Very short-lived halogens over the Western PacificRobyn Butler

Paul Palmer and Liang Feng

Very Short-Lived Halogens

• Very short-lived halogens (VSLH) • Natural oceanic source

• CHBr3, CH2Br2, CH2ICl, CH2BrCl etc.

• Lifetime < 6 months

Scientific Assessment of Ozone Depletion: 2010, WMO, Chapter 1

CHBr3 + hv/OH → 3Br + other products

• Coordinated Airborne Studies in the Tropics (CAST)• Jan/Feb 2014• CONTRAST (NCAR)• ATTREX (NASA)

Emissions in GEOS-ChemC

HB

r 3C

H2B

r 2

Standard GEOS-Chem emissions:Top down estimate

Liang

Additional emissions estimate:Bottom up estimate

Ziska

1.6

0.8

0.0

1.2

0.6

0.0

1.6<

0.8

0.0

1.2<

0.6

0.0

VSL

H f

lux

/ 1

0-1

3kg

m-2

s-1

Overestimation of observations using standard GEOS-Chem Liang emissions (red).

Observational comparisons

CHBr3CH2Br2

Ziska emissions fluxes (green) are directly comparable.

The opposite can be said for CH2Br2.

>8

4

>1

Alt

itu

de

/ km

Online Emissions Scheme

• Online calculation of CHBr3 and CH2Br2 flux

𝐹𝑙𝑢𝑥 = 𝑘𝑤(𝐶𝑜𝑐𝑒𝑎𝑛 − 𝐶𝑔𝑎𝑠 × 𝐻)

• Use an ocean concentration map of CHBr3 to calculate local flux of both VSLH

y = 0.15x + 0.85

y = 0.20x + 0.97

y = 0.09x + 12.46

y = 0.12x + 2.24

0.1

1

10

100

1 10 100 1000

CH

2B

r 2s

w (

pm

ol/

l)

CHBr3 sw (pmol/l)Schall et al., 1997, N. Atlantic open ocean Class et al., 1986, Atlantic open oceanMoore and Tokarczyk, 1993, NW Atlantic open ocean Carpenter et al., 1999b, NE Atlantic coastAbrahamsson and Klick, 1990, S. Atlantic open ocean Reifenhauser and Heumann, 1992, South Polar SeaSchall and Heumann, 1993, Arctic coast Schall and Heumann, 1993, Arctic macroalgal fieldSturges et al., 1993, Antarctic ice algae

𝒍𝒏 𝑪𝑯𝟐𝑩𝒓𝟐 𝒐𝒄𝒆𝒂𝒏 = 𝟎. 𝟕𝟖 𝒍𝒏 𝑪𝑯𝑩𝒓𝟑 𝒐𝒄𝒆𝒂𝒏 − 𝟎. 𝟕𝟔

Courtesy of L. Carpenter, University of York

Ocean concentration maps

Coastal average of 20 pmolCHBr3 concentration

Inversion of Ziska emission flux

Just coastal driven emissions of VSLH?

Can daily flux represent monthly average?

CH

Br 3

con

cen

trat

ion

/ p

mo

lL-1

20

10

0

20

10

0

CHBr3: Additional emission source in open ocean? Or underestimation of magnitude?

CH2Br2: Is this a mainly coastal driven emission? Or overestimation from CH2Br2/CHBr3

relationship?

ResultsZiska

inversio

n m

ap2

0 p

mo

lcoastal m

ap

CHBr3 mixing ratio / pptv CH2Br2 mixing ratio / pptv

>8

4

>1

>8

4

>1

Alt

itu

de

/ km

Spatial variation of model difference

Mo

del

–O

bse

rved

VSL

H /

pp

tv

Standard GEOS-Chememissions

Online Ziska inversion flux

Overestimate of open ocean contribution

Underestimate at coastal regions

CHBr3

Spatial variation of model difference

Mo

del

–O

bse

rved

VSL

H /

pp

tv

CH2Br2Ziska emission flux Online Ziska inversion flux

Are we seeing different emissions mechanisms for these two related gases?

General overestimate

Summary

• Difficult to replicate observed VSLH mixing ratios over the region• Are their emission mechanisms different than previously thought?

• Is it not a linear relationship between CHBr3 and CH2Br2?

• Next steps:• Tagged-VSLH model run

• Inversion modelling

• Use nested version of the model

• What can be learnt from the data?

Tagged emissions of CHBr3 from Guam.