Trends in methane and sulfur hexafluoride at a tropical coastal site, Thumba (8.6°N, 77.°E), in India

Measurements of methane (CH4) and sulfur hexafluoride (SF6) have been made using gas chromatographic technique at a coastal site, Thumba (8.6°N, 77°E), India from the flask air samples collected during 1996–2000. The growth rates of CH4 and SF6 are observed to be 30 and 0.25 pptv yr−1 respectively. The growth rate of CH4 at Thumba is found to be much higher than its average growth rate of about 7 ppbv yr−1 at other tropical monitoring sites. The observed growth rate of SF6 at Thumba is similar to that found at other tropical stations. Seasonal amplitudes in CH4 and SF6 are higher at Thumba when compared to the other sites. The large growth rate of CH4 at Thumba could be attributed to the increase in regional sources, and the greater seasonal amplitude is shown to be caused by the change in dynamical structure from Northern Hemisphere summer to winter.