I had basically completed part 4 of this series for posting in May, and my home state (California) reversed course on several pending actions. This is because expected actions by the silly federal chief executive, who opposes any stricter air quality regulations (he must not need to breathe).
My home state (California) has an interesting relationship with air. We have more vehicles than any other state, and also drive more miles. Many of these miles are driven in very long daily commutes in four of the largest metropolitan areas in the US. Our air quality problems date back to the 1950s. Since then, we have had the most aggressive pollution control measures in the U.S. In fact, in recent years a group of states with similar issues have adopted our solutions. One other thing, the governmental body that defines the rules to remedy our air quality issues is the California Air Resources Board (CARB).