Thermodynamics of the formation of atmospheric organic particulate matter by accretion reactions—Part 1: aldehydes and ketones

The term “accretion reactions” is introduced here to refer to the large collection of reactions by which atmospheric organic molecules can add mass, especially as by combination with other organic molecules. A general thermodynamic approach is developed for evaluating the tendency of atmospheric constituents (e.g., C10 aldehydes) to undergo accretion reactions (e.g., dimerization) and thereby form less volatile molecules (e.g., aldol condensation products) that may subsequently condense and so contribute to the levels of organic particulate matter (OPM) observed in the atmosphere. As an example, gaseous compounds A and B may contribute to OPM formation by the net overall reaction Ag+Bg=Cliq. This reaction may occur according to any of three kinetic schemes. Scheme I: (1) Ag+Bg=Cg (accretion in the gas phase): then (2) Cg=Cliq (condensation of the accretion product); Scheme II: (1) Bg=Bliq (condensation of B); then (2) Ag+Bliq=Cliq (heterogeneous accretion reaction of gaseous A with condensed B); or Scheme III: (1) Ag+Bg=Aliq+Bliq (condensation of A and B); then (2) Aliq+Bliq=Cliq (accretion of A with B within the PM phase). For all three schemes, the net overall reaction remains Ag+Bg=Cliq. The overall thermodynamic tendency of the net reaction remains the same regardless of the actual predominating kinetic mechanism. If an accretion reaction between two atmospheric components is to produce significant new OPM, appreciable amounts of the product C must form, and the vapor pressure of C must be relatively low so that a significant proportion of C can condense into the multicomponent liquid OPM phase. This study considers the thermodynamics of accretion reactions of atmospheric aldehydes including: (a) hydration, polymerization (i.e., oligomer formation), hemiacetal/acetal formation; and (b) aldol condensation. It was concluded regarding OPM formation that: (1) the reactions in the first group are not thermodynamically favored, either in the atmosphere, or in the “acid-catalyzed” chamber experiments of Jang and Kamens (Environ. Sci. Technol. 35 (2001b) 4758) with n-butanal, n-hexanal, n-octanal, and n-decanal; (2) aldol condensation is not thermodynamically favored for the conditions of the Jang and Kamens (2001b) experiments with those four aldehydes; (3) the mechanism for any observed OPM formation from n-butanal, n-hexanal, and n-octanal in those experiments remains unknown, and may also have been involved in the “acid-catalyzed” experiments with n-decanal; (4) whether Jang and Kamens (2001b) observed the consequences of aldol condensation in their n-decanal experiments remains unclear due in part to uncertainties in the free energy of formation (ΔGf0) values for the aldol products of n-decanal; (5) analogous refinement in the quality of needed ΔGf0 values is required to clarify the potential importance of aldol products of pinonaldehyde in the formation of ambient OPM; and (6) the possibility that photo-assisted mechanisms may compensate for unfavorable thermodyamics in the formation of accretion products in the atmosphere should be considered.