<p>Direct air capture of CO₂ is needed to mitigate past emissions and those of persistent and difficult-to-abate sources. Current liquid-sorbent-based direct air capture relies on large-scale air handling and coupled sorbent–solid chemical loops, but the complexity and cost of this approach are barriers to scaling. Here we report a departure from established capture mechanisms in which ultraconcentrated KOH solutions (&gt;9 M) achieve rapid CO₂-to-carbonate crystallization at the air interface. On the basis of this finding, we develop a carbonate crystallizer that leverages evaporation to concentrate KOH on a wicking substrate, enabling the stable, passive capture of atmospheric CO₂ directly into a solid form. This approach achieves a capture flux over sixfold that of conventional systems, with regeneration demonstrated via a subsequent electrochemical step. A module with 100 such crystallizers achieved an average capture flux over threefold that of conventional contactors, with sustained operation over seven cycles and 25 days. This passive, single-chemical-loop approach has the potential to reduce capital and levelized costs by approximately 42% and 32%, respectively, compared with conventional liquid-based direct air capture systems.</p><p></p>