Aerosol Impacts on Climate

Climate Fellows WorkshopUsing Technology to Promote Climate Literacy

North Carolina Botanical GardensChapel Hill NCJune 21 2011

Uma ShankarUNC–Institute for the Environment

Outline

What are atmospheric aerosols? What are their major sources? What are their physical, chemical and optical

properties? How do they affect climate – what is the

whitehouse effect? What regions of the world are most affected? What do we do?

So what do aerosols look like?The view from close up…

•Ionic map of atmospheric sulfate (green) and methane sulfonate (blue) particles that would be typical of a marine environment

…and from far away

Haze from particulate pollution off the coast of the eastern United States, August 6, 2007 Haze over the

Himalayas, Jan 17, 2007

Dust storm over the Persian Gulf on March 7, 2007

Major Aerosol Sources: Natural

Sea spray (Na, Cl, S)

Terpenes from pine, isoprene from oak oxidize to form secondary organic aerosol (SOA)

Volcanic eruptions

Wind-blown dust

Major Aerosol Sources: Man-made

Power plants (SOx)Auto exhaust (NOx, soot)

Oil refineries (VOCs)

Human-caused forest fires and, yes, even rural cookstoves (organic and black carbon)

Aerosol Size Distributions

Accumulation mode is most important for scattering sunlight in the UV-to-visible wavelengths, and for formation of cloud droplets

AitkenAccumulation

Coarse

Processes Affecting Aerosol Physical Properties

Coagulation Nucleation Condensation and evaporation of volatile species

on/off particle surface Dry deposition (including gravitational settling) Cloud processes (including precipitation) Direct emissions

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Atmospheric Chemical Reactions

Emissions of SO2 undergo oxidation by OH radical and combine with water vapor to form H2SO4 particles (binary nucleation)

– If NH3 is present it condenses on particles to neutralize H2SO4 to varying degrees of neutralization

– Any HNO3 present (terminal product of photochemical NOx reaction cycle) will also condense on neutralized sulfate to form NH4NO3

– In marine environments reactions of Na and Cl with these constituents also occur

Secondary organic aerosols (SOA) form a vast array of species by oxidation of VOC aerosol precursors

– Olefins, alkanes, alkenes, monoterpenes, sesquiterpene, isoprene…– Typically oxidants are OH-, NO3

- and O3

– inorganic aerosol can mediate formation of some SOA (oligamers)

Direct Radiative Impacts

Aerosols directly attenuate solar radiation in the short and visible wavelengths

– Direct scattering of solar radiation cools the earth’s surface – Known as the direct radiative forcing of aerosols

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Radiative Transfer – Beer’s Law

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Attenuated beamI - dI

dz

dI= -IKN ds

I= initial intensity of a monochromatic beamK= scattering or absorption (or extinction) efficiency (unitless) = areal cross-section of particle or molecule (m2)N = number concentration (m-3)

Integrating over the depth of the atmosphere, we get

I = IT, where

transmissivity T= e-sec ,

and = z KN dz

is the normal optical depth

ds = dz sec

Radiation incident on an infinitesimal atmospheric layer of thickness dz

I

Radiative Transfer (cont’d) In the absence of scattering the absorptivity is:

= 1- T= 1- e-sec

● The contributions of various substances to the respective efficiencies are additive, as are the contributions of scattering and absorption efficiencies to total extinction, i.e.,

K N = (K)1 N1 1 + (K)2 N2 2 + …

K(extinction) = K(scattering) + K(absorption)

● Can calculate Kfor aerosols based on models of scattering by a sphere of radius r

• Kis a function of size parameter x = 2r / andthecomplex refractive index = r + ii

• r represents the scattering and i the absorbing component• Mie theory covers .1 ≤ x ≤ 50 (atmospheric aerosol sizes)

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Indirect Radiative Impacts Aerosols activate as Cloud Condensation Nuclei (CCN), given

the proper conditions of atmospheric saturation, particle solubility (or wettability), and size

CCN increase the number of cloud droplets, increasing cloud cover, and BRIGHTNESS, i.e., the scattering of sunlight back to the atmosphere

- The whiter atmosphere is visible from space (sometimes called the ‘whitehouse’ effect)

- Known as the First Indirect Effect Pollutants in cloud droplets reduce the droplet size, and

activated droplets do not grow to rain drop size, increasing cloud LIFETIME

– Known as the Second Indirect Effect Semi-direct effect (heating or cooling) of Black Carbon (BC)– Absorbing aerosol; below cloud layer can heat the air, reduce relative

humidity and decrease low-lying cloud cover– Above cloud layer BC can reduce the magnitude of this heating

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Climate Impacts Tropical troposphere feels greatest impact of

Atmospheric Brown Clouds (ABCs)– Mixture of SO4, NO3, BC, OC, dust, etc. that forms haze layer– Mostly wash out in extra-tropical troposphere

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BC emis. (contained

combustion)

BC emis.(open

combustion)

SO4 emis.

RF (BC+SO4)

Climate Impacts (cont’d)

ABCs inhibit vertical transport of moisture– Increase humidity above surface– Inhibit surface evaporation

Have reversed the large-scale circulation, which lofts warm, humid air north of equator and causes subsidence south of equator and brings rainfall to Northern China and Ganges valley

– Projected to cause increased droughts in these regions– Increased flooding expected in Yangtze Valley

Persistent, produce a positive feedback loop Threaten food and water security in the most

populous parts of the globe

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In the grand scheme of things• Radiative “forcing” of aerosols on climate has very large

uncertainties, making climate change prediction very difficult

Concluding Remarks

Aerosols have clearly had an impact on the global radiation budget thus far

Health considerations are making it imperative to control particulate matter, reduce impact on cardio-pulmonary function

Reductions in the surface cooling effects of aerosols will add to the heating of the atmosphere

Reduction strategies for various types of aerosol sources MUST pay attention to the delicate balance between the absorbing and scattering aerosol properties

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Direct radiative forcing from aerosolBASE CASE

WARMING

COOLING

Reductions in sulfate due to future air

quality policy will result in warming

WARMING

COOLING

Carbon control policy leads to decreased coal combustion and sulfate emissions, which results

in further short-term warming

Direct radiative forcing from aerosolControl CO2 CASE

WARMING

COOLING

Implementation of BC emission controls on

transportation sources can offset remaining

warming

Direct radiative forcing from aerosolAdd black carbon controls

Questions?

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OMI L2G vs. Regridded (36-km) AOD

• Developed a tool to remap the satellite data to the model grid for quantitative comparisons against CMAQ

• Can remap global grid as well as swath data

L2G (0.25°) Regridded (36-km)

CMAQ 36-km AOD vs. OMI

CMAQ at 2 PM MDT OMI L2G (regridded)

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Grid-based Air Quality Modeling

Evaluation and Iterative refinements

Meteorological model

AQ model

Emissionsmodel

Observations

Other Inputs

Improved Predictions for

Impact Assessments,

Decision-making

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Air Quality Data Needs Models need to be evaluated against

observations• to quantify biases, characterize uncertainty in predictions

• to reduce uncertainty and increase reliability by improving relevant process algorithms

• to help define requirements for future observations (what, when, where)

Most air quality model evaluations use ground-based measurements

• Models are 3-D

No detailed understanding of model behavior aloft

• Sparse spatial and temporal coverage in most ground-based observational networks

• Intensive field campaigns are usually short and sporadic due to high cost

Knowledge gaps

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Satellite Data Use in AQMs

To fill some of these gaps Satellite data products can be used in areas where

surface data are sparse or unavailable for model evaluation, to verify spatial distribution of pollutants

Satellite data are increasingly being assimilated in air quality models

• to improve such inputs as photolysis rates and lateral boundary conditions, especially over the oceans

• to constrain emissions inputs (e.g., biomass burning, dust)

This course covers the use of satellite data in the analysis of model results

PM Treatment in CMAQ Characteristics of atmospheric aerosol modes

• Aitken mode (up to ~ 0.1 m) (typically for freshly produced particles)• Accumulation mode (0.1 – 2.5 m) (typically for aged particles)• Coarse mode (2.5 – 10 m)• PM10 is the sum of the mass concentrations in all three modes

Primarily Emitted• AORG_P (Primary organic aerosol)• AEC (Primary elemental carbon)• A25 (Unspeciated fine PM/dust)

Secondarily Produced• ASO4 (Sulfate aerosol)• ANH4 (Ammonium aerosol)• ANO3 (Nitrate aerosol)• AORG_S (Secondary organic aerosol)

Ref: Binkowski et al., JGR 2003

CMAQ Post-processing of AOD Fast optics calculation parameterized from Mie

theory reproduces theoretical results to very good accuracy *• Constructs for water soluble, insoluble, soot and sea salt

aerosol and water from a LUT at discrete spectral intervals

• Currently configured for AOD calculation at 550 nm and the CB05_Hg_Tx_ae5 mechanism used in the Middle East

• Also calculates single scattering albedo and asymmetry parameter

• Useful for comparison with these other RS metrics

Uses vertical layer and model grid inputs from met files along with aerosol species and volume concentration from CMAQ concentration files

AOD is output hourly

* Binkowski et al., J. Appl. Met. Clim., 200729

Usage and Caveats

● Very important to reconcile satellite overpass time and the time at which model output is “sampled”

• Satellite data reported at local overpass time

• Different satellites have different overpass times

• Model output reported in UTC

• Vijayaraghavan et al. (2007) recommended multi-day averages to improve sampling statistics

● Accuracy of satellite data can vary depending on the wavelength at which data are used

• OMI AOD are retrieved at 388 nm and converted to values at 500 nm using an aerosol model – room for uncertainty

● Accuracy of satellite data varies with product

30Vijayaraghavan et al., Environ. Sci. Technol, 2008

Usage and Caveats (cont’d)

Accuracy of comparison to model output critically depends on the spatial extent of data sampling

• Tools based on the CMAS Spatial Allocator to now available to regrid satellite data and project them to the model grid

• Quality of regridded data depends on the model resolution

• Can be seen by comparing the .gif image from OMI against the regridded data on the 36-km model grid

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Other Column Data Products

CMAQ calculates concentrations and mixing ratios at each model layer

– Tropospheric column calculations for comparison with satellite data need post-processing from model output concentrations

● CMAS has added a new interactive tool to m3tools in the I/O-API

• Performs unit conversion from species units (ppm or g/m3) to molecules/cm3 for all the species on the file

• Integrates the concentrations through all the model layers

• Outputs an hourly I/O-API netCDF file that can be further processed to create a daily average file

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