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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