1. Remote Sensing of Aerosols
Zhanqing Li
& R. Dickerson
Dept. of Atmos. & Oceanic Science University of Maryland

2. What are aerosols and where are they from ?

3. Pollution over India The view from Tibet Sea of pollution Over India
Image from the Shuttle

4. 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.

6. Pollution/dust in China
Smoke
Pollution?
Saharan dust
Sea Salt
Pollution/dust in India
Dust and smoke are Transported to the North-East Atlantic. From MODIS

7. Natural biogenic aerosol particles – Wherever you have trees or vegetation, these particles are there. These are from Brazil.
P.S.: Are these particles spherical? Are they Sulfates? Soil Dust? Sea Salt?
photos from Gunther Helas, MPIC

8. Aerosol Size Distribution
It presents 3 modes :
- « nucleation  »: radius is between 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

9. Aerosol Observations

10. 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

11. Aerosol Observation Programs
AERONET

12. 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 (cloud-screened), and Level 2.0 (cloud-screened and quality-assured).

13. NASA’s AERONET

14. 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.
Sun photometers are spectral radiometers
Cimel Spectral Radiometer

15. 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

16. 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.

17. Sun Photometer Tutorial

18. On NASA AERONET site
Find a nearby Cimel sun photometer
Determine the annual average AOD and seasonal cycle
Look at a few individual days.
July 2011 was dry and hot; August 2011 was rainy. Can you see a difference?
Pick a polluted location and a pristine one – examine the differences.
Repeat for water vapor, Angstrom exp.

19. Aerosol Climatology from AERONET
Cooling
Hansen et al. (1997)
Heating
FIG. 1. The averaged optical properties of different types of tropospheric aerosol retrieved from the worldwide AERONET network of ground-based radiometers. Urban–industrial, biomass burning, and desert dust aerosols are shown for text(440) = Oceanic aerosol is shown for text(440) = 0.15 since oceanic background aerosol loading does not often exceed Also, v0(l) and the refractive index n shown for Bahrain was obtained only for the cases when a <= 0.6 [for higher a, v0(l) and refractive index n were very variable due to a significant presence of urban–industrial aerosol].
Dubovik, O., B. Holben, T. F. Eck, A. Smirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, 2002: Variability of absorption and optical properties of key aerosol types observed in worldwide locations. J. Atmos. Sci., 59, 590–608.
Dubovik, O., B. Holben, T. F. Eck, A. Smirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, 2002: Variability of absorption and optical properties of key aerosol types observed in worldwide locations. J. Atmos. Sci., 59, 590–608.
256 citations

20. Aerosol Climatology from AERONET
Cooling
However, we show the particle size distribution (below) representing all observations in Bahrain (complete range of a). Angstrom parameter a is estimated using optical thickness at two wavelengths: 440 and 870 nm.
Hansen et al. (1997)
Heating
Dubovik, O., B. Holben, T. F. Eck, A. Smirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, 2002: Variability of absorption and optical properties of key aerosol types observed in worldwide locations. J. Atmos. Sci., 59, 590–608.
Dubovik, O., B. Holben, T. F. Eck, A. Smirnov, Y. J. Kaufman, M. D. King, D. Tanré, and I. Slutsker, 2002: Variability of absorption and optical properties of key aerosol types observed in worldwide locations. J. Atmos. Sci., 59, 590–608.
256 citations

21. 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”

22. MISR Provides New Angle on Haze
September 7, 2000
Nadir
46° Forward
60° Forward
70° Forward

23. Aerosol Retrieval over the Appalachians
September 7, 2000
Nadir
70° Forward
Aerosol optical thickness