Measurements of carbon dioxide on a very tall tower



We present a continuous, 2-year long record of carbon dioxide (CO2) mixing ratio at three altitudes up to 496 m above the ground on a television transmitter tower in the southeastern United States. The data show strong diurnal and seasonal variations, and large vertical gradients. The diurnal cycles are modulated by surface uptake and release by vegetation and soils, emissions from fossil fuel combustion, and by the diurnal development of the planetary boundary layer. Gradients of 1-2 ppm between 496 m and 51 m are typically observed during summertime afternoons, due to vigorous photosynthetic uptake. With increasing altitude the magnitude of the diurnal cycle is damped, and daily average mixing ratios decrease, caused by coincident changes in the sign and magnitude of the surface flux, and changes in vertical stability of the boundary layer over the course of the day. Measurements at 496 m give an approximate measure (within a few tenths of a ppm) of the afternoon mean mixing ratio in the convective boundary layer. Vertical gradients between 51 m and 496 m are typically close to zero, and monthly mean mixing ratios increase slowly between November and April, indicating that biological activity is minimal during this period. The amplitude of the seasonal cycle of CO2 is larger at the tower site than at marine boundary layer and mountaintop sites which are at nearly the same latitude, because of the proximity of the tower site to terrestrial sources and sinks. Comparison of our continental tower data with data from “background” sites should provide a strong constraint for regional and global models of terrestrial CO2 fluxes. We also present data from weekly flask samples taken from the 496 m level and which have been analyzed for methane (CH4), carbon monoxide (CO), and the stable isotopes of carbon in CO2 (δ13C). The flask data provide further information about the processes that drive observed changes in CO2 mixing ratio at the tower.