Unexpected Growth Coordinate in Large Clusters Consisting of Sulfuric Acid and C8H12O6 Tricarboxylic Acid.

Abstract

The exact participating compounds and especially the involvement of multifunctional organic molecules in atmospheric new particle formation remain highly uncertain. Using 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) as a representative multifunctional organic molecule, this paper investigates the formation of large atmospheric molecular clusters consisting of up to six sulfuric acid (SA) molecules and four MBTCA molecules utilizing quantum chemical methods. The formation free energies of the clusters are calculated using density functional theory with the PW91 functional and a 6-31++G(d,p) basis set. Using cluster kinetics calculations, it is identified that the lowest free energy barrier in the system is along the MBTCA coordinate and that MBTCA by itself is capable of forming new particles without the assistance of sulfuric acid, if present in sufficient concentration. Employing the Atmospheric Cluster Dynamics Code (ACDC), the new particle formation rates of the (MBTCA) a(SA) b clusters were simulated and compared to the sulfuric acid-dimethylamine system and ambient measurements. The findings suggest that new particle formation in realistic atmospheric conditions originates from two different decoupled mechanisms: One consisting purely of organic acids and the other consisting of sulfuric acid and bases.