Impact of humidity on the ozone initiated oxidation of limonene, Δ3-carene, and α-pinene

The effect of water on the initial secondary organic aerosol (SOA) formation from gas-phase ozonolysis of limonene, Delta(3)-carene, and a-pinene (similar to 3 and similar to 1.5 x 10(11) molecule cm(-3) reacted) has been investigated in a flow reactor at controlled relative humidity (RH), temperature (298 +/- 0.4 K), and reaction time (270 +/- 2 s). Low amounts of terpene converted minimize the impact of secondary reactions. A comparison of the SOA formation from the three terpenes was made for initial rate of reactions being around 7.5 x 10(8) and 15 x 10(8) molecule cm-3 s(-1). The most efficient species in producing SOA was limonene, while alpha-pinene was the least efficient. The results showed that an enhancement in water vapor concentration (< 2-85% RH) caused an increase in both integrated mass (M10-300nm) and total number (N10-300nm). The effect on number and mass were a factor of 2-3 and 4-8, respectively. Physical water up-take can partly explain the increase in mass, but not the observed increase in number. Therefore it was concluded that the increase in water concentration must, by a gas-phase reaction, produce more low volatility product(s).