Environ Monit Assess (2012) 184:1857–1863 DOI 10.1007/s10661-011-2084-9 Ground level volume mixing ratio of methane in a tropical coastal city George Thomas · E. J. Zachariah Received: 27 November 2010 / Accepted: 11 April 2011 / Published online: 25 May 2011 © Springer Science+Business Media B.V. 2011 Abstract Urban regions are hotspots of green- house gas emissions which include CO 2 , CH 4 , N 2 O, etc. Methane is a strong greenhouse gas which is produced from a number of sources in- cluding fossil fuel combustion, municipal waste, and sewage processing, etc. Ground level mixing ratio of methane in the tropical coastal city of Thiruvananthapuram in South India, during calm early morning period was measured. Measure- ments were done during both winter and summer seasons. Concentrations were significantly higher than global average value. Intra-city variation in ground level mixing ratio was also significant. Ground level methane concentration at Thiru- vananthapuram urban area showed maximum value of 3.16 ppmV. Under stable atmospheric conditions in early morning, ground level mixing ratio of methane was 2.79 ppmV in winter and G. Thomas (B ) · E. J. Zachariah Atmospheric Sciences Division, Centre for Earth Science Studies, PB 7250, Thuruvikkal PO, Thiruvananthapuram, 695031, India e-mail: geonce@gmail.com E. J. Zachariah e-mail: ejzach@vsnl.com 2.54 ppmV during summer. The spatial distribu- tion of methane concentration shows correlation with urban heat island. Keywords Methane · Urban regions · Coastal city · Urban heat island Introduction Urban regions are known to be hotspots of green- house gas emissions. These greenhouse gases include carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), carbon monoxide (CO), and a variety of industrial gases including halo-carbons, sulfur compounds, etc. Main rea- sons are the concentration of industries, auto- mobiles, household emissions, municipal waste, and sewage accumulation and processing, etc. Methane is the second largest contributor among greenhouse gases to global warming through ra- diative forcing of the Earth’s climate system. Industrialization and urbanization leads to the increase in concentration of greenhouse gases. This eventually alters the radiative balance of the climate system. The net change in the irradi- ance at the upper atmospheric boundary between troposphere and stratosphere in a given climate system is termed radiative forcing (IPCC 2001). Pre-industrial era has been taken as the base- line for the radiative forcing calculations. The