Download CCN (~100 nm) Other particles (aerosols)

Aerosols and Climate
Peter J. Adams and Neil Donahue
Center for Atmospheric Particle Studies (CAPS)
Carnegie Mellon University
CEDM Annual Meeting
May 25, 2017
Outline
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Aerosols 101
Their Climate Forcings
Climate Sensitivity and Aerosols
Why Aerosol Forcings are Uncertain
Health Effects and Hidden Warming
Black carbon (soot) as short-term climate
mitigation
Recall Definitions
• Aerosol = collection of particles (solid or liquid),
suspended in a gas (=atmosphere for us)
• Particulate matter (PM) = aerosol
• Two National Ambient Air Quality Standards
(NAAQS)
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PM2.5: the mass concentration of particles whose
aerodynamic diameter is <= 2.5 microns
PM10: … same but <= 10 microns
PM2.5 is more likely to penetrate deeper in the lungs
than particles larger than 2.5 microns
Composition Is Complex
• Inorganic ions: sulfate, nitrate, ammonium,
others
Biggest contributors
to PM2.5 (fine) mass
• Organic carbon
conc.
• Elemental carbon (“soot”)
Health effects?
• Trace metals: Pb, V, Fe, Ni, Cu, etc.
• Crustal/mineral dust species: Si, Ca, Al, Fe, Ti,
etc
Coarse PM, key
natural sources,
• Sea spray: NaCl (and some organics) obvious in satellite
• Biological particles: pollen, spores, etc. imagery
• Fly ash
Size Ranges
• Atmospheric particles vary greatly in size
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1 nm
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< 1 nm, you are probably a gas molecule
> 10 mm, you won’t be in atmosphere very long
(gravitational settling)
10 nm
100 nm
1 mm
10 mm
factor of 104 in diameter, 1012 in volume
from a cluster of a few molecules … to a cloud
droplet
Dp
(diameter)
Terminology
• *for regulatory purposes, 2.5 mm is cutoff
between “fine” and “coarse”
Anthropogenic
aerosol forcing
Nucleation
mode
1 nm
10 nm
“Aitken”
mode /
ultrafine
mode
“Fine” mode /
accumulation
mode
100 nm
Submicron
1 mm*
Natural, windblown sea spray,
mineral dust
“Coarse”
mode
10 mm
Supermicron
Dp
(diameter)
Key Processes: Particle Formation
Making big particles
• 1) wind-blown emission (primary particles)
out of bigger
• high winds lift dust and sea spray into atmosphere
“particles” (the Earth)
• 2) mechanically generated (primary particles)
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construction, abrasion of tires on roadway etc.
3) regional nucleation (secondary particles)
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high (super-saturated) levels of H2SO4(g)
… plus “other stuff” (ammonia, amines, oxidized organics …
a research question)
Makingstill
small
• gas molecules cluster until a stable particle is formed
particles out of
•gas 4)
near-source nucleation (“primary” particles)
molecules
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at point of emission (e.g. tailpipe of car), emissions are gases
but rapid cooling causes nucleation and growth of particles
near source
sometimes in source: e.g. combustion soot
Particle Evolution
• Condensation (and evaporation)
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recall gas-phase reactions that produce: H2SO4,
HNO3, semi-volatile organic compounds
these tend to condense onto particles, causing them
to grow (1-20 nm/h is typical during day)
• Coagulation
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particles collide and stick due to Brownian (“random
walk”) diffusive motion
mostly a mechanism for ultrafine and nucleation
mode particles to add to larger particles
• It should be clear that particles are mixtures of
lots of stuff, not all from same source
Particle Removal
• Dry deposition
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mostly for super-micron particles (tD is ~1 day)
• Wet deposition
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mostly for Dp > 100 nm (tD is 4-8 days)
• *In some sense, coagulation is also “removal”
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(decreases N conc., M conc. remains the same)
Super-micron vs Sub-micron
Nucleation
mode
“Ultrafine”
mode
“Fine” mode /
accumulation
mode
Freshly nucleated
particles are smallest (by
definition)
1 nm
10 nm
Near-source nucleation
(from combustion) are a
little bigger
“Coarse”
mode
Windblown and
mechanically generated
particles are bigger
100 nm
Submicron
1 mm*
10 mm
Supermicron
Dp
(diameter)
Super-micron vs Sub-micron
• This drives a big difference in composition
between sub-micron / super-micron
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Super-micron: wind-blown soil, sea spray,
mechanically generated dust, tire wear, etc.
Sub-micron: primary combustion particles and all
secondary material produced by gas-phase
chemistry
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inorganics: sulfate, nitrate, ammonium
organics: POA, SOA (primary/secondary organic aerosol)
black carbon (elemental carbon / soot)
Life Cycle of Super-micron (Boring)
Nucleation
mode
“Ultrafine”
mode
“Fine” mode /
accumulation
mode
• Particle emitted
• Negligible evolution due to
condensation/coagulation
•1 nm
Removed10usually
by100dry
nm
nm
deposition
Submicron
“Coarse”
mode
Windblown and
mechanically generated
particles are bigger
1 mm*
10 mm
Supermicron
Dp
(diameter)
Life Cycle of Sub-Micron Particle (Interesting)
Nucleation
mode
“Ultrafine”
mode
“Fine” mode /
accumulation
mode
Freshly nucleated
particles are smallest (by
definition)
condensation
1 nm
10 nm
Near-source nucleation
(from combustion) are a
little bigger
100 nm
Submicron
“Coarse”
mode
• Particles grow by
condensation (1-20
nm/h)
• Ultrafine/nucleation
mode particles lost by
Dp
coagulation on timescale
(diameter)
of hours
1 mm*
10 mm
• Both processes drive
mass to “accumulation”
mode (slow dry
deposition)
Super• Eventual
micronremoval by
precipitation
Aerosols Scattering Sunlight
Dust and smoke over Australia (Terra)
Aerosols Absorbing Sunlight (Direct)
photo courtesy of Jay Apt
Kuwaiti oil fires
Clouds and Climate
•fluxes are W m-2
•Width of arrow proportional to flux
Atmosphere
342
77
67
(reflected)
(absorbed)
30
168
Earth
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23% of incoming sunlight reflected by atmosphere (mostly by clouds)
Without cloud reflection, the Earth would be ~15º C warmer
Aerosol Cloud Reflectivity Effect
Clean air mass
•Lower CCN concentration
•Higher Transmittance
•Lower reflectivity (albedo)
•Better chance of
precipitation?
•Shorter cloud lifetime?
Polluted air mass
•Higher CCN concentration
•Lower Transmittance
•Higher reflectivity
•Less precipitation?
•Longer cloud lifetime?
Cloud Optics: Surface Area
For a given amount of liquid water (or ice):
• More pollution/CCN → More cloud droplets → More surface area
• → More scattering → Brighter cloud → Cooler Earth
Darker clean
cloud
Brighter polluted
cloud
(Few CCN)
(More CCN)
photo courtesy of Amy Sage
Cloud Condensation Nuclei (CCN)
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In a particle-free atmosphere, a strong supersaturation
(~400% relative humidity) is required to nucleate new
liquid droplets
Instead, cloud water condenses onto pre-existing
particles: cloud condensation nuclei (CCN)
Clear Sky
(RH < 100%)
Activation:
Cloudy Sky
(RH > 100%)
water condenses
on CCN to form
cloud droplets
Other
particles
(aerosols)
CCN
(~100 nm)
Cloud
droplets
(~10 mm)
Aerosol Activation
 “Activation” = formation of cloud
droplet
 involves a competition between solute
and surface tension effects
Number
Depends on number
concentration above
“critical diameter”
Diameter
Aerosols and Climate: Semi-Direct Effect
High RH
Atmospheric Heating
Low RH
• Atmospheric heating from absorptive aerosol
• Warmer air  lower RH  less cloud formation
How direct is direct?
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Direct effect: scattering/absorbing sunlight
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Semi-direct effect:
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aerosol absorption heats atmospheric layer
warmer air → lower relative humidity → less/no cloud
forcing or feedback?
Indirect effect: modifying cloud properties
• “brightness (first) effect”
• “lifetime (second) effect”
Other Forcings
• BC on snow/ice surfaces
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Makes surface darker and more absorbing
Relatively small global forcing
May be regionally important
• BC as ice nuclei
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Ice nuclei are important triggers of precipitation
May change cloud cover, affect hydrological cycles
Almost nothing very concrete known about this effect
Long-lived GHGs:
+3 W/m2 (+/- 20%)
Aerosol effects:
mostly cooling,
highly uncertain
Radiative Forcing (W m-2)
Climate Change Uncertainty
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“Climate sensitivity” is a key parameter
global average
temperature
change
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global average
radiative forcing
Key parameter is l,“climate sensitivity”
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T  lF
0.3 to 1 °C per W/m2
1.5 - 4.5 °C for doubling of CO2
Implicit assumption that l is same for all kinds of forcings
(absorbing aerosols are known exception)
Boundary between “forcing” and “feedback” is fuzzy sometimes
In climate models, representation of cloud feedback is largest
source of uncertainty
In retrospective studies, knowledge of aerosol forcing is lacking
Aerosols and Climate Uncertainty
3.5
GHG forcing
High
sensitivity
Temperature Change (K)
3.0
2.5
Aerosol (haze)
+ GHG forcing
2.0
1.5
Low
sensitivity
1.0
20th century T
increase
0.5
0.0
0.0
0.5
1.0
1.5
2.0
Forcing (W m -2)
2.5
3.0
3.5
Climate Models: Sensitivity / Aerosols
Figure from Kiehl et al., GRL v34
doi:10.1029/2007GL0313832007
Climate Models: Sensitivity / Aerosols
Figure from Kiehl et al., GRL v34
doi:10.1029/2007GL0313832007
Challenges
• Need to characterize particle
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mass/number concentration
size distribution: ~10 nm to 10 mm
chemical composition: >hundreds compounds
mixing state
interactions with clouds (sub-grid)
separate anthropogenic from natural
• Highly variable in space and time:
intrahemispheric
mixing
Mean
aerosol
residence
hourly
daily
monthly
annual
Mean CO2
residence
NH/SH
mixing
decadal
century
Aerosol Variability
Uncertainty in pre-industrial/natural baseline matters
20th Century Forcings
Temperature Reconstruction
Temperature Reconstruction
Temperature Reconstruction
Temperature Reconstruction
Temperature Reconstruction
PM and Life Expectancy: 1980
PM and Life Expectancy: 2000
Global Distribution of PM2.5
Global Mortality from PM2.5
Hidden Warming from China’s Aerosols
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China is ~25% of world emissions for anthropogenic
aerosol precursors
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Global aerosol forcing is
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-0.8 W/m2 (best guess) … so China is -0.2 W/m2
-2 W/m2 (coolest) … so China is -. 5 W/m2
Climate sensitivity
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IPCC AR5 Table 7.1
1.5 to 4.5 deg C for 2xCO2 (~4 W/m2)
0.3 to 1.0 K/(W/m2)
Multiplying by climate sensitivity…
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Best guess: China is -0.2 W/m2 x 0.6 K/W/m2 = -0.12 deg C
Worst case: so China is -. 5 W/m2 x 1.0 K/W/m2 = -0.5 deg C
Won’t Climate Response Take a Long Time? … No
Surface air T
Instantaneous global
desulfurization (red)
Y2000 levels of sulfate and
GHGs held constant (blue)
Won’t Climate Response Take a Long Time? … No
Surface air T
Surface layer of ocean adjusts
quickly (0.8 K warming in one
decade)
More gradual response as deep
ocean equilibrates
Black Carbon as Climate Mitigation?
Warming
Cooling
Sunlight Absorption
Cloud Brightening
Cloud Burnoff
Co-emitted Reflectors
Snow/Ice Darkening
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Black carbon sources
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contain
cooling/scattering
aerosol
contribute to cloud
condensation nuclei
How much cooling goes
with the warming?
Kuwaiti oil fires (photo courtesy of Jay Apt)
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Timing
• BC mitigation would lead to fast benefits
Source: UNEP report, 2011
Climate Effects
BC 0.05 to 0.55
W/m2 (IPCC) but
maybe ~1 W/m2?
Not easily
separable from
cooling aerosols
Source: IPCC AR4
Co-Emitted Species: Early Debate
• Response from Joyce Penner (selected quotes)
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he obtains a warming from the combined direct and indirect
effects of f.f. BC + OM that is inadequately documented
warming may disagree with the results of published models by
the aerosol-climate community.
Penner et al. [2003] … obtain a forcing for f.f. BC + OM that is
not significantly different from zero
Jacobson [2002] has an inferred forcing for f.f. BC + OM that is
0.5 W/m2
Co-Emitted Species
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Most recent comprehensive analysis (Bond et al., 2012) shows coemitted cooling could completely offset BC warming
Source: Bond et al., 2012
Co-Emitted Species
• Different BC sources have different warming
potential
A, H, RC all different models
Warming
more
likely
than not
Source: Kopp and Mauzerall, 2010
Net Warming/Cooling By Source
Diesel: + ~.15 W/m2
“high confidence in net positive total climate forcing is possible
only for black-carbon source categories with low co-emitted
species, such as diesel engines.” (Bond et al.Biofuel
2013) cooking: + but small
Residential coal: + but small
Biomass burning: net cooling
~.2 W/m2 from right sources
→ 0.06 to .2 K
BC Controls Reduce CDNC
In global annual average,
50% FF: CDNC reduced by 4.6%
50% CARB: CDNC reduced by 8.7%
CDNC
[cm-3 ]
Smax
[%]
Reff
[μm]
195.6
0.26
8.27
Ratio of cloud droplet
number (CDNC)
50% FF
Base case
Black Carbon “Warming and Drying”
Aerosol Forcing (W m-2)
-0.5 W m-2
Governs global
mean T
+2.0 W m-2
Reduces
evaporation
-2.5 W m-2
Ramanathan et al., Science 294, 2119-2124, 2001.
Summary
• Estimating aerosol climate forcing is tough
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Similar/related to cloud feedback problem
• Estimates of aerosol forcing and climate
sensitivity are correlated
• Strong aerosol cooling is bad news
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Implies high climate sensitivity, lots of hidden
warming (e.g. China)
• Mitigating black carbon is worthwhile
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but expectations for climate benefits should be
limited