Precise CO₂ concentration measurements at marine stations and tall towers are crucial for quantifying global trends in atmospheric CO₂ concentrations. We propose that measurements in the continental planetary boundary layer—the poor cousin of the clean background stations—can be used to understand trends in, and controls, of atmospheric CO₂ concentrations at local and regional scales as well as global scales. The key is choosing appropriate time scales of integration for the data. In the US Southern Great Plains, we are measuring precise CO₂ concentrations continuously at 2–60 m and weekly at 300 and 3300 m above ground level (agl). CO₂ flux is measured in individual crop fields and pastures (4 m towers) and at 60 m. The precise CO₂ concentrations show strong continental influence in both diurnal and seasonal cycles. In continental regions, atmospheric CO₂ profiles are strongly influenced by atmospheric dynamics as well as ecosystem and anthropogenic fluxes. Relating site level measurements or atmospheric profiles to regional CO₂ budgets requires methods to represent or evaluate these influences. We observe inter-annual differences in the climatology of diurnal cycles (seasonal average diurnal cycles). Using the several years’ data for boundary layer concentrations, the annual trend in CO₂ growth nearly matches the value estimated by National Oceanic and Atmospheric administration (NOAA) Climate Monitoring Diagnostic Laboratory for our latitude band.

CO₂ emissions from fossil-fuel combustion can be estimated at the state or monthly level even when full data on fuel combustion are not available. Our hypothesis is that a representative proxy can accurately estimate the pattern of CO₂ emissions if a sufficient fraction of the total can be represented, even if the dataset used does not cover all energy consumption sectors. Our approach employs monthly sales data for each state from the U.S. Department of Energy’s Energy Information administration (EIA). This is used to estimate the relative proportions of solid, liquid and gaseous fossil fuels for each state for each month.

Measurements of CO₂ concentration are carried out on a weekly basis since 1992 on the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean. Measurements are based at the Station for Climate Observations, which rests on a rocky plateau (45 m asl) on the North-Eastern coast of the island, and are made with a NDIR analyzer. World Meteorological Organization (WMO) reference standards are used for calibrations.  Continuous measurements were started in 1998; they were interrupted in early 2003, and activated again in 2005. The continuous observations show evidence of a small daily cycle (amplitude < ±1 ppm) only during the months of June, July, and August. Mean annual cycles derived from weekly flask measurements show a dependency on the wind origin: the annual cycle and the annual CO₂ mean are smaller for winds originating from the Southern sectors, than for winds from Northern sectors. The continuous measurements were combined with daily backward airmass trajectories to identify the dependency of the CO₂ amount on the airmass origin. Trajectories provided by National Oceanic and Atmospheric administration / Air Resources Laboratory (Hysplit) are used. During winter, low CO₂ is generally connected to Southern/South-Eastern airmasses. In summer airmasses from North often display lower CO₂ content, due to the influence of the European sink.