school of earth and environment institute for climate & atmospheric science is the arctic the...
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School of Earth and EnvironmentINSTITUTE FOR CLIMATE & ATMOSPHERIC SCIENCE
Is the Arctic the most uncertain aerosol environment in the world?
Ken Carslaw, Jo Browse, Lindsay Lee, Graham Mann Kirsty Pringle, Carly Reddington, Graham Mann,
Dom Spracklen, Alex Rap, Piers Forster, Ben DrummondUniversity of Leeds, UK
Jeff PierceColorado State University
Funding: NERC AEROS, GASSP and ACCACIA projects,
EU PEGASOS, the National Centre for Atmospheric Science
Model predictions of Arctic aerosol
Black
Carbon
SulfateCO
Shindell et al., 2008
Annual mean sulfate
Mean concentration and diversity of the “central two-thirds” of 12 AEROCOM microphysics model
AEROCOM microphysics intercomparison: Sulfate diversity
SO4 concentration
SO4 diversitySO4 diversity
1
10
30
1
10
30
Mann et al., ACPD, 2013
AEROCOM microphysics intercomparison: BC diversity
Annual mean BC
Mean concentration and diversity of the “central two-thirds” of 12 AEROCOM microphysics model
Mann et al., ACPD, 2013
BC concentration
BC diversityBC diversity
1
10
30
1
10
30
AEROCOM microphysics intercomparison: Particle concentration diversity
Mann et al., ACPD, 2013
N30 concentration N100 concentration
N30 diversity N100 diversity
1
10
30
1
10
30
Two “aerosol seasons” with very different processes
Svalbard observations show a transition from “Arctic haze” in winter to very clean summer
Korhonen et al., ACP, 2008
Browse et al., ACP, 2012
December-April June-August
Garrett et al (2010): “In the Arctic, the freezing point appears to serve as a ‘scavenging point’ that is only passed in the mid-summer months
A meteorological “scavenging point” temperature
6
8
2
1
6
8
2
1
Observed ratio of winter/summer BC
Winter Spring
Fraction of precip as snow
Browse et al., ACP, 2012
“Fixing” the inverse seasonal cycle problem
Switching off scavenging at -15C, adding summer drizzle
Browse et al., ACP, 2012
More comprehensive sampling of the model uncertainty
Model response surface in one grid box
Model runs
Essentially using the interpolated parameter space to enable a very
dense Monte Carlo sampling of the modelParameter 1
Parameter 2
Ou
tpu
t (e.
g.,
CC
N)
Extend to 28 dimensions with 168 runs
Lee, L.A. et al., Emulation of a complex global aerosol model to quantify sensitivity to uncertain parameters, ACP 2011.
Lee, L.A. et al., The magnitude and causes of uncertainty in global model simulations of cloud condensation nuclei, ACP 2013.
28 simultaneously perturbed parameters (1/2)
Parameter Lower UpperBCOC mass emission rate (fossil fuel) 0.5 2.0
BCOC mass emission rate (biomass burning) 0.25 4.0
BCOC mass emission rate (biofuel) 0.25 4.0
Sea spray mass flux (coarse/acc) 0.2x 5.0x
SO2 emission flux (anthropogenic) 0.6x 1.5x
SO2 emission flux (volcanic) 0.5x 2.0x
Biogenic monoterpene production of SOA 5 Tg/a 360Tg/a
Anthropogenic VOC production of SOA 2Tg/a 112Tg/a
DMS mass flux 0.5x 3.0x
BCOC mode diameter (fossil fuel) 30 nm 80 nm
BCOC mode diameter (biomass burning) 50 nm 200 nm
BCOC mode diameter (biofuel) 50 nm 200 nm
Subgrid conversion of SO2 to SO4 ("primary SO4“) 0% 1%
Mode diameter of "primary SO4" 20 nm 100 nm
Particle and precursor gas emission rates
Properties of emitted particles
A probability distribution for each parameter was also elicited
Parameter Lower UpperBL nucleation rate k[H2SO4] 4E-7 2E-04
FT nucleation rate (BHN) x0.01 X10
Ageing "rate" from insol to sol (monolayer) 0.3 5
Modal width (accumulation) 1.2 1.8
Modal width (Aitken) 1.2 1.8
Mode separation diameter (nucleation/Aitken) 9 nm 20 nm
Mode separation diameter (Aitken/accumulation) x1.5 x3
Microphysical rates
Model “structural choices”
Cloud drop activation dry diameter 30 100
Reaction SO2 + O3 in cloud water (clean) pH=4 pH=6.5
Reaction SO2 + O3 in cloud water (polluted) pH=3.5 pH=5
Nucleation scavenging dry D (above activation) 0 nm 100 nm
Nucleation scavenging fraction (T> -15C) 0.2 0.99
Dry deposition velocity (Aitken) x0.5 X2.0
Dry deposition velocity (accumulation) X0.1 X10.0
Cloud processing
Dry and wet deposition
28 simultaneously perturbed parameters (2/2)
Current projects extending to host model physics parameters
PDFs of mean CCN concentration in every grid box (July)
Parametric uncertainty in BC and sulfate in one model
Standard deviation divided by the mean (December)
Uncertainty due to 28 parameters
Crude comparison with multi-model ensemble:
Parametric +s/-s ~ 4 compared to +MME/-MME ~ 20
Arctic dominated by model structural uncertainty?
BC uncertainty SO4 uncertainty
0.1
1
2
0.5
0.1
1
2
0.5
BC fraction of variance (January)
BC fraction of variance (July)
CCN fraction of variance (July)
BL nuc
Dry dep
Aitken width
BB ems
BB diam
1750-2000 indirect forcing fraction of variance (July)
BB diam
DMS ems
SO2 ems
CCN and indirect forcing uncertainties are not caused by the same factors
See Carslaw et al., Nature, 2013
Response of marine emissions to loss of sea ice
Present day DMS DMS (no ice)
Sea spray (no ice)Sea spray
Browse et al., The complex response of Arctic cloud condensation nuclei to retreat of sea ice, ACPD 2013.
Response of Arctic aerosol to loss of sea ice
Change in CCN Change in N3
Browse et al., The complex response of Arctic cloud condensation nuclei to retreat of sea ice, ACPD 2013.
In a scavenging dominated environment, the response of CCN to changes in emissions can be complex!
Minimal impact of future Arctic shipping
Future Arctic shipping could contribute locally 10% to BC deposition on snow and ice
There are likely to be much larger changes due to meteorology
Browse et al., Impact of future Arctic shipping on high-latitude black carbon deposition, GRL 2013.
Conclusions
• The AEROCOM global aerosol microphysics multi-model ensemble shows huge diversity in Arctic BC, SO4 and CCN (factor 30 between central 2/3 of models versus <factor 4 elsewhere)
• The parametric uncertainty of aerosol in a single model (GLOMAP) peaks in the Arctic (factor 4 versus <factor 2 elsewhere)
• Structural uncertainty dominates? (although correction of some gross model errors would reduce this substantially)
• The factors controlling the uncertainty in Arctic CCN are different to the factors that control indirect forcing
• Retreat of Arctic ice seems to cause a complex response in CCN
• Changes in Arctic shipping not a major factor for future BC
• The answer is YES: the Arctic is the most uncertain aerosol environment on Earth
Confronting the model with observations
See gassp.org.uk