Influence of biotic exchange and combustion sources on atmospheric CO2 concentrations in New England from observations at a forest flux tower

Hourly data for concentrations and fluxes of CO 2 at 30 m in Harvard Forest (Petersham, Massachusetts) are analyzed using linear modeling to obtain regionally representative CO 2 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 (C 2 H 2 )), and measurements of indicators of proximate canopy exchange (CO 2 flux and momentum flux). Results are compared to observations at Cold Bay, Alaska. Combustion contributes on average 4–5 ppm to ambient CO 2 at Harvard Forest in winter and 2–3 ppm in summer. Regional biotic emissions elevate daily mean CO 2 by 4–6 ppm in winter, and the covariance of the biotic cycle of uptake and emission with PBL height enhances daily mean CO 2 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 CO 2 can be determined at continental sites if suitable correlates (tracers, fluxes of CO 2 , and momentum) are measured simultaneously with CO 2 itself.