Formation of secondary organic aerosol during the dark ozonolysis of α-humulene

Sesquiterpenes (C15H24) are a class of biogenic volatile organic compounds that are significant secondary organic aerosol (SOA) precursors due to their high reactivity with oxidants and their high SOA yields. Previous studies have focused almost exclusively on β-caryophyllene, and there is relatively little known about the other sesquiterpenes. In this study, we focus on another major sesquiterpene, α-humulene, which has three endo-cyclic double bonds. A series of experiments quantified the SOA production during the ozonolysis of α-humulene in the Carnegie Mellon atmospheric simulation chamber. The experiments resulted in high SOA yields ranging from 30 to 70% for SOA concentration in the range 10 to 100 μg m-3. Most of the SOA had effective volatility equal or less than 1 μg m-3 at 298 K and the average SOA effective vaporization enthalpy was 115 ± 23 kJ mol-1. The SOA aerosol mass spectrometer (AMS) spectrum was slowly evolving during experiments, which suggested modest differences, from the AMS point of view, between the SOA compounds produced initially and the SOA compounds produced towards the end of the experiment. The α-humulene SOA mass spectrum resembled that of βcaryophyllene SOA but it was less similar to α-pinene SOA.