Transcript Pollutants and Their Effect on the Water and Radiation Budgets

Pollutants and Their Effect on
the Water and Radiation
Budgets
Burton Gray
Khara Lombardi
Dana Lowes
Aerosol Indirect Effect
The impact of aerosols on cloud
radiative properties
Dana Lowes
What is the aerosol Indirect
effect?
• The climatic impact of aerosols on cloud
properties is called the aerosol indirect effect
• A high concentration of aerosols overseed
cloud droplets to generate highly
concentrated, narrowly distributed cloud
droplet spectra
• This can increase the cloud albedo up to 30%
reducing the amount of radiation reaching the
surface
• Narrowly distributed cloud droplet spectra
prevent the formulation of precipitation and
could increase cloud lifetime that further
cools the Earth’s surface (Matsui et al., 2004)
Conant et al., 2002
Effects of changing optical depth
• Extinction takes place when the
intensity of the radiation is decreased.
• The optical depth () of extinction is a
measure of the radiation lost over a
cloudy path
• Optical depth is proportional to Liquid
Water Path (LWP) and indirectly
proportional to effective radius (r)
(Curry and Webster, 1999).
Optical Depth
= 3(LWP)
2r
The aerosol indirect effect is associated
with higher values of optical depth due to
higher values of LWP and lower values of
r that can be attributed to anthropogenic
emissions (Schwartz et al., 2001)
Example (Black Carbon)
• Black Carbon (BC) is an anthropogenic
aerosol that is an effective absorber of solar
radiation.
• When BC becomes a part of the CCN
population it can prevent CCN from activating
and becoming a cloud droplet.
• BC does this by releasing the heat that it has
absorbed and increasing the critical saturation
of some CNN ( Conant et al.,2002)
Example (Black Carbon)
• The effect of heating from BC delays activation of the
aerosol in a cloud drop (Conant et al.,2002)
Example (Black Carbon)
• BC effectively slows droplet growth to
decrease their size and ability to form
drizzle
• This causes an increase in cloud droplet
number concentration (smaller r and
larger LWP)
• This cloud is now more reflective and
persistent (Nenes et al.,2002).
Possible Black
Carbon heating
scenarios
(Nenes et al.,
2002)
Example (Sulfate)
• Sulfate is another anthropogenic aerosol source that
is has been found to influence LWP and effective
radius.
• Increase in sulfate concentration over two different
time periods has been shown to reduce the effective
radius (r).
• In the first case r was reduced from approx. 15.7 μm
to approx. 7.6 μm and in the second case r was
reduced from approx. 16.3 μm to approx. 8.8 μm.
• LWP was also shown to increase during both periods
of increase in sulfate concentration (Schwartz et al.,
2001)
Fig. 3. Pixel-average cloud optical depth [tau]c as a function of vertical cloud LWP for eight
satellite overpasses over the study area 50-55{degrees}N, 25-30{degrees}W, for April 2-8, 1987
Schwartz, Stephen E. et al. (2002) Proc. Natl. Acad. Sci. USA 99, 1784-1789
Copyright ©2002 by the National Academy of Sciences
Example (Sulfate)
• Albedo can also be included in the cloud
radiative properties affected by anthropogenic
aerosols (primarily sulfate) in the form of
cloud-top spherical albedo.
• Cloud-top spherical albedo, “a(sph)”, is the
ratio of reflected to incident (solar) flux
averaged over all angles of incident radiation.
• During peak sulfate concentration it is shown
that a(sph) increases (Schwartz et al., 2001)
Fig. 4. Pixel-average cloud spherical albedo as a function of vertical cloud LWP, for three satellite
overpasses for the first episode (Left), study area 25-30{degrees}W, 50-55{degrees}N, and for the
second episode (Right), study area 20.25-23.625{degrees}W, 43.875-47.25{degrees}N, for indicated
dates in April, 1987
Schwartz, Stephen E. et al. (2002) Proc. Natl. Acad. Sci. USA 99, 1784-1789
Copyright ©2002 by the National Academy of Sciences
Summary
• The aerosol indirect effect impacts climate by
preventing radiation from reaching the Earth’s
surface by increasing optical depth and cloud
persistence.
• The studies of anthropogenic CCN
demonstrate a higher liquid water path and
shorter effective radius affect optical depth.
• Over time the effects of anthropogenic CCN
on cloud radiative properties could counteract
the Greenhouse effect.
Pollutants and Cloud
Formation/Precipitation
Khara Lombardi
Aerosols and Cloud Formation
• Clouds are formed on pre-existing aerosol particles
by condensation of water vapor on the particles.
• The aerosols interact with the water by the
dissolution of water soluble compounds.
• This can start at humidities well below saturation for
several atmospheric aerosol constituents: sulphates,
nitrates, etc.
• At supersaturated conditions, the growing liquid
particles may surpass the critical radius (B.
Martinsson et al., 1999).
Aerosols and Cloud Formation
• The capability for an aerosol to supply cloud
condensation nuclei (CCN) is dependant on
the particle number concentration and size
distribution of the particles.
• Also, an aerosol’s capability to supply CCN is
relative to its chemical composition and
mixing state of the cloud (B. Martinsson et
al., 1999).
Aerosols and Saturation
***The lower the amount of solute, the higher the supersaturation.***
(Seinfeld and Pandis, 1998)
Types of Aerosols
• Natural Sources
• Dust
• Volcanic emissions (carbon)
• Anthropogenic Sources
•
•
•
•
Sulphates
Nitrates
Organic Carbon
Black carbon (soot)
(B. Liepert et al., 2004)
Aerosol Effects on CCN
• Anthropogenic sources of sulphur and carbon have the
potential to significantly increase the capability of the
aerosol to supply CCN.
• This elevates the droplet number concentration (B.
Martinsson et al., 1999).
• Langner et al. (1992) found that, at most, 6% of
anthropogenic sulphur emission forms new particles,
while 44% adds mass to existing sulphate particles
previously activated in clouds.
Second Indirect Effect of
Aerosols
• Studies have shown that urban and industrial air
pollution suppresses precipitation-forming processes
in convective clouds.
• Although the pollution supplies aerosols for CCN, they
serve as very small CCN.
• Thus, the cloud has a large number of very small
CCN and this prolongs the time required to convert
the cloud water into large hydrometeors that can
precipitate out (A. Givati and D. Rosenfeld, 2004).
Aerosol Indirect Effect
Clean Cloud: Few CCN.
Polluted Cloud: Many CCN.
Few, but large cloud droplets.
Many, small droplets.
Precipitates easily.
Precipitation suppressed.
Aerosol Indirect Effect
Anthropogenic aerosol example using ship emissions:
(Johnson et al., 1996)
Aerosol Indirect Effect
(Hobbs, 1993)
Aerosol Indirect Effect
Image taken from the MODIS satellite
Previous Findings
• (Borys et al., 2003) found that the addition of 1µg m-3
of anthropogenic aerosols to clean air can reduce the
orographic snowfall rate in the Colorado Rockies by up
to 50%.
• Suppression is stronger in shallower clouds with
warmer cloud tops, in which, satellite observations
have depicted that pollution can completely shut off
precipitation from clouds that have temperatures
greater than -10ºC at their tops.
• Greatest suppression found in short-lived clouds (A.
Givati and D. Rosenfeld, 2004).
Differences in Suppression
Rates
Weak Updraft: Low
water content = lower
precipitation rate.
Strong Updraft: High
water content = heavy
precipitation rate.
So, What Happens to the
Water in the Clouds???
• The precipitation is transported downwind from
the urban area or pollution source.
• The aerosols still affect precipitation rates in these
areas, though.
• A. Givati and D. Rosenfeld (2004) found that the
downwind areas in which compensatory
enhancement occurs have a much lower absolute
amount of precipitation (approximately 25%).
Summary
• Aerosols act as CCN to create clouds.
• Aerosols in extreme amounts (i.e., volcano eruptions,
anthropogenic sources, etc.) can create many very
small CCN.
• Extremely small CCN have small effective radii
(activation radii), and thus, activation of the water
drops take longer to occur (if at all). Thus,
precipitation is suppressed.
• Precipitation will occur in longer lived clouds, but in
smaller amounts.
Aerosols and Drought
Burton Gray
Aerosols and Precipitation
• Effects of Aerosol Presence
• Direct negative radiative forcing
• Indirect decrease in cloud droplet effective
radius
• Indirect radiative impact due to decrease in
precipitation efficiency
Sulfates and Sahelian Drought
• Sahelian Drought From 1970 to 1985
• Possibly the Result of “Indirect Effect”
of Anthropogenic Sulfate Emissions
• Results From Rotstayn and Lohmann
AGCM
• Southward shift in tropical rainfall
• Model results correlate well with
increase/decrease sulfate emissions
North Drought, South Flooding
• East Chinese Summer Monsoon Rainy
Belt Moving South Since Late 1970s
• Sharp Decrease in Measured
Summertime Radiative Forcing Since
1959 (“Direct Effect”)
• Correlates Well With Rapid Rise in Coal
Consumption
Should We Worry?
• Indo-Asian Haze, Annually From
December to April
• Indian Ocean Experiment (INDOEX)
• Polluted Northern Air Juxtaposed With
Clean Southern Air
• Pollution Travels to 60 S (> One Week)
• Implication: Pollution Reaches MidPacific and North America
References
• Conant, Nenes, Seinfeld,2002: Black carbon radiative heating
effects on cloud microphysics and implications for the
aerosol inderect effect 1. Extended Kohler theory, J. Geo. Res.,
Vol. 107, NO. D21, 4604, doi:10.1029/2002JD002094.
• Conant, Nenes, Seinfeld,2002: Black carbon radiative heating
effects on cloud microphysics and implications for the
aerosol inderect effect 2. Cloud Microphysics, J Geo Res, Vol.
107, NO. D21, 4605,
doi:10.1029/2002JD002101.
• Curry, Webster, 1999: Thermodynamics of Atmospheres and
Oceans, Vol. 65, 223-225.
• Matsui, Masunaga, Pielke, Tao, 2004: Impact of aerosols and
atmospheric thermodyanmics on cloud properties within the
climate system, Geo. Res. Lett., Vol. 31, L06109,
doi:10.1029/2003GL019287,2004.
• Schwartz, Harshvardhan, Benkovitz, 2002: Influence of
anthropogenic aerosol on cloud optical depth and albedo
shown by satellite measurements and chemical transport
modeling, PNAS, Vol. 99, NO 4, 1784-1789.
References (cont.)
• Boyrs, R. D., D. H. Lowenthal, S. A. Cohn, and W. O. J. Brown, 2003:
Mountain and radar measurements of anthropogenic aerosol effects on
snow growth and snowfall rate, Geophys. Res. Lett., 30,
doi:10.1029/2002GL016855.
• Givati, A. and D. Rosenfeld, 2004: Quantifying precipitation suppression due
to air pollution, J. Appl. Met., 43, 1038 – 1056.
• Langner, J., H. Rodhe, P. Crutzen, P. Zimmerman (1992), Nature, 359, 712
– 715.
• Liepert, B., J. Feichter, U. Lohmann, and E. Roeckner, 2004: Can aerosols
spin down the water cycle in a warmer and moister world?, Geophys.
Res. Lett., 31, doi:10.1029/2003G019060.
• Martinsson, B., G. Frank, S. Cederfelt, E. Swietlicki, O. Berg, J. Zhou, K.
Bower, C. Bradbury, W. Birmili, F. Stratmann, M. Wendisch, A.
Wiedensohler, and B. Yuskiewicz, 1999: Droplet nucleation and growth
in orographic clouds in relation to the aerosol population, Atm. Res.,
50, 289 – 315.
References (cont.)
• Ramanathan, V., et al, 2001: Indian Ocean Experiment: An integrated
analysis of the climate forcing and effects of the great Indo-Asian haze, J.
Geophys. Res., Vol. 106, No. D22, 28371 – 28398.
• Rotstayn, L., and Lohmann, U., 2002: Tropical rainfall trends and the
indirect aerosol effect, J. of Climate, Vol. 15, No. 15, 2103 – 2116.
• Xu, Q., 2001: Abrupt change of the mid-summer climate in central east
China by the influence of atmospheric pollution, Atmospheric Environment,
Vol. 35, 5029 – 5040.