Atmospheric fluxes and photo-oxidation of methane in the mangrove environment of the Sundarbans, NE coast of India; A case study from Lothian Island

Abstract

Atmospheric fluxes of CH4 across different boundaries and its photo-oxidation were measured in the Sundarbans mangrove forest during June 2010–December 2011. The annual mean CH4 emission rate from intertidal sediment and adjacent estuarine surfaces were 7.06 and 0.14mgm−2d−1, respectively. Together these sources contribute 10.89Gg of CH4 annually to the atmosphere, of which 99.17% was from sediment. The atmospheric CH4 mixing ratio varied between 1.693 and 2.251ppmv, having an average of 2.004ppmv. CH4 exchange flux from this mangrove biosphere to the atmosphere was 0.086mgm−2d−1, contributing 0.30Gg of CH4 annually. The annual average rate of CH4 photo-oxidation in the forest atmosphere was 3.25×10−9mgcm−3d−1 with maximum oxidation occurring during the monsoon period and minimum oxidation occurring during the post-monsoon period. Total CH4 photo-oxidation within the atmospheric boundary layer of the Sundarbans was 9.26Gg annually, which acts as sink for 85.03% of the emitted CH4. The HCHO and O3 mixing ratios in the forest atmosphere ranged between 2.14±0.06–4.08±1.61ppbv and 14.66±1.88–37.90±0.91ppbv, respectively, having maximal pre-monsoon and minimal monsoon periods. The CH4 photo-oxidation mediated productions of HCHO and O3 within the atmospheric boundary layer were 17.5 and 139Ggyr−1, respectively. A ‘sources and sinks dependent’ mathematical model for atmospheric CH4 proposed in this study can explain the seasonal variability of CH4 and showed significant correlation with the observed values of this regional atmosphere.