Interaction of aerosol particles composed of protein and saltswith water vapor: hygroscopic growth and microstructural rearrangement

Abstract

Abstract. The interaction of aerosol particles composed of the protein bovine serum albumin (BSA) and the inorganic salts sodium chloride and ammonium nitrate with water vapor has been investigated by hygroscopicity tandem differential mobility analyzer (H-TDMA) experiments complemented by transmission electron microscopy (TEM) and Köhler theory calculations (100-300 nm particle size range, 298 K, 960 hPa). BSA was chosen as a well-defined model substance for proteins and other macromolecular compounds, which constitute a large fraction of the water-soluble organic component of air particulate matter.Pure BSA particles exhibited deliquescence and efflorescence transitions at ∼35% relative humidity (RH ) and a hygroscopic diameter increase by up to ∼10% at 95% RH in good agreement with model calculations based on a simple parameterisation of the osmotic coefficient. Pure NaCl particles were converted from near-cubic to near-spherical shape upon interaction with water vapor at relative humidities below the deliquescence threshold (partial surface dissolution and recrystallisation), and the diameters of pure NH 4 NO 3 particles decreased by up to 10% due to chemical decomposition and evaporation.Mixed NaCl-BSA and NH 4 NO 3 -BSA particles interacting with water vapor exhibited mobility equivalent diameter reductions of 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 porous agglomerates (envelope void fractions up to 50%) due to ion-protein interactions and electric charge effects on the one hand, and by compaction of the agglomerate structure due to capillary condensation effects on the other. The size of NH 4 NO 3 -BSA particles was apparCorrespondence to: U. Pöschl (ulrich.poeschl@ch.tum.de) ently also influenced by volatilisation of NH 4 NO 3 , but not as much as for pure salt particles, i.e. the protein inhibited the decomposition of NH 4 NO 3 or the evaporation of the decomposition products NH 3 and HNO 3 . The efflorescence threshold of NaCl-BSA particles decreased with increasing BSA dry mass fraction, i.e. the protein inhibited the formation of salt crystals and enhanced the stability of supersaturated solution droplets.The H-TDMA and TEM results indicate that the protein was enriched at the surface of the mixed particles and formed an envelope, which inhibits the access of water vapor to the particle core and leads to kinetic limitations of hygroscopic growth, phase transitions, and microstructural rearrangement processes.The Köhler theory calculations performed with different types of models demonstrate that the hygroscopic growth of particles composed of inorganic salts and proteins can be efficiently described with a simple volume additivity approach, provided that the correct dry solute mass equivalent diameter and composition are known. A parameterisation for the osmotic coefficient of macromolecular substances has been derived from an osmotic pressure virial equation. For its ...

Hygroscopicity of organic surrogate compounds from biomass burning and their effect on the efflorescence of ammonium sulfate in mixed aerosol particles

“…Kerminen (1997) estimated the necessary residence time for achievement of water equilibrium of aqueous droplets to be between 0.005 and 0.1 s (water uptake coefficient α w = 0.001, 25 • C) for 100 and 500 nm particles, respectively. Therefore, the typical residence time of a few seconds in the humidification or dehumidification section in a HTDMA measurement is assumed to be sufficient for most equilibrium hygroscopicity measurements (Brooks et al, 2004;Mikhailov et al, 2004). Moreover, our HGF results for the three pure organic components are in good agreement with data by Mochida and Kawamura, (2004), Brooks et al (2004), and conducted with different techniques and/or residence times.…”

“…The organic particles measured are likely always in the effloresced, i.e., crystalline, state apparently even at high RH. The apparent increase in diameter during dehumidification may be explained by particle shape restructuring since the (poly)crystalline particles are likely nonspherical at dry conditions but may become more sphere-like in shape when exposed to higher RH (Mikhailov et al, 2004). Also, no ERH of 4-hydroxybenzoic acid in the dehydration mode was observed during the experiments; the likely reason is that the highest RH reached in the humidifier was approximately 98 %, which may be below the ERH of 4-hydroxybenzoic acid, reported as above 98 % RH in another study (Mochida and Kawamura, 2004).…”

Intercomparison study of six HTDMAs: results and recommendations

“…On the other hand volatile aerosol particles may suffer from evaporation artefacts inside the HTDMA if the residence time exceeds a few tens of seconds. and Mikhailov et al (2004) demonstrated that this artefact can occur for ammonium nitrate particles. It can be expected that also other compounds (e.g.…”

Hygroscopicity distribution concept for measurement data analysis and modeling of aerosol particle mixing state with regard to hygroscopic growth and CCN activation

“…Moreover, κ values determined by HTDMA measurements can be different from κ values determined by CCN measurements, because of the general dependence of κ and equivalent hygroscopicity parameters (van't Hoff factor and osmotic coefficient) on solute concentrations (Mikhailov et al, 2004(Mikhailov et al, , 2009Rose et al, 2008;Reutter et al, 2009) and potential solubility effects (Petters and Kreidenweis, 2007).…”