg, and easy to carry. Performance tests of the REA system were conducted to determine collection efficiency of atmospheric terpenoids. When suction resistance was largely different between sampling and dummy lines, the recovery ratio of isoprene standard gas decreased with an increase in frequency of line switching. In case of no difference in the suction resistances, the recovery ratio was more than 90%, even if the frequency of line switching was fast (5 Hz). Carbon dioxide fluxes above a Japanese larch (Larix kaempferi) forest were determined simultaneously with the REA method using this devise and with the widely-used eddy covariance method. A good agreement was observed between the REA- and EC-CO2 fluxes (r2 = 0.89). Using the REA sampling devise terpenoid fluxes above the larch forest were measured during summer campaign. Mean isoprene and monoterpene fluxes during the daytime were 1.2 nmol m-2 s-1 and 0.65 nmol m-2 s-1, respectively. This monoterpene flux value is similar to those reported for Japanese red pine and Scots pine forests.