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Volume concentrations of steady‐state secondary organic aerosol (SOA) were measured in several hydrocarbon/NO x irradiation experiments after passing through a constant temperature heated tube. Higher temperatures resulted in greater loss of particle volume. Negatives of the measured effective enthalpies of vaporization (Δ H vap , eff 25–300 ) for SOA range from 11 to 44 kJ mol −1 , and depend upon the reactant hydrocarbon (α‐pinene , toluene, or 1,3,5‐trimethylbenzene). Significant negative correlations between Δ H vap , eff 25–300 and NO x are observed for SOA formed from α‐pinene or toluene. The Δ H vap , eff 25–300 of SOA formed in the photooxidation of toluene is similar to that generated by the nebulization of glyoxal, indicating that the oligomers formed by nebulizing glyoxal may represent a surrogate for the chemical mixture formed in toluene photooxidation. These laboratory measurements suggest that the thermodynamic behavior of SOA depends upon the reactant hydrocarbon mixture and NO x concentration.
Offenberg et al. (Wed,) studied this question.