Elevated Trimethylarsine Oxide and Inorganic Arsenic in Northern Hemisphere Summer Monsoonal Wet Deposition

For
arsenic speciation, the inputs for wet deposition are not well
understood. Here we demonstrate that trimethylarsine oxide (TMAO)
and inorganic arsenic are the dominant species in monsoonal wet deposition
in the summer Indian subcontinent, Bangladesh, with inorganic arsenic
dominating, accounting for ∼80% of total arsenic in this medium.
Lower concentrations of both species were found in monsoonal wet deposition
in the winter Indian subcontinent, Sri Lanka. The only other species
present was dimethylarsinic acid (DMAA), but this was usually below
limits of detection (LoD). We hypothesize that TMAO and inorganic
arsenic in monsoonal wet deposition are predominantly of marine origin.
For TMAO, the potential source is the atmospheric oxidation of marine
derived trimethylarsine. For inorganic arsenic, our evidence suggests
entrainment of water column inorganic arsenic into atmospheric particulates.
These conclusions are based on weather trajectory analysis and on
the strong correlations with known wet deposition marine derived elements:
boron, iodine, and selenium. The finding that TMAO and inorganic arsenic
are widely present and elevated in monsoonal wet deposition identifies
major knowledge gaps that need to be addressed regarding the understanding
of arsenic’s global cycle.