Diurnal and seasonal variations in surface methane at a tropical coastal station: Role of mesoscale meteorology

Highlights: • First time observation of the impacts of mesoscale meteorology on the temporal variation of methane at a coastal site • Mesoscale circulation namely sea-land breeze and the boundary layer height (BLH) significantly modulates the diurnal variation • Less homogeneity in the vertical distribution of methane over tropics • The contributions of the land breeze transported component and the changes in the BLH were delineated In view of the large uncertainties in the methane (CH₄) emission estimates and the large spatial gaps in its measurements, studies on near-surface CH₄ on regional basis become highly relevant. This paper presents the first time observational results of a study on the impacts of mesoscale meteorology on the temporal variations of near-surface CH₄ at a tropical coastal station, in India. It is based on the in-situ measurements conducted during January 2014 to August 2016, using an on-line CH₄ analyzer working on the principle of gas chromatography. The diurnal variation shows a daytime low (1898–1925 ppbv) and nighttime high (1936–2022 ppbv) extending till early morning hours. These changes are closely associated with the mesoscale circulations, namely Sea Breeze (SB) and Land Breeze (LB), as obtained through the meteorological observations, WRF simulations of the circulations and the diurnal variation of boundary layer height as observed by the Microwave Radiometer Profiler. The diurnal enhancement always coincides with the onset of LB. Several cases of different onset timings of LB were examined and results presented. The CH₄ mixing ratio also exhibits significant seasonal patterns being maximum in winter and minimum in pre-monsoon/monsoon with significant inter-annual variations, which is also reflected in diurnal patterns, and are associated with changing synoptic meteorology. This paper also presents an analysis of in-situ measured near-surface CH₄, column averaged and upper tropospheric CH₄ retrieved by Atmospheric Infrared Sounder (AIRS) onboard Earth Observing System (EOS)/Aqua which gives insight into the vertical distribution of the CH₄ over the location. An attempt is also made to estimate the instantaneous radiative forcing for the measured CH₄ mixing ratio.