Kinetics feasibility study of alcohol sulfate esterification reactions in tropospheric aerosols

Sulfate esters have recently been identified in the secondary organic aerosol (SOA) formed in the photooxidation of biogenic hydrocarbons both in laboratory and under ambient atmospheric conditions. In the present study, the kinetics feasibility of direct reactions between alcohols and sulfuric acid to form sulfate esters in aerosol particles is explored. Nuclear magnetic resonance methods were used to monitor the bulk reaction kinetics of sulfate esterification reactions for a number of simple alcohols. The experiments were carried out at various sulfuric acid concentrations and a range of temperatures in order to estimate the rate constants of such reactions in aerosols under the previously studied laboratory conditions and under ambient atmospheric conditions. The measured rate constants did not depend greatly on the identity of the reactant alcohol, but increased strongly as a function of the sulfuric acid concentration, as predicted by excess acidity theory. Because of the strong temperature dependence of the rate constants for the direct reaction of alcohols with sulfuric acid, it appears that these reactions are kinetically infeasible for low temperature upper tropospheric sulfuric aerosols. For lower tropospheric SOA, it appears that the aerosol acidity is rarely high enough such that these reactions are likely to be responsible for the presence of sulfate esters in SOA.