atmospheric chemistry
DESCRIPTION
atmospheric chemistryTRANSCRIPT
Atmospheric Chemistry
• Photochemical Pollutants - Ozone Formation and Degradation
• Global Warming• Visibility
Dr. Steven Japar
Ford Motor Co. – Retired
March 29, 2005
TROPOSPHERIC CHEMISTRYJohn H. Seinfeld, Spyros N. Pandis, “Atmospheric Chemistry and Physics” (John
Wiley & Sons, 1998)
Photochemical Cycle of NO2, NO and O3
• NO2 + hν (λ< 424 nm) NO + O (jNO2)
• O + O2 + M O3 + M (k1) • O3 + NO NO2 + O2 (k2)
• d[NO2]/dt = k2[O3][NO] - jNO2[NO2 ]
For O atoms, O3 (very reactive), invoke pseudo-steady-state approximation, i.e., rate of formation = equals rate of loss
• d[O3]dt = k1[O][O2][M] - k2[O3][NO] ~ 0• d[O]/dt = jNO2 [NO2 ] – k1[O][O2][M] ~ 0
jNO2[NO2] = k1[O][O2][M]• d[O3]dt = jNO2 [NO2] - k2[O3][NO] ~ 0
Photostationary state relationship[O3]ss = jNO2[NO2] /k2[NO]
Atmospheric Chemistry of Carbon Monoxide and NOx
• O3 + hν O + O2
• O(1D) + O2
• O(1D) + M O + M• O + O2 + M O3
• O(1D) + H2O 2 OH• CO + OH CO2 + H• H + O2 + M HO2 + M • CO + OH + O2 CO2 + HO2 • HO2 + NO NO2 + OH
Atmospheric Catalytic Oxidation of COCO + OH + O2 CO2 + HO2
HO2 + NO NO2 + OHNO2 + hν NO + OO + O2 + M O3 + M
CO + 2O2 + hν CO2 + O3
• HO2, OH not consumed in this cycle• Net formation of O3: NO NO2 is accomplished
by HO2
• The chain terminating step:OH + NO2 + M HNO3 + M
The Hydroxyl Radical• Most important reactive radical species in
the atmosphere.• Measurements, theoretical estimates --
average tropospheric [OH] =– Daytime (summer) 5-10 x 106 molecules cm-3
– Daytime (winter) 1- 5 x 106
– Nighttime <2 x 105
• Nighttime reservoirOH + NO + M HONO + M
• Early morning jump startHONO + hν OH + NO
Peroxyacyl Nitrates • CH3CHO + OH CH3CO + H2O• CH3CO + O2 + M CH3C(O)O2 + M• CH3C(O) O2 + NO CH3C(O)O + NO2
• CH3C(O)O CH3 + CO2
• CH3C(O)O2 + NO2 <==> CH3C(O)OONO2
Peroxyacetyl nitrate (PAN)CH3C=O
OONO2
Peroxyacyl Nitrates• Lower troposphere -- relatively
unreactive– Lifetime determined by thermal
dissociation• PAN: ~ 30 minutes at 298 K; 8 hours at
273 K; months in upper troposphere• Upper troposphere – lifetime
determined by photolysis, OH• Mechanism for long-range transport of
reactive NOx
Hydrocarbon Oxidation in the Atmosphere
CH4 + 4O2 + 2NO + OH HCHO + 2O3 + OH + 2NO2 + H2O
CH3CH3 + 2NO + OH + O2 CH3CHO + 2NO2 + OH
C2H4 + OH + NO + 2O2 NO2 + 1.44 HCHO + 0.28 HOCH2CHO + OH
Urban Atmosphere on a Smoggy Day
NOX/Hydrocarbon/OzoneRelationships in the Atmosphere
Urban-Suburban
O3: 100-400 ppb
Rural
O3: 50-120 ppb
Remote
O3: 20-40 ppb
Marine
O3: 20-40 ppb
Ozone Isopleths• Graphical representation of the
dependence of O3 formation on initial [VOC] and [NOx]
• Simple box model representation of the atmosphere– After initialization, nothing enters or leaves the
box
• Implications for Control of O3 All VOC/NOx regimes are not equal
• Ridge line: low VOC/NOx vs. high VOC/NOx
• Above ridge lineDecreased [NOx]
increased [O3]Decreased [VOC]
decreased [O3]
• Below ridge line:Decreased [NOx]
decreased [O3]Decreased [VOC]
no change in [O3]
VOC-limited
NOx-limited
Michigan Air Pollutionhttp://www.deq.state.mi.us/documents/deq-aqd-aqe-ozone-bumpdown-
westmich.pdf
• Air quality in Michigan has been improving since the mid-1980’s
Visibility
Extinction Coefficient - bext
• Measure of atmospheric transparency • Measure of the fraction of light energy lost
from a collimated beam of energy E in traversing a unit thickness of atmosphere
• The extinction coefficient has dimensions of inverse length (e.g., Mm-1)
• bRay is light scattering by gas molecules known as Rayleigh scattering – Gas scattering is almost entirely attributable
to oxygen and nitrogen molecules in the air. – It is unaffected by pollutant gases and is 12 x
10-6 m-1 (Mm-1) at the wavelength of 550nm at sea level. (Vr ~300 km)
• bsp is light scattering by particles – Dominated by fine particles in the size range
of 0.1~1.0 μm
apagspRayabsscatext bbbbbbb
• bag is light absorption by gases – NO2 is the only common atmospheric gas that
significantly absorbs light
• bap is light absorption by particles – Absorption arises nearly entirely from
elemental carbon particles
• 0.1 2 µm diameter particles scatter the most light per unit mass.
• Sulfates ~ two-thirds of the visibility reduction in the Appalachian Mts.
• In southern California, nitrates are the greatest contributor to haze, with organic carbon also very important.
Dry
Wet
Visual Range – Koschmieder Equation
• Distant objects are perceived in terms of contrast against the background (usually the sky) – At increasing distances, both bright and
dark objects fade and approach the horizon of the brightness
– Apparent contrast relative to the horizon decreases
• Initial object contrast (Co) = ratio of the object brightness minus the horizon brightness divided by the horizon brightness.
• For a homogeneous atmosphere (pollutant concentration, sky brightness), the apparent contrast decreases with increasing object-observer distance;
C = Coexp(bextx)bext is the extinction coefficient
x is the observer-object distance.
• For a large black object Co = -1; assume the contrast threshold for human perception is 0.02
0.02 = - exp(bextVr) and Vr = 3.912/bext
• Pristine coastal air: Vr ~ 160-200 km• Remote continental air: Vr ~ 80-120 km• Urban Plume: Vr ~ 5-20 km
Smoke from multiple wildfires in Canada blanketed the eastern U.S. with a smoke plume nearly 200 miles wide, affecting air quality from
New York to Washington D.C in July 2002. CREDIT: NASA/GSFC.
1981-1985 1986-1990 1991-1995
bext derived from Vr
B. A. Schichtel, et al, Atmospheric Environment 35, 5205-5210 (2001)
Visibility Trends in the Eastern U.S.
Trends in improving visibility in the eastern U.S. correlate well with the decrease in SO2 emissions (precursor to particle sulfate in the atmosphere) in the U.S.
Air Emissions Trends - Continued Progress Through 2003http://epa.gov/airtrends/econ-emissions.html
National Air Pollutant Emissions Estimates for Major Pollutants
This image shows ocean-crossing aerosols as dust from the Sahara desert is carried over the Atlantic Ocean. Dust and pollution from Asia floats toward the Pacific Northwest. CREDIT: NASA/GSFC.
Global Warming
Definitions
• Weather: Look out the window– High short-time variability
• High, low pressure systems; meandering jet stream
– High spatial variability (E. Lansing vs. Detroit)• Climate
– Weather averaged over large areas (sub-continental global) and long time periods (decades centuries)
– Recent climate change in Michigan?
What Determines Climate?
• Physics and Chemistry of the Atmosphere– Greenhouse gases; aerosols– Feedbacks
–Water Vapor and Clouds• The Sun• Interactions between Biosphere and
Atmosphere
• Natural Climate Variability
Facts• Since ~1800
– Earth has warmed 0.8oC (since 1880)– GHG atmospheric concentrations have
increased • CO2: 280 ppm 370 ppm• CH4: 0.7 ppm 1.75 ppm• N2O: 270 ppb 320 ppb
• Climate is controlled by the Greenhouse Effect
The Natural Greenhouse Effect345 Wm-2
105 Wm-2
Atmospheric Absorption 68 Wm-2
Contribution to the Natural Greenhouse
Water 90-98%Carbon Dioxide 3-5%Methane <1%Nitrous Oxide <1%
Contribution to the Natural GreenhouseWater 90-95%
Carbon Dioxide 5-7%
Methane <1%
Nitrous Oxide <1%
Physics and Chemistry of the Atmosphere
• Greenhouse gas concentrations– Radiative forcings
• Feedbacks– Chemistry, physics, meteorology– Water vapor and clouds
Atmospheric CO2 ConcentrationsCurrently ~372 ppm
Aerosols/Particles
• Major impact on climate• Atmospherically inhomogeneous, short lifetimes
– unlike GHGs• Direct Effects – Fairly straight-forward
– Particles scatter and absorb solar radiation• Light scattering cools (all particles)• Light absorption warms (primarily BC and iron-
containing dust)• Indirect Effects – Very difficult to quantify
– Cloud formation (condensation nuclei)– Cloud properties (water droplet size; cloud water
content)
Feedbacks
• 2x CO2 Direct warming ~0.5-1.0oC• Predictions ~5oC require positive
climate system FEEDBACKS that amplify the direct warming from the extra GHGs. – Water vapor– Clouds
• Inherent uncertainties in feedback mechanisms
Water Vapor and CloudsICE CRYSTALS
WATER DROPLETS
CLOUDS
PRECIPITATION
WATERVAPOR
SEA SURFACE - EVAPORATION
• On average, clouds cover 40-45% of the Earth’s surface• Additional 2-3% cloud cover offsets warming from man-made GHG (+2.5
Wm-2)• Model grid scale requirements make it impossible to directly model
clouds and their climate effects
8-Day Atmospheric Water Cycle
The Sun and Global Warming
Correlation Between Solar Cycle and Surface Temperature
Courtesy of George Wolff (GM)
Dashed line is length of sun’s magnetic cycle..
Solar Hypothesis
• Excellent correlation between solar activity and temperature for past 30,000 years
• Solar activity greatest in last 8000 yearsS. Solanki, et al., Nature (28 Oct. 2004)
• Change in direct solar forcing ~ 10% of the observed temperature variation
• Solar/climate theory– High solar activity strengthens magnetic barrier which
deflects cosmic particles away from earth (known)– Cosmic particles enhance cloud formation (limited
recent data)
Global Average Radiative ForcingBaseline: Pre-Industrial Revolution
TAR (IPCC, 2001)
Major Forcing Uncertainties
• Black carbon – Very short-lived, strong solar energy absorber– As important as CO2?
• Aerosol indirect forcing– Aerosols impact cloud formation, cloud characteristics– Offset much of the GHG warming?
• Land use changes– Changes planetary surface albedo
• Solar influence– An important warming component?
Natural Climate Variability
Natural Climate Variability• What do we “know”?
– 140 years of “real” data, paleo-data, GCM predictions: +0.2-0.3oC
• 100 Years of Consensus– Example: Sargasso Sea Temperatures
Medieval Warm Epoch
Little Ice Age
Natural Climate Variability
• “Consensus” challenged in 1998 (IPCC 2001) by the Hockeystick
• Minimized the importance of Medieval Warm Epoch; Little Ice Age – regional rather than global event
Regional Climate Cycles
El Nino
-40
-30
-20
-10
0
10
20
30
Jan-81 Jan-86 Jan-91 Jan-96 Jan-01 Jan-06
Date
SOI
Australian Bureau of Meteorology
• El Nino – hot and dry on west coast of Americas
• La Nina – cold and rainy on west coast of Americas; intense drought in Australia
• Global connections
Existing Issues• Radiative forcing – clouds, aerosols• Natural climate variability
– Cycles within Cycles within Cycles …• Ocean response - “Instantaneous” climate
shifts freezing in the greenhouse?• Sea level – ice melt, thermal expansion; will the
glaciers grow?• Carbon cycle - terrestrial sinks; deforestation• Land use• Extreme weather – floods, droughts, hurricanes• Regional climate change - “winners and losers”
Global Average SurfaceTemperatures
GISS Annual Average Surface Temperatures[Relative to the 1951-1980 Average Annual Temperature]
-60
-40
-20
0
20
40
60
80
1880 1900 1920 1940 1960 1980 2000
Year
Tem
pera
ture
Ano
mal
y (D
eg. C
x
100)
GISS Annual Average Surface Temperatures[Relative to the 1951-1980 Average Annual Temperature]
-40
-20
0
20
40
60
80
1940 1950 1960 1970 1980 1990 2000 2010
Year
Tem
pera
ture
Ano
mal
y (D
eg. C
x
100)
Great Pacific Climate Shift1975-1980 - 0.23 deg. C
Since 1976, temperaturehas increased 0.17
deg./decade
25-Year Temperature Trendshttp://www.junkscience.com/MSU_Temps/CD-Trends.GIF
Arctic Temperature TrendsGraph from JunkScience.com
Arctic PaleoclimateOverpeck, et al., Science 278, 1251 (1997)
-1.5
-1
-0.5
0
0.5
1
1600 1725 1850 1975Year
Tem
pera
ture
(deg
. C
)
Summer-weighted, Arctic-wide (29 sites)Annual Temperatures: Proxy Data
T = -0.942 + 0.00313(Year-1600)For 1600-1800
~0.016 deg./yr for
1880-1950
CommentaryOn the Basis of the Facts:
• Anthropogenic impacts on climate are occurring• We cannot accurately quantify those impacts• Prediction of future climate is very difficult
– Significant issues involving the representation of climate science, including feedbacks, in global climate models
– Unverifiable assumptions about societal actions and technological evolution• Linear extrapolations from non-linear
systems
Kyoto Protocol Requirements2008-2012 Timeframe
(vs. 1990 CO2 Emissions)• EU
– Goal: -8%– Chance of success: uncertain, even with major
emissions cuts associated with UK conversion to natural gas and the shutdown of East German industry after 1990
– Population trends (UN, 1998): -3% (2010); -5% (2020)
• US– Goal: -7%– Current status: +32% projected for 2010– Population trends: +14% (2010); +21% (2020)
• Developing Countries (China, India, Indonesia, S. Korea) -- NO REQUIREMENTS
Growth In Developed/ Developing Nations
Billion Metric Tons C/yr(IPCC Scenario IS92a)
02468
101214161820
1990 1992 2000 2005 2010 2025 2100
Rest of WorldFormer Soviet Bloc
Other OECDUnited States
Accumulated CO2 emissions 1990 to 2100 is 1,500 bmt C.
Results of the Kyoto Protocol • 2020 vs 1990
– US, OECD CO2 emissions: 3 GT to 2.8 Gt– RoW CO2 emissions: 1.8 Gt to 3.0 Gt– Net Effect: Global CO2 emissions increase from 6
to 8 Gt (vs. 8.2 Gt) -- no measurable progress• 2100 vs 1990
– US, OECD CO2 emissions 0 Gt– RoW - emissions increase from about 1.8 to 13.2 Gt– Net Effect: Global emissions increase from 6 to
15.5 Gt (vs. 20 Gt)• CO2 emissions are 2.5 times 1990 levels• Temperature rises about 1.8oC vs. 2.1oC for “no
reductions” T.M.L. Wigley, Geophys. Res. Lett., 25, 2285 (1998)
2300220021002000-1
1614121086420
550 ppm350 ppm
Emission Pathways(Fossil Fuels, Deforestation, & Cement Calcination Included)
Man
’s E
mis
sion
s (B
illio
n M
Ton
s Car
bon/
Yea
r)
No Controls Assumed
300 Years
Kyoto Commitment Period2008 TO 2012
EmissionPaths to Achieve:
Constant 1990 Emissions
T. Wigley, R. Richels, and J. Edmonds, Nature 379, 240 (1996)
Stabilization of Atmospheric CO2
Stabilization of Atmospheric CO2 Levels
• Massive controls on CO2 emissions are needed for stabilization as far as 300 years into the future.– Stabilization at 550 ppm emissions reduced ~70%
from current levels– Stabilization at 350 ppm 85-90% reduction in
emissions (with a period of zero CO2 emissions around the year 2100).
– The specifics of the short-term (25-50 years) emissions control scenarios have very little impact on long-term stabilization.
• It is certain that society is on a path that will lead us to atmospheric CO2 concentrations of at least 550 ppm.
Regional Climate Cycles
• NAO: Dominates winter climate variability in the N. Atlantic region from central North America to Europe and into Northern Asia.– Positive Phase: N. Atlantic intense winter storms; warm, wet
winters in Europe, eastern US• AO: Controlled by sea level pressure in the Arctic
– “High index” or “warm phase”: Below normal Arctic SLP, enhanced upper level westerlies in the N. Atlantic; warm US winters; warm, wet winters in N. Europe; thinning Arctic sea ice.
• PDO: Long-term ocean fluctuation of the Pacific Ocean– Major impacts in the N. Pacific, especially along N. America– Positive (warm) phase: SSTs cool in central, north Pacific, warm
along N. America coast
• NAO: Controlled by Icelandic low pressure vs. subtropical high pressure
– Dominates winter climate variability in the N. Atlantic region central North America to Europe and into Northern Asia.
– Positive Phase: N. Atlantic intense winter storms; warm, wet winters in Europe, eastern US
– Negative Phase: Fewer N. Atlantic storms; cold winters (snow) in Europe, eastern US.
http://www.ldeo.columbia.edu/NAO/• AO: Controlled by sea level pressure in the Arctic
– “High index” or “warm phase”: Below normal Arctic SLP, enhanced upper level westerlies in the N. Atlantic; warm US winters; warm, wet winters in N. Europe; thinning Arctic sea ice.
– “Low index” or “cool phase”: Above normal Arctic SLP, weak upper level westerlies; cold US, N. Europe winters; robust Arctic sea ice.
http://jisao.washington.edu/wallace/natgeo/ArcticSubart.pdf• PDO: Long-term ocean fluctuation of the Pacific Ocean
– Major impacts in the N. Pacific, especially along N. America– Negative (cool) phase: SSTs warm in central, north Pacific, cool along N.
America coast– Positive (warm) phase: SSTs cool in central, north Pacific, warm along N.
America coasthttp://sealevel.jpl.nasa.gov/science/pdo.html