Organic acid-ammonia ion-induced nucleation pathways unveiled by quantum chemical calculation and kinetics modeling: A case study of 3-methyl-1,2,3-butanetricarboxylic acid

Abstract

Nucleation of organic acids (OAs) and H2SO4 is an important source for new particle formation in the atmosphere. However, it is still unclear whether organic acids can produce nanoparticles independent of H2SO4. In this study, 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) was adopted as a model of OAs. Pathways of clustering from MBTCA, ammonia and ions (NH4+ and NO3−) to form a 1.9 nm nucleus were investigated by quantum chemical calculation and kinetic modeling. Results show recombination of charged clusters/ions plays an essential role in the nucleation processes. Cluster formation rates increase by a factor of 103 when NH3 increases from 2.6 × 108 molecules·cm−3 (under clean conditions) to 2.6 × 1011 molecules·cm−3 (under polluted conditions), as NH3 can stabilize MBTCA clusters and change ion compositions from H3O+ to NH4+. Although the proposed new mechanism cannot compete with H2SO4–NH3–H2O or H2SO4-OA nucleation currently, it may be important in the future with the decline of SO2 concentration.