Aerosol Lifetimes at High Latitudes
Betty Croft1, Jeff Pierce1,2 and Randall Martin1,3
1Dalhousie
University, Halifax, Canada
2Colorado State University, Fort Collins, USA
3Harvard-Smithsonian Center for Astrophysics, Cambridge, USA
NETCARE Workshop 2013
November 18, 2013
Aerosol Lifetimes are Poorly Constrained in Global Models
Global mean BC lifetime [days]
• Black carbon (BC) global mean lifetime range: 3.3 and 10.6 days (Bond et al., 2013)
Similarly, Arctic BC
lifetimes are poorly
constrained due to
several uncertain
processes
(e.g. Browse et al., 2012;
Wang et al., 2011;
Koch et al., 2009).
• Direct radiative forcing (DRF) depends on lifetime (Schultz et al., 2006)
E: emissions rate; L: lifetime governed by removal;
MACBC : mass absorption cross-section; AFE: absorption forcing efficiency
• Lifetime also controls geographic distribution and indirect aerosol forcing.
Radionuclides as a Constraint on Aerosol Lifetime
simulated with GEOS-Chem model
Croft et al. (submitted), ACP
Surface Layer 137Cs/133Xe
137Cs
137Cs
Column Burden
 Fukushima Dai-Ichi
accident site (white)
 CTBTO measurement
sites (black)
 Dalhousie-CTBTO
contract for access to
radionuclide
measurements
[Bq m-2]
Regionally Varying Lifetime Bias at High Latitude Sites
Croft et al. (submitted), ACP
Simulated 137Cs / 133Xe Surface Layer E-folding Times
•
Bias reflects
uncertainty in
aerosol removal
simulation,
particularly at
high latitudes
April-May, 2011
•
Fit over days 20-80 after the
March 11, 2011 earthquake
•
Circles: Measurement values
•
Site-mean measurement
e-folding time: 13.9 days
Site-mean simulated:
e-folding time: 16.7 days
Global mean simulated 137Cs
lifetime: 1.8 days
•
•
[days]
Lifetime Gradients Reflect Differences in Removal Processes
Zonal Mean Lifetime with respect to Removal: May-August 2011
137Cs
Altitude [km]
Black Carbon
Latitude oN
Latitude oN
0
•
3
10
30
100 [days]
Rapid transition of removal efficiency with altitude north of 60oN  Need measurements with altitude
Challenges in Simulating Aerosol Scavenging in Arctic
Simulated Column Black Carbon Lifetime with respect to Removal
Winter
Mixed phase and ice
cloud uncertainties:
• ice nuclei number
and behavior
• impaction
scavenging
• aging
• seasonal changes
• influence of
processes outside
the Arctic
Summer
0
3
10
30
100 [days]
60oN-90oN Simulation:
Summer Winter
Burden:
9.8 Gg
5.4 Gg
Removal:
150 Gg
42 Gg
(85% wet) (80% wet)
Lifetime (removal):
5.9 d
11.6 d
Measurements Needed to Constrain Uncertain Processes
Arctic Scavenging Sensitivity Studies
Black carbon
Aerosol fraction in cloud
liquid and ice is important
for scavenging.
75N-90N 180W-30W
Typical assumptions:
• 100% over given T ranges
or
• Verheggen et al. (2007)
for T-dependent fraction
Winter
Summer
•
Need to understand variation of
cloud-borne aerosol fraction
with temp, size, and composition
(how BC and OC differ).
• Concentration vertical profiles are strongly sensitive to transitions between several
uncertain processes related to scavenging.
Ice Nuclei Number Strongly Sensitive to Aging
Arctic Scavenging Sensitivity Studies
Black carbon
75N-90N 180W-30W
• Measurements of aerosol
composition/mixing state are
needed to better understand
aging.
• Need measurements outside
Arctic . What happens in the
Arctic depends on what
happens outside the Arctic!
(e.g. winter aging sensitivity)
- even tropical convective
scavenging affects Arctic aerosol
concentrations (Croft et al.,2012)
Winter
Summer
Aging e-folding times:
• 1.15 day
• Reimer et al. 2007 (1.15 days if no sun and 2 hours if sunlit)
• Lee et al. (2013) assigned
low uncertainty to aging
for CCN prediction, but IN
prediction is a different
story.
Aerosol Scavenging is Strongly Size Dependent
• Size-resolved models (e.g. GEOS-Chem TOMAS and APM) allow closer coupling between
cloud microphysics and scavenging.
• Need measurements of aerosol size and composition in interstitial and condensed phase.
Impaction by ice crystals
Impaction by rain and snow
• Ongoing need
for
measurements
of collision and
collection
efficiency
(Croft et al., 2010)
(Croft et al., 2009)
Vertical Profiles Highly Sensitive to Scavenging – Need Constraints
Black: Measurements
Color: ECHAM5-HAM
prescribed fraction (0.75)
size-dependent scavenging
with aerosol processing
Black carbon concentration ng kg-1
(Croft et al., 2010)
• Common to see two-order-of-magnitude inter-model differences in predicted
black carbon vertical profiles in Arctic (e.g. Koch et al., 2009).
• Similar situation for organic carbon.
• Differences are strongly controlled by scavenging parameterizations.
Outlook and Future Research Directions
• Scavenging plays a critical role in Arctic aerosol concentrations, but remains poorly
constrained in global models.
• Measurements are needed to:
1) improve of aerosol scavenging parameterizations and understanding of processes
(measurements of interstitial and condensed phase aerosol concentration,
composition, size, CCN, IN, deposition and their variation with altitude,
temperature and cloud properties)
2) evaluate global model simulations.
• Scientific focus:
- Understanding and parameterizing aerosol size-dependent removal in the Arctic.
- Understanding impacts of scavenging on predicted radiative forcing.
- Coupling GEOS-Chem TOMAS (two moment aerosol sectional) microphysics model
simulations with upcoming measurement campaigns.
Should Cloud-Borne Fractions Differ Between OC and BC?
Arctic Scavenging Sensitivity Studies
75N-90N 180W-30W
Organic Carbon
Winter
Summer