DODO RESULTS: Campaign Averages & BAe-146 Nephelometer Findings Claire McConnell Ellie Highwood Acknowledgements: Paola Formenti, Met Office, FAAM

DODO Flights DODO 1 Feb 2006 DODO 2 Aug 2006

DODO Average Properties DODO1DODO2 ObservationsMieObservationsMie ω ± ± Refractive index i i g σ ext 550 /m E-03 ± 1.88E E-04 ± 1.02E-04 k ext 550/ m 2 g

Under-sampling in the Nephelometer Coarse particles are lost in the inlet of the nephelometer During SHADE (2000) AODs derived from scattering on the C-130 required multiplication of 1.5 to agree with AERONET AODs, implying an inlet cut-off between 1.3 and 1.9μm radius. (Haywood et al., 2003) Experiments on BAe-146 nephelometer: 1.The intercomparison between the BAe-146 and the NASA DC-8 during DODO2 allowed a comparison between the nephelometers 2.Size distributions measured during DODO & Mie code allow an analysis of the under-sampling 3.Under-sampling can be estimated through comparisons with ground-based sites These processes allow an estimation of the factor of sampling missed and of the cut-off radius on the BAe-146 nephelometer

Intercomparison with NASA DC-8 BAe-146 and DC-8 show same spatial variability in scattering –correlation coefficients vary between Difference in absolute scattering measured –average ratio of DC-8/BAe-146 uncorrected scattering = 2.1 Hypothesis: The BAe-146 nephelometer is underestimating the total DODO dust scattering at 550nm by a factor of 2-3, and is only measuring submicron (diameter) scattering Uncorrected nephelometer scattering for 3 straight level runs – bold=BAe-146, light=DC-8

Correlation plots of 146 vs DC-8 uncorrected scattering for 3 wavelengths for 3 different SLRs performed wingtip- wingtip. Each panel has a different horiz/vertical scales, but has same range on both x & y axes. Black = 550nm with 146 data scaled by a factor of 2.1

Comparison with AERONET AODs for DODO2 calculated by integrating BAe-146 profile data AERONET AODs for the same times are a factor of 3.2 larger than those from the nephelometer profiles (but factor varies between 2 and 4.5) Results consistent with the DC-8 intercomparison FlightProfileUncorrectedAERONET τ 550 Correction factor τ 440, Level 2.0 B236P B237P B238P B238P B242P B242P11/P Average4.3 AODs from BAe-146 profiles and from AERONET station at Dakar

Estimation of Nephelometer Cut-off Radius The size distribution was measured by the PCASP and SEM sizing during DODO ‘Cut-off test’ – average size distribution successively ‘cut-off’ at decreasing radii to mimic the loss of the coarse mode Each size distribution run through Mie code to obtain scattering/absorption properties ‘Cut-off Factor’ (C) calculated – factor required to match the ‘cut- off’ scattering/absorption/extinction with that of the full size distribution Size distributions available for use in the cut-off test

Results from Cut-off Tests Submicron sampling (r<0.5μm) results in a scaling factor of 3 for the nephelometer ω 0 is considerably lower if the full size distribution is considered

Conclusions on Cut-off Radius on BAe-146 Nephelometer Submicron sampling on the 146 neph would result in a cut-off factor of 3 The AERONET-derived scaling factor suggests a cut-off of 0.5μm DC-8 intercomparison-derived scaling factor suggests a cut off of 0.8-1μm Suggested cut-off, around r=0.5μm, appears to be smaller than experienced for dust during SHADE 2000 Caveats Mie work assumes spherical particles Cut-off factors may change with different size distributions (e.g. more/less coarse mode present)

Future Work Observations Results from radiometers Results from Edwards & Slingo radiation code Chemistry –Iron Content, for deposition –Refractive indices, for comparison with derived values Specific case studies, comparison to remote sensing Modelling DODO – dust modelling, comparisons to observations, deposition estimates