| Abstract
| - Using an electrodynamic balance, we determined the relative humidity (RH) at which aqueous inorganic−malonic acid particles crystallized, with ammonium sulfate ((NH4)2SO4), letovicite ((NH4)3H(SO4)2), orammonium bisulfate (NH4HSO4) as the inorganic component. The results for (NH4)2SO4−malonic acid particlesand (NH4)3H(SO4)2−malonic acid particles show that malonic acid decreases the crystallization RH of theinorganic particles by less than 7% RH when the dry malonic acid mole fraction is less than 0.25. At a drymalonic acid mole fraction of about 0.5, the presence of malonic acid can decrease the crystallization RH ofthe inorganic particles by up to 35% RH. For the NH4HSO4−malonic acid particles, the presence of malonicacid does not significantly modify the crystallization RH of the inorganic particles for the entire range of drymalonic acid mole fractions studied; in all cases, either the particles did not crystallize or the crystallizationRH was close to 0% RH. Size dependent measurements show that the crystallization RH of aqueous (NH4)2SO4 particles is not a strong function of particle volume. However, for aqueous (NH4)2SO4−malonic acidparticles (with dry malonic acid mole fraction = 0.36), the crystallization RH is a stronger function of particlevolume, with the crystallization RH decreasing by 6 ± 3% RH when the particle volume decreases by anorder of magnitude. To our knowledge, these are the first size dependent measurements of the crystallizationRH of atmospherically relevant inorganic−organic particles. These results suggest that for certain organicmole fractions the particle size and observation time need to be considered when extrapolating laboratorycrystallization results to atmospheric scenarios. For aqueous (NH4)2SO4 particles, the homogeneous nucleationrate data are a strong function of RH, but for aqueous (NH4)2SO4−malonic acid particles (with dry organicmole fraction = 0.36), the rates are not as dependent on RH. The homogeneous nucleation rates for aqueous(NH4)2SO4 particles were parametrized using classical nucleation theory, and from this analysis we determinedthat the interfacial surface tension between the crystalline ammonium sulfate critical nucleus and an aqueousammonium sulfate solution is between 0.053 and 0.070 J m-2.
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