In this paper, the level of air quality in ten highly polluted cities in California US state is investigated by looking at persistence andseasonality properties of the time series based on fractional integration framework. Five pollutants, which are carbon monoxide(CO), nitrogen dioxide (NO2), ozone (O3) and fine and coarse particulate matters (PM2.5and PM10), for ten selected cities,including Bakersfield, Los Angeles, and El Centro, are considered. The levels of CO, O3and PM2.5are above the exceedancelimits for these ten cities, while the results of fractional integration show the possibility of mean reversion for PM2.5whereas COlevels are still in non-mean reverting range. Levels of pollutants are found to be seasonal in all pollutant cases and across thecities. Since all pollutants pose serious concern to inhabitants of the cities, the California government should intensify more effortin making appropriate policy to reduce pollutant levels in the state.