The formation of organic sulfate esters in the limonene ozonolysis secondary organic aerosol (SOA) under acidic conditions

Secondary organic aerosol (SOA) formation from the gas phase ozonolysis of limonene and the influence of particle acidity were investigated in a series of indoor aerosol chamber experiments. The particle acidity was adjusted in three stages using Na2SO4 (neutral), (NH4)2SO4/H2SO4 (acidic) and H2SO4 (strongly acidic) seed particles. Detected low molecular weight carboxylic acids (Mw<300) were found to account only for a smaller fraction of produced SOA mass at all particle acidities. Although SOA components with molecular weights between 300 and 900 were detected regardless of the seed particle acidity, the intensities of these compounds were much higher for acidic seed particle experiments. Compounds with mass to charge ratios (m/z) 281, 465 and 481 were only detected in the presence of sulfuric acid. MSn and TOFMS analyses showed a strong fragment of m/z 97 and elemental compositions of C10H17O7S-, C20H33O10S- and C20H33O11S- suggesting organic sulfate structures, possibly formed by a heterogeneous acid catalyzed reaction of limonene oxidation products and sulfuric acid in the particle phase. The high intensities of organic sulfate esters suggest that these compounds contribute at least as much as first and second generation oxidation products to the SOA mass.