Chasing quicksilver:: Modeling the atmospheric lifetime of Hg0(g) in the marine boundary layer at various latitudes

The lifetime of elemental mercury in the marine boundary layer (MBL) has been studied using AMCOTS (Atmospheric Mercury Chemistry Over The Sea), a box model of MBL photochemistry including aerosols and detailed mercury chemistry. Recently measured Hg-(g)(0) oxidation reactions have been included, and the studies were performed as a function of latitude, time of year, boundary layer liquid water content (LWC) and cloud optical depth. The results show that Hg has the shortest lifetime when air temperatures are low and sunlight and deliquescent aerosol particles are plentiful. Thus the modeled lifetime for clear-sky conditions is actually shorter at mid-latitudes and high latitudes than near the equator, and for a given latitude and time of year, cooler temperatures enhance the rate of Hg oxidation. Under typical summer conditions (for a given latitude) of temperature and cloudiness, the lifetime (tau) of Hg-(g)(0) in the MBL is calculated to be around 10 days at all latitudes between the equator and 60degrees N. This is much shorter than the generally accepted atmospheric residence time for Hg-(g)(0) of a year or more. Given the relatively stable background concentrations of Hg-(g)(0) which have been measured, continual replenishment of Hg-(g)(0) must take place, suggesting a "multihop" mechanism for the distribution of Hg, rather than solely aeolian transport with little or no chemical transformation between source and receptor. Inclusion of an empirical Hg-(g)(0) emission factor related to insolation was used to stabilize the Hg-(g)(0) concentration in the model, and the emission rates necessarily agree well with estimated emission fluxes for the open ocean.