I worked as Air Quality Engineer in Arizona for over 10 years. For five years before that I analyzed data for various pollutants in the air. Every summer Ozone is a problem in the Phoenix metro area (Maricopa county). Frequently we have HPA (High Pollution Advisory). Some days people are encouraged to car pool or use public transportation because NOx from car exhaust help create ground level Ozone, which is bad for our health. The Ozone in the upper atmosphere is 'good' because it protects us against the sun.
Here is a helpful link about the basics about Ozone.
Learn the difference between good (stratospheric) and bad (tropospheric) ozone, how bad ozone affects our air quality, health, and environment, and what EPA is doing about it through regulations and standards.
www.epa.gov
I am not surprised that big cities have bad air. More traffic, industry contribute to it. The Maricopa County Air Quality Department, where I used to work, published Air Quality Index (AQI) daily to help us understand if the air that day is good/moderate/dangerous, etc.
In the USA,
www.airnow.gov tells you about the air quality at different locations.
Right now it is good where I am. It has not been too hot, which helps. It will change of course as the summer temperatures climb.