TY - JOUR
T1 - Vapor Condensation and Coating Evaporation Are Both Responsible for Soot Aggregate Restructuring
AU - Enekwizu, Ogochukwu Y.
AU - Hasani, Ali
AU - Khalizov, Alexei F.
N1 - Funding Information:
This work was supported by the National Science Foundation (AGS-1463702 to A.F.K.) and the Mid-Atlantic States Section of the Air and Waste Management Association (MASS-A&WMA) Air Pollution Educational and Research Grant Program (APERG scholarship to O.Y.E.). The authors acknowledge Mary McGuinness for initial calibration measurements of the EPC and Ella Ivanova for estimating condensation times of intermediate volatility chemicals in menisci.
Publisher Copyright:
© 2021 American Chemical Society
PY - 2021/7/6
Y1 - 2021/7/6
N2 - Fresh soot is made of fractal aggregates, which often appear collapsed in atmospheric samples. A body of work has concluded that the collapse is caused by liquid shells when they form by vapor condensation around soot aggregates. However, some recent studies argue that soot remains fractal even when engulfed by the shells, collapsing only when the shells evaporate. To reconcile this disagreement, we investigated soot restructuring under conditions ranging from capillary condensation to full encapsulation, also including condensate evaporation. In these experiments, airborne fractal aggregates were exposed to vapors of wetting liquids, and particle size was measured before and after coating loss, allowing us to isolate the contribution from condensation toward restructuring. We show the existence of three distinct regions along the path connecting the initial fractal and final collapsed aggregates, where minor restructuring occurs already at zero vapor supersaturation due to capillary condensation. Increasing supersaturation increases the amount of condensate, producing a more notable aggregate shrinkage. At even higher supersaturations, the aggregates become encapsulated, and subsequent condensate evaporation leaves behind fully compacted aggregates. Hence, for wetting liquids, minor restructuring begins already during capillary condensation and significant restructuring occurs as the coating volume increases. However, at this time, we cannot precisely quantify the contribution of condensate evaporation to the full aggregate compaction.
AB - Fresh soot is made of fractal aggregates, which often appear collapsed in atmospheric samples. A body of work has concluded that the collapse is caused by liquid shells when they form by vapor condensation around soot aggregates. However, some recent studies argue that soot remains fractal even when engulfed by the shells, collapsing only when the shells evaporate. To reconcile this disagreement, we investigated soot restructuring under conditions ranging from capillary condensation to full encapsulation, also including condensate evaporation. In these experiments, airborne fractal aggregates were exposed to vapors of wetting liquids, and particle size was measured before and after coating loss, allowing us to isolate the contribution from condensation toward restructuring. We show the existence of three distinct regions along the path connecting the initial fractal and final collapsed aggregates, where minor restructuring occurs already at zero vapor supersaturation due to capillary condensation. Increasing supersaturation increases the amount of condensate, producing a more notable aggregate shrinkage. At even higher supersaturations, the aggregates become encapsulated, and subsequent condensate evaporation leaves behind fully compacted aggregates. Hence, for wetting liquids, minor restructuring begins already during capillary condensation and significant restructuring occurs as the coating volume increases. However, at this time, we cannot precisely quantify the contribution of condensate evaporation to the full aggregate compaction.
KW - condensate
KW - electrostatic particle classifier
KW - encapsulation
KW - structural collapse
KW - vapor supersaturation
KW - volatility
KW - wetting
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U2 - 10.1021/acs.est.1c02391
DO - 10.1021/acs.est.1c02391
M3 - Article
C2 - 34128645
AN - SCOPUS:85110057393
SN - 0013-936X
VL - 55
SP - 8622
EP - 8630
JO - Environmental Science & Technology
JF - Environmental Science & Technology
IS - 13
ER -