Ben KravitzTuesday, November 10, 2009

AERONET

What is AERONET?

AErosol RObotic NETwork

Worldwide collection of sun photometers

What is a sun photometer?

A device used to measure

solar irradiance

Solar Irradiance

Optical Depth

Iλ=Iλ,0e-τλm

Iλ,0 = incident solar radiation (at the top of the atmosphere)Iλ = solar radiation that reaches the surfaceτλ = optical depthm = atmospheric mass (how much of the atmosphere the radiation is passing through)

note the wavelength dependence

!

Beer-Lambert-Bougner Law

Optical Depth

τ=τR+τO+τA+τX

R = RayleighO = OzoneA = Aerosol (what we really want to know)X = other absorbers (second order effect)

Rayleigh Scatter

Proportional to λ-4

400 nm (blue) 700 nm (red)

Plugging these in, blue light is scattered about 10 times more effectively than red light, which

is why the sky is blue

Measure this at 415 nm

Ozone, Miscellaneous

Ozone is measured at 600-700 nm or is given from other ozone monitoring sensors

Miscellaneous absorbers include water vapor, NOx, and other greenhouse gases. This is a

second order effect (which means we usually just ignore it).

So what’s left?

Determining aerosol optical depth is why we use sun photometers.

This is usually measured around 850-950 nm.

Why do we care about aerosols?

AERONET

These are sometimes co-located with LIDARS to get very accurate profiles of aerosols. Sun photometers can be used to determine the

LIDAR constant.

Issues with Sun Photometers

•Calibration

•Corrections for solar progression

•Uncertainties in irradiance

Calibration

Uses a tipping angle calibration and a Langley plot (like we saw in microwave sounding)

Iλ=Iλ,0e-τλm ln(Iλ)=-τm+ln(Iλ,0)

Plot this on a log plot withslope -τm and y-intercept ln(Iλ,0)

Langley Plot

•We create a Langley plot and extrapolate back to atmospheric mass 0 to get a calculated value of I0.

•We know what I0 should be (top of the atmosphere radiation).

•This tells us how to calibrate the sun photometer.

Calibration

Why is this measured at Mauna Loa?

Why is this measured at Mauna Loa?

Mauna Loa has very clean, dry air, so there are very few aerosols in our measurements. We know Rayleigh and Ozone optical depths very well, which means we know all of the terms in

the equation.

•The distance between the Earth and the sun modulates solar irradiance.

•This results in a 6% variation of I0 over the course of a year.

Corrections

Iλ=Iλ,0e-τλm

Iλ=Iλ,0e-τλm

I0 is the term that gives us the most trouble in this equation. Uncertainties in I0 can be up to

5% for some wavelengths.

Solar Irradiance Curves

Shadowband Radiometer

Measures diffuse

radiation

Angstrom Exponent

AOD = aerosol optical depth

Describes how AOD changes with wavelength

Angstrom Exponent

Some typical values:

•Very coarse mode (large) aerosols: α is small (much less than 1) - e.g. dust and sea salt

•Accumulation mode (very small) aerosols: α is large (more than 1, and sometimes more than 2 - e.g. fresh smoke, urban aerosols

•Clouds: α=0

Angstrom Exponent

This is highly dependent upon which wavelengths you choose to calculate the

Angstrom exponent

Urban Aerosols

Biomass Burning (smoke)

Desert Dust

Deliquescence

The process of certain aerosols growing as they

collect water

This changes the optical characteristics of the aerosols