Interaction of aerosol particles composed of protein and saltswith water vapor: hygroscopic growth and microstructural rearrangement
[摘要] The interaction of aerosol particles composed of the proteinbovine serum albumin (BSA) and the inorganic salts sodium chlorideand ammonium nitrate with water vapor has been investigated byhygroscopicity tandem differential mobility analyzer(H-TDMA) experiments complemented by transmission electronmicroscopy (TEM) and Köhler theory calculations (100-300nmparticle size range, 298K, 960hPa). BSA was chosen as awell-defined model substance for proteins and other macromolecularcompounds, which constitute a large fraction of the water-solubleorganic component of air particulate matter.
Pure BSA particles exhibited deliquescence and efflorescencetransitions at 
35% relative humidity (
) and ahygroscopic diameter increase by up to 10% at 95% in good agreement with model calculations based on a simpleparameterisation of the osmotic coefficient. Pure NaCl particleswere converted from near-cubic to near-spherical shape uponinteraction with water vapor at relative humidities below thedeliquescence threshold (partial surface dissolution andrecrystallisation), and the diameters of pure NH4NO3 particles decreased by up to 10% due to chemical decompositionand evaporation.Mixed NaCl-BSA and NH4NO3-BSA particles interactingwith water vapor exhibited mobility equivalent diameter reductionsof up to 20%, depending on particle generation, conditioning,size, and chemical composition (BSA dry mass fraction 10-90%).These observations can be explained by formation of porousagglomerates (envelope void fractions up to 50%) due toion-protein interactions and electric charge effects on the onehand, and by compaction of the agglomerate structure due tocapillary condensation effects on the other. The size ofNH4NO3-BSA particles was apparently also influencedby volatilisation of NH4NO3, but not as much as forpure salt particles, i.e. the protein inhibited the decompositionof NH4NO3 or the evaporation of the decompositionproducts NH3 and HNO3. The efflorescencethreshold of NaCl-BSA particles decreased with increasing BSA drymass fraction, i.e. the protein inhibited the formation of saltcrystals and enhanced the stability of supersaturated solutiondroplets.The H-TDMA and TEM results indicate that the protein was enrichedat the surface of the mixed particles and formed an envelope,which inhibits the access of water vapor to the particle core andleads to kinetic limitations of hygroscopic growth, phasetransitions, and microstructural rearrangement processes.The Köhler theory calculations performed with different typesof models demonstrate that the hygroscopic growth of particlescomposed of inorganic salts and proteins can be efficientlydescribed with a simple volume additivity approach, provided thatthe correct dry solute mass equivalent diameter and compositionare known. A parameterisation for the osmotic coefficient ofmacromolecular substances has been derived from an osmoticpressure virial equation. For its application only the density andmolar mass of the substance have to be known or estimated, and itis fully compatible with traditional volume additivity models forsalt mixtures.
