Kinetics of CCN activation and Droplet growth observed in recent field campaigns

Fan Mei (fan.mei@pnnl.gov) and Jian Wang (jian@bnl.gov)

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*