Structural analysis and flux associations of CO2, H2O, heat and ozone over cotton and grape canopies

Micrometeorological tower data, collected over grape and cotton canopies as part of the California ozone deposition experiment (CODE) during the summer of 1991, are used to examine the temporal association between fluxes, and the physical characteristics of the coherent structures which dominate transport for both stable nighttime and unstable daytime conditions. Flux was calculated using the eddy covariance technique and the dominant modes of flux transport determined by quadrant analysis. The mean flux densities for both the cotton and grape site showed the surface acting as a sink for CO2 and ozone and a source of heat and H2O during the day, as would be expected, while during the night it became a source for CO2 and a sink for heat, but remained a sink for ozone and a source of H2O. The flux association results indicated a single vegetated ozone sink for the grape site, but a vegetated as well as a non-vegetated sink for the cotton site. For both sites, structures simultaneously transporting significant flux contributions of CO2, H2O, heat and ozone dominate during unstable conditions, but differed during stable conditions, where unmixed single flux structures dominated over cotton but not over grape. Structure sizes were less than 10 m during nighttime conditions and ranged from 3 to 69 m during the day. The results of this study contribute empirical evidence about the relationship between ozone uptake and the physical and physiological state of vegetation, as well as the limitations placed on eddy scales in simulation models.