A preliminary assessment of the sensitivity of air quality in California to global change

A regional air quality model was used to quantify the effect of temperature, humidity, mixing depth, and background concentrations on ozone (O 3 ) and airborne particulate matter during three air quality episodes in California. Increasing temperature with no change in absolute humidity promoted the formation of O 3 by +2 to +9 ppb K −1 through increased reaction rates. Increasing temperature with no change in relative humidity increased predicted O 3 concentrations by +2 to +15 ppb K −1 through enhanced production of hydroxyl radical combined with increased reaction rates. Increasing mixing depth promoted the formation of O 3 in regions with an over-abundance of fresh NO emissions (such as central Los Angeles) by providing extra dilution. Increasing temperature with no change in absolute humidity reduced particle water content and promoted the evaporation of ammonium nitrate at a rate of −3 to −7 μg m −3 K −1 . Increasing temperature with no change in relative humidity maintained particle water content and moderated ammonium nitrate evaporation rates to a maximum value of −3 μg m −3 K −1 during warmer episodes and increased ammonium nitrate condensation by +1.5 μg m −3 K −1 during colder episodes. Increasing mixing depth reduced the concentration of primary particulate matter but increased the formation of secondary particulate matter in regions with an over-abundance of fresh NO emissions. O 3 transported into California from upwind areas enhanced the formation of particulate nitrate by promoting the formation of N 2 O 5 and HNO 3 at night. A 30 ppb increase in background O 3 concentrations (roughly doubling current levels) increased maximum PM 2.5 concentrations by +7 to +16 μg m −3 even when temperature was simultaneously increased by +5 K with no change in absolute humidity (most unfavorable conditions for nitrate formation).