QUESTIONS
1. Is hexane more or less reactive with OH than propane?
2. Is pentene or isoprene more reactive with OH?
LARGE SUPPLY OF BIOGENIC VOCs –
unrecognized until the 1990s
Switches polluted areas in U.S. from NOx-saturated to NOx-limited regime!
recognized in Revised Clean Air Act of 1999
Anthropogenic VOCs
Isoprene (biogenic VOC)
Jacob et al., [1993]
Isoprene (C5H8) and monoterpenes (C10H16) are oxidized by OH, O3 and NO3
(generally analogous to alkene rxn)  secondary organic aerosol (SOA)
LATEST INVENTORIES OF BIOGENIC vs. ANTHROPOGENIC
VOCs
…notice difference in scale!
Millet et al. [2007]
GLOBAL DISTRIBUTION OF ISOPRENE EMISSIONS
E = f (T, h)
MEGAN biogenic emission model (Guenther et al., 2006)
CONSTRAINT ON VOC EMISSIONS
FROM SPACE OBSERVATIONS OF FORMALDEHYDE
GOME satellite observations (July 1996)
2.5x1016
molecules
cm-2
2
1.5
1
0.5 detection
limit
South
Atlantic
Anomaly
(disregard)
High values are associated with biogenic emissions (eastern US),
anthropogenic emissions (China), fires (Africa, Siberia)
0
-0.5
SIMULATED SENSITIVITY OF SURFACE OZONE
TO EMISSION CONTROLS
U.S is NOx-limited!
Jacob et al., [1993b]
U.S. GROWTH MEASURES (2008)
U.S. EMISSIONS OF OZONE PRECURSORS
and trends over past 20 years
• Vehicles
• Fires
Fuel combustion
• vehicles
• power plants
Flat/down
Down 40%
Anthropogenic VOCs
•Fuel combustion
& transport
• Solvents
Down 30%
Isoprene (biogenic VOC)
Vegetation
Flat
OZONE TRENDS IN U.S.
http://www.epa.gov/airtrends/
National trend
Fort Collins trend
TREND IN 4th-HIGHEST 8-HOUR OZONE,
2004-2006 vs 1990-1992
DEPENDENCE OF OZONE PRODUCTION
ON NOx AND HYDROCARBONS
NET: RH + 4O2  R’CHO + 2O3 + H2O
O3
HOxfamily
RO2
RH
NO
5
RO
4
O3
PHOx
6
O2
7
OH
NO
NO2, M 9
HNO3
2k4 PHOx [ RH ]
P (O3 ) 
k9 [ NO2 ][ M ]
“NOx- saturated” or
“hydrocarbon-limited” regime
HO2
8
O3
H2O2
PHOx 1/ 2
P (O3 )  2k7 (
) [ NO ]
k8
“NOx-limited” regime
ALTHOUGH THE O3 PRODUCTION RATE IS ~ [NOx],
THE TOTAL O3 PRODUCED IS HYDROCARBON-DEPENDENT
AND [O3] = f(ENOx) IS STRONGLY NONLINEAR
P(O3)
L(NOx)
HO2,RO2,O3
OH, O3
NO
hv
NO2
Emission
HNO3
Deposition
Define ozone production efficiency (OPE) as the total number of O3 molecules
produced per unit NOx emitted.
Assuming NOx steady state, efficient HOx cycling, and loss of NO2 by
reaction with OH:
P (O3 )
2k7 [ HO2 ][ NO ] 2k4 [ RH ]
OPE =


L( NOx ) k9 [ NO2 ][OH ]
k9 [ NO2 ]
OPE m
as NOx k
e
strong nonlinearity
1999-2004 NOx EMISSION REDUCTIONS
AND SIMULATED EFFECTS ON SURFACE OZONE
Hudman et al. [2008]
50% decrease in power plant emissions
20% decrease in total U.S. emissions
TRENDS IN 4th-HIGHEST 8-HOUR OZONE
AT NATIONAL PARKS, 1992-2001 [EPA, 2003]
OBSERVED TREND IN OZONE BACKGROUND OVER
CALIFORNIA IN SPRING SUGGESTS 10-15 ppbv INCREASE
OVER PAST 20 YEARS
Jaffe et al. [2003]
Trend: 0.5-0.8 ppbv yr-1
Background: concentration that would be present in absence of local
anthropogenic emissions
RISING OZONE BACKGROUND IN EUROPE
Hohenpeissenberg/
Payerne
polluted
3-5 km
background
Naja et al. [2003]
Changes in anthrop. NOx emissions
Mace Head, 1987-2004
[Simmonds et al., 2004]
HEMISPHERIC OZONE POLLUTION:
IMPLICATIONS OF ENHANCED OZONE BACKGROUND
FOR MEETING AIR QUALITY STANDARDS (AQS)
Europe AQS
(8-h avg.)
Europe AQS
(seasonal)
U.S. AQS
(8-h avg.)
U.S. AQS
(1-h avg.)
Was here
until 2008!
0
Preindustrial
ozone
background
20
40
60
Present-day ozone
background at
northern midlatitudes
80
100
120 ppbv
GLOBAL OZONE BACKGROUND:
METHANE AND NOx ARE THE LIMITING PRECURSORS
Sensitivity of global tropospheric ozone inventory (Tg) to 50% global reductions
in anthropogenic precursor emissions
GEOS-Chem model [Fiore et al., 2002]
1995 base case
330
320
50% methane
310
300
50% NOx
290
280
270
260
250
50% NMVOCs
50%
NOx+NMVOCs
50% CO
240
50% all
Anthropogenic methane enhances surface ozone natural
by 4-6 ppbv worldwide
PROJECTIONS OF GLOBAL NOx EMISSIONS
Anthropogenic
NOx emissions
[IPCC, 2001]
2000
“Optimistic”
IPCC scenario:
OECD, U.S. m20%,
Asia k 50%
2020
109 atoms N cm-2 s-1
EFFECT OF INCREASING SIBERIAN FOREST FIRES ON
SUMMER SURFACE OZONE IN PACIFIC NORTHWEST
Observations
GEOS-Chem ozone enhancements
Siberian fires
Ozone
Mean summer 2003 enhancement
of 5-9 ppbv (9-17 ppbv in events)
Jaffe et al. [2004]
EFFECT OF CLIMATE CHANGE ON OZONE AIR QUALITY
Probability of max 8-h O3 > 84 ppbv
vs. daily max. T
Ozone exceedances of 90 ppbv,
summer 2003
Lin et al. [Atm.
Env. 2001]
Correlation of high ozone with temperature is driven by
(1) stagnation, (2) biogenic hydrocarbon emissions, (3) chemistry
EFFECT OF CLIMATE CHANGE ON REGIONAL STAGNATION
GISS GCM simulations for 2050 vs.
present-day climate using pollution
tracers with constant emissions
weather map illustrating
cyclonic ventilation of the eastern U.S.
2045-2052
summer
1995-2002
Pollution episodes double in duration in 2050 due to decreasing frequency of
cyclones ventilating the eastern U.S; expected result of greenhouse warming.
Mickley et al. [2004]