Seasonal and diurnal variations of methane and carbon dioxide in the Kathmandu Valley in the foothills of the central Himalaya
Authors: Mahata Khadak Singh , Panday Arnico Kumar , Rupakheti Maheswar , Singh Ashish , Naja Manish , Lawrence Mark G. 
Published: March 2017

Abstract. The SusKat-ABC (Sustainable Amosphere for the Kathmandu Valley- Atmospheric Brown Clouds) international air pollution measurement campaign was carried out during December 2012–June 2013 in the Kathmandu Valley and surrounding regions in Nepal. The Kathmandu Valley is a bowl-shaped basin with a severe air pollution problem. This paper reports measurements of two major greenhouse gases (GHGs), methane (CH4) and carbon dioxide (CO2), that begun during the campaign and extended for a year at the SusKat-ABC’s supersite in Bode, a semi-urban location in the Kathmandu Valley. Measurements were also made at a nearby rural site (Chanban), ~ 25 km (aerial distance) to the southwest of Bode, on the other side of a tall ridge. The ambient mixing ratios of methane (CH4), carbon dioxide (CO2), water vapor, and carbon monoxide (CO) were measured with a cavity ring down spectrometer (Picarro G2401, USA), along with meteorological parameters for a year (March 2013–March 2014). Simultaneous measurements were also made at Chanban from 15 July to 3 October 2015. These measurements are the first of their kind in the central Himalayan foothills. At Bode, the annual average mixing ratios of CO2 and CH4 were 419.4 (±23.9) ppm and 2.193 (±0.224) ppm, respectively. These values are higher than the levels observed at background sites such as Mauna Loa, USA (CO2: 396.8 ppm, CH4: 1.831 ppm) and Waliguan, China (CO2: 397.7 ppm, CH4: 1.879 ppm) during the same period, and at other urban/semi-urban sites in the region such as Ahmedabad and Shadnagar (India) and Nanjing (China). They varied slightly across the seasons at Bode, with seasonal average CH4 mixing ratios being 2.157 (±0.230) ppm in the pre-monsoon season, 2.199 (±0.241) ppm in the monsoon, 2.210 (±0.200) ppm in the post-monsoon, and 2.214 (± 0.209) ppm in the winter season. The average CO2 mixing ratios were 426.2 (±25.5) ppm in pre-monsoon, 413.5 (±24.2) ppm in monsoon, 417.3 (±23.1) ppm in post-monsoon, and 421.9 (±20.3) ppm in winter season. The maximum seasonal mean mixing ratio of CH4 in winter was only 0.057 ppm or 2.6 % higher than the seasonal minimum during the pre-monsoon period, while CO2 was 12.8 ppm or 3.1 % higher during the pre-monsoon period (seasonal maximum) than during the monsoon (seasonal minimum). On the other hand, the CO mixing ratio at Bode was 191 % higher during the winter than during the monsoon season. The enhancement in CO2 mixing ratios during the pre-monsoon season is associated with additional CO2 emissions from forest fire and agro-residue burning in northern South Asia in addition to local emissions in the Kathmandu Valley. Published CO / CO2 ratios of different emission sources in Nepal and India were compared with the observed CO / CO2 ratios in this study. This comparison indicated that the major sources in the Kathmandu Valley were residential cooking and vehicle exhaust in all seasons except winter. In winter, the brick kiln emissions were a major source. Simultaneous measurement in Bode and Chanban (15 July–3 Oct 2015) revealed that the mixing ratio of CO2, CH4 and CO mixing ratios were 3.8 %, 12 %, and 64 % higher in Bode than Chanban. Kathmandu Valley, thus, has significant emissions from local sources, which can also be attributed to its bowl shaped geography that is conducive to pollution build-up. All three gas species in Bode showed strong diurnal patterns, whereas CH4 and CO at Chanban did not show any noticeable diurnal variations. These measurements provide the first insights into diurnal and seasonal variation of key greenhouse gases and air pollutants and their local and regional sources, which are important information for the atmospheric research in the region.