Local influences on the composition of precipitation on Bermuda

The understanding of atmospheric biogeochemical cycles in marine areas depends heavily on the collection and analysis of precipitation samples. However, collection of marine precipitation is difficult due to constraints on sampling locations. For example, the island of Bermuda has been used to characterize the composition of precipitation over the western north Atlantic Ocean. However, Bermuda is a small, heavily populated island and the possibility exists that local processes can influence the composition of precipitation. Using 2 years of data on the composition of precipitation at 2 locations on Bermuda, this paper documents that enrichments of sea-salt in Bermuda precipitation, relative to open ocean precipitation, can occur due to ocean-island turbulence. Smaller enrichments of non-sea-salt (nss) Ca⁺⁺, nss SO⁼₄, NH⁺₄ and H⁺ are also observed. At present it is not possible to determine whether the enrichments are due to island sources or spatial variability in the precipitation scavenging process. By assuming the latter effect to be negligible, the upper limits of the effect of island sources on the concentrations of the nss components (Ca⁺⁺, SO⁼₄, NO^-₃, H⁺) are ~ 5-30%, depending on the species. Precipitation associated with westerly airflow is the least affected by local sources.