Carboxylic acids have been detected in particles collected in various regions of the world. Here, we use experiments and Born-Oppenheimer molecular dynamics simulations to better understand the mechanism of particle formation from gas phase mixtures of formic acid (HCOOH), (CH 3 ) 3 N, and water vapor. A flow reaction cell coupled to two scanning mobility particle sizers has been used to measure particle size, absolute number of particles and kinetics of particle formation. Experimental results

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... show that the addition of (CH 3 ) 3 N to a mixture of HCOOH and water vapor results in a dramatic increase in particle formation. Simulation results indicate that the ion-pair formation on the water surface involves direct proton transfer between HCOOH and (CH 3 ) 3 N. The HCOO -··(CH 3 ) 3 NH + ion-pair remains at the air-water interface due to hydrogen bonding and the interfacial hydration shell. This experiment-theory study shows the formation of aerosol particles from the organic acidamine interactions, which may aid in understanding the role of organics in haze and cloud droplet formation and nanoparticle growth.