Incorporating Diurnal and Meter-Scale Variations of\nAmbient CO<sub>2</sub> Concentrations in Development of Direct Air\nCapture Technologies

To be implemented\non climate-relevant scales, direct air capture\nof CO<sub>2</sub> (DAC) will require large capital-intensive facilities\nand careful attention to cost minimization. In making decisions among\npotential sites for DAC facilities, all of the factors that will impact\nprocess cost and efficiency should be considered. In this paper we\nfocus on a factor that has previously received little attention in\nthe DAC community, namely variations in atmospheric conditions on\nhourly time scales and length scales of meters. We present data curated\nfrom extensive previous studies of biosphere-atmosphere fluxes with\nobservations of CO<sub>2</sub> concentration, temperature, and relative\nhumidity (RH) with hourly resolution from many sites in North America.\nThese include locations where typical diurnal variations in CO<sub>2</sub> concentration during summer months exceeds 150 ppm. These\nvariations are larger than the seasonal variations that exist between\naveraged CO<sub>2</sub> concentrations in winter and summer, and they\nare highly correlated with diurnal variations in temperature and RH.\nDiurnal variations are dependent on the height above ground at which\nCO<sub>2</sub> concentrations are measured, with smaller variations\nexisting at heights of 10 m or more than at ground level. We illustrate\nthe potential implications of these short-term variations for the\noperation and optimization of a DAC process with process-level calculations\nfor a specific adsorption-based process using amine-rich adsorbents.