Large injection of carbon monoxide into the upper troposphere due to intense biomass burning in 1997

Air samples at 8–13 km were collected regularly using a commercial airliner to obtain long‐term measurements of carbon monoxide (CO) mixing ratio in the upper troposphere over the western Pacific between Australia and Japan during April 1993‐December 1997. The measurements in 1997 clearly reveal an anomalous CO increase during September to November in the Southern Hemisphere, with a maximum of 320–380 ppb around 20°S in October. Tropical biomass burning, not urban/industrial emissions, was the main source for the enhanced CO in 1997. A similar southern‐spring increase due to biomass burning was observed in previous years. The peaks showed a large interannual variation associated with the El Niño/Southern Oscillation (ENSO) events. The largest CO spring peak appeared during the strong El Niño event in 1997, while the weak La Niña year of 1996 was marked by a largely suppressed CO spring peak. The outgoing longwave radiation (OLR) anomaly is largest during the El Niño events indicating that the events cause a longer drought in the tropics and significantly influence the enlargement of biomass burning in tropical Southeast Asia. Thus the most likely cause for the ENSO‐cycle CO variability is a year‐to‐year change of biomass‐burning emissions mainly from Southeast Asia. The appearance of the CO spring peak in the southern subtropics is discussed on the basis of the possible long‐range transport of biomass‐burning CO from Southeast Asia to the upper troposphere over the western South Pacific.