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?