What Is Required for Highly Oxidized Molecules To Form Clusters with Sulfuric Acid?

We have studied the
specific requirements of a given neutral organic
molecule to act as a stabilizer in sulfuric acid induced new particle
formation. Based on an analysis of the reaction Gibbs free energies
between simple functional groups and sulfuric acid, carboxylic acid
groups are identified to show the strongest hydrogen bonding interaction
with sulfuric acid. The free energy associated with the hydrogen bonding
between sulfuric acid and 14 different carboxylic acids of atmospheric
relevance reveal that the binding strength is very dependent on the
ability of sulfuric acid to form an additional hydrogen bond via its
vacant S–OH group to a γ-carbonyl group in the organic
molecule. Extending the analysis to monoterpene oxidation products
and further to large dimer esters, we identify the following necessary
criteria for a given organic oxidation product to efficiently stabilize
sulfuric acid clustering: (1) weak or no intramolecular hydrogen bonds
in the isolated monomer; (2) more than two carboxylic acid groups.
As a proof of concept we show that these requirements correspond to
the docking of a sulfuric acid molecule between two non-interacting
carboxylic acid groups in the organic molecule. These findings suggests
that, for a given organic oxidation product to participate in the
initial steps in new particle formation involving sulfuric acid, very
distinct molecular features are required.