A Centralized Photobioreactor Network for Algae-Based Urban Air Purification: Targeting Low-Canopy, High-AQI City Zones

In cities, neighborhoods with the most polluted air often have the fewest trees. This connection is not coincidental and has been confirmed by several independent datasets. However, there is a lack of effective methods to address this overlap directly, especially in areas where planting trees isn't practical. This paper explores one potential solution: a network of microalgae photobioreactors based on a hub-and-spoke model. In this setup, a single production facility cultivates algae under controlled conditions and distributes it to small reactors attached to bus shelters, medians, and building facades. A formula scores ten cities using 2024 PM2.5 readings and canopy-cover estimates. Jakarta, Cairo, and Delhi score highest (6.40, 5.32, and 4.24), while London and New York score near zero. A comparison of four candidate organisms, Chlorella vulgaris, Chlorella sorokiniana, Scenedesmus obliquus, and Spirulina, shows no clear winner in terms of fixation rate, pollutant removal, and outdoor hardiness. Therefore, a two-species combination of C. vulgaris and S. obliquus is proposed for a practical reactor design. This is not presented as a final technology. The gap between what these organisms achieve in a lab and their performance in real air is significant. This uncertainty is addressed rather than overlooked.