Influence of relative humidity and temperature on the production of pinonaldehyde and OH radicals from the ozonolysis of α-pinene

The ozonolysis of alpha-pinene has been investigated under dry and humid conditions in the temperature range of 243-303 K. The results provided new insight into the role of water and temperature in the degradation mechanism of alpha-pinene and in the formation of secondary organic aerosols (SOA). The SOA yields were higher at humid conditions than at dry conditions. The water induced gain was largest for the lowest temperatures investigated (243 and 253 K). The increase in the SOA yields was dominated by water (and temperature) effects on the organic product distribution, whilst physical uptake of water was negligible. This will be demonstrated for the example of pinon-aldehyde (PA) which was formed as a major product in the humid experiments with total molar yields of 0.30+/-0.06 at 303 K and 0.15+/-0.03 at 243 K. In the dry experiments the molar yields of PA were only 0.07+/-0.02 at 303 K and 0.02+/-0.02 at 253 K. The observed partitioning of PA as a function of the SOA mass present at 303 K limited the effective vapour pressure of pure PA p(PA)(0) to the range of 0.01-0.001 Pa, 3-4 orders of magnitude lower than literature values. The corresponding mass partitioning coefficient was determined to K-PA=0.005+/-0.004 m(3) mu g-1 and the total mass yield alpha(PAtotal)=0.37+/-0.08. At 303 K PA preferably stayed in the gas-phase, whereas at 253 K and 243 K it exclusively partitioned into the particulate phase. PA could thus account at