Modeling of tropospheric halogen (Cl-Br-I) chemistry:cycling, debromination, and impact

Lei Zhu1, Daniel Jacob1, Sebastian Eastham1,2, Melissa Sulprizio1, Xuan Wang1, Tomás

Sherwen2, Mat Evans2, and Johan Schmidt3

GCA105/21/2018

1Harvard; 2MIT; 3University of York; 4University of

Copenhagen Gas phase; Photolysis; Heterogeneous reactions

Global annual mean inventory of tropospheric inorganic bromine (Bry)Gg Br [ppt]; Read 4.98(3) as 4.98x103 Gg Br a-1

Results based on one-year simulation (v1102-d, 2012, 4x5, MERRA2)

TORERO, 2012, Jan.-Feb.

[Theys et al. 2011; Wang et al., 2015]

Increasing evidence for widespread bromine in the troposphere

• Tropospheric daytime BrO background: ~1ppt

• Bromine radicals (Br and BrO) play important roles in tropospheric chemistry by: • depleting ozone and OH • oxidizing elemental mercury and VOCs

GOME-2 tropospheric BrO column

Background

Probably impact your work!

SURFACE

CHBr3

400CH2Br2

55CH3Br

Marine biosphere

(Metabolism)

Sea-salt aerosol debromination

(50% of 1-10 µm particles)

STRATOSPHERE

TROPOSPHERE

62

27

Bry (2.2 ppt)

Volcanoes

5-15

Deposition

Lifetime ~ 1 week

820

• Global source of tropospheric halogens is mainly natural and from the oceans

• Bromoform (CHBr3) is major source in the free troposphere

• Conundrum: Sea salt aerosol (SSA) is the dominant global source, but caused too high BrO

in previous model studies

[Gg Br a-1]

BrO (0.7 ppt) Br

HBr HOBr

BrNO2 BrNO3

Br2 BrCl IBr

Global tropospheric budget for inorganic halogen (Bry)

Modeling of sea salt aerosol (SSA) debromination

NMB = -10% Observed EF

GEOS

-Che

mEF

!" Br =( Br

'Na* ),,-

( Br'

Na* ),./ 0/1.2

• Observations indicate a 50% depletion of bromide in SSA relative to seawater composition

• Less bromide depletion (i.e., larger EF) over the Southern Ocean - debromination only occurs in acidified SSA

EF(Br) data are from Sander et al. [2003] and Newberg et al. [2005]; Results based on one-year simulation (2012, 4x5, MERRA2)

r = 0.91

SSA Br- + H+ + HOBr ® Br2 + H2O

• BrO in the marine boundary layer are maintained at relatively low levels by adding:

• Uptake of HBr by the sea salt aerosol becomes the major sink of Bry

CONTRAST 2014, Jan.-Feb.

TORERO 2012, Jan.-Feb.

Observational constraints on BrO

HOBr(aq) + HSO3-/SO3

2- ® HBr + HSO4-/SO42-

Br + CH3CHO ® HBr + CH2CHO

NMB = -13%NMB = -58%

NMB = +37%NMB = -75%

[Millet et al. 2010; Chen et al., 2017]

NMB = -4%NMB = -36%

MBL

Effect of halogen chemistry in tropospheric ozone and OH

• Halogen chemistry results in decreases in tropospheric mean ozone (15%) and OH (11%)• SSA debromination will be off in v11-02 because it breaks ozone

Zonal averageTropospheric

meanMean=5.6 ppb

• A cycling Bry family• Sea salt aerosol as a source and a sink of bromine• To make SSA debromination consistent with ozone

w/o halogen – w/ halogen