New Mechanistic Pathways for the Reactions of Formaldehyde with Formic Acid Catalyzed by Sulfuric Acid and Formaldehyde with Sulfuric Acid Catalyzed by Formic Acid: Formation of Potential Secondary Organic Aerosol Precursors

Formaldehyde (HCHO) plays an important role in the formation of secondary organic aerosols. Here, we report new mechanistic pathways for the reactions of formaldehyde with formic acid catalyzed by sulfuric acid and formaldehyde with sulfuric acid catalyzed by formic acid by using quantum chemical methods and reaction rate theory. We have found that sulfuric acid and formic acid have a strong catalytic role in the HCHO···HCOOH + H2SO4 and HCHO···H2SO4 + HCOOH reactions because the calculated energy barriers of the sulfuric acid-catalyzed reaction of formaldehyde with formic acid and formic acid-catalyzed reaction of formaldehyde with sulfuric acid are decreased by 21.17 and 14.81 kcal/mol, respectively. The calculated rates show that the reaction rate of HCHO···HCOOH + H2SO4 is faster than those of HCHO···H2SO4 + HCOOH and HCHO + HCOOH···H2SO4. Moreover, the HCHO + HCOOH + H2SO4 → H2C(OH)OCOH + H2SO4 reaction readily forms the carboxylic acid ester (H2C(OH)OCOH); this is a new mechanistic pathway for the reaction of formaldehyde with formic acid, which is expected to extend to other aldehydes' reaction with carboxylic acids. Additionally, the present findings also suggest that formic acid-catalyzed formaldehyde and sulfuric acid reaction leads to the formation of an organosulfate as the nucleation precursor. The present results not only help understand the mechanisms for the initial processes of atmospheric nucleation but also are expected to extend other aldehydes and acids in the atmosphere.