Aircraft measurements of trace gases between Japan and Singapore in October of 1993, 1996, and 1997

Carbon dioxide (CO 2 ), methane (CH 4 ), and carbon monoxide (CO) mixing ratios were measured in discrete air samples from aircraft between Japan and Singapore in October. The mixing ratios of all trace gases at 9–12 km were enhanced over the South China Sea in 1997 compared with those in 1993 and 1996. Vertical distributions of all trace gases over Singapore in 1997 also showed largely elevated mixing ratios at all altitudes. These distributions indicate a wide outflow of trace gases from intense biomass burning in the southeast Asia regions in the very strong El Niño year. The enhanced trace gases showed a strong linear correlation between CH 4 and CO, and between CO and CO 2 , with the regression slopes of 0.051 (ΔCH 4 ppb/ΔCOppb) and 0.089 (ΔCOppb/ΔCO 2 ppb). The emission ratios are characteristic of fires with relatively lower combustion efficiency from the tropical rain forest and peat lands in Kalimantan and Sumatra of Indonesia.