We present a conceptually simple method for optimizing the design of a gas-liquid contactor for capture of carbon dioxide from ambient air, or 'air capture'. We apply the method to a slab geometry contactor that uses components, design and fabrication methods derived from cooling towers. We use mass transfer data appropriate for capture using a strong NaOH solution, combined with engineering and cost data derived from engineering studies performed by Carbon Engineering Ltd, and find that the total costs for air contacting alone-no regeneration-can be of the order of $60 per tonne CO(2). We analyse the reasons why our cost estimate diverges from that of other recent reports and conclude that the divergence arises from fundamental design choices rather than from differences in costing methodology. Finally, we review the technology risks and conclude that they can be readily addressed by prototype testing.