Reactive gaseous mercury formation in the North Pacific Ocean's marine boundary layer: A potential role of halogen chemistry

[1] Reactive gaseous mercury (RGHg), atmospheric elemental mercury (Hg-0), and ozone (O-3) along with other ancillary parameters have been measured simultaneously in the marine boundary layer (MBL) over the North Pacific Ocean during the 2002-IOC cruise between Osaka, Japan, and Honolulu, Hawaii. Atmospheric Hg-0 concentrations varied from 1.6 to 4.7 ng m(-3) with an average of 2.5 ng m(-3) and did show significant variations along the cruise track with a slight diel cycle in Hg-0 concentrations. RGHg concentrations varied from 0.15 to 92.4 pg m(-3) with an average of 9.5 pg m(-3). The data strongly suggest photochemical in situ formation of RGHg with increasing concentrations in the tropical regions concomitant to a dramatic decrease in O-3 ( from over 50 to less than 5 ppbv). A distinct diurnal variation in RGHg concentrations with maxima at midday suggests a photochemically driven oxidation of marine boundary layer Hg-0. In the tropical area, enhanced RGHg formation and daily variation in O-3 could both be related to reactive halogen chemistry. Flux calculations also emphasize the role of the tropical marine boundary layer in mercury cycling over the North Pacific Ocean.