Chemistry of polar ice (part II)

• S & N cycles from ice core studies

Robert DELMAS

YESTERDAY• Chemical information is located in the ice matrix

itself• Basic features of glaciochemistry- soluble vs insoluble- ion balance • Primary aerosol species- Sea salt. May be modified in ice records. Strong

interaction with secondary sulfate aerosol- Continental dust: very high in glacial conditions

Sulfur cycle at high southern latitudes

SULFATESULFATE

• MAJOR COMPONENT OF THE GLOBAL AEROSOL LOAD

• CLIMATIC ROLE: Direct & indirect

• DEPOSITED AS AN AEROSOL

• AFFECTED BY « DRY DEPOSITION » EFFECT

Excess-sulfate or nssSO4 : [nssSO4 ] = [SO4] - 0.25 [Na]

nssSULFATEORIGINS FOR CENTRAL ANTARCTICA

• MARINE BIOGENIC ACTIVITY (gaseous DMS emission)

• together with MSA

• VOLCANIC ACTIVITY

• Continuous or sporadic

• Stratospheric pathway

• Tropospheric pathway (South America)

• Antarctic volcanoesIn glacial conditions: an additional source (e.g. gypsum: CaSO4)?

A tool to differentiate origins: S & O isotope measurements

About Antarctic nsssulfate…

• H2SO4 is formed from SO2 in gaseous or in liquid phase (see next)

• H2SO4 may be scavenged by sea salt aerosol

• Are sea salt and sulfate aerosol transported separately or internally mixed?

Oxidation ways of SOOxidation ways of SO2 2

(investigated by O isotope (investigated by O isotope measurements)measurements)

2 Gas-phase:

SO2 + OH new aerosol particle

1 Heterogeneous phase:

SO2 + O3/H2O2 growth of existing aerosol particle, in particular sea salt

Alexander, B., J. Savarino, N.I. Barkov, R.J. Delmas, and M.H. Thiemens, 2002

Alexander, B., M.H. Thiemens, J. Farquhar, A.J. Kaufman, J. Savarino, and R.J. Delmas, 2003

Two kinds of sulfate in the Antarctic

10Be is attached to background aerosol

Methanesulfonic acid (HCH3SO3)

• Directly derived from DMS• Aerosol or gas?• Specific tracer of marine biogenic activity (from

DMS)• Tracer of El Niño events?• Ratio MSA/nssSO4 commonly used• Strong post-deposition effect• Concentrations generally high in glacial

conditions

Volcanic sulfate

ECM: ElectroConductometric Measurement

• Sulfuric acid peaks

Tambora period (1800-1820)

•Sulfuric acid peaks

Volcanic eruptions recorded at various Antarctic sites

South Pole1964-65

1259 AD

Volcanism recorded at Vostok Ash layers 1259 AD eruption:

sulfate and fluoride

Sulfate in Antarctica

Sulfate in Greenland

The turn of the century in Greenland

Volcanic eruptions in the Northern Hemisphere

Sulfate and MSA in Antarctic coastal regions

• In James Ross Island snow

Antarctic Peninsula

Seasonal variations in South Pole snow

• MSA is labile in the upper firn layers

MSA at South Pole

El Niño events ?

MSA: important loss in the upper firn layers

• VOSTOK

• MSA is released to the interstitial air but remains stored in the firn layers

• It is then entrapped again by ice below close-off

MSA in Antarctic ice cores

Are this data reliable?

In Greenland

Isotope measurements related to the sulfur cycle

• S-isotopes in SO4

• O isotopes in SO4

Dronning Maud Land

(german core)

Depth

Years AD

Fluctuation of S-isotopic composition over 2 centuries

10

12

14

16

18

1988 1973 1951 1930 1899 1849 1791

Année Moyenne

34S

(‰

)

d34exc

d34Stot

FB1 FB2 FB3 FB4 FB5 FB6 FB8

Annual mean

Dronning Maud Land

0%

20%

40%

60%

80%

100%

FB1 FB2 FB3 FB4 FB5 FB6 FB8

fter/volc

fbm

fsm

% S

ou

rce

s

A

0%

20%

40%

60%

80%

100%

FB1 FB2 FB3 FB4 FB5 FB6 FB8

fcont

fbm

fsm

% S

ou

rce

s

B

Continental source only volcanic

A continental source + a volcanic source

18001990

NITROGEN CYCLE

• UP TO NOW, NOT UNDERSTOOD• There are two major species in polar ice related

to this cycle: NO3 and NH4

• MAY EXIST in the ATMOSPHERE as a GAS (HNO3) or an AEROSOL

• VERY COMPLEX TRANSFER FUNCTION for HNO3

• IMPORTANT ENVIRONMENTAL ISSUES like O3 hole, biomass burning or photochemistry (in-situ production)

Strong decrease in upper firn layers

During ice ages, nitrate is attached to dust

0 200 400 600M ETE R S

4

3.5

3

2.5

2

1.5

1

Acc

um

ula

tio

n c

m/y

r

0 200 400 600

0

0.4

0.8

1.2

NO

3 µ

Eq

/l

EPICA

EPICA

Dome F

Biomass burning?

NITRATE IN ANTARCTIC CORES

Anthropogenic pollution in Greenland

Lead pollution in Greenland

N-isotope measurements in NO3-

Ammonium

• Samples easily contaminated

• Extremely weak in central Antarctic snow (<1 ppb)

• In coastal regions higher concentrations linked to penguins

Greenland

Carboxylic acids at Summit

Conclusions (1)• Glaciochemical work is much more sophisticated

and difficult than water stable isotope measurements and gas measurements

• Prioritiy recently given to aerosol research could give a boost to glaciochemistry

• It can be envisaged to investigate in the future viruses, bacteria, microorganisms … which are attached to aerosol particles, in particular in non-polar regions

• More ice cores in tropical and mid-latitude mountains to understand continental aerosol and source regions of polar dust

Conclusions (2)

• Glaciochemistry is still a very open domain • Processes occurring in firn have to be confirmed in

particular for NO3, Cl and MSA• The interaction between sea salt and sulfate

aerosol has to be taken into account• The role of glacial dust on atmospheric chemistry

has to be investigated • Na as an indicator of sea ice extent in the past• CaNO3 as a tracer of biomass burning in Antarctica