> is there any data that shows the was a greater improvement in cities with monitors than those without?

Yes, the article discusses that:

> To study this, they used satellite data to look at air pollution in 466 cities in 136 countries, of which 50 cities across 36 countries received the air-quality monitors by 2020. They looked at the air pollution in all of these cities in the years before and after the monitors were installed, using the years prior to installation and the cities that did not receive monitors as controls.

> [...] compared to their air quality before getting the monitor and to other similar cities that do not have a monitor.