The variability of methane, nitrous oxide and sulfur hexafluoride in Northeast India

Abstract

High-frequency atmospheric measurements of methane (CH[subscript 4]), nitrous oxide (N[subscript 2]O) and sulfur hexafluoride (SF[subscript 6]) from Darjeeling, India are presented from December 2011 (CH[subscript 4])/March 2012 (N[subscript 2]O and SF[subscript 6]) through February 2013. These measurements were made on a gas chromatograph equipped with a flame ionization detector and electron capture detector, and were calibrated on the Tohoku University, the Scripps Institution of Oceanography (SIO)-98 and SIO-2005 scales for CH[subscript 4], N[subscript 2]O and SF[subscript 6], respectively. The observations show large variability and frequent pollution events in CH[subscript 4] and N[subscript 2]O mole fractions, suggesting significant sources in the regions sampled by Darjeeling throughout the year. By contrast, SF[subscript 6] mole fractions show little variability and only occasional pollution episodes, likely due to weak sources in the region. Simulations using the Numerical Atmospheric dispersion Modelling Environment (NAME) particle dispersion model suggest that many of the enhancements in the three gases result from the transport of pollutants from the densely populated Indo-Gangetic Plains of India to Darjeeling. The meteorology of the region varies considerably throughout the year from Himalayan flows in the winter to the strong south Asian summer monsoon. The model is consistent in simulating a diurnal cycle in CH[subscript 4] and N[subscript 2]O mole fractions that is present during the winter but absent in the summer and suggests that the signals measured at Darjeeling are dominated by large-scale (~100 km) flows rather than local (<10 km) flows.