Guanidine: A Highly Efficient Stabilizer in Atmospheric New-Particle Formation

The role of a strong organobase,
guanidine, in sulfuric acid-driven
new-particle formation is studied using state-of-the-art quantum chemical
methods and molecular cluster formation simulations. Cluster formation
mechanisms at the molecular level are resolved, and theoretical results
on cluster stability are confirmed with mass spectrometer measurements.
New-particle formation from guanidine and sulfuric acid molecules
occurs without thermodynamic barriers under studied conditions, and
clusters are growing close to a 1:1 composition of acid and base.
Evaporation rates of the most stable clusters are extremely low, which
can be explained by the proton transfers and symmetrical cluster structures.
We compare the ability of guanidine and dimethylamine to enhance sulfuric
acid-driven particle formation and show that more than 2000-fold concentration
of dimethylamine is needed to yield as efficient particle formation
as in the case of guanidine. At similar conditions, guanidine yields
8 orders of magnitude higher particle formation rates compared to
dimethylamine. Highly basic compounds such as guanidine may explain
experimentally observed particle formation events at low precursor
vapor concentrations, whereas less basic and more abundant bases such
as ammonia and amines are likely to explain measurements at high concentrations.