HUMAN INFLUENCE ON OZONE IN THE GLOBAL TROPOSPHERE

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HUMAN INFLUENCE ON OZONE IN THE GLOBAL
TROPOSPHERE
Daniel J. Jacob

• Climate response from increasing ozone
• What is background ozone in the U.S.?
• Mapping of ozone precursor emissions from space

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INTERCONTINENTAL TRANSPORT vs. HEMISPHERIC
POLLUTION
For ozone, background enhancement by hemispheric
pollution is critical issue for PM,
intercontinental transport events are more
important
Tropopause
HEMISPHERIC POLLUTION BACKGROUND
Mixing
Free troposphere
Direct intercontinental transport
PBL
boundary layer advection
WCB
subsidence
N. America
Europe
Asia
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QUANTIFYING OZONE BACKGROUND IN U.S. - current
round of revision of NAAQS -
Background ozone present in absence of N
American anthrop. emissions

• What background concentrations should be used to
  assess risk?
• Do frequent occurrences of high background
  compromise achievability of NAAQS, or preclude
  the imposition of a tougher NAAQS?

• Fiore et al. JGR 2002, GRL 2002, using
  GEOS-CHEM
• Mean present-day ozone background in U.S. is
  20-35 ppbv
• Background includes 5-10 ppbv enhancement from
  anthropogenic emissions outside North America,
  with equal contributions from CH4 and NOx
  emissions
• Background decreases to 10-15 ppbv during
  pollution episodes (gt80 ppbv O3)

• Lefohn et al. JGR 2001, using obs stats at
  remote sites and backtrajectories

• Frequent occurrences of ozone above 50-60 ppbv
  observed at remote sites in the U.S. in spring do
  not appear to be associated with regional
  production
• ? Suggests current 25-45 ppbv EPA background
  definition is inadequate
• ? Implies that NAAQS may be unattainable via
  domestic emissions reductions

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GEOS-CHEM SENSITIVITY SIMULATIONS FOR OZONE
SOURCE ATTRIBUTION

• Standard simulation..2o x2.5o GEOS-CHEM, 48
  sigma levels
• 2001
• Backgroundno anthrop. NOx, CO, NMVOC
  emissions from N. America
• Natural O3 level.no anthrop. NOx, CO, NMVOC
  emissions globally CH4 700 ppbv
• Stratospheric.tagged O3 tracer simulation

Regional Pollution Standard
Background Hemispheric Pollution
Background Natural O3 level
Conduct 2001 ozone season simulation, evaluate
with stats from CASTNET sites
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Seasonal cycle in mean afternoon (1-5 p.m.)
ozone in surface air, 2001
Stratospheric ozone 0-10 ppbv, highest in
spring Natural ozone 15-25 ppbv Hemispheric
pollution enhancement 5-15 ppbv, highest in
spring
Fiore et al., JGR in prep.
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Ozone Time Series at CASTNet stations in spring
2001
1-5 pm daily data
Model reproduces structure regional pollution
is 1 factor, hemispheric pollution
also significant
Fiore et al., JGR in prep.
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High-O3 Haywood County event in North Carolina
in spring 2000

Regional pollution
D

D
Hemispheric pollution
APR-MAY 2000
APR-MAY 2001

Events are explained in model by regional
pollution, not transport from stratosphere
Fiore et al., JGR in prep.
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Cumulative probability distributions for daily
mean afternoon O3 at CASTNET sites, spring-summer
2001
Apr-May
Jul-Aug
11 remote sites (western U.S.)
34 polluted sites (eastern U.S.)
Regional pollution
Background and pollution increments can be
quantified along probability distribution for
different seasons background is never gt 50 ppbv
except perhaps under exceptional circumstances
Fiore et al., JGR in prep.