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

thornton@atmos.uw.edu

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