Oxidizing Capacity and Urban Air Pollution
• Regional and Global Impacts
– Background Chemistry and Composition
– Climate
• Public and Environmental Health Concerns
– Elevated levels of toxic compounds
Joel Thornton
Associate Professor
506 ATG
The Atmosphere’s Oxidative Capacity
oxidation
emission
CH4 CO2 OH
O3 + hv
O(1D) + H2O
O3
O3 + hv 2OH H2O
Stratosphere
CO CH4
1-2 x1013
moles yr-1 2-4 x1013
moles yr-1
6-10 x1013
moles yr-1 3 x1013
moles yr-1
OH only from strat O3 would be titrated
X X
Troposphere
OH Titration Problem
NOx = NO + NO2
NO NO2
O3
~ 100 s
NOx Emissions Increase Oxidative Capacity
oxidation
emission
NOx
HOx
CH4 CO2
H2O
Photochemical Smog Formation
OH
HO2
RO2
RH
CO
O3
h
NO2
NO
R’(O)H
NOx, Air Quality, and Climate Forcings
Catalyze ozone
production
Regulates OH
abundance
Shindell, et al Science 2009
NOx
NOx emissions couple air quality and climate concerns
Question
If NOx is important for sustaining the oxidizing capacity of
the atmosphere, how was it sustained before human
activities came to dominate the sources of NOx?
Houston, TX
Aug. 2000
Smog – “Smoke” + “Fog” •Coined due to reduced visibility associated with pollution episodes
Major components: •―invisible‖: O3, CO, SO2 •―visible‖: PM (aerosols)
Smog Ingredients •Sun (photochemistry) •Stagnation •Sources of NOx, SO2, PM and VOC
The Urban Smog Problem: Amplified Oxidation
NOx = NO + NO2 CO/VOC hv
Fuel Combustion and Use
Biogenic Activity 0 5 10 15 20 25-20
0
20
40
60
80
100
120
Hour of Day
part
s p
er
bill
ion (
ppb)
O3
NO
NO2
Evolution of NOx and O3 in Nashville, TN June 1999
Chemical Production of O3: Main Ingredients
0 10 20 30 40 50 60 70 800
10
20
30
40
50
60
70
Modeled NO2/NO Ratio
Observ
ed N
O 2/N
O R
atio
1. NO + O3 NO2 + O2 k1 2. NO2 + hv NO + O k2 3. O + O2 + M O3 k3
2 1 3
2
NO k O
NO k
predict
2 2
OBS MODEL
NO NO
NO NO
NO + XO2 NO2 + XO
implies
Often (not always)
Cycling of HOx And NOx Leads to NET O3
Ozone Damage
Needle damage (tip necrosis) is a common sign of ozone stress on pines. Often observed in forests downwind of major urban areas—Sierra Nevada, New England, Mexico City, etc.
O3 – Carbon Cycle Couplings
Indirect radiative forcing of climate change through ozone
effects on the land-carbon sink (Nature, 2007)
Blue – ―earth system‖ model assumes plants are relatively less sensitive to ozone Red – ―earth system‖ model assumes plants are relatively more sensitive to ozone
Large Subset of U.S. Population Exposed
8-hr Avg O3 < 75 ppb
24-hr Avg PM2.5 < 60 g/m3 Annual Avg PM2.5 < 15 g/m3
2008 U.S. NAAQS**
#of people living where pre-2008 NAAQS not attained
**http://www.epa.gov/air/criteria.html
8hr avg > 85 ppb
1hr avg > 125 ppb
Violation of O3 NAAQS By Region
Surface O3 and Transport
90th percentile O3 concentrations for
summers 1991-1995 and mean 850hPa
winds on days when O3 > 90th percentile
•Stagnation enhances chemistry
•Persistent stagnation in regions of strong subsidence: LA, Mexico City, Athens
•Air pollution is not just an urban problem
1. The rate limiting step in NET O3 production is the conversion of NO to NO2 by peroxy radicals. What is the rate expression for photochemical O3 formation?
2. Ronald Reagan famously noted that ―trees pollute too‖. What did he mean by this statement?
Questions
In U.S. isoprene emissions > total anthropogenic hydrocarbon emissions
+ OH RO2
O2
[1012 atoms C cm-2 s-1]
GEIA Isoprene Emissions
In July, Guenther, et al
Important source of peroxy radicals: enhance O3 production
Biogenic VOC: Major role in O3/PM Pollution
A vast majority of NOx is emitted at the surface, and 75% of NOx is a result of human activity. Given that NOx catalyzes O3 production, it is important to consider the global impact of anthropogenic NOx emissions.
1. NOx is removed from the atmosphere primarily by the
reaction NO2 + OH HNO3, k ~ 1x10-11 cm3 molec-1 s-1 what is a typical timescale for NOx removal ? ([OH] ~ 1x106 molec cm3)
2. Is this timescale sufficient to allow NOx to be efficiently transported away from an urban area to the remote troposphere?
Questions
Urban areas (mega-cities) are major point sources
Courtesy of P. Weiss-Penzias and D. Jaffe, UWB
Global pollution transport makes meeting local air quality standards more challenging
Global Impacts of Urban Air Pollution
CO - MOPPIT
NO2 - OMI
Questions
Isoprene + OH RO2 O2
k ~ 1x10-11 cm3 molec-1 s-1
Propane + OH RO2 + H2O k ~ 1x10-13 cm3 molec-1 s-1 O2
1. An important biogenic hydrocarbon is isoprene. Isoprene is very reactive towards OH. Given that [Isoprene] ~ 1 ppb, and [propane] ~ 3 ppb outside of Atlanta, would it make sense to regulate propane emissions to combat O3 production?
RO2 + NO NO2 + RO
NO2 + hv + O2 O3 + NO