Effect of Mixing Ammonia and Alkylamines on Sulfate Aerosol Formation

Sulfate
aerosols’ cooling effect on the global climate has
spurred research to understand their mechanisms of formation. Both
theoretical and laboratory studies have shown that the formation of
sulfate aerosols is enhanced by the presence of a base like ammonia.
Stronger alkylamine bases such as monomethylamine (MMA), dimethylamine
(DMA), and trimethylamine (TMA) further increase aerosol formation
rates by many orders of magnitude relative to that of ammonia. However,
recent lab measurements have found that the presence of ammonia and
alkylamines together increases nucleation rates by another 1–2
orders of magnitude relative to the stronger alkylamines alone. This
work explores that observation by studying the thermodynamic stability
of clusters containing up to two sulfuric acids and two bases of the
same or different type. Initial configurational sampling is performed
using genetic algorithm (GA) interfaced to semiempirical methods to
find a large number of low-energy configurations. These structures
are then subject to quantum mechanical calculations using PW91, M06-2X,
and ωB97X-D functionals and MP2 with large basis sets. The thermodynamics
of formation is reviewed to determine if it rationalizes why mixed
base systems yield higher rates of aerosol formation than single base
ones. The gas phase basicity of the bases in a cluster is the main
determinant of binding strength in smaller clusters such as those
in the current study while aqueous phase basicity is more important
for larger particles. Besides thermodynamic considerations, the differences
in aerosol formation mechanisms as a function of size and between
the gas and particle phases are discussed.