Chemical and Dynamical Processes Affecting the Composition of the

Tropical Pacific Troposphere

Amanda Staudt

Ph.D. Thesis Defense

May 2001

The oxidizing capacity of the atmosphere

• The ability of the atmosphere to remove many pollutants

• OH is the most important oxidant; formation of OH depends strongly on sunlight and water vapor:

O3 + hv O2 + O(1D)

O(1D) + H2O 2 OH

Maximum OH is found in the tropics

How do biomass burning, fossil fuel combustion, and lightning perturb the chemical composition of the tropical Pacific troposphere?

1. Large impact of biomass burning emissions on the chemical composition of the tropical Pacific troposphere

Major findings

3. Pathway for interhemispheric exchange above the eastern Pacific

4. NOx over the tropical Pacific is not in chemical steady state

2. European and Asian fossil fuel emissions have a large and comparable impact on CO in the lower troposphere over the North Pacific

Pacific Exploratory Missions to the Tropics

Species measured:•O3

•NO and NO2

•CO, CO2, and CH4

•OH and HO2

•PAN and HNO3

•peroxides

•CH2O•aerosols•many hydrocarbons

PEM-Tropics ASeptember 1996

PEM-Tropics BMarch-April 1999

Biomass fire counts from the Along Track Scanning Radiometer (ATSR)

PEM-Tropics BMarch-April 1999

North Pacific

High

South Pacific

High

North Pacific

High

South PacificHigh

ITCZ

PEM-Tropics ASeptember 1996

and SSM/I rainrates

outgoing longwave

OzoneColumns

Emissions

FSU Global Spectral Model• horizontal velocities• pressure, temperature, and relative humidity• optical depth• convective activity• precipitation

• 4° latitude x 5° longitude• 14 vertical layers• 1 hour timestep• 24 tracers

ECMWFassimilated

meteorologicalanalysis

radiation (OLR)

Harvard 3-D Model of

• detailed O3-NOx-hydrocarbon chemistry• tracer advection• convective transport• wet and dry deposition

Chemistry and Transport

Chemistry and TransportHarvard/FSU 3-D Model of

• Biomass burning pollution from South America and Africa during PEM-Tropics A

• Biomass burning and fossil fuel pollution from northern hemisphere continents during PEM-Tropics B

• Pathways for interhemispheric exchange in the upper troposphere during PEM-Tropics B

• Chemical cycling of NOx with HNO3 and PAN during PEM-Tropics A and B

Outline

• Biomass burning pollution from South America and Africa during PEM-Tropics A

• Biomass burning and fossil fuel pollution from northern hemisphere continents during PEM-Tropics B

• Pathways for interhemispheric exchange in the upper troposphere during PEM-Tropics B

• Chemical cycling of NOx with HNO3 and PAN during PEM-Tropics A and B

Outline

PEM-Tropics A biomass burning plumes

The strong correlation between O3 and CO demonstrates the pervasive influence of biomass

burning over the South Pacific.

Observations

Model

PEM-Tropics A observations from 0-30oS and 4-8 km

Southern

Hemisphere

Tropical

South Pacific

Fossil Fuel and Biofuel

14% 14%

Biomass Burning in South America

10% 13%

Biomass Burning in Africa

18% 14%

Biomass Burning in Australia and

Indonesia

5% 5%

CO Contributions during PEM-Tropics A

• Biomass burning pollution from South America and Africa during PEM-Tropics A

• Biomass burning and fossil fuel pollution from northern hemisphere continents during PEM-Tropics B

• Pathways for interhemispheric exchange in the upper troposphere during PEM-Tropics B

• Chemical cycling of NOx with HNO3 and PAN during PEM-Tropics A and B

Outline

PEM-Tropics B CO (ppb)

River of pollution

River of pollution

Interhemispherictransport

Interhemispherictransport

BiomassBurning

BiomassBurning

Observed

Simulated

Column CO (1017 molecules cm-2)

NorthAmericanfossil fuel

7%

Europeanfossil fuel

12%

Asianfossil fuel

15%

Biomass burning

27%

1. Large impact of biomass burning emissions on the chemical composition of the tropical Pacific troposphere

During September 1996, biomass burning in South America and Africa contributed 27% of simulated CO over the South Pacific

Biomass burning in southeast Asia and India contributed 27% of simulated CO over the North Pacific during March-April, 1999

Major findings

CO (ppb) observed during PEM-Tropics B

CO (ppb) at 400 m altitude

NorthAmericanfossil fuel

11%

Europeanfossil fuel

19%

Asianfossil fuel

19%

Biomass burning

17%

1. Large impact of biomass burning emissions on the chemical composition of the tropical Pacific troposphere

During September 1996, biomass burning in South America and Africa contributed 27% of simulated CO over the South Pacific

Biomass burning in southeast Asia and India contributed 27% of simulated CO over the North Pacific during March-April, 1999

Major findings

2. European and Asian fossil fuel emissions have a large and comparable impact on CO in the lower troposphere over the North Pacific

• Biomass burning pollution from South America and Africa during PEM-Tropics A

• Biomass burning and fossil fuel pollution from northern hemisphere continents during PEM-Tropics B

• Pathways for interhemispheric exchange in the upper troposphere during PEM-Tropics B

• Chemical cycling of NOx with HNO3 and PAN during PEM-Tropics A and B

Outline

Transport of CO in the upper troposphere (8-12 km altitude)

Longitudinal gradients for 0-20oS and 8-12 km altitude

Convectiveoutflow

Convectiveoutflow

Westerlyduct

32%25% 18%

Simulated transport fluxes of European fossil fuel CO during PEM-Tropics B

10-13.5 km altitude

Westerly duct = a region of upper tropospheric westerlies

Zonal winds (m/s) in the upper troposphere during PEM-Tropics B

Easterlies Easterlies

Westerlies

Westerlies

Westerly duct

The westerly duct is strongest during:• northern hemisphere winter and spring• La Nina conditions

1. Large impact of biomass burning emissions on the chemical composition of the tropical Pacific troposphere

During September 1996, biomass burning in South America and Africa contributed 27% of simulated CO over the South Pacific

Biomass burning in southeast Asia and India contributed 27% of simulated CO over the North Pacific during March-April, 1999

Major findings

3. Pathway for interhemispheric exchange above the eastern Pacific

The westerly duct was the dominant conduit for northern hemispheric air into the southern hemisphere during March-April, 1999

2. European and Asian fossil fuel emissions have a large and comparable impact on CO in the lower troposphere over the North Pacific

• Biomass burning pollution from South America and Africa during PEM-Tropics A

• Biomass burning and fossil fuel pollution from northern hemisphere continents during PEM-Tropics B

• Pathways for interhemispheric exchange in the upper troposphere during PEM-Tropics B

• Chemical cycling of NOx with HNO3 and PAN during PEM-Tropics A and B

Outline

NO NO2

PAN

HNO3

• deposition

• scavenging

• photolysis

• thermal decomposition

• NO2+CH3COO2+M

• NO2+OH+M

• N2O5+H2O

• photolysis

• HNO3+OH

Primary sources:• lightning• biomass burning• fossil fuel• soils• biofuel burning• aircraft

NOx budget

NO NO2

PAN

HNO3

• deposition

• scavenging

• photolysis

• thermal decomposition

• NO2+CH3COO2+M

• NO2 +OH+M

• N2O5+H2O

• photolysis

• HNO3+OH

Primary sources:• lightning• biomass burning• fossil fuel• soils• biofuel burning• aircraft

NOx budget

Loss of NOx

Production of NOx

1Chemical steady state:

LNOx/PNOx

during PEM-Tropics A

Harvard/FSU ModelAverage

LNOx/PNOx=1.6

Observed Average

LNOx/PNOx=1.9-

2.4

6-12 km, 10-30oS

PEM-Tropics B

Sensitivity simulations:

• Source of NOx from lightning increased by 50%

• Convective scavenging of HNO3 by ice turned off

• Acetone concentrations set to a minimum of 400 ppt

• Acetaldehyde concentrations set to a minimum of 80 ppt

No scavengingby ice

Standardsimulation

Fixedacetone

Fixedacetaldehyde

NOx chemical budget during PEM-Tropics B

1. Large impact of biomass burning emissions on the chemical composition of the tropical Pacific troposphere

During September 1996, biomass burning in South America and Africa contributed 27% of simulated CO over the South Pacific

Biomass burning in southeast Asia and India contributed 27% of simulated CO over the North Pacific during March-April, 1999

Major findings

3. Pathway for interhemispheric exchange above the eastern Pacific

The westerly duct was the dominant conduit for northern hemispheric air into the southern hemisphere during March-April, 1999

4. NOx over the tropical Pacific is not in chemical steady state

Emissions of NOx from lightning maintain a chemical imbalance in the tropical upper troposphere

New observations of elevated carbonyl concentrations over the Pacific raise questions about PAN chemistry

Scavenging of HNO3 can also cause a chemical imbalance

2. European and Asian fossil fuel emissions have a large and comparable impact on CO in the lower troposphere over the North Pacific