Atmospheric elemental mercury and reactive gaseous mercury (RGHg) concentrations, as well as ancillary parameters and meteorological data, were collected during a cruise in the North Atlantic Ocean between Bermuda and Barbados and at two land-based sites: the Chesapeake Biological Laboratory (CBL), a semirural, coastal site, and a site within the city of Baltimore, Maryland. There were two deployments at CBL, including a 6-month deployment in 2003/2004. Wet deposition samples were collected and analyzed for mercury where possible. A diurnal change in RGHg concentration was found at both CBL and over the North Atlantic Ocean, with maxima in the afternoon that coincided with maximum UV radiation, demonstrating the importance of in situ formation of RGHg. The maxima in RGHg concentration did not coincide with elevated Hg0 but were often found under conditions of low wind speed, and over the ocean, under low ozone conditions. In contrast, at the urban site there was more evidence of local sources. The data collected at the sites are contrasted and compared and used to examine the factors controlling the formation of RGHg in open ocean and nearshore locales and to estimate the importance of the dry deposition of RGHg to surface waters. Finally, the data collected at CBL in 2002--2004 were compared to earlier data collected in 1997--2000 to examine trends in concentration with time and to contrast these with the open ocean data sets to examine changes in Hg0 at the hemispheric scale.