DISTRIBUTION OF TROPOSPHERIC METHANE OVER SIBERIA IN JULY 1993

Airborne observations of atmospheric methane (CH4) mixing ratio were c arried out over Siberia during July 15-30, 1993, using a continuous me asurement system. The distribution of CH4 in the upper troposphere at an altitude of about 7 km was highly variable (1.72-1.91 parts per mil lion, or ppm), being affected by intrusion of stratospheric air, long- range transport of CH4 from the source region, and a synoptic-scale mi xing process. During flights along nearly constant latitude between Ya kutsk (62.10 degrees N, 129.50 degrees E) and Nizhnevartovsk (60.57 de grees N, 76.40 degrees E) on July 19 and 28 we observed large-scale en hancements of CH4 from near 90 degrees to 100 degrees E with a horizon tal scale of more than 500 km. Accompanied by high humidity and low mi xing ratios of CO2 and O-3, the air masses with enhanced CH4 mixing ra tio were considered to be affected by lower tropospheric air. The prob able source region of the CH4 was inferred to be the West Siberian Low land, which contains vast wetlands and huge oil/gas fields. The distri bution of CH4 in the lower troposphere (0.1-3 km) over the boreal wetl ands in the West Siberian Lowland, ranging from 1.85 ppm to 2.75 ppm, showed large accumulations near the ground in the morning. The shortte rm temporal variations in the vertical profile of CH4 mixing ratio rev ealed that the accumulated CH4 was transported to the free troposphere with the growth of the mixed layer. On the other hand, the distributi on of CH4 (0.1-1 km) over the tundra areas in the Siberian Arctic was less variable (1.81-1.84 ppm), and little accumulation was observed.