This Week—Tropospheric Chemistry

READING: Chapter 11 of text

Tropospheric Chemistry Data Set Analysis

The Atmosphere: Oxidizing Medium

EARTHSURFACE

Emission

Reduced gasOxidized gas/aerosol

Oxidation

Uptake

Reduction

O(1D) Production in Troposphere?

30o equinoxmiddaySolar spectrum

Actinic Flux vs wavelength (O1D production requires < 320nm)

3Oj I d

O(1D) production in the Troposphere?

OH Radical: Main Tropospheric Oxidant

O3 + hv O2 + O(1D) (1)

O(1D) + M O + M (2)

O(1D) + H2O 2OH (3)

Primary source:

Sink: oxidation of reduced species

CO + OH CO2 + H

CH4 + OH CH3 + H2O

HCFC + OH H2O + …

Major OH sinks globally

GLOBAL MEAN [OH] = 1.0x106 molecules cm-3

Questions

1. How would a thinning of the stratospheric ozone layer affect the source of OH in the troposphere?

2. How might global warming affect the source rate of OH in the troposphere?

Wetlands Termites Other Natural Natural Gas Livestock Rice Paddies Other Anthropogenic

Natural Sources

Anthropogenic Sources

Global Sources of Methane

Global Distribution of MethaneNOAA/CMDL surface air measurements

Historical Trends In Methane

Historical methane trend

Recent methane trend

Fossil Fuel Combustion Biomass Burning Vegetation Ocean Oxidation of Methane Oxidation of other hydrocarbons

Global Sources of CO

Satellite Measurements

Lower Tropospheric CO

Upper Tropospheric CO

Satellite Measurements

Global Distribution of CO

NOAA/CMDL surface air measurements

O3

O3 + hv 2OHH2O

Stratosphere

CO CH4

1-2 x1013

moles yr-1 2-4 x1013

moles yr-1

6-10 x1013

moles yr-13 x1013

moles yr-1

OH only from strat O3 would be titrated

X X

Troposphere

OH Titration Problem

OHCH3O2

CH3OHO2

NO

CH4

O2

NO

H2CO

O3NO

NO2

CH3O2

HO2

h

O O2

O3 + h

H2CO + h

H2O

O2

COO2

HOx and NOx Catalyzed O3 Production

Questions

1. NO also reacts with O3 (as in the stratosphere) to produce NO2. In fact, the rate of conversion of NO to NO2 is dominated by this reaction in the troposphere. What is the effect of this reaction on O3 production and HOx cycling?

2. Loss of NOx in the troposphere takes place by NO2+OHHNO3, same as in the stratosphere. What is the effect of this reaction on tropospheric ozone?

3. What’s the maximum number of O3 that can be produced from the oxidation of 1 CH4?

Tropospheric NOx Sources (Tg N yr-1)

FOSSIL FUEL 23.1

AIRCRAFT 0.5

BIOFUEL 2.2

BIOMASSBURNING 5.2

SOILS 5.1

LIGHTNING 5.8

STRATOSPHERE 0.2

Probably bigger according to recent UW ATMSCI research

Tropospheric NO2 from Space

Martin et al. [2002]

NOx Lifetime < 1 day! Fraction transported away from source regions very low

Lightning Flashes From Space

DJF

JJA

Long-Range Transport of NOx

By NOx Reservoir: Peroxyacetyl Nitrate (PAN)

HO2NO2

RO2NO2

NO

h

O3

NO2

HO2

RO2

HORO

CORH

CO2

NOx Sources in Remote Regions

Questions

1. How would you calculate the chemical lifetime of NOx?

2. Sketch a typical vertical profile for NOx.

3. Anthropogenic activity has lead to the increase of NOx, CO, and CH4 emissions, with roughly factors of 2-4 increases of each over pre-industrial times. How have these changes affected OH concentrations?

[OH]global ~ constant

CO and CH4 tend to decrease OH

NOx and O3 tend to increase OH

Predicted Change Since Pre-Industrial Times

Global Distribution of Ozone

January

August

What are the main controls on tropospheric O3?

O3

O3

Stratosphere

400-1100 Tg yr-

1

Flux from Stratosphere3400-5700 Tg yr-

1

o o

Troposphere

Chemical ProductionDuring NOx/HOx catalyzed photo-oxidation of CO, CH4

500-1500 Tg yr-1

Deposition

Chemical LossHOx catalyzed destructionO(1D) + H2O

3000-4200 Tg yr -1

Global Tropospheric O3 Budget