Mixing states and related hygroscopicity of aerosol particles over West Africa:
AMMA aircraft campaign in summer 2006
O20
A. Matsuki 1,3, A. Schwarzenboeck 1, S. Crumeyrolle 1,2, B. Quennehen 1, L. Gomes 2,
H. Venzac 1, P. Laj 1, O. Laurent 1, G. Momboisse 1,2, T. Bourrianne 2
1 LaMP,
Observation
Clermont-Ferrand, France, 2 CNRM, Météo-France, Toulouse, France
3 FSO, Kanazawa, Japan
cloud residual  clear sky particles
ATR42 aircraft
CVI:
CAI:
Counterflow Virtual Impactor
Community aerosol inlet
(installed only during SOP2a2)
Dp < 5 µm
Dp > 5 µm
Niamey, Niger (13º
(13º 30' N, 02º
02º 05' E)
Special Observation Periods (SOP)
SOP1 (Dry Period) : June 2006
SOP2a1 (Transition) : July 2006
SOP2a2 (Monsoon) : August 2006
Transport patterns (METEX, NIESNIES-CGER, 5days backward, 3D mode)
Instruments:
•Cascade impactor (for TEM, SEMSEM-EDX analysis)
•Scanning Mobility Particle Sizer SMPS
•CCN counter (University of Wyoming, 100100-B)
only during SOP2a1 and SOP2a2
•Condensation Particle Counter CPC (TSI, 3025)
Accumulation mode particles
AirAir-mass trajectories arriving the points of sampling. The average height
height of the planetary boundary layer
(PBL) decreased from 1.6km (SOP1), 1.4km (SOP2a1) down to 0.85km (SOP2a2). SouthSouth-westerly flow
(Monsoon flux) predominant in the PBL, while (north(north-) easterlies take over in the Saharan Air Layer (SAL)
influenced by the African Easterly Jet (AEJ).
(0.1 µm < Dp < 1.0 µm)
6
1.0
4
SOP2a1PBL
3
SOP2a2PBL
SOP2a1SAL
2
SOP2a2SAL
1
0
Submicron dust particles were in fine flakes of clay like minerals.
minerals. Biomass burning particles contained K and S
elements along with Carbon (> background peak). They often showed
showed inclusions of soot like aggregates.
Sulfate particles were predominantly nonnon-acidic over land.
(n=100)
(n=8)
(n=37)
(n=112)
(n=8)
(n=29)
(n=100)
(n=30)
(n=25)
(n=20)
(n=19)
(n=26)
(n=134)
(n=13)
(n=19)
(n=66)
SOP2a2SAL
SOP2a1PBL
0.4
SOP2a2PBL
0.2
0.2
0.4
0.6
CCN0.4% / CN
0.8
1
0.0
0.2
0.4
0.6
0.8
1.0
CCN0.4% / CN
CCN / CN ratio generally higher in SAL than
Higher CCN / CN ratio can be partly
PBL. Exceptionally high during SOP2a2 in SAL.
SAL. explained by the larger fraction of particles
in CCN relevant sizes (Dp
(Dp > 120nm).
Exceptionally high CCN0.4% / CN ratio found
during SOP2a2 in SAL can be partly due to
the presence of supposedly aged particles
(i.e. large fraction of sulfate and biomass
burning particles with high S/(S+K) ratio).
Relative abundance of major submicron particles collected in PBL and SAL during three SOPs. Error bars
indicate 95% confidence intervals. Numbers in the parenthesis are
are the total number of particles analyzed by
TEMTEM-EDX. Significant decrease in dust fraction was observed.
Coarse mode particles
SOP2a1SAL
0.6
Vertical variation of CCN / CN ratio at 0.4%
CN121
121--288nm / CN: fraction of particles in
super saturation measured during SOP2a1
sizes 121121-288nm (by SMPS) relative to total
(Pre(Pre-monsoon) and SOP2a2 (Monsoon) periods. CN concentration (Dp
(Dp > 3nm).
Electron micrographs and XX-ray spectra of representative particles collected during AMMA campaign.
campaign.
(n=51)
0.8
0.0
0
(n=35)
CN121-288nm / CN
Altitude, km
5
Even the supposedly insoluble dust particles
were found as actual cloud residues
(stratocumulus type, water clouds 0.50.5-3km)
during SOP2a2 along with biomass burning
and sulfate.
Aged biomass burning particles are suggested to be more hygroscopic
hygroscopic than
freshly emitted biomass burning particles.
(Dp > 1.0 µm)
Processed Calcite
mineral dust (spherical)*
These three types accounted for most of the coarse particles.
Their relative abundance varied significantly in time and space.
•Sea salt particles were transported to Niamey by the prevailing
monsoon flux.
Aged Sea salt
TEM image of Coarse
particles collected via CVI
•Mineral dust > Sea salt in cloud residues over Niamey. Important
role of mineral dust as CCN.
*Spherical particles enriched in Calcium (c) were assumed to be
processed Calcite (and/or dolomite) mineral dust. Chemical test
using nitron coated film showed presence of NO3- in most of such
particles. Spherical shape most likely results from deliquescence
deliquescence of
extremely hygroscopic Ca(NO3)2.
Interestingly, irregularly shaped calcite particles were hardly
found even outside of clouds. Significant fraction of dust can be
present in spherical form  Impact on dust optical properties.
Uptake of acidic gases on mineral dust: dependence on mineralogy
Detection frequencies of S and Cl elements as well as NO3- ion among the individual mineral dust
particles of different mineralogy. The Fe rich and Ti containing fraction (Aluminosilicate
(Aluminosilicate II) was
distinguished from the major aluminosilicate particles (Aluminosilicate
(Aluminosilicate I) for comparison. The detection
frequencies are compared between the clearclear-sky (gray area) and inin-cloud (solid black line) samples.
Number of total investigated particles is given in parenthesis.
Calcite and Dolomite are particularly reactive with acidic gases (SO2, HCl,
HCl, HNO3 etc.).
dependence on relative humidity
100
Detection frequency %
Silicate
mineral dust
(irregular)
%
,y 75
c
n
e
u
q
re
f 50
n
o
ti
c
e
t
e 25
D
S clear-sky
Cl clear-sky
S in-cloud
Cl in-cloud
0
0
25
50
RH, %
75
100
Detection frequencies of S and Cl elements in silicate
particles (Quartz, Aluminosilicate I + II) plotted against the
relative humidity at which they were collected. Samples
collected inin-cloud and clearclear-sky conditions are compared.
High RH% and eventual immersion into droplet  increased
uptake coefficient of acidic gases on mineral dust.
Both Silicate and Calcite particles were found as cloud
residues. It is highly probable that the observed formation
of soluble materials enhanced their cloud nucleating abilities.