title: Liquid-liquid phase separation in aerosol particles: imaging at the nanometer scale.
abstract: Atmospheric aerosols can undergo phase transitions including liquid-liquid phase separation (LLPS) while responding to changes in the ambient relative humidity (RH). Here, we report results of chemical imaging experiments using environmental scanning electron microscopy (ESEM) and scanning transmission X-ray microscopy (STXM) to investigate the LLPS of micrometer-sized particles undergoing a full hydration-dehydration cycle. Internally mixed particles composed of ammonium sulfate (AS) and either: limonene secondary organic carbon (LSOC), α, 4-dihydroxy-3-methoxybenzeneaceticacid (HMMA), or polyethylene glycol (PEG-400) were studied. Events of LLPS were observed for all samples with both techniques. Chemical imaging with STXM showed that both LSOC/AS and HMMA/AS particles were never homogeneously mixed for all measured RH's above the deliquescence point and that the majority of the organic component was located in the outer phase. The outer phase composition was estimated as 65:35 organic: inorganic in LSOC/AS and as 50:50 organic: inorganic for HMMA/AS. PEG-400/AS particles showed fully homogeneous mixtures at high RH and phase separated below 89-92% RH with an estimated 70:30% organic to inorganic mix in the outer phase. These two chemical imaging techniques are well suited for in situ analysis of the hygroscopic behavior, phase separation, and surface composition of collected ambient aerosol particles.
authors: †Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.
authors: Rachel E
authors: RE
authors: O'Brien
authors: ‡William R. Wiley Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
authors: Bingbing
authors: B
authors: Wang
authors: †Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.
authors: Stephen T
authors: ST
authors: Kelly
authors: †Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.|§Department of Chemistry, University of California, Berkeley, California 94720, United States.
authors: Nils
authors: N
authors: Lundt
authors: ⊥Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1 Canada.
authors: Yuan
authors: Y
authors: You
authors: ⊥Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1 Canada.
authors: Allan K
authors: AK
authors: Bertram
authors: †Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.|§Department of Chemistry, University of California, Berkeley, California 94720, United States.|∥Department of Physics, University of California, Berkeley, California 94720, United States.
authors: Stephen R
authors: SR
authors: Leone
authors: ‡William R. Wiley Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
authors: Alexander
authors: A
authors: Laskin
authors: †Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-8198, United States.
authors: Mary K
authors: MK
authors: Gilles
mesh_terms: D000336:Aerosols / Q000737:chemistry*; D000645:Ammonium Sulfate / Q000737:chemistry; D053138:Cyclohexenes / Q000737:chemistry; D000077222:Limonene; D008855:Microscopy, Electron, Scanning; D008956:Models, Chemical; D010316:Particle Size; D052638:Particulate Matter / Q000737:chemistry; D044367:Phase Transition; D011092:Polyethylene Glycols / Q000737:chemistry; D013729:Terpenes / Q000737:chemistry
chemical_list: D000336:Aerosols; D053138:Cyclohexenes; D052638:Particulate Matter; D013729:Terpenes; D011092:Polyethylene Glycols; D000077222:Limonene; D000645:Ammonium Sulfate