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Urban Air Pollution
• Public and Environmental Health Concerns– Elevated levels of toxic compounds
• Regional and Global Impacts– Background Chemistry and Composition– Climate
Joel Thornton, Asst. Professor
ftp.atmos.washington.edu/thornton/atms501
Houston, TXAug. 2000
Smog – “Smoke” + “Fog”•Coined due to reduced visibility associated with pollution episodesMajor components:•“invisible”: O3, CO, SO2
•“visible”: PM (aerosols) + some gases (NO2)
Ingredients to Make Smog•Sun (photochemistry)•Stagnation•Sources of NOx, SO2, PM and VOC
The Urban Smog Problem
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.
Large Subset of U.S. Population Exposed
1-hr Avg O3 < 125 ppb8-hr Avg O3 < 85 ppb
24-hr Avg PM2.5 < 60 g/m3
Annual Avg PM2.5 < 15 g/m3
U.S. NAAQS
#of people living where NAAQS are not attained
8hr avg > 85 ppb
1hr avg > 125 ppb
Violation of O3 NAAQS By Region
Annual Average PM2.5 in Urban Areas
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
NOx = NO + NO2
CO/VOC
hv = uv-vis radiation
Fossil Fuel Combustion and Use
Biogenic Activity 0 5 10 15 20 25-20
0
20
40
60
80
100
120
Hour of Day
pa
rts
pe
r b
illio
n (
pp
b)
O3
NONO
2
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
Ob
se
rve
d N
O 2/NO
Ra
tio
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 kpredict
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
Net O3 Production
NO2
NOO3
h
HO2 CO2
COHO
O3
O2
NO2
NOO3
h
The Null Cycle
The Net O3 Production Cycle
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 Regan 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 EmissionsIn July, Guenther, et al
Important source of peroxy radicals: enhance O3 production
Biogenic VOC: Major role in O3/PM Pollution
Urban areas (mega-cities) are major point sources
Courtesy of P. Weiss-Penzias and D. Jaffe, UWB
Global pollution transport makes meeting own air quality standards more challenging
Global Impacts of Urban Air PollutionCO - MOPPIT
NO2 - OMI
A vast majority of NOx is emitted at the surface, and most 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 lifetime for NOx w.r.t. this process? ([OH] ~ 1x106 molec cm3)
2. Is this lifetime sufficient to allow NOx to be transported away from an urban area to the remote troposphere?
3. Do anthropogenic emissions of NOx reach the remote troposphere at all?
Questions
Questions
+ OH RO2
O2
k ~ 1x10-11 cm3 molec-1 s-1
CH3CH2CH3 + 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] < ~ 1 ppb outside of Atlanta, would it make sense to try to regulate propane emissions to combat O3 production?