Gas-phase formation of water-soluble organic compounds in the atmosphere: A retrosynthetic analysis

Atmospheric particles contain a myriad of organic compounds, including many multifunctional, water-soluble organic compounds. Many of these compounds are postulated to be secondary of origin. This work investigates the possible precursors of several classes of multifunctional, water-soluble secondary organic compounds by analyzing the pathways leading to their formation, based on known gas-phase reactions. The analysis is termed `retrosynthetic' due to the backward direction of the analysis, from products to precursors. Pathways for multi-functional compounds were generated combinatorially, considering the formation of one functional group at a time. Many multifunctional organic compounds with carboxylic acid, carbonyl, and hydroxy functional groups were found to be first- or second-generation products of common anthropogenic and biogenic volatile organic compounds such as alkanes, alkenes, aromatics, and cyclic alkenes. The estimated yields of water-soluble organic compounds from primary precursors ranged from less than 1% to over 10%, based on stoichiometric considerations. The SOA formation index, which combines the concepts of yields and rates, was used to compare the feasibility of the retrosynthetic pathways. Many of the candidate pathways involve the isomerization reaction of alkoxy radicals and oxygenated intermediate products such as monocarboxylic acids and hydroxyaldehydes.