Cite abstract as Author(s) (2009), Title, European Aerosol Conference 2009, Karlsruhe, Abstract T043A14
Chemical and physical properties of oxalic acid and oxalate aerosol particles
A. A. Mensah1, A. Buchholz1, A. Kiendler-Scharr1, Th. F. Mentel1
1
Institut für Chemie und Dynamik der Geosphäre, ICG-2, Forschungszentrum Jülich, Germany
Keywords: AMS, Hygroscopic growth, H-TDMA, Oxalic acid.
a sealed container was free of ammonium. If
atmospheric air was allowed to enter the container,
oxalic acid had an ammonium content of 10% and
higher within less then 1 day. This means, special
care (protective gas) is essential for the storage of
oxalic acid solid or in aqueous solution to ensure the
purity of oxalic acid.
The deliquescence and the efflorescence
branch of the growth curve of all four substances
were measured and deliquescence (DRH) and
efflorescence points (ERH) were determined with the
HTDMA (Table 1). KOx and NaOx show normal
behavior of slightly soluble inorganic salts. In
contrast, OxAc and AmOx exhibited some
irregularities at the theoretical points of
efflorescence (Figure 1). The particles grew instead
of shrank with decreasing RH. This behavior can be
explained by the formation of crystal hydrates.
Additionally, Oxalic acid particles did not deliquesce
at a specific RH but grew continuously.
Table 1. Measured deliquescence (DRH) and
efflorescence points (ERH) of the investigated salts.
KOx
NaOx
AmOx
OxAc
DRH (%)
84.8±1.0
92.2±1.4
93.3±1.4
continuous growth
ERH (%)
50.5±1.0
83.3±1.0
irregular
irregular
2.0
1.9
1.8
1.7
AmOx DRH
AmOx ERH
OxAc DRH
OxAc ERH
1.6
gf
We present aerosol mass spectrometric and
hygroscopic growth measurements of oxalic acid and
some of its derivates. A Hygroscopic Tandem
Differential Mobility Analyzer (HTDMA) and two
Aerodyne Aerosol Mass Spectrometers (AMS,
(Jayne, J. T. et al., 2000)) were used to investigate
physical and chemical properties of oxalic acid
(OxAc, H2C2O4 x 2 H2O), ammonium oxalate
(AmOx, (NH4)2C2O4 x 1 H2O), potassium oxalate
(KOx, K2C2O4 x 1 H2O), and sodium oxalate (NaOx,
Na2C2O4). The chemical composition, especially the
crystal water content was determined by use of the
two AMSs. Hygroscopic growth properties in the
range of 0% to 95% relative humidity (RH) were
determined by an HTDMA.
Aqueous solutions of the oxalates were
atomized, dried, and size selected prior to the
measurements. MilliQ-water with a total organic
carbon content below 10 ppb and a resistance of
18.2 MΩ was used to prepare the solutions. The salts
were of purity grade puriss. (>99%, Sigma Aldrich).
Aerosol particles were generated in an atomizer
(TSI 3076) with filtered (< 1 #/cm3) and dried
(< 0.2% RH) synthetic air. The aerosol stream was
dried with a silica gel and a nafion drier to below 1%
RH. Aerosol particles were size selected by a DMA
(TSI 3071) operated at a RH below 1%.
Two experimental setups were used. First,
size selected particles were directly introduced into
the AMS and the Scanning Mobility Particle Sizer
(SMPS) of the HTDMA. The complete aerosol path
was held below 1% RH. Second, dry size selected
particles were humidified at different RH in a nafion
humidifier before introduction into the AMS and
SMPS. The sheath air of the SMPS was kept at the
same RH as the humidifier.
The chemical composition of all four oxalates
was determined by a Quadrupole-AMS (Q-AMS)
and a High-Resolution Time of Flight AMS (HR-ToF
AMS). Basic analysis of the oxalates with the QAMS showed expected composition and water
content except for oxalic acid. High resolution
analysis of oxalic acid revealed a significant fraction
of the cat ions to be ammonium instead of hydrogen.
Additionally, and in agreement with the
interpretation of substantial ammonium uptake, the
measured crystal water content of oxalic acid was
substantially lower then theoretically expected.
A series of experiments of oxalic acid was
performed to investigate the absorption of
ammonium. Only oxalic acid salt taken directly from
1.5
1.4
1.3
1.2
1.1
1.0
0.00
0.20
0.40
0.60
0.80
1.00
RH
Figure 1. hygroscopic growth curves for AmOx
(squares) and OxAc (triangles). (Error bars are
smaller than symbols (∆RH = ±1.0, ∆gf = 0.005)
Jayne, J. T., D. C. Leard, et al., (2000). Aerosol
Science and Technology, 33, 49-70.