Efflorescence upon humidification? X‐ray microspectroscopic in situ observation of changes in aerosol microstructure and phase state upon hydration

Abstract

The phase and mixing state of atmospheric aerosols is a central determinant of their properties and thus their role in atmospheric cycling and climate. Particularly, the hygroscopic response of aerosol particles to relative humidity (RH) variation is a key aspect of their atmospheric life cycle and impacts. Here we applied X-ray microspectroscopy under variable RH conditions to internally mixed aerosol particles from the Amazonian rain forest collected during periods with anthropogenic pollution. Upon hydration, we observed substantial and reproducible changes in particle microstructure, which appear as mainly driven by efflorescence and recrystallization of sulfate salts. Multiple solid and liquid phases were found to coexist, especially in intermediate humidity regimes. We show that X-ray microspectroscopy under variable RH is a valuable technique to analyze the hygroscopic response of individual ambient aerosol particles. Our initial results underline that RH changes can trigger strong particle restructuring, in agreement with previous studies on artificial aerosols.

Hygroscopic behavior of NaCl–MgCl<sub>2</sub> mixture particles as nascent sea-spray aerosol surrogates and observation of efflorescence during humidification

“…5b). Efflorescence during humidification was once reported for Amazonian rain forest aerosols (Pöhlker et al, 2014), where it was claimed that the impacted ambient organic-inorganic mixed aerosols initially had amorphous or poly-crystalline structures and underwent restructuring through kinetic water and ion mobilization, resulting in the crystallization of inorganic salts during hydration. During the humidification process, the MgCl 2 q 4H 2 O-dominant eutonic solids in the effloresced type B particles of X NaCl = 0.1 and 0.2 dissolve at MDRH 1 (i.e., 15.9 (±0.3) %) and some moisture is also adsorbed on the surface of the crystalline NaCl moiety.…”

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…This commonly happens when BC is emitted by BB, since it is co-emitted with large amounts of organic vapors that would condense on BC particles (Saleh et al, 2014). Black carbon particles can also obtain a secondary organic aerosol (SOA) coating during advection over the rain forest as well as inorganic coatings, which has been previously observed at the ATTO site (Pöhlker et al, 2014). It has been found that the coating mass significantly affects the absorption enhancement of BC cores but no significant changes are caused by a different coating's O:C ratio (Tasoglou et al, 2017).…”

Measuring Mass-Based Hygroscopicity of Atmospheric Particles through in Situ Imaging

“…STXM/NEXAFS has previously been used to quantify the hygroscopicity of laboratory 387 generated particles of known composition 29,30,32,33 and to observe the hygroscopic behavior of 388 ambient particles of unknown composition. 21 Here, we demonstrated that the combination of 389 STXM/NEXAFS and SEM/EDX could quantitatively determine the hygroscopic behavior of 390 both homogeneous particles with known composition and mixed atmospheric particles of 391 unknown composition. Mass-based growth factors of mixed atmospheric particles of unknown 392 composition were obtained on a per-particle basis by combining SEM/EDX measurements with 393 STXM/NEXAFS characterizations.…”