It may be there’s no way for cities in the western United States to improve their air quality without an international accord:
According to a new study in Nature that analyzed large sets of ozone data captured since 1984, springtime ozone levels above western North America are rising primarily due to air flowing eastward from the Pacific Ocean, a trend that is largest when the air originates in Asia. These increases in ozone could make it more difficult for the United States to meet Clean Air Act standards for ozone pollution at ground level.
The study focused on springtime ozone in a slice of the atmosphere from two to five miles above the surface of western North America, far below the protective ozone layer but above ozone-related, ground-level smog that is harmful to human health and crops. Ozone in this intermediate region constitutes the northern hemisphere background or baseline level of ozone in the lower atmosphere. The study was the first to pull together and then analyze the nearly 100,000 ozone observations gathered in separate studies by instruments on aircraft, balloons, and other platforms.
How much influence ozone levels in the region two to five miles up have on ground level ozone is unknown. But it’s pretty clear that air pollution is more than just a local problem. Which means that controlling is probably not amenable to local solutions.