# Presentation: 185940

## Intro to AERONET

- PI: Brent Holben, NASA GSFC

- Aerosol Robotic Network
- http://aeronet.gsfc.nas.gov

## AERONET: The “ground” based satellite

- Some sites since 1993; currently a dozen sites in India

## Sunphotometry

- Measurement of the sun direct beam
- Direct application of Beer-Bouguer-Lambert’s extinction law
- L = F0, exp[-m]
-    where  is wavelength, m=m(0) is “air mass” (function of solar zenith angle, 0), L=L(0) is radiance, F0=F0(0) is solar irradiance and is total optical depth.
- Total optical depth is sum of contributions from molecular scattering, gas absorption, aerosol scatter/absorption, and cloud scattering/absorption.
- Depending on wavelength (), different assumptions can be made to neglect one or more of the contributing optical depths.
- Sunphotometer measurements date back to early 1900’s via pioneering work of Ångstrom.

## Hundreds of Nearly Identical Sunphotometers (CE-318)

- Manufactured by Cimel Electronique (France)
- CE-318-1: Standard Model (with 1020, 870, 675, 440, 936, 340 and 380 nm filters)
- CE-318-2: Polar Model (with 1020, 870P1, 675, 440, 870P2, 870, 936, and 870P3 nm filters)
- CE-318-N (after 2004): Models with varying # of filters, wavelength ranges, polarization, etc.
- Robot mount for automatic sun tracking
- Includes satellite transmitter for remote deployment
- Owned by P.I.
- Operated and deployed by GSFC AERONET team
- Carefully calibrated at GSFC before and after field deployment.
- Products quality controlled at GSFC
- Primarily to retrieve aerosol properties, but also cloud properties, gas and H2O absorption.
- Originally, AERONET intended for “ground truth” for EOS.

## Two types of measurements

- Direct ‘sun’
- Approx 4 per hour
- Measurements of direct solar extinction in ≥4 
- Assumes molecular and gas extinction.
- Derives spectral aerosol optical depth (AOD)
- --> relative aerosol size
- Derives H2O optical depth from 936 nm.

- Diffuse ‘sky’
- Approx 1 per hour, + specific times
- Measurements of sky radiance in 4 (1020, 870, 670 and 440 nm)
- At discrete azimuth angles along sun’s ‘alumcantur’ (constant 0)
- At discrete zenith angles along sun’s ‘principle plane’ (constant 0)
- Uses RT (LUT) to invert spectral/angular radiance to retrieve aerosol size, shape and composition.

## More about direct ‘sun’

- Measurements are taken in “triplets” to ensure homogeneity
- “Level 1” = Unscreened data
- “Level 1.5” = Cloud Screened
- “Level 2.0” = Post deployment (re-calibrated)
- If available: Level 2 is usually preferred

- Products =
-  Spectral AOD () (accuracy:  = ±0.01 for L2)
-  Angstrom Exponent ()
-  Total column water vapor (w)
-  Statistics: Daily averages, monthly averages

## Kanpur Site: 2001-present

- 2006

- Aerosol Optical Depth

- 5
- 4
- 3
- 2
- 1
- 0

- Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

## More about diffuse ‘sky’

- Measurement of spectral sky reflectance
- Plus assumptions of surface optical properties at site (MODIS surface albedo products)
- Results
- Size distribution in 22 size bins --> comparable aerosol lognormal properties (assuming two modes)
- Spectral absorption (and single scattering albedo, 0)
- Refractive Index (complex)
- Phase function (+ Assymetry Parameter)
- Downward and upward radiative fluxes
- Percentage of spherical and spheroid particles
- High Accuracy requires some constraints
- large aerosol signal (e.g., > 0.4 at 0.44 m)
- large sun angle (Θ0 ≥ 50°)
- Minimum number of “symmetric” angles (homogenous sky)
- “Level 2” product is quality controlled
- Statistics (daily and monthly averages), Error bars

## Retrieval (Pune, India: 4 Mar 2008)

- Size distribution

- SSA

## http://aeronet.gsfc.nasa.gov

## One stop shopping for all things AERONET

- Data available (Level 1.5) within hours of measurement

## TIGERZ

- 4-year campaign to measure aerosol in India, beginning April, 2008
- In India: Dept of Science & Technology, Ministry of Earth Sciences, IIT-Kanpur, IIT-Khargpur, ISRO.
- From Elsewhere: AERONET/Calipso, PHOTONS (France), AEROCAN (Canada), Finnish Met (Finland)
- Along Calipso track: Sunphotometers + Rain Gauge + Ground Lidars + Etc
- Begin measuring of pre-monsoon conditions in relationship to ITCZ (India Tropical Convergence Zone) experiment.
- Links to data and protocals from AERONET web site

## AERONET: The “ground” based satellite