Hourly data for concentrations and fluxes of CO2 at 30 m in Harvard Forest (Petersham, Massachusetts) are analyzed using linear modeling to obtain regionally representative CO2 concentrations at a continental site. The time series is decomposed into contributions due to regional combustion, local canopy exchange, monthly average regional biotic exchange (as modulated by the daily cycle of growth and decay of the planetary boundary layer (PBL)), and the regional monthly background concentration. Attributions are derived using time series analysis, data for a tracer for combustion (CO or acetylene (C2H2)), and measurements of indicators of proximate canopy exchange (CO2 flux and momentum flux). Results are compared to observations at Cold Bay, Alaska. Combustion contributes on average 4--5 ppm to ambient CO2 at Harvard Forest in winter and 2--3 ppm in summer. Regional biotic emissions elevate daily mean CO2 by 4--6 ppm in winter, and the covariance of the biotic cycle of uptake and emission with PBL height enhances daily mean CO2 by 1--2 ppm in summer; minimum values in late afternoon average 10 ppm lower than at Cold Bay in summer. The study shows that regionally representative concentrations of CO2 can be determined at continental sites if suitable correlates (tracers, fluxes of CO2, and momentum) are measured simultaneously with CO2 itself. ¿ 1999 American Geophysical Union