2005 SOLAS Summer School

Introduction to Marine Aerosols

Eric S. SaltzmanEarth System ScienceUniv. of CA, Irvine

What driving marine aerosol research?

• geochemical cycles

– metals, nutrients, organics

• acidification (sulfur, nitrogen)

• climate change

– direct/indirect effects

– aerosol optical properties, aerosol/cloud interactions

• nutrients

– N deposition

– coastal, HNLC

– desert dust and iron deposition

• human health

– air quality, airborne pathogen transport

Earth’s energy balance...

Global mean radiative forcing of climatefor year 2000 relative to 1750 (IPCC)

Aerosol effects on radiation budget radiation

Direct effects (cloud-free):•scatter cooling•absorption heating

Indirect effects (clouds):•more (but smaller) droplets scatter (Twomey)•more droplets longer cloud lifetime (Albrecht)•absorption heating evaporates clouds

The life cycle of marine aerosols...

•What are marine aerosols made of ?

•Where do they come from?

•How long do they stay in the atmosphere? How are they removed?

•How do they evolve while in the atmosphere?

•How do they interact with the climate system?•forcing•feedback

Note on observational constraints: intensive – many param., poor coverage network – good coverage, few param. satellite – excellent coverage, limited interpretability...

Things we “know we know”... (or don’t)...

Unknown knowns...?

• “nucleation is rare”

• “sulfate is the source of CCN (cloud condensation nucleii) over the oceans”

• “sea spray doesn’t make submicron aerosols"

“...there are known knowns. These are things we know that we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns. These are things we don't know we don't know.”

Donald Rumsfeld

Terminology...

•aerosol - a dispersion of solid and liquid particles suspended in gas (air).

note: in common practice, “aerosol” is used to refer to the particles only!

•primary aerosol - emitted directly into the atmosphere.

•Saharan dust, sea spray, pollen, plant waxes, soot

•secondary aerosol - created by nucleation of new particles, aggregation of existing particles, or growth of preexisting particles from gas phase molecules (gas to particle conversion).

•either type → natural, anthropogenic, or both

•internal vs. external mixtures – in an internally mixed aerosol all particles have the same composition

•CN – condensation nucleii - aerosols that scatter light at very high supersaturation levels

•CCN – cloud condensation nucleii - aerosols that scatter light at very low superaturation levels

How much aerosol is there?

typically ~10’s 100’s of ug/m3 (air density ~1kg/m3)

Aerosol size distributions...

numberdistribution

surface areadistribution

volume distribution

(Seinfeld and Pandis)

the log-normal aerosol size distribution...

numberdistribution

surface areadistribution

volume distribution

Aitken mode

accumulation mode

coarse mode

(Seinfeld and Pandis)

The aerosol modes...

• Aitken mode – 0.01-0.1 um

• accumulation mode – 0.1-1 um

• coarse mode - >1 um

and sometimes, the elusive

• nucleation mode <0.01 um

(C. Leck)

• hygroscopic aerosols grow/shrink with RH (with hysteresis!)

Humidity and aerosol size...

• aerosol size strongly affects light scattering cross-section

deliquescence

efflorescence

a process-oriented view of the size distribution ...

... reflecting competition between production/transformation/removal

no ultrafines here...at the time, there was no instrumentation to detect them

why “accumulation” mode?

impaction, settling

diffusion, coagulation

removal mechanisms... gravitational settling

• 10 um particle 1000 cm hr-1

• 1 um particle 10 cm hr-1

coarse particles

Diffusion ...

fine particles

Diameter (μm) Distance diffused in 1 s (cm) .001 0.2 .01 0.02 .1 .002 1 .0004 10 .0001

distance (cm)= Dt

You can estimate the distance a particle will diffuse in a given time from the equation:

where D is the diffusion coefficient

The chemical perspective ... a chemical size distribution

1. chemical size distributions resemble mass, not number

2. sulfate and organics dominate the accumulation mode, but there’s a surprising amount of seasalt

3. there are a lot of unidentified organics

4. the coarse mode has the expected mechanically generated aerosols, but also nitrate and sometimes sulfate

Mas

s

(C. Leck)

Marine aerosol system...

cloud processingnucleation

Dust (mineral aerosols)diameter size: 2-300 µmmain material: sand, silt, clayincludes essential trace metals such as Feconsists of insoluble and soluble fractions

Mineral Dust

Seasalt aerosols...

seasalt production via bubble bursting...

• film drops (many, small, drags along surface organics)

• jet drops (fewer, larger, bulk)• spume drops (larger still)

wind bubbles spray

whitecap coverage W α U3+!

Seasalt aerosols...

Jet Drops

10 m1 m80 nm20 nm Particle Diameter(Dry)

NumberFilm Drops

(from C. Leck)

Seasalt – number and mass as a function of surface wind speed

(compiled by Lewis and Schwartz, 2004)

seasalt particle number seasalt mass

Seasalt – mass fraction as a function of size

(Lewis and Schwartz, 2004)

Marine aerosol mass fractions...

The sulfur story (in brief) ...• emissions: fossil fuel SO2, volcanic SO2, oceanic DMS

• DMS oxidation ... gas phase ... complex!

(mod. from Yin et al., 1990)

Happily, SO2 oxidation in the gas phase is simple...

but most SO2 oxidation occurs in the aqueous phase...

some basics...

M

M

SO OH HOSO

HOSO O HO SO

SO H O H O H SO H O

2 2

2 2 2 3

3 2 2 2 4 2

pK ~

pK ~

SO (g ) SO (aq )

SO H O HSO H

HSO H O SO H

1

2

2 2

42 2 3

83 2 3

heterogeneous oxidation of SO2

• in-cloud oxidation– weakly buffered, pH ~4

– oxidation by H2O2

– growth of CN, split Aitken mode

• seasalt aerosols– strongly buffered by carbonate system

– rapid oxidation by O3

– slower oxidation by H2O2 (also OH, halogen radicals...)

– growth of existing particles, inhibits nucleation of new particles

(Chameides and Stelson, 1992)

the single-particle view...

Murphy et al., 1998; Buseck and Posfai, 1999

Organic aerosols - burning

“brown carbon”:sugars

alcohols

aromatics

di/tri acids

ketoacids

hydroxyacids

soot – “elemental carbon”formed in flames

little spectral abs. dependence

carbon-only

Marine-derived organic aerosols...

(O’Dowd et al., 2004)

Bacterium

Bacterium

DiatomOrganic filmVirus like

Organic cluster

C. Leck

A surprise from the Arctic sea ice...

More organics in marine aerosols ...

Negative TOF-SIMS spectra of marine aerosol.

mass 255 C15H31COOH (palmitic acid)

SEM micrograph of an Arctic marine aerosol sample. Single particles (diameter 1 μm) and groups of particles are lying between the fibers of the filter. About half of the particles are coated by an organic layer.

Tervahattu et al., 2002

C14 C15 C17 C18

C15

Iodine and nucleation...

• coastal I emissions

I2O5

• macroalgae

• CH2I2, I2 ?

• broader importance?

O’Dowd et al., 2002a,b

Multi-year time series of bulk aerosol chemistry at island stations...

eg. Prospero et al., 2003

Modeled aerosol sources...

IPCC, 2001

annual average sources (kg km-2 hr-1)

Aerosol properties from space...

Data from POLDER-1 (IPCC, Deuze et al., 1999)

•mineral dust•urban pollution•biomass burning

•aerosol size info

Understanding forcing/feedback requires realistic aerosols in climate models...

What’s left to do...?

• in situ aerosol/cloud radiation experiments!

• shape – non-sphericity• mixing state• “life cycle” of marine aerosol organics, marine microlayer...• gas phase aerosol chemistry (sulfur, iodine, organics) • aerosol gas phase chemistry (organics, halogens)• depositional fluxes• coupled aerosol/chemistry/climate models

• marine aerosols are woefully undersampled!

The end.