Carbon dioxide (CO₂), an important greenhouse gas, is transferred from most estuaries to the atmosphere at a poorly known but potentially large rate. The difficulty in accurately quantifying the transfer is the lack of data of the amount of CO₂ in estuarine surface waters. We evaluated the proposition that the abundance of historical water quality measurements of pH could be used to fill this data gap by conducting an analysis of CO₂ and its transfer to the atmosphere in surface waters of the mainstem of the Chesapeake Bay, a large estuary of eastern United States. The analysis is unusual because of the large number of observations over many years that were used to calculate the CO₂ concentration in estuarine surface waters. CO₂ was found to vary greatly throughout the estuary, from season to season, and from year to year. Using measurements of temperature, salinity, and oxygen, we determined that photosynthesis and respiration had a large influence on surface water CO₂. Averaged over 1998–2018, the mainstem of the bay released CO₂ to the atmosphere. Our analysis suggests that the approach of using historical water quality measurements should be applied elsewhere to more accurately determine how much CO₂ is exchanged between world estuaries and the atmosphere.