Secondary organic aerosol production from terpene ozonolysis. 2. Effect of NOx concentration

We report secondary organic aerosol (SOA) yields from the ozonolysis of alpha-pinene in the presence of NO and NO2. Experimental conditions are characterized by the VOC]0/ NOx]0 ratio (ppbC/ppb), which varies from approximately 1 to approximately 300. SOA yield is constant for VOC]0/NOx]0 > approximately 15 and decreases dramatically (by more than a factor of 4) as VOC]0/NOx]0 decreases. Aerosol production is completely suppressed in the presence of NO for VOC]0/NOx]0 < or = 4.5. Fouriertransform IR analysis of filter samples reveals that nitrate-containing species contribute significantly to the total aerosol mass at low VOC]0/NOx]0. Yield reduction is a result of the formation of a more volatile product distribution as VOC]0/NOx]0 decreases; we propose that the change in the product distribution is driven by changes in the gas-phase chemistry as NOx concentration increases. We also present two-product model parameters to describe aerosol production from the alpha-pinene/0/NOx system under both high- and low-NOx conditions.