Processes Controlling the Seasonal Cycle of Arctic Aerosol Number and Size

B. Croft1, J. R. Pierce2, R. V. Martin1,3, R. Leaitch4, T. Breider5, A. Morrow1 and S. D’Andrea2

1Dalhousie University, Halifax, Canada2Colorado State University, Fort Collins, USA

3Harvard-Smithsonian Center for Astrophysics, Cambridge, USA4Science and Technology Branch, Environment Canada, Toronto, Canada

5Harvard University, Cambridge, USA

NETCARE Workshop November 4, 2014

Aerosol Number and Size are Important to Aerosol Effects on Climate

• Aerosol size and number also contribute to regional air quality.

CCN and IN

• Scattering and absorption of radiation, and cloud property modification.

Strong Seasonal Cycle in Alert SMPS Measurements

Similar cycle at Zeppelin 2008 and Alert 2012

Dp [nm] Dp [nm]

Alert 2012 SMPS Measurements

dN/d

logD

p [cm

-3]

dN/d

logD

p [c

m-3

]

Accumulation Mode

Aitken Mode

Effective Diameter Cycle

Processes Controlling Aerosol Size Distributions

Wet Scavenging• Below clouds• In clouds

New Particle Formation and Emissions

Aerosol Growth• Coagulation• Condensation

Cloud Evaporation

Precipitation Formation

Activation, Collisions,FreezingTransport

Deposition

1

2

3

Initial GEOS-Chem Simulation Over-Predicts Aerosol Number at Alert (similar findings for Zeppelin and ARCTAS 2008)

Model over-predicts accumulation mode number in all seasons

Poorest accumulation mode simulation in summer

DJF MAM

JJA SON

dN/d

logD

pdN

/dlo

gDp

Dp [nm]Dp [nm]

Model Revision 1: High Efficiency Wet Scavenging Vigorous Wet Scavenging Reduces Accumulation Mode in Summer

Model improvement only in summer.Non-summer number over-prediction continues.

AlertIntegrated SMPS [cm-3]

Model still has too many aerosols, except summer is improved.

Num

ber [

cm-3

]

Month

DJF MAM

JJA SON300

Arctic has Strong Seasonal Cycle of Aerosol Lifetime

GEO

S-Ch

em Z

onal

Mea

n

Arctic haslowest lifetimes insummer

Revised scavenging reduces lifetimes

Arctic has greatest inter-seasonal range of lifetimes Solid: New scavenging

Dashed: Original

Model Revision 2: Sensitivity Test: Shut off new particle formation (NPF) globally in model

>75% of summernumberdue to new particle formation

Also, not physically reasonable to shut off all new particle formation

Problem: Over-erosion of accumulation mode

Nucleation change does not effectively fix what is wrong in model.

DJF MAM

JJA SON

No NPF

Original NPF

Alert Integrated SMPS

Contribution of New Particle Formation to Aerosol Number

Alert Integrated SMPSN

umbe

r [cm

-3]

250

Month

No new particle formation

Standard new particle formation

Measurements

New particle formation

Hypothesis: Aerosol Coagulation by Cloud Processing is Needed

Scenario 1:

Outside of clouds:

Scenario 2:

Inside clouds:Activated 80 nm aerosol behaves like it is 10 μm, not 80nm!

areas of persistent super-cooled cloud of low efficiency wet scavenging.This cloud processing is expected to be relevant in

40 nm aerosol

80 nm aerosol

Inefficient coagulation

Efficient coagulation

Model Revision 3: Cloud-Fraction-Weighted Coagulation Kernels

Ωi,j(new) = (1 – α) Ωi,j(original) + α Ωi,j(activated_size)

where Ω = coagulation kernel between aerosol i and j α = cloud fraction from met fields

Assumption: In a cloud, aerosols >80 nm act as if 10 μm for coagulation kernel calculation.

Geographic impact Global % Change in Total Number of Aerosols > 3 nm in diameter (N3)

GEOS-Chem November Mean with Revised Coagulation

Reductions in N3 of > 40% in Arctic, a region of persistent, extensive super-cooled liquid clouds and low efficiency scavenging.

Revised Coagulation: Closer Agreement with Alert SMPS

Winter, spring and fall simulation agrees better with Alert SMPS

Original Coagulation

New Coagulation

Improved Agreement with Alert Effective Diameter from SMPS

Revised coagulation and scavenging

Revised scavengingonly

No nucleation

Original Simulation

Effec

tive

Dia

met

er [n

m]

Month

300

140

Revised Model Agrees Better with Alert Integrated SMPS

Revised coagulation and scavenging

Revised scavenging

Num

ber [

cm-3

]

Month

300

Summary and Outlook

• Wet scavenging efficiency has a strong seasonal cycle in the Arctic.

• Efficient wet scavenging in summer reduces accumulation mode and allows for new particle formation (NPF: >75% of total number).

• Aerosol-aerosol coagulation in clouds in seasons of low efficiency wet scavenging may be an important sink for the Aitken mode.

• Future work: Evaluation of number and size simulation with NETCARE aircraft and ship measurements.

Accurate simulation of aerosol number and size is important for prediction of CCN and IN.