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

• Atmospheric fluxes of methane in the mangrove biosphere.
• Seasonal variability of methane mixing ratio in the mangrove atmosphere.
• Atmospheric methane photo-oxidation and its byproducts.
• Mathematical model to predict and explain variability of methane mixing ratio.

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 0.14 mg m−2 d−1, respectively. Together these sources contribute 10.89 Gg of CH4 annually to the atmosphere, of which 99.17% was from sediment. The atmospheric CH4 mixing ratio varied between 1.693 and 2.251 ppmv, having an average of 2.004 ppmv. CH4 exchange flux from this mangrove biosphere to the atmosphere was 0.086 mg m−2 d−1, contributing 0.30 Gg of CH4 annually. The annual average rate of CH4 photo-oxidation in the forest atmosphere was 3.25 × 10−9 mg cm−3 d−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 9.26 Gg 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 ± 1.61 ppbv and 14.66 ± 1.88–37.90 ± 0.91 ppbv, 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 139 Gg yr−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.