Main Topic: Vertical Characterization of Aerosols Sub-topic: Tropospheric and Stratospheric Aerosol Erin Robinson, July5, 2010

Sub-Project Topic: Tropospheric and Stratospheric Aerosol AOT Purpose of : Estimate the AOT for tropospheric and stratospheric aerosol layers to enable using the column measurements to estimate surface AOT

Background: Vertical Structure of Atmospheric Aerosols LT ~ years LT ~ weeks LT ~ days

Approach: Use multi-sensory obs, starting with Aeronet spectral AOT data Observe seasonal pattern Separate layers based on spectral and seasonal characteristics Assumptions: Each aerosol layer has different seasonal and/or spectral characteristics

NASA AERONET Federated Sun Photometer Network ~ 40 site over N. America, ~100+ over the world Standard Instrument CIMEL - France Data Handling: Standard Data Processing Central Data Archive Retrieval: Spectral Optical Depth Columnar Size distribution Applications: Satellite Ground Truth Aerosol Properties Holben et al., 1998, 2001;

Separating Stratosphere Assume that the stratosphere and troposphere have a constant spectral profile year-round Assume it is constant over the entire world. Use Mauna Loa to determine stratospheric component because it is at 4.2 km above sea level (only tropo and strato components).

Separating Stratosphere/Troposphere Unknowns: τ Trop (λ), τ Strat (λ) τ Tot (λ) = τ Trop (λ) + τ Strat (λ) Angstrom Exponent: – τ(λ) = C*λ -α – α = -ln(τ(λ 1 )/τ(λ 2 ))/ln(λ 1 /λ 2 )

Mauna Loa Based on the assumption that W,S,F are constant and just stratosphere: Assume AOT values for stratosphere at peak (day 120) Subtract those values from total AOT to find Trop Calculate the Angstrom Exponent for troposphere and stratosphere Assume these Angstrom Exponents are constant

Mauna Loa Since we know τ Tot (λ) for λ=380, 500 and 675 nm we can chose two wavelengths substitute and solve for τ Trop (λ), τ Strat (λ) Iteration is needed to establish the correct stratosphere background

Separating Troposphere/Surface Use island sites to avoid complication with continental sources Started with Midway Island because it is at sea level, it is a few km 2 and could be considered a ‘stationary platform’ Assume that the surface and troposphere have a constant spectral profile year-round Surface AOT is predominantly sea salt

Separating Troposphere/Surface Unknowns: τ Trop (λ), τ Surface (λ) τ Tot (λ) = τ Trop (λ) + τ Surface (λ) + τ Strat (λ) Angstrom Exponent: – τ(λ) = C*λ -α – α = -ln(τ(λ 1 )/τ(λ 2 ))/ln(λ 1 /λ 2 )

Midway Island Spring Peak in March-May Rest of year is spectrally white

Midway Island - Results

Discussion Guessing the spectral profile of the surface and subjectively determining the ‘best fit’ Only using two wavelengths. Would bringing in a third wavelength help?

Future work: Based on Smirnov et al (2003) work, incorporate surface wind speed into model to determine if aerosol is sea salt on surface or elevated Based on Prospero et. al (2001), add chemical speciation on the surface from the in order to better estimate spectral profile Incorporate relative humidity into model. Sea salt is hydroscopic, so high humidity may mean bigger particles Look at additional stations in the Pacific. Once we’ve separated the surface and the troposphere components we can subtract this troposphere from other continental sites in the same latitude belt Validate the MODIS and MISR AOT with Aeronet we will be able to make this same subtraction from the satellite and characterize AOT both horizontally and vertically

Method Pick Aeronet sites in Pacific where there isn’t a lot of pollution Extract three wavelengths (380, 500, 675) and plot We assume that the only components in the pacific column are sea salt at the surface and the additional is from the troposphere. – AOT Surf + AOT Trop = AOT Tot

Angstrom Exponent The other information we have about the aerosol is the angstrom exponent – Bext = C(Lambda)^-A – A= - ln(Tau1/Tau2)/ln(wavelength 1/wavelength 2) – A is the slope between two wavelengths and indicates the color of the aerosol For the pacific island sites we made the assumption that the surface angstrom exponent = 0 and that the troposphere would have a constant A

6 Equations/ 6 Unknowns We have six unknowns, AOT Surf and Trop for 380, 500 and 675 We know that AOT surf + AOT trop = AOT tot – This gives us 3 equations Since the AngsExponent on the surf is 0 we can say that AOTsurf 380 = AOTsurf 500= AOTsurf 675 – This gives us 2 independent equations AOT380=AOT 500 and AOT380 = AOT 675

Last Equation At the peak we assume a surface AOT and then calculate the tropospheric AOT and the trop Angstrom Exponent We assume that Angs Exponent is fixed in the troposphere, so then we can substitute and solve for Surf AOT 380 We then get two curves – Surf and trop We iterate to find the best sea salt concentration