Title: Distribution of tropospheric methane over Siberia in July 1993
Abstract: Airborne observations of atmospheric methane (CH 4 ) mixing ratio were carried out over Siberia during July 15–30, 1993, using a continuous measurement system. The distribution of CH 4 in the upper troposphere at an altitude of about 7 km was highly variable (1.72–1.91 parts per million, or ppm), being affected by intrusion of stratospheric air, long‐range transport of CH 4 from the source region, and a synoptic‐scale mixing process. During flights along nearly constant latitude between Yakutsk (62.10°N, 129.50°E) and Nizhnevartovsk (60.57°N, 76.40°E) on July 19 and 28 we observed large‐scale enhancements of CH 4 from near 90° to 100°E with a horizontal scale of more than 500km. Accompanied by high humidity and low mixing ratios of CO 2 and O 3 , the air masses with enhanced CH 4 mixing ratio were considered to be affected by lower tropospheric air. The probable source region of the CH 4 was inferred to be the West Siberian Lowland, which contains vast wetlands and huge oil/gas fields. The distribution of CH 4 in the lower troposphere (0.1–3 km) over the boreal wetlands in the West Siberian Lowland, ranging from 1.85 ppm to 2.75 ppm, showed large accumulations near the ground in the morning. The shortterm temporal variations in the vertical profile of CH 4 mixing ratio revealed that the accumulated CH 4 was transported to the free troposphere with the growth of the mixed layer. On the other hand, the distribution of CH 4 (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.