Large-scale Distribution of CH4 in the Western Pacific: Sources and transport from the Asian Continent

Methane (CH4) is a long-lived atmospheric trace gas that has both anthropogenic and natural sources which vary seasonally. The TRACE-P mission, a component of NASA's Global Tropospheric Experiment (GTE), was flown off the coast of the Asian continent and over the western North Pacific Ocean during March and April of 2000, a period when sources are expected to be dominated by anthropogenic contributions. In agreement with this, CH4 mixing ratios sampled during TRACE-P were in general well correlated with other trace gases such as carbon monoxide (CO) and ethane (C2H6) that have predominantly anthropogenic sources. For example, in air sampled in the highly polluted Shanghai plume, CH4 mixing ratios were as high as 2000 ppbv and the CH4/CO ratio was 0.26 (r2=0.86), characteristic of a mix of industrial and combustion anthropogenic sources. The CH4/C2H6 ratio in this air mass was 0.04 (r2=0.83). As expected, near-surface CH4 mixing ratios (0 - 2 km) were significantly elevated over background levels sampled by the CMDL network during this timeframe over the Pacific basin. Enhancements were greatest above 15oN and ranged from 25 - 38 ppbv higher. At latitudes less than 15oN, only minimal enhancement over background levels was observed. These trends were similar to those seen during the PEM West B mission, flown during roughly the same time of year in 1994. At that time, near-surface CH4 was enhanced 41 - 57 ppbv over background between 15 and 25oN, with somewhat lower elevations to the north and south. Comparision of the TRACE-P and PEM West B mixing ratios, averaged by latitude and longitude suggests that increases in the near-surface were in line with changes seen in background levels. At higher altitudes, comparisions are more difficult, but ranged from 15 - 40 ppbv (2 - 6 km) and 20 - 60 ppbv (above 6 km), generally similar to changes seen in the near-surface.