Ben Kravitz Tuesday, 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 I λ =I λ,0 e -τ λ m I λ,0 = incident solar radiation (at the top of the atmosphere) I λ = solar radiation that reaches the surface τ λ = optical depth m = atmospheric mass (how much of the atmosphere the radiation is passing through) note the wavelength dependence! Beer-Lambert-Bougner Law

Optical Depth τ=τ R +τ O +τ A +τ X R = Rayleigh O = Ozone A = Aerosol (what we really want to know) X = other absorbers (second order effect)

Rayleigh Scatter Proportional to λ 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 nm or is given from other ozone monitoring sensors Miscellaneous absorbers include water vapor, NO x, 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 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) I λ =I λ,0 e -τ λ m ln(I λ )=-τm+ln(I λ,0 ) Plot this on a log plot with slope -τm and y-intercept ln(I λ,0 )

Langley Plot

We create a Langley plot and extrapolate back to atmospheric mass 0 to get a calculated value of I 0. We know what I 0 should be (top of the atmosphere radiation). This tells us how to calibrate the sun photometer. Calibration

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 I 0 over the course of a year. Corrections

I λ =I λ,0 e -τ λ m

I 0 is the term that gives us the most trouble in this equation. Uncertainties in I 0 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