Meteorologisches Observatorium Hohenpeißenberg, www.dwd.de/mohp

Sulfuric acid is a key components in new particle formation in the atmosphere.

Nucleation rate and the number concentration of freshly nucleated particles are both

observed to strongly depend on sulfuric acid concentration (Kerminen et al., 2010,

Paasonen et al., 2010).

The only available technique to measure sulfuric acid in atmosphere (some 105–107

molecules/cm³) is Chemical Ionization Mass Spectrometry (CIMS) (Fig. 1).

Fig. 1: Scheme of a CIMS instrument ((Berresheim et al., 2000)

H2SO4 measurement by CIMS needs indirect calibration, generally, photolysis of

water vapor is used to produce OH radicals, which are then titrated by excess SO2 to

yield H2SO4 (Eisele and Tanner, 1993). Table 1 gives typical uncertainties of

calibration, OH measurement and H2SO4 measurements.

Table 1: 2-sigma uncertainties for CIMS measurements

Chemical artifacts in photolysis and titration zone in ambient air calibration:

Between photolysis, titration, and the addition of an OH-scavenger (rear injector)

(several 10 ms each) OH and HO2 radicals are not in equilibrium and inter-conversion

with artifact OH loss or H2SO4 formation may occur –> Periods short (<50ms),

chemical model for corrections (typically 10%)

Artifacts due to matrix effects using synthetic air flow tube calibrators:

Conditions in titration, flow tube and ion-molecule reaction zone are different from

ambient air and ion-molecule cluster formation (acid ions with water and neutral acids

and other polar compounds) is suppressed which might result in different instrument

sensitivity for ambient air matrix and synthetic air matrix -> occasional changes in

CIMS sensitivity in ambient air are observed and are not understood

Chemical artifacts in ambient air measurements – same as above

Matrix effect in ambient air measurements:

Neutral molecule clusters of H2SO4 with polar molecules (water, acids, ammonia,

amines,…) may form (Kurten et al., 2010). Depending on the clusters, the ionization

reaction with nitrate-ion clusters in the ion-molecule reaction zone might be

suppressed. Accordingly, cluster bound H2SO4 is (depending on the clusters) not

quantitatively detected – but: CIMS is for measurement of non-clustered H2SO4, yet,

the question remains which form of H2SO4 prevails in the atmosphere

Production of artifact H2SO4 in the CIMS system:

Impurities in the used process gases, e.g. t/c-2-pentene in propane, might react with

ambient ozone and produce Criegee radicals which then can form H2SO4 from

ambient SO2. Another issue is impurities in the nitric acid added to the sheath gas.

Malfunction of valves and flow controllers, plugged injectors … are other sources of

problems which are often hard to detect –> multiple problems hard to resolve

Sulfuric Acid Measurements by CIMS – Uncertainties and Consistency between Various Data Sets

C. PLASS-DÜLMER1, T. ELSTE1, P. PAASONEN2, and T. PETÄJÄ2

1Hohenpeissenberg Meteorological Observatory, Deutscher Wetterdienst, Germany, and 2Department of Physics, University of Helsinki, Finland

To check for consistency between different data sets, the calculated H2SO4 from

balance equations (assuming stationary state) can be compared to the measured data:

d/dt [H2SO4] = kOH [OH] [SO2] – CS [H2SO4] => [H2SO4] = kOH [OH] [SO2] / CS

This approach compares measured H2SO4 with calculated H2SO4 from OH measured

by the same CIMS, thus, uncertainties are substantially reduced. Data from the DWD-

CIMS from various campaigns and from different CIMS in Hyytiälä are compared.

Fig. 2: H2SO4 data obtained by the

DWD CIMS during EUCAARI

Sulfuric acid calculated from balance is generally lower than measured sulfuric acid

(Eisele and Tanner, 2003, Petäjä et al., 2009), on average by factors of 1-2, indicating

either an overestimation of the CS or a missing production pathway for sulfuric acid.

Fig. 4: Median diurnal cycles of k-UV=[H2SO4]meas / ([SO2] UV / CS) with UV=integrated irradiation 280-

320nm (left) and of k-OH-calc.=[H2SO4]meas / ([OH]meas [SO2] / CS); CIMS by UHEL , MPI , DWD

Using the UV radiation instead of measured OH in k-UV (Fig. 4) allows to compare

H2SO4-meas with independently measured quantities. At noon-time, k-UV varies on

average between 6 10-7 and 1.7 10-6 m²/J, e.g. factor 2.5 between min and max (Forest

Fire data were excluded for contaminated conditions). The same comparison for k-

OH-calc yields 6 10-13(min) and 3 10-12cm³/(molec.s) (max), which is more variable

probably due to additional uncertainties in OH measurements.

• H2SO4 data sets are comparable (median, noon) by better than factor 2 (k-UV)

• other H2SO4 sources than OH + SO2 in the morning/evening (Criegee+SO2 ?)

• overestimation of condensation sink? (H2SO4+H2O+… clusters / accomodation)

ACKNOWLEDGEMENTS

We wish to thank many contributors at DWD, UHEL, Melpitz (IFT), San Pietro Capofiume (CNR), and

NCAR for support, data provision and cooperation. Financial support was given by the EUCAARI project.

REFERENCESBerresheim et al., Int. J. Mass Spectrom., 202, (2000) 91. Boy et al., Atmos. Chem. Phys. 5, 863, 2005.

Eisele and Tanner, J. Geophys. Res. 98 (1993) 9001. Kerminen et al., Atmos. Chem. Phys., 10, 10829, 2010.

Kurten et al., Atmos. Chem. Phys. Disc., 10, 30539, 2010. Paasonen et al., Atmos. Chem. Phys., 10, 11223, 2010

Petäjä et al., Atmos. Chem. Phys., 9, 7435, 2009.

DW

D -

10

/20

10

Melpitz

random systematic total random systematic total

Calibration OH-meas.

Photonflux 184.9 nm 5% 15% 16% count statistics (30 s) 25% 25%

v (inlet-flow) 4% 6% 7% wind+chem+cal.factor 7% 16% 31%

[H2O] 3% 10% 10% total OH (30 s) 26% 16% 40%

s-H2O 3% 3% total OH (5 min) 11% 32%

wind ind. turbulence 5% 10% 11%

chemical artifacts 5% 10% 11% H2SO4

meas. Reproduc. 5-10% count statistics (30 s) 13% 13%

calibration factor 26% wind+cal.factor 5% 10% 31%

total H2SO4 (30 s) 13% 10% 33%

total H2SO4 (5 min) 6% 31%

Fig. 3: Sulfuric acid

concentration calculated

versus measured H2SO4 for

different CIMS instruments

in different campaigns.

Upper row by DWD-CIMS:

San Pietro Capofiume, Italy

Melpitz, Germany

Hohenpeissenberg

lower row all Hyytiälä by:

MPI-Heidelberg CIMS (Boy

et al., 2005)

UHEL-CIMS

NCAR-CIMS oper. by

UHEL (Petäjä et al., 2009)

y = 0.98x

R2 = 0.46

0.0E+00

5.0E+06

1.0E+07

1.5E+07

0.0E+00 5.0E+06 1.0E+07 1.5E+07

H2SO4-meas, molec/cm³

H2S

O4-

calc

, m

olec

/cm

³

QuestII_03-04/2003_MPI Heidelberg

1:1

y = 0.78x

R2 = 0.79

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06

H2SO4-meas, molec/cm³

H2S

O4-

calc

, m

olec

/cm

³

HUMPPA_07-08/2010_UHEL

1:1

y = 0.48x

R2 = 0.75

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

0.0E+00 2.0E+06 4.0E+06 6.0E+06 8.0E+06

H2SO4-meas, molec/cm³

H2S

O4-

calc

, m

olec

/cm

³

EUCAARI_03-06/2007_UHEL w NCAR

1:1

y = 0.77x

R2 = 0.87

0.0E+00

2.0E+07

4.0E+07

6.0E+07

0.0E+00 2.0E+07 4.0E+07 6.0E+07

H2SO4-meas, molec/cm³

H2S

O4-

calc

, mol

ec/c

m³

1:1

San Pietro Capofiume, June/July 2009

y = 0.73x

R2 = 0.87

0.0E+00

2.0E+07

4.0E+07

6.0E+07

0.0E+00 2.0E+07 4.0E+07 6.0E+07

H2SO4-meas, molec/cm³H

2SO

4-c

alc

, mol

ec/c

m³

1:1

Melpitz, May 2008

y = 0.66x

R2 = 0.64

0.0E+00

1.0E+07

2.0E+07

3.0E+07

4.0E+07

0.0E+00 1.0E+07 2.0E+07 3.0E+07 4.0E+07

H2SO4-meas, molec/cm³

H2S

O4-

calc

, m

olec

/cm

³

1:1

Hohenpeißenberg, 1998-99

1.0E+05

1.0E+06

1.0E+07

1.0E+08

01.01.07 01.01.08 01.01.09 01.01.10

sulp

hu

ric

aci

d, m

ole

c/cm

³ X

daily mean d(10-14:59)d(22-2:59) monthly meanm(10-14:59) m(22-2:59)

San Pietro Capofiume

Melpitz

1.0E-07

1.0E-06

1.0E-05

1.0E-04

0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00

med

ian

k-U

V,

m²/

J

EUC_03-4/07

EUC_05/07

EUC_06/07

Ffire_04-05/09

HUM_07-8/10

QuestII_03-4/03

QuestIV_04-5/05

HPB_01-7/08

SPC_07/09

Melpitz_05/09

1.0E-13

1.0E-12

1.0E-11

1.0E-10

0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00

med

ian

k-O

H-c

alc,

cm

³/(m

olec

s)

k(OH)=9.2e-13, DeMore et al., JPL, 1997