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Abstract. Bromine radicals influence global tropospheric chemistryby depleting ozone and by oxidizing elemental mercury and reduced sulfurspecies. Observations typically indicate a 50 % depletion of sea saltaerosol (SSA) bromide relative to seawater composition, implying that SSAdebromination could be the dominant global source of tropospheric bromine.However, it has been difficult to reconcile this large source with therelatively low bromine monoxide (BrO) mixing ratios observed in the marineboundary layer (MBL). Here we present a new mechanistic description of SSAdebromination in the GEOS-Chem global atmospheric chemistry model with adetailed representation of halogen (Cl, Br, and I) chemistry. We show thatobserved levels of SSA debromination can be reproduced in a mannerconsistent with observed BrO mixing ratios. Bromine radical sinks from theHOBr + S(IV) heterogeneous reactions and from ocean emission ofacetaldehyde are critical in moderating tropospheric BrO levels. Theresulting HBr is rapidly taken up by SSA and also deposited. Observations of SSA debromination at southern midlatitudes in summer suggest that modeluptake of HBr by SSA may be too fast. The model provides a successfulsimulation of free-tropospheric BrO in the tropics and midlatitudes in summer,where the bromine radical sink from the HOBr + S(IV) reactions iscompensated for by more efficient HOBr-driven recycling in clouds compared toprevious GEOS-Chem versions. Simulated BrO in the MBL is generally muchhigher in winter than in summer due to a combination of greater SSA emissionand slower conversion of bromine radicals to HBr. An outstanding issue inthe model is the overestimate of free-tropospheric BrO in extratropicalwinter–spring, possibly reflecting an overestimate of the HOBr∕HBr ratiounder these conditions where the dominant HOBr source is hydrolysis ofBrNO3.
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Random Close Packing as a Dynamical Phase Transition
Sphere packing is an ancient problem. The densest packing is known to be a face-centered cubic (FCC) crystal, with space-filling fraction ϕFCC = pi/sqrt(18) ≈ 0.74. The densest “random packing,” random close packing (RCP), is yet ill defined, although many experiments and simulations agree on a value ϕRCP ≈ 0.64. We introduce a simple absorbing-state model, biased random organization (BRO), which exhibits a Manna class dynamical phase transition between absorbing and active states that has as its densest critical point ϕcmax ≈ 0.64 ≈ ϕRCP and, like other Manna class models, is hyperuniform at criticality. The configurations we obtain from BRO appear to be structurally identical to RCP configurations from other protocols. This leads us to conjecture that the highest-density absorbing state for an isotropic biased random organization model produces an ensemble of configurations that characterizes the state conventionally known as RCP.
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- Award ID(s):
- 1832291
- PAR ID:
- 10481295
- Editor(s):
- Kamien, Randall; Thomas, Jessica; Chaté, Hugues; Garisto, Robert; Agarwal, Abhishek; Dalena, Serena; Mitra, Samindranath
- Publisher / Repository:
- American Physical Society
- Date Published:
- Journal Name:
- Physical review letters
- Volume:
- 127
- Issue:
- 3
- ISSN:
- 0031-9007
- Subject(s) / Keyword(s):
- colloids
- Format(s):
- Medium: X Other: PDF-A
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1103/PhysRevLett.127.038002
