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| Titel |
Rapid aqueous phase SO2 oxidation in winter fog in the Indo-Gangetic Plain |
| VerfasserIn |
Himanshu Sachan, Chinmoy Sarkar, Baerbel Sinha |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250072178
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| Zusammenfassung |
| Sulphate and sulphur dioxide play an important role in environmental chemistry and climate.
The majority of anthropogenic sulphur is released directly as SO2, and a significant fraction
of biogenic and natural sulphur emissions are also either directly released as SO2 or oxidised
to SO2 in the atmosphere (e.g. H2S, OCS, DMS). Around 50% of global atmospheric
sulphur dioxide is then oxidised to sulphate, while the rest is lost through dry and
wet deposition. The pathway by which SO2 is oxidised to sulphate is critical in
determining the climate forcing and environmental effects of sulphate. Gas-phase
oxidation of SO2 by OH radicals or criegee intermediates produces H2SO4 (g), which
plays an important role in controlling new particle formation in the troposphere
and also modifies the surface properties of hydrophobic particles such as soot and
mineral dust. Heterogeneous oxidation of SO2 is considered to occur primarily in
cloud droplets, although oxidation on sea salt aerosols and mineral dust surfaces
are considered to be regionally important. Heterogeneous oxidation leads to the
formation of fewer and larger particles with shorter atmospheric lifetime. The major
oxidation pathways which are considered to contribute to sulphate formation in
the aqueous phase are oxidation by H2O2 and oxidation by O3 and the lifetime of
SO2 with respect to all known loss processes combined is considered to be 1-2
days.
Here we report measurements of SO2 measurements from IISER Mohali –
Ambient Air Quality Station (30.67°N, 76.73°E), a station located at a suburban site in
the Indo Gangetic Basin (IGB) during wintertime (10th Dec. 2011 to 29th Feb.
2012). We use a strong point source of SO2 with known SO2/CO emission ratio
(brick kiln) located 6.5 km east of our measurement site to estimate the loss rate of
SO2 in wintertime fog in the IGB. We consider the transport from the source to
the receptor site to be Lagrangian and use the measured CO concentration at the
receptors site to account for changes in the emission intensity (activity pattern) and the
dilution of the plume during transport. We see a linear correlation between the
measured SO2/CO ratio and the transport time. Binning the data on the basis of relative
humidity and applying first order kinetics to SO2 loss within each humidity bin we
find the SO2 loss rate with respect to aqueous phase oxidation at our sites varies
between > 2.2 x 10-3 mol/cm3/s at 96 % RH and 3.8 x 10-4 mol/cm3/s at 47 %
RH.
Simple box model calculations reveal that neither oxidation by H2O2 nor oxidation by O3
can account for such rapid SO2 oxidation in the fog water. Considering the high mineral dust
loadings are our station (PM 10 typically > 300 μg/m3) we propose that transition metal
catalysis by TMI leached from natural mineral dust and resuspended road dust may be
responsible for the rapid oxidation of SO2 in the fog water. However, the observed lifetime
with respect to aqueous phase oxidation in wintertime fog is a factor 150-800 times shorter
than the lifetime of SO2 with respect to TMI catalyzed oxidation currently implemented in
global atmospheric chemistry models.
During 2012-2013 winter season we will measure TMI concentrations in the fog water
and verify the rates coefficients estimated from the ambient observation by conducting
controlled experiments both using collected fog water and different TMI mixtures. If
confirmed through laboratory studies our findings have major implications for the SO2
lifetime over the IGP (and possibly other regions with high mineral dust loadings) and will
significantly alter the regional direct and indirect aerosol forcing estimates due to
anthropogenic SO2 emissions.
Acknowledgement: We thank the IISER Mohali Atmospheric Chemistry Facility for data
and the Ministry of Human Resource Development (MHRD), India and IISER Mohali for
funding the facility. Himanshu acknowledges the DST-INSPIRE Fellowship program.
Chinmoy Sarkar thanks the Max Planck-DST India Partner Group on Tropospheric OH
reactivity and VOCs for funding the research. |
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