With the 2011 promulgation of the Mercury and Air Toxics Standards by the U.S. Environmental Protection Agency, and the successful negotiation by United Nations Environment Programme of the Minamata Convention, global emissions of mercury (Hg) to the atmosphere from anthropogenic sources are expected to decline.  Recent reports suggest regional gaseous Hg declines have already begun well before they were anticipated; however, providing independent evidence for the drivers of such declines is difficult.  To address this challenge, the U.S. Geological Survey and the National Atmospheric Deposition program (NADP) have initiated a national-scale effort to establish a baseline of total gaseous mercury (TGM) and Hg stable isotopic compositions at 31 sites distributed across North America over a two-year period (March 2016 – May 2018).  Collaboration with national scale air quality monitoring networks, such as the Mercury Deposition Network (MDN) and the Atmospheric Mercury Network (AMNet), has provided the backbone for linking the baseline measurements of TGM stable isotopic compositions to long-term wet Hg deposition and gaseous Hg monitoring. Approximately two thirds of the bulk air samplers were operated at MDN or AMNet sites. The network includes a highly diverse set of sites ranging from remote (e.g., Denali National Park, Alaska and Mauna Loa, Hawaii) to highly urbanized locations (e.g., Bronx, New York and Boston, Massachusetts). For all sites, the average odd isotope mass independent fractionation (MIF, D¹⁹⁹Hg), an indicator of photochemistry, was -0.20±0.07‰ and had a range of -0.43 to 0.01 ‰. Mass dependent fractionation (MDF, d²⁰²Hg), the commonly used isotope ratio for source tracking, was generally positive with a mean value of 0.45±0.40‰, although intermittent negative MDF was also observed at some sites. Urban sites were consistently lighter in d²⁰²Hg (0.34‰) compared to remote background sites (0.65‰), potentially indicating differences between emission sources. The site at Oak Ridge National Lab, near the contaminated Y12 plant, had a mean d²⁰²Hg of -0.04±0.36‰, indicating an industrial Hg end member with very little odd MIF. Though regional differences in odd MIF and MDF were small, we observed a trend showing enrichment of d²⁰²Hg in the northeastern United States, from the Ohio River Valley to remote sites in northern Maine and Nova Scotia, suggesting a shift from localized emission sources to globally transported Hg across this region.  The long-term goal of this effort is to help provide a better understanding of regional trends in TGM isotopic composition and evaluate the success of new regulations.