Heterogeneous nucleation of the efflorescence of (NH₄)₂SO₄ particles internally mixed with Al₂O₃, TiO₂, and ZrO₂

Accurate models of atmospheric processes are currently hindered by poor microphysical models of particle phase transitions, including efflorescence. For aqueous particles, it is possible that insoluble crustal inclusions behave as heterogeneous nuclei to induce efflorescence. This paper reports on the results of laboratory experiments concerning the efflorescence of (NH 4 ) 2 SO 4 particles that are internally mixed with TiO 2 , Al 2 O 3 , or ZrO 2 particles. The particles are prepared by condensing SO 3 and H 2 O vapors in the presence of insoluble metal oxide particles, followed by neutralization with NH 3 . A Fourier transform infrared spectrometer is employed to observe the aqueous‐to‐solid phase change as relative humidity decreases. Efflorescence occurs at 35% relative humidity (±2%) in the absence of heterogeneous nuclei. In contrast, internally mixed particles effloresce at 57% (Al 2 O 3 ), 59% (ZrO 2 ), and 65% (TiO 2 ) at 298 K.