Biogenic hydrogen sulphide emissions and non-sea sulfate aerosols over the Indian Sundarban mangrove forest

Abstract

Temporal variations in atmospheric hydrogen sulphide concentrations and its biosphere-atmosphere exchanges were studied in the World’s largest mangrove ecosystem, Sundarbans, India. The results were used to understand the possible contribution of H2S fluxes in the formation of atmospheric aerosol of different size classes (e.g. accumulation, nucleation and coarse mode). The mixing ratio of hydrogen sulphide (H2S) over the Sundarban mangrove atmosphere was found maximum during the post-monsoon season (October to January) with a mean value of 0.59 ± 0.02 ppb and the minimum during pre-monsoon (February to May) with a mean value of 0.26 ± 0.01 ppb. This forest acted as a perennial source of H2S and the sediment-air emission flux ranged between 1213 ± 276 μg S m−2 d−1(December) and 457 ± 114 μg S m−2 d−1 (August) with an annual mean of 768 ± 240 μg S m−2d−1. The total annual emissions of H2S from the Indian Sundarban were estimated to be 1.2 ± 0.6 Tg S. The accumulation mode of aerosols was found to be more enriched with non-sea salt sulfate with an average loading of 5.74 μg m−3 followed by the coarse mode (5.18 μg m−3) and nucleation mode (1.18 μg m−3). However, the relative contribution of Non-sea salt sulfate aerosol to total sulfate aerosol was highest in the nucleation mode (83%) followed by the accumulation (73%) and coarse mode (58%). Significant positive relations between H2S flux and different modes of NSS indicated the likely link between H2S, a dominant precursor for the non-sea salt sulfate, and non-sea sulfate aerosol particles. An increase in H2S emissions from the mangrove could result in an increase in enhanced NSS in aerosol and associated cloud albedo, and a decrease in the amount of incoming solar radiation reaching the Sundarban mangrove forest.