Infrared solar spectroscopic measurements of free tropospheric CO, C2H6, and HCN above Mauna Loa, Hawaii: Seasonal variations and evidence for enhanced emissions from the Southeast Asian tropical fires of 1997-1998

High spectral resolution (0.003 cm(-1)) infrared solar absorption measureme nts of CO, C2H6, and HCN have been recorded at the Network for the Detectio n of Stratospheric Change station on Mauna Loa, Hawaii, (19.5 degrees N, 15 5.6 degrees W, altitude 3.4 km). The observations were obtained on over 250 days between August 1995 and February 1998. Column measurements are report ed for the 3.4-16 km altitude region, which corresponds approximately to th e free troposphere above the station. average CO mixing ratios computed for this layer have been compared with flask sampling CO measurements obtained in situ at the station during the same time period. Both show asymmetrical seasonal cycles superimposed on significant variability. The first 2 years of observations exhibit a broad January-April maximum and a sharper CO min imum during late summer. The C2H6 and CO 3.4-16 km columns were highly corr elated throughout the observing period with the C2H6/CO slope intermediate between higher and lower values derived from similar infrared spectroscopic measurements at 32 degrees N and 45 degrees S latitude, respectively. Vari able enhancements in CO, C2H6, and particularly HCN were observed beginning in about September 1997. The maximum HCN free tropospheric monthly mean co lumn observed in November 1997 corresponds to an average 3.4-16 fim mixing ratio of 0.7 ppbv (1 ppbv = 10(-9) per unit volume), more than a factor of 3 above the background level. The HCN enhancements continued through the en d of the observational series. Back-trajectory calculations suggest that th e emissions originated at low northern latitudes in southeast Asia. Surface CO mixing ratios and the C2H6 tropospheric columns measured during the sam e time also showed anomalous autumn 1997 maxima. The intense and widespread tropical wild fires that burned during the strong El Nino warm phase of 19 97-1998 are the likely source of the elevated emission products.