The Thermal-Stability of Oligomers in Alpha-Pinene Secondary Organic Aerosol

Recent studies characterizing laboratory generated secondary organic aerosol (SOA) have shown that 50% or more of the particle mass is made up of oligomeric species, which based on molecular weight and structure should be nonvolatile. On the other hand, about 75% the particle mass is lost when SOA is passed through a denuder heated to 100 degrees C, suggesting that it is semi-volatile. To study the reason for this discrepancy, the chemical effects of a thermal denuder (TD) on oligomers in laboratory SOA formed by ozonolysis of alpha-pinene were examined with high-resolution molecular mass spectrometry. Both monomers and oligomers in the particle phase were lost to the gas phase upon heating to 100 degrees C. Additionally, when the species released to the gas phase during heating were allowed to readsorb onto the remaining particle mass, almost all of the readsorbed material was monomeric. Both of these observations suggest that oligomers decompose when heated in a TD. Chemical analysis of the remaining particle mass after passing through the TD showed formation of many new compounds that corresponded to oligomer decomposition products. While similar experiments should be performed with other aerosol types, these results suggest that care must be taken when attempting to use a TD at an elevated temperature to infer the volatility of SOA at ambient temperature.