Post on 14-Jan-2016
description
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