I find it highly unlikely they'd make much impact outdoors. 1 million cubic feet/hour is 16,666CFM. Wikipedia says a Manhattan city block is 264 by 900 feet. So

I find it highly unlikely they'd make much impact outdoors. 1 million cubic feet/hour is 16,666CFM. Wikipedia says a Manhattan city block is 264 by 900 feet. So even if we assume 10' ceiling (so 2.3M CuFt) and little airflow, we'd still need several of these towers to clean it. Except outdoors are higher than 10 feet, and even light wind (avg in NY is 9MPH) will replace the air several times an hour. More realistic are small filters like this one[1] where you have a relatively limited semi-enclosed space the air flows around. So perhaps in, say, small pavilions and seating areas in a park or something, it might be slightly useful if the surrounding area is rather polluted. But sticking a few in a "filthy rich neighborhood" isn't gonna accomplish much (apart from making money or making people feel smug). Maybe on a city-scale, using real industrial-strength devices all over would work. But one would assume that cities with smog issues must have already looked at filtering systems and there must be some serious limitation or they'd be deployed already. 1: https://www.techinasia.com/outdoor-air-purifier-wait-bus-40-smoggy/ https://www.techinasia.com/outdoor-air-purifier-wait-bus-40-...