Kinetics of CCN activation and Droplet growth observed in recent field campaigns
Fan Mei ([email protected]) and Jian Wang ([email protected])
Introduction & Motivation Aerosol indirect effects, which describe the influences of aerosol
on climate through modifying cloud properties, remain the most uncertain components in forcing of climate change over the industrial period.
The formation of cloud droplets from aerosol particles is kinetically controlled by the availability of water vapor, equilibrium water vapor pressure above the growing droplet surface, and both the gas phase and aerosol phase mass transfer resistances.
It has been hypothesized that the formation of surface organic films or the delay in dissolution of solute could significantly delay the growth of cloud droplets, therefore influence the cloud formation, life time and precipitation.
In this study, we examine the droplet growth kinetics using data collected in three recent field campaigns.
General Approach Results
Sampling Locations and Periods
Acknowledgements Atmospheric System research program
ARM climate research facility
NOAA, CEC and Caltech
Conclusions When Sint = S*, the average droplet size for size-selected ambient
particles with the same S* and αC is larger than that for (NH4)2SO4 particles (due to higher σ(Sc)/S*).
This may mask reduced droplet growth due to lower αC value (i.e, formation of compressed organic film).
Comparing the droplet sizes at Sint > S* + 0.1% allows us to better isolate the impact of αC on average droplet size and identify potential slow droplet growth.
By comparing the average droplet sizes of size-selected ambient particles to those of (NH4)2SO4 calibration particles (for the same Sint and S*), we show that there were no significant delay in droplet growth for the aerosol particles from a variety of sources observed during the three field campaigns.
Droplet growth inside CCN counter
Results (field observations, Sint = S*)
Results (field observations, Sint > S*)
Results
When Sint = S*, for aerosols with identical S*, but different σ(Sc)/S*, the average droplet sizes are different, which may mask the difference in droplet sizes due to potential growth delay.
When Sint = S*+0.1%, the average droplet sizes are essentially the same for aerosols with identical S*, but different σ(Sc)/S*.
For particles from various representative sources sampled during the three field campaigns, the average droplet sizes were essentially the same as those of (NH4)2SO4 calibration aerosol (with the same Sint and S*), suggesting no significant delay in droplet growth.
Measurements of size-resolved CCN spectrum
Cloud droplet growth rate inside CCN counter
Equilibrium S (Seq) over growing droplet, a function of D and Sc of CCN
Experimental setup http://pubs.acs.org/cen/coverstory/89/8906cover.html
CalNex-LA site •05/16/2010 -- 06/05/2010
CARES site •06/10/2010 -- 06/28/2010
ALC-IOP site •07/04/2011 -- 08/15/2011
CPC
aerosol
sheath
hum
idifi
er
OPC CCN counter
Aerosol sample
DMA
NCN (Particle Conc.)
NCCN (CCN Conc.)
NCCN /NCN (Activation fraction of size selected particles
as a function of S)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.2
0.4
0.6
0.8
1
Instrument supersaturation (%)
NC
CN
/NC
N
0.5 maximumactivated fraction
S*
Maximum Activatedfraction
Determine characteristic supersaturation S* and σ(Sc)/S*, which reflects heterogeneity in critical supersaturation of size selected particles (e.g., due to difference in particle composition)
( )),
(in
C
t ceqS DdDdt
SD
SG α
−=
⋅
Instrument supersaturation
Mass transfer resistance is a function of water accommodation coefficient αc Use (NH4)2SO4 particles (no
organic film) as standards.
To isolate the potential impact of αC on droplet growth rate, we need to compare the droplet size of ambient particles to that of (NH4)2SO4 particles with the same Sc, and exposed to the identical Sint.
0 0.2 0.4 0.6 0.8 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Instrument supersaturation (%)
Activ
atio
n fra
ctio
n
S*
Sint = S*
Sint = S*+0.1%
Ambient particles σ(Sc)/S*=0.2
(NH4)2SO4 particles σ(Sc)/S*=0.1
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.41
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Instrument supersaturation, Sint (%)
Aver
age
drop
let d
iam
eter
( µm
)
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.51
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Instrument supersaturation, Sint (%)
Aver
age
drop
let d
iam
eter
( µm
)
CalNex σ(Sc)/S* <1.1
CARES σ(Sc)/S* <0.3
0.05 0.1 0.15 0.2 0.25 0.31
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
Characteristic supersaturation, S* (%)
Aver
age
drop
let d
iam
eter
( µm
)
Exposed to 0.13 %
Exposed to 0.18 %
Exposed to 0.22 %
Exposed to 0.29 %
Exposed to 0.38 %
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.451.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
Characteristic supersaturation, S* (%)
Aver
age
drop
let d
iam
eter
( µm
)
Exposed to 0.19 %Exposed to 0.23 %Exposed to 0.30 %Exposed to 0.39 %Exposed to 0.45 %
0 0.1 0.2 0.3 0.4 0.5 0.6 0.71
2
3
4
5
6
7
8
Characteristic supersaturation, S* (%)
Aver
age
drop
let d
iam
eter
( µm
)
Exposed to 0.15 %
Exposed to 0.20 %
Exposed to 0.25 %
Exposed to 0.32 %
Exposed to 0.41 %
Exposed to 0.50 %
Exposed to 0.79 %
CARES CalNex
ALC-IOP
When Sint=S*, ambient particles sampled at the CalNex-LA site often grew to substantially larger average sizes (blue circles) than (NH4)2SO4 particles with the same Sint and S* (indicated by black line with uncertainty). This is due to much larger values of σ(Sc)/S* for ambient particles and is consistent with the simulation above. This also suggests that examining droplet sizes under Sint > S* is a better approach to identify potential delay in droplet growth.
For particles sampled during CARES, σ(Sc)/S* was much lower, and the average droplet sizes were mostly in agreement with those of (NH4)2SO4 particles.
Slope gives σ(Sc)/S*
Results (simulations of droplet growth )
0.1 0.2 0.3 0.40
2
4
6
8
Sc (%)
Drop
let d
iam
eter
at C
CNC
exit
( µm
)
Sint
=0.4%
αC=0.01
αC=0.02
αC=0.05
αC=0.1 αC=0.2 αC=0.5 αC=1
0.1 0.2 0.3 0.4 0.50
2
4
6
8
Sc (%)
Drop
let d
iam
eter
at C
CNC
exit
( µm
)
αC=0.05
Sint=0.4%
Sint=0.5%
Sint=0.3%
Sint=0.2%
0.2 0.4 0.6 0.83
4
5
6
7
8
9
10
Instrument supersatuation, Sint (%)
Aver
age
drop
let d
iam
eter
(µm
)Simulated droplet size, αC=0.05
σ(Sc)/S*=0.10
σ(Sc)/S*=0.20
σ(Sc)/S*=0.50
σ(Sc)/S*=1.0
Sint = S*+0.1%
0.2 0.4 0.60
2
4
6
8
10
Instrument supersatuation, Sint (%)
Aver
age
drop
let d
iam
eter
(µm
)
Simulated droplet size, αC=0.05
σ(Sc)/S*=0.10
σ(Sc)/S*=0.20
σ(Sc)/S*=0.50
σ(Sc)/S*=1.0
Sint = S*
