Global 3-D simulation of reactive bromine chemistry

T. Canty, Q. Li, R.J. Salawitch

Jet Propulsion Laboratory, Caltech, Pasadena CA

Tim.Canty@jpl.nasa.gov

Title

Measurements of column BrO from GOMEmuch higher than standard stratospheric modeled values

What’s the problem?

Tropospheric BrO ?Missing stratospheric BrO?

due to Arctic BL spring bloom

Hypotheses

• Discrepancy resolved by global, ubiquitous, background level of ~2 ppt of tropospheric BrO (Platt and Hönninger, Chemosphere, 2003 & references therein)

– But: Schofield et al. (JGR, 2004) report upper limit of 0.9 ppt for tropospheric BrO over Lauder, NZ

• Discrepancy may be resolved by: ~ 1 ppt of tropospheric BrO (perhaps consistent with UL of Schofield et al., JGR, 2004) ~ 8 ppt of stratospheric of Bry in the lowermost stratosphere

(Salawitch et al., GRL, 2005)

• Stratospheric bromine supplied by decomposition of VSL (very short lived) organics not considered in most global models as well as tropospheric BrO (Salawitch et al., GRL, 2005) • Excess bromine in UT and LS has important consequences for: – mid-latitude ozone trends (Salawitch et al., GRL, 2005) – tropospheric ozone photochemistry (Boucher et al., ACP, 2003; von Glasow et al., ACP, 2004; Lary, ACP, 2004) – polar ozone loss (Salawitch and Canty, in preparation, 2005) – chemistry - climate coupling (Carpenter and Liss, JGR, 2000; Hollwedel et al., ACP, 2004; Quack et al., GRL, 2004)

Enhanced Arctic BL BrO

GOME Satellite data:

• BrO Enhancements over Hudson Bay

& Arctic ice shelf every spring

• BrO column abundances of ~41013 cm-2

seen at NH mid-latitudes year round

• Unlikely spring bloom BrO supplies all of the global tropospheric background, but may contribute

Chance, GRL 1998

BryTROP = 0 ppt Bry

TROP = 8 ppt

AER Model Time Slice: 47°N, March 1993

Implications for Stratospheric Ozone Photochemistry

Enhanced Bromine: lower stratospheric ozone depletion due to BrO+ClO cycle BrO+HO2 cycle becomes significant O3 sink below 16 km, extending into upper troposphere (BrO+HO2 does not drive O3 depletion because VSL source is assumed constant over time)

Salawitch et al., GRL 2005

• Tropospheric ozone: – zonal mean 6 to 18% for a high-latitude VSL source

– local up to 40%, maxim. in SH free trop during summer (von Glasow et al., ACP, 2004)

• DMS: – DMS + BrO becomes significant sink

– DMS to SO2 conversion efficiency dramatically (von Glasow et al., ACD, 2003) (Boucher et al., ACP, 2003)• NOx: – BrONO2 hydrolysis significant source of HNO3

(Lary, ACP, 2004)

Implications for Tropospheric Ozone Photochemistry

MacroalgealOceanSource VMR Surface Lifetime Main Loss

(ppt) (days) Process

CHBr3 Bromoform 2.0 – 20 26 JCH2Br2 Dibromomethane 0.8 – 3.4 120 OHCH2BrCl Bromochloromethane 0.1 – 0.3 150 OHC3H7Br n-propyl bromide 0.1 – 1.0 13 OHC2H5Br Ethyl bromide 0.0 – 2.0 48 OHCHBr2Cl Dibromochloro- 0.1 – 0.5 69 OH & J methaneC2H4Br2 Ethylene dibromide 0.1 – 1.0 84 OH

Possible VSL organic sources

Location Surface Water(mean;median)

pmol/L

Atmosphere(mean;median)

ppt

Global near shore(<2 km from shore)

934; 946 25; 3.3

Global shelf 71.7; 40 5.4; 2.2

Global open ocean 18.3; 16.6 1.9; 1.2

Global ocean Range0.6 - 2770

Range0.2 - 460

mostly Atlantic ocean mostly Pacific ocean

Oceanic and atmospheric bromoform

from Quack et al., JGR, 2003

Adding CHBr3 to GEOS-Chem

• Create bromine_mod.f• Add ocean source of bromoform

1. Determine shore, shelf, and open ocean2. Create ocean bromoform “mask”

GEOS-CHEM v7-01-01• GEOS-Strat• 4º x 5º grid

Land Ocean

NearShore

CoastalShelf

OpenOcean

300 m

2 km

LowCHBr3

Ocean graph

HighCHBr3

Use U.S. Navy bathymetry measurements of ocean depth (5x5 min.)Use focean to determine near shore region Create a “mask” file of ocean bromoform

Adding CHBr3 to GEOS-CHEM

• Create bromine_mod.f• Add ocean source of bromoform

1. Determine shore, shelf, and open ocean2. Create ocean bromoform “mask”

• Add bromoform chemistry1. Photolysis2. Reaction with OH

Bromoform Chemistry

RO2, NO

CHBr3

CBr3

OH

O2

HOOCBr3

HO2

OH

hv

OOCBr3

OCBr3

C(O)Br2

O2NOOCBr3

NO2

, hv

hv

C(O)Br2

RO2, HO2

RO2, NO

CHBr3

CHBr2

hv

O2

HOOCHBr2

HO2

OH

hv

OOCHBr2

OCHBr2

C(O)HBr

O2NOOCHBr2

NO2

, hv

hv

C(O)HBrRO2, HO2

1/3 of the time 100 days

2/3 of the time 36 days

“Fast J”

Little or no kinetic studies

total 26 daysFig. 2-6, WMO 2003

PEM Tropics-A results

Lat = 18ºSLon = 145ºW

Need to understand CHBr3

as prerequisite forunderstanding BrO

PEM Tropics-A results

Lat = 18ºSLon = 145ºW

“Perfect World Scenario”

Woohoo! Everything compares well.

Lat = 18ºSLon = 145ºW

PEM Tropics-A results

D’Oh! Model does not seem to be affected by the ocean source.

“Real World Scenario”

Conclusions

• Evidence for global, ubiquitous ~1 to 2 ppt of tropospheric BrO• Potential important consequences for tropospheric:

– O3

– DMS oxidation – HNO3 production

• Tropospheric BrO likely supplied by VSL organics • Have begun to examine link between tropospheric BrO and biogenic, VSL organics using the GEOS-CHEM model – much work remains!!!

Future work

• Determine why modeled CHBr3 is so low – identify and remove bugs

• Implement full CHBr3 chemistry:

– agreement between measured and modeled CHBr3

– how much BrO is supplied to UT/LS by CHBr3

– fate of decomposition products: aerosol uptake, heterog rxns

• Incorporate other VSL species