1. Page 1© Crown copyright Aircraft observations of mineral dust

2. Page 2© Crown copyright Flight tracks of dust flights Low-level dust plumes were encountered on all flights. These three flights went searching for dust northeast of Niamey.

3. Page 3© Crown copyright HYSPLIT 120 hr back trajectories (from Niamey) B160 21 st January 2006 B165 30 th January 2006

4. Page 4© Crown copyright Vertical structure of extinction coefficient SLRs dust BB aerosol Similar magnitude of BB layers More variable magnitude, depth and structure of dust layers dust clear slot dust

5. Page 5© Crown copyright 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.

6. Page 6© Crown copyright Extinction vs particle size Mode fitting aims to reduce errors in extinction curves

7. Page 7© Crown copyright 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

8. Page 8© Crown copyright 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?

9. Page 9© Crown copyright 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

10. Page 10© Crown copyright 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 %

11. Page 11© Crown copyright 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

12. Page 12© Crown copyright Vertical structure of ozone and CO CO O3O3 Why is CO > 100ppbv in pure dust layers? Vertical distribution of gases not always correlated with aerosol layers. Top of dust from  ext

13. Page 13© Crown copyright B160 & B161 1 Hz scatter plot: destruction of O 3 ? BB dust ‘mixed’ Relationship breaks down in dust layers Angstrom exponent (α) from Nephelometer

14. Page 14© Crown copyright Aircraft vs. 20 yr January mean from UM  a/c = 0.48  model = 0.21 ‘Niamey’ region 450 550 700 670550440

15. Page 15© Crown copyright Aircraft vs. 20 yr January mean from UM DODO region: 20-12  W, 12.5-17.5  N  a/c = 0.26  model = 0.21 HadGEM2-A run with 2000 emissions

16. Page 16© Crown copyright 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 O 3 depletion.