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Aircraft observations of mineral dust

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Flight tracks of dust flights

Low-level dust plumes were

encountered on all flights.

These three flights went searching for dust northeast of

Niamey.

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HYSPLIT 120 hr back trajectories (from Niamey)

B160 21st January 2006 B165 30th January 2006

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Vertical structure of extinction coefficient

SLRs

dust

BB aerosolSimilar magnitude of BB layers

More variable magnitude, depth and structure of dust layers

dust

clear slot

dust

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Aerosol size distributions

5 lognormals

2 lognormals

Wing PCASP + CVI PCASP

DABEX mean dust:

2 mode fit probably more useful for modelling, though less accurate.

CVI for particles > 1.5 m radius. Efficiency unknown, but used in

preference to AERONET.

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Extinction vs particle size

Mode fitting aims to reduce errors in extinction curves

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Volume distributions: Banizoumbou AERONET vs aircraft

Shape of Version 2 better than Version 1,

but weight of accumulation mode (BB)

too great ?

Better agreement here

+BAe-146

AERONET vs1

AERONET vs2

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Variation of 0 with height near Niamey/Banizoumbou

Large spread in BB

Height of top of dust

variable

Small amount of absorption in

dust near Niamey: dust or

industrial aerosol?

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Optical properties of dust from DABEX (2006), DODO (2006) and SHADE (2000)

• Dust almost purely scattering in DABEX and DODO, but more absorbing in SHADE

• Variations in ‘required’ refractive index

• Large decrease in specific extinction by adding coarse mode

AM = Accumulation mode

CM = Coarse mode

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Aerosol optical depth through DABEX

Aircraft data from neph+PSAP profiles

Some agreement, but tendency for larger aircraft values

Dust contribution varied between 6 % and 75 %

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Comparisons of AERONET optical depth, single scatter albedo and Angstrom exponent with aircraft profiles (a/c)

• agreement fairly poor in B160 and B165; good in B161

• 0 in good agreement with AERONET Vs 1; too much absorption in Vs 2

• Trends in good; consistency of aircraft obs with Mie calcs suggest AERONET values too high

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Vertical structure of ozone and CO

COO3

Why is CO > 100ppbv in pure dust layers?

Vertical distribution of

gases not always

correlated with aerosol layers.

Top of dust from ext

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B160 & B161 1 Hz scatter plot: destruction of O3 ?

BB

dust

‘mixed’Relationship

breaks down in dust layers

Angstrom exponent (α) from Nephelometer

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Aircraft vs. 20 yr January mean from UM

a/c = 0.48

model = 0.21

‘Niamey’ region

450550700

670 550 440

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Aircraft vs. 20 yr January mean from UMDODO region: 20-12W, 12.5-17.5N

a/c = 0.26

model = 0.21

HadGEM2-A run with 2000 emissions

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Summary

Dust was from the Sahara (North of Niamey)

Dust was mainly in lowest 1-2km.

Dust almost non-absorbing (ω ~ 0.99).

AERONET and aircraft agree reasonably well on optical depth and single scattering albedo, but differ on size distributions.

Evidence of O3 depletion.