Crystallization Pathways of Sulfate−Nitrate−Ammonium Aerosol Particles

Crystallization experiments are conducted for aerosol particles composed of aqueous mixtures of
(NH4)2SO4(aq) and NH4NO3(aq), (NH4)2SO4(aq) and NH4HSO4(aq), and NH4NO3(aq) and NH4HSO4(aq).
Depending on the aqueous composition, crystals of (NH4)2SO4(s), (NH4)3H(SO4)2(s), NH4HSO4(s),
NH4NO3(s), 2NH4NO3·(NH4)2SO4(s), and 3NH4NO3·(NH4)2SO4(s) are formed. Although particles of
NH4NO3(aq) and NH4HSO4(aq) do not crystallize even at 1% relative humidity, additions of 0.05 mol fraction
SO42-(aq) or NO3-(aq) ions promote crystallization, respectively. 2NH4NO3·(NH4)2SO4(s) and
(NH4)3H(SO4)2(s) appear to serve as good heterogeneous nuclei for NH4NO3(s) and NH4HSO4(s), respectively.
2NH4NO3·(NH4)2SO4(s) crystallizes over a greater range of aqueous compositions than
3NH4NO3·(NH4)2SO4(s). An infrared aerosol spectrum is provided for each solid based upon a linear
decomposition analysis of the recorded spectra. Small nonzero residuals occur in the analysis because aerosol
spectra depend on particle morphology, which changes slightly across the range of compositions studied. In
addition, several of the mixed compositions crystallize with residual aqueous water of up to 5% particle
mass. We attribute this water content to enclosed water pockets. The results provide further insights into the
nonlinear crystallization pathways of sulfate−nitrate−ammonium aerosol particles.