Transcript Document

GLOBAL SULFUR BUDGET [Chin et al., 1996]
(flux terms in Tg S yr-1)
cloud
42
SO2
4
NO3
18
t = 3.9d
OH
t = 1.3d 8
SO42-
H2SO4(g)
OH
(CH3)2S
DMS
t = 1.0d
10
64
dep
27 dry
20 wet
22
Phytoplankton
Volcanoes
Combustion
Smelters
dep
6 dry
44 wet
GLOBAL SULFUR EMISSION TO THE ATMOSPHERE
2001 estimates (Tg S yr-1):
Industrial 57 Volcanoes 5
Ocean
15 Biomass burning 1
[Chin et al. 2000]
FORMATION OF SULFATE-NITRATE-AMMONIUM AEROSOLS
Thermodynamic rules:
H 2O
H 2 SO4 ( g ) 
 SO42  2 H 
H 2O
NH 3 ( g ) 
 NH 4  OH 
H 2O
HNO3 ( g ) 
 NO3  H 
Sulfate always forms an aqueous aerosol
Ammonia dissolves in the sulfate aerosol
totally or until titration of acidity, whichever
happens first
Nitrate is taken up by aerosol if (and only if)
excess NH3 is available after sulfate
titration
NH 3 ( g )  HNO3 (Highest
g)
NH
4 NO3 (aerosol )
concentrations
in industrial Midwest
(coal-fired power plants)
Observed
aerosol
acidity in US
HNO3 and excess NH3
can also form a solid aerosol
if RH is low
GLOBAL EMISSIONS OF AMMONIA
[Bouwman et al., 1997]
GLOBAL
55
Ammonia,
Tg N yr-1
Livestock
Fertilizer
Humans
Industry
Biofuels
Soils/vegetation
Oceans
Biomass burning
UNITED STATES
2.8
SULFATE-NITRATE-AMMONIUM AEROSOLS IN U.S.
(2001)
Sulfate
Ammonium
Nitrate
Highest concentrations
in industrial Midwest
(coal-fired power plants)
STRATOSPHERIC AEROSOL
Injection of
volcanic ash
(SiO2, Al2O3, Fe2O3)
as well as gases
(H2S, SO2, HCl)
PSCs (nitric acid /
water vapor)
TROPOPAUSE
Transport of
long-lived S
gases (eg. COS)
Aerosols in the stratosphere are long-lived due to absence of precipitation
and “layered” transport (due to stability)
HOW COMPOSITION AND SIZE FIT TOGETHER…
Image from: C. Leck
SURFACE AEROSOL NUMBER CONCENTRATION
GLOMAP: 2 moment sectional model simulating sulfuric acid / sea salt
Dec
July
Continental: > 250 cm-3
Urban/polluted: > 2000 cm-3
Marine BL: ~ 200 cm-3
[Spracklen et al., 2006]
RAOULT’S LAW
o
H 2O, SAT
P
water saturation vapor pressure
over pure liquid water surface
PH 2O,SAT = x
o
H 2O H 2O,SAT
P
water saturation vapor pressure
over aqueous solution of water
mixing ratio xH2O
An atmosphere of relative humidity RH can contain at equilibrium
PH 2O,SAT
aqueous solution particles of water mixing ratio
xH 2O =
PHo 2O,SAT
=
RH
100
HOWEVER, AEROSOL PARTICLES MUST ALSO
SATISFY SOLUBILITY EQUILIBRIA
Consider an aqueous sea salt (NaCl) particle: it must satisfy
xNa  xCl   K s (solubility equilibrium)
xNa  = xCl  (electroneutrality)
xNa   xCl   xH 2O = 1 (closure)
This requires:
1
2
RH  100(1  2Ks ) "deliquescence RH"
At lower RH, the particle is solid at equilibrium, though it can
also remain in metastable aqueous state
UPTAKE OF WATER BY AEROSOLS
RELATIVE HUMIDITIES FOR
DELIQUESCENCE/CRYSTALLIZATION OF AEROSOLS
IN CONTRAST TO OZONE, HEMISPHERIC AEROSOL BACKGROUND
IS NOT AN AIR QUALITY ISSUE (wrt NAAQS)
…because of efficient precipitation scavenging in continental outflow
TRACE-P aircraft observations over NW Pacific (Mar-Apr 2001)
and GEOS-Chem model simulations
P3B DATA over NW Pacific (30 – 45oN, 120 – 140oE)
[Park et al. 2005]
HOWEVER, DESERT DUST CAN BE TRANSPORTED ON
INTERCONTINENTAL SCALES
clear day
April 16, 2001: Asian dust!
Glen
Canyon,
Arizona
Annual mean PM2.5 dust (mg m-3), 2001
Asia
Sahara
Most fine dust in the U.S. (except in southwest) is of intercontinental origin
LONG RANGE TRANSPORT OF DUST FROM
AFRICA TO THE AMAZON (2008)
Model simulation of the African dust plume
Timeseries of dust @ field site N of Manaus
[Prenni et al., 2009]
AEROSOL CLIMATE FORCING
[IPCC 2007]
SCATTERING OF
RADIATION
BY AEROSOLS:
“DIRECT EFFECT”
Scattering efficiency is
maximum when
particle diameter = l
particles in 0.1-1 mm
size range are efficient
scatterers of solar
radiation
By scattering
solar radiation,
aerosols
increase the
Earth’s albedo
EVIDENCE OF AEROSOL EFFECTS ON CLIMATE:
0
Temperature decrease following large volcanic eruptions
Observations
Temperature
Change (oC)
-0.6
-0.4
-0.2
+0.2
NASA/GISS general
circulation model
1991
1992
1993
Mt. Pinatubo eruption
1994
SCATTERING vs. ABSORBING AEROSOLS
Scattering sulfate and organic aerosol
over Massachusetts
Partly absorbing dust aerosol
downwind of Sahara
Absorbing aerosols (black carbon, dust) warm the climate by absorbing solar
radiation
AEROSOL “INDIRECT EFFECT” FROM CLOUD CHANGES
Clouds form by condensation on pre-existing aerosol particles (“cloud
condensation nuclei”) when RH>100%
clean cloud (few particles):
large cloud droplets
• low albedo
• efficient precipitation
polluted cloud (many particles):
small cloud droplets
• high albedo (1st indirect)
• suppressed precipitation (2nd indirect)
EVIDENCE OF INDIRECT EFFECT: SHIP TRACKS
N ~ 100 cm-3
W ~ 0.75 g m-3
re ~ 10.5 µm
N ~ 40 cm-3
W ~ 0.30 g m-3
re ~ 11.2 µm
from D. Rosenfeld
 Particles emitted by ships increase concentration of cloud condensation nuclei (CCN)
 Increased CCN increase concentration of cloud droplets and reduce their avg. size
 Increased concentration and smaller particles reduce production of drizzle
 Liquid water content increases because loss of drizzle particles is suppressed
 Clouds are optically thicker and brighter along ship track
SATELLITE IMAGES OF SHIP TRACKS
NASA, 2003
Atlantic, France, Spain
AVHRR, 27. Sept. 1987, 22:45 GMT
US-west coast
OTHER EVIDENCE OF CLOUD FORCING:
CONTRAILS AND “AIRCRAFT CIRRUS”
Aircraft condensation trails (contrails) over France, photographed from the Space Shuttle (©NASA).