Lower tropospheric COS as main source of stratospheric background aerosol, a CCM study

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21 Φεβ 2014 (πριν από 3 χρόνια και 6 μήνες)

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Lower tropospheric COS as main source of stratospheric background

aerosol, a CCM study

Christoph Brühl (1), Holger Tost (2), Paul Crutzen (1), Jos Lelieveld (1), and Francois Benduhn (1)

(1) Max
-
Planck
-
Institut für Chemie, Mainz, Germany, (2) Universität Mainz, Germany

Abstract:

The modular atmospheric chemistry circulation model EMAC with the aerosol module gmxe and high vertical resolution has been u
sed

for
multiyear simulations of atmospheric aerosol. As lower boundary condition observed concentrations of COS and other long lived

so
urce
gases are used, together with emissions of shorter lived gas and aerosol species. The parameters for the aerosol size distrib
uti
on in the 7
modes are the same for troposphere and middle atmosphere. Evaporation of sulfate particles in the middle atmosphere is taken
int
o
account. We show that the oxidation of COS transported into the stratosphere explains most of the stratospheric background ae
ros
ol
(Junge layer) and SO
2

observed by satellites (SAGE, Space Shuttle), including the modulation by the (internally calculated) Quasi
-
Biennial
Oscillation. Tropospheric SO
2

has a minimum below the tropopause due to scavenging and cannot be a large contribution to stratospheric
sulfur.

We show also, that the model is able to build up the aerosol from a volcanic SO
2

injection, and discuss some radiation effects. For both, the
background, and the volcanic aerosol, the sedimentation is critical.


The model


EMAC (ECHAM5/MESSy Atmos.Chem. circulation model), ATF
-
version 1.9 (Jöckel et al, 2006; Pringle et al, 2010)


Resolution T42/L90 and T31/L39 (to 1Pa), up to 7 years


Mecca1 chemistry module, modified +
GMXe

aerosol module (4 soluable and 3 unsoluable modes with
eqsam
) +
scav

(cloud chemistry)


In
mecca1

extended SOx gas phase chemistry with HS, S, SO, SO
2
, SO
3


In
GMXe

evaporation

of sulfate particles added using Vehkamäki vapor pressure formula (essential to avoid
particles in upper stratosphere), mode change due to shrinking possible; optical properties diagnosed.


Modes: Nucleation < 6nm < Aitken < 70nm < Accumulation < 1000nm < Coarse (r)





1.59



1.59
1.49

1.7 (sigma)


Photolysis of H
2
SO
4

and SO
2

in UV and visible (vibrational overtones) included


PSCs included, with sulfate sedimentation in solid particles


Long lived tracers including COS nudged to observations at surface (Montzka and ALE/GAGE)


Internally calculated QBO, consistent with observations


Optical properties of aerosol from lookup
-
table (Mie based)


More see

www.messy
-
interface.org

References
: Jöckel et al., The atmospheric chemistry general circulation model ECHAM5/MESSy1: consistent simulation of ozone from the s
urf
ace to the
mesosphere. Atmos.Chem.Phys. 6, 5067
-
5104, 2006.

Pringle et al., Description and evaluation of GMXe: a new aerosol submodel for global simulations (v1). Geosci.Model Dev.. 3,

39
1
-
412, 2010.

Vehkamäki et al., An improved parameterization for sulfuric acid

water nucleation rates for tropospheric and stratospheric condi
tions. J.Geoph.Res. 107,
D22, 4622, 2002.

Production of total anorganic sulfur from oxidation of COS

(proposed first by Crutzen, 1976)



QBO west to east (1999)

QBO east to west (2000)

COS, observed by ACE satellite (Barkley et
al, 2008, corresponding season and QBO)

COS, observed by ACE satellite (Barkley et
al, 2008, corresponding season and QBO)

References:

Barkley et al, Global distributions of carbonyl sulfide in the upper troposphere and
stratosphere, Geoph.Res.Lett. 35, L14810, 2008.

Crutzen, The possible importance of CSO for the sulphate layer of the stratosphere.
Geophys.Res.Lett. 3, 73
-
76, 1976.

Watts, The mass budgets of carbonyl sulfide…, Atm.Environ. 34, 761
-
779, 2000.

EMAC

EMAC

Altitude, km

30

10

30

10

Altitude, km

SOx, and total S, ppbv. Observed by
ATMOS on Spacelab 3 at 1hPa
about
0.1ppbv SO
2

(Rinsland et al,
1995, Geoph.Res.Lett.)

Particulate sulfate, ppbv.

Black contours: Derived from SAGE
observations

Grey contours: Zonal wind (QBO,
m/s):
-
30,
-
10E, +10, +30W

Modif. Walcek sedimentation

Modif. Walcek sedimentation

Trapezoid sedimentation

Background aerosol, Junge layer
(after 3years of spinup from zero sulfur in the middle atmosphere)

log(p/hPa)

log(p/hPa)

log(p/hPa)

log(p/hPa)

Sulfate, ppbv, April 2001.

Black contours: SAGE fitting to QBO,
grey: fitting to season.

Tropical pipe better reproduced with
(
modified
) Walcek sedimentation
scheme

log(p/hPa)

log(p/hPa)

Trapezoid

Walcek

Observed SO
2

by ATMOS on


Spaceshuttle in March 92


at 1hPa
0.4ppbv


(Rinsland et al 1995)

Mostly via gaseous H
2
SO
4

and subsequent photolysis

Pinatubo volcanic aerosol, ppbv (log), injection of SO
2

at 1 Sept, different sedimentation schemes and resolution, 1
°
N

Conclusions


Oxidation of COS explains most of the stratospheric Junge layer (about 75%) and stratospheric
SO
2
, as observed. The stratospheric aerosol layer builds up in about 1 year if initialized from zero.


The Quasi
-
Biennual
-
Oscillation of the zonal wind modulates the Junge layer in EMAC, in
agreement with satellite data


Calculated extinctions agree with satellite data if Mie calculations are consistent with size
distribution parameters. Aerosol radiative heating rates are simulated realistically.


In the lowermost stratosphere aerosol water, dust and organic carbon are also important for
radiative effects.


It is possible to use the 7
-
mode scheme for troposphere and stratosphere applying the same
parameters, including volcanic aerosol (here decay somewhat too fast in the first year).


To obtain optimal agreement with observed SO
2

in the upper stratosphere, the use of a
sedimentation scheme with small numerical diffusion is important (e.q. modified Walcek);
resolution also matters.


L39/T31, modified Walcek

L90/T42 modif. Walcek

SAGE observations

L90/T42, trapezoid

L39/T31, upwind

Numerical diffusion

Sulfate

Aitken
-
mode, soluable

Accumulation
-
mode, soluable

N, 1/cm³

r, µm

log(p/hPa)

log(p/hPa)

log(p/hPa)

log(p/hPa)

log(p/hPa)

log(p/hPa)

log(p/hPa)

EMAC

SAGE (WMO)

Extinction at 1µm, 1/km (log)

Extinction, 1/km, at equator

Heating rates, K/d, at equator

Reference
: WMO, SPARC Assessment
of Stratospheric Aerosol Properties,
WRCP
-
124, 2006

SOx, 1
°
N

SAGE observations

EMAC

COS


sources


sinks(Mt/yr)

Ocean

0.30+/
-
0.25

Oxic soils

1.05+/
-
0.36

DMS ox

0.17+/
-
0.04

Vegetation

0.56+/
-
0.10

CS2 ox

0.42+/
-
0.12

Photochem

0.18+/
-
0.03

Anthrop. 0.12+/
-
0.06

Biom.B.

0.07+/
-
0.05

Precip.

0.13+/
-
0.08


Watts (2000)

Other

0.10+/
-
0.08


Montzka

Best
agreement
with Walcek
and L90/T42

Maximum shifted to higher layers

Modif. Walcek sedimentation