Remote Sensing of
Aerosols
Zhanqing Li
& R. Dickerson
Dept. of Atmos. & Oceanic
Science University of Maryland
What are aerosols and where are
they from ?
Pollution over
India
The view from
Tibet
Sea of pollution
Over India
Image from the
Shuttle
Aerosol Types and Origin
•
Aerosol particles larger than about 1 mm in
size are produced by windblown dust and
sea salt from sea spray and bursting bubbles.
•
Aerosols smaller than 1 µm are mostly
formed by condensation processes such as
conversion of sulfur dioxide (SO2) gas
(released from volcanic eruptions) to sulfate
particles and by formation of soot and
smoke during burning processes.
•
After formation, the aerosols are mixed and
transported by atmospheric motions and are
primarily removed by cloud and
precipitation processes.
Pollution/dust in China
Smoke
Pollution?
Saharan dust
Sea Salt
Pollution/dust in India
Dust and smoke are
Transported to the NorthEast Atlantic. From MODIS
Natural biogenic aerosol particles – Wherever
you have trees or vegetation, these particles
are there. These are from Brazil.
Fungal hyphae
P.S.: Are these particles spherical? Are they Sulfates? Soil Dust? Sea Salt?
photos from Gunther Helas, MPIC
Aerosol Size Distribution
It presents 3 modes :
- « nucleation »: radius is
between 0.002 and 0.05 mm.
They result from combustion
processes, photo-chemical
reactions, etc.
- « accumulation »: radius is
between 0.05 mm and 0.5 mm.
Coagulation processes.
- « coarse »: larger than 1 mm.
From mechanical processes like
aeolian erosion.
« fine » particles (nucleation and
accumulation) result from anthropogenic
activities, coarse particles come from
natural processes.
0.01
0.1
1.0
10.0
Aerosol Observations
Aerosol Observation Programs
In recent years, a great deal of effort has gone into improving
measurements and data sets:
It is feasible to shift the estimate of aerosol radiative effects from
largely model-based to increasingly measurement-based.
MISR
Aerosol Observation Programs
AERONET
NASA’s AERONET
• The AERONET (AErosol RObotic NETwork) program: a
federation of ground-based remote sensing aerosol networks.
• AERONET provides global observations of spectral aerosol
optical depth (AOD), inversion products, and precipitable water.
• Quality levels: Level 1.0 (unscreened), Level 1.5 (cloudscreened), and Level 2.0 (cloud-screened and quality-assured).
NASA’s AERONET
Instrumentation
• The Cimel Electronique 318A spectral
radiometer is a solar-powered, weather-hardy,
robotically-pointed sun and sky spectral sun
photometer.
• A sensor head points the sensor head at the
sun according to a preprogrammed routine.
• The Cimel controller, batteries, and Vitel
satellite transmission equipment are usually
deployed in a weatherproof plastic case.
Cimel Spectral Radiometer
Which of the three levels would you use
to have the highest confidence in your
data set? Which has the most data?
Sun photometer system
Spectral Radiometer
 The radiometer makes two basic measurements-either direct
sun or sky-both within several programmed sequences.
 The direct sun measurements are made in eight spectral bands
requiring approximately 10 seconds
 at wavelengths of 340, 380, 440, 500, 670, 870, 940 and 1020 nm
 the 940 nm channel is used for column water abundance determination
 A preprogrammed sequence of measurements is taken by these
instruments from 7 am-7pm daily.
Sun Photometer Tutorial
http://calipsooutreach.hamptonu.edu/sunphoto-sim/photometer.html
http://aeronet.gsfc.nasa.gov/
http://aeronet.gsfc.nasa.gov/cgibin/type_one_station_opera_v2_new?site=GSFC&nachal=2&level=1
http://aeronet.gsfc.nasa.gov/new_web/DRAGONUSA_2013_Houston.html
On NASA AERONET site
1.Find a nearby Cimel sun photometer
2.Determine the annual average AOD and seasonal
cycle
3.Look at a few individual days.
4.July 2011 was dry and hot; August 2011 was rainy.
Can you see a difference?
5.Pick a polluted location and a pristine one –
examine the differences.
6.Repeat for water vapor, Angstrom exp.
Aerosol Climatology from AERONET
Heating
Hansen et al. (1997)
Cooling
Dubovik, O., B. Holben, T. F. Eck, A. Smirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, 2002: Variability
citations
of absorption and optical properties of key aerosol types observed in worldwide locations. J. Atmos.256
Sci., 59,
590–608.
Summary
• Remote sensing with sun photometers is a powerful tool
for monitoring AOD and other aerosol properties.
• Industrial aerosols tend to be small and highly reflective in
developed countries.
• Mineral dust is larger and absorbs more radiation.
• Biomass burning can be sootier and absorb even more
radiation.
• AERONET provides a world-wide network.
• Only works when you can see the sun.
• This is ok for satellite “validation”
MISR Provides New Angle on
Haze
September 7, 2000
Nadir
46°
Forward
60°
Forward
70° Forward
Aerosol Retrieval over the Appalachians
September 7, 2000
Nadir
70°
Aerosol optical
Forward
thickness