Using sulfur isotope fractionation to understand the atmospheric oxidation of SO 2
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Abstract
Sulfate aerosol plays an important but uncertain role in cloud formation and radiative forcing of the climate, and is also important for acid deposition and human health. The oxidation of SO2 to sulfate is a key reaction in determining the impact of sulfate in the environment through its effect on aerosol size distribution and composition. This thesis presents a laboratory investigation of sulfur isotope fractionation during SO2 oxidation by the most important gas-phase and heterogeneous pathways occurring in the atmosphere. The fractionation factors are then used to examine the role of sulfate formation in cloud processing of aerosol particles during the HCCT campaign in Thuringia, central Germany.
The fractionation factor for the oxidation of SO2 by ·OH radicals was measured by reacting SO2 gas, with a known initial isotopic composition, with ·OH radicals generated from the photolysis of water at -25, 0, 19 and 40°C (Chapter 2). The product sulfate and the residual SO2 were
collected as BaSO4 and the sulfur isotopic compositions measured with the Cameca NanoSIMS 50. The measured fractionation factor for 34S/32S during gas phase oxidation is αOH = (1.0089 ± 0.0007) − ((4 ± 5) × 10−5 )T (°C). Fractionation during oxidation by major aqueous pathways was measured by bubbling the SO2 gas through a solution of H2 O2 , and by bubbling SO2 and O3 gas through water. The total fractionation factor for SO2 (g) →→ sulfate (aq) with O3 or H2 O2 as the terminating oxidant is αaq = (1.0167 ± 0.0019) − ((8.7 ± 3.5) × 10−5 )T (°C). Measurements at different pH values were used to constrain the fractionation factor for each individual step of this oxidation pathway (Chapter 3). Phase transfer made the smallest contribution to the total fractionation, while hydration accounted for the majority of fractionation (αphase = 1.00256 ± 0.00024 and αhydration = 1.0105 ± 0.0037 at 19°C). Fractionation factors were also measured in the droplet phase, and it was found that droplet microphysical
nprocesses do not alter the fractionation factors from their values in bulk aqueous phase.
Mineral dust and sea salt both represent a large proportion of total atmospheric aerosol loading, and play a very important role in heterogeneous SO2 oxidation, which shifts the size distribution of sulfate towards coarse particles, and limits H2 SO4 (g) production and new particle nucleation. Thus, oxidation pathways associated with these two aerosol types were examined in more detail. The alkalinity of sea salt aerosols is critical for heterogeneous sulfate production: SO2 oxidation by ·Cl catalysis and O3 only occurs at high pH and is thus limited by the neutralisation capacity of the aerosol, while oxidation by transition metal catalysis and by hypohalous acids can continue once aerosols are acidified.
Thus, the partitioning between these oxidation pathways is extremely important for the extent of sulfate production in the marine boundary layer. The isotopic fractionation factors for these pathways were
measured by generating aerosol from pure water, NaOCl, and sulfate-free sea salt solutions (Chapter 3). Oxidation by the alkalinity-limited pathways, O3 and ·Cl catalysis, was found to favour the heavy isotope (α34 = 1.0163 ± 0.0018 at low pH and α34 = 1.0199 ± 0.0024 at high pH at 19°C) while oxidation by the nonlimited pathways, transition metal catalysis and hypohalous acids, favours the light isotope (αTM−cat = 0.9905 ± 0.0031 and αOCl = 0.9882 ± 0.0036 at 19°C). Fractionation during oxidation in the synthetic sea salt aerosol (αseasalt = 1.0124 ± 0.0017 at 19°C) showed that hypohalous acids contributed 29% of total sulfate production on the short timescale of the experiments, while O3 and ·Cl catalysis accounted for the remaining 71%, highlighting the potential importance of the hypohalous acid pathway in the environment.
The measured fractionation factors, particularly when used in combination with the ∆17 O isotope anomaly, will allow quantification of the different sulfate production pathways in
the marine boundary layer.
Oxidation of SO2 on mineral dust aerosol, which represents the dominant mass fraction of particulate matter in the atmosphere, can occur in the aqueous phase as leached transition metals catalyse a radical chain reaction oxidation pathway, or on the surface of dust following SO2 chemisorption. Although the former is much faster and dominates globally, high humidities are usually first encountered only after dust has travelled for several days, thus surface oxidation can be regionally important in dust source regions. Sahara dust
from the Cape Verde Islands was used to examine these two oxidation pathways (Chapter 4). Aqueous phase oxidation was investigated by bubbling SO2 through leachate from the dust, and resulted in a fractionation factor of αleachate = 0.9917±0.0046 at 19°C. Although this is equal to isotopic fractionation in a solution of Fe2+ /Fe3+ (αFe = 0.9894 ± 0.0043), the reaction in the dust leachate was several orders of magnitude faster than in the pure Fe
solution, showing that interactions between multiple metal ions are critical for oxidation
rate. Oxidation on the dust surface was examined by passing SO2 over dust mounted on a filter under various combinations of humidity, irradiation and O3 exposure for 6-9 hours at 19°C. A multivariate analysis model was used to investigate the isotopic composition of the sulfate produced in relation to the chemical composition of the dust and the reaction conditions. The overall fractionation factor for surface oxidation on Sahara dust is αsurface = 1.0096 ± 0.0036. The majority of oxidation was due to iron and titanium oxides in the dust, however the clay fraction of dust accounted for 12% of sulfate production and had a very distinct fractionation factor of αclay = 1.085 ± 0.013. These fractionation factors will be very useful to understand sulfate production on mineral dust in field studies, particularly the importance of surface vs. aqueous oxidation and the role of clay minerals. Sulfur isotopes were measured in
SO2 and H2 SO4 gas and sulfate-containing particulate matter upwind, in-cloud and downwind of an orographic cloud during the Hill Cap Cloud Thuringia (HCCT) campaign in central Germany in Autumn, 2010 (Chapter 5).
The sulfur isotopic composition of gas-phase sulfur was measured following precipitation to BaSO4 , while for particulate matter combined SEM and NanoSIMS analyses allowed δ 34 S values to be resolved for particle size and type. The fractionation factors measured in the laboratory were used to investigate in-cloud sulfate production and particle processing, which modifies particle size and hygroscopicity and thus plays an important role in determining the magnitude of direct and indirect aerosol radiative forcing. In contrast to
modelling studies, which predict that H2 O2 is the dominant in-cloud oxidant for SO2 , the change in the isotopic composition of SO2 gas following the cloud showed that transition metal-catalysed oxidation was the most important pathway for SO2 removal in the cloud.
The results show that this pathway is underestimated in models, which either approximate its rate with the much slower rate of Fe-catalysed oxidation or omit the pathway entirely. Although oxidation of SO2 is responsible for the majority of sulfate production in clouds, the single particle δ 34 S measurements showed that condensation of sulfuric acid gas and coagulation of ultrafine particulate dominate sulfate addition to fine particulate, and are therefore the most important in-cloud sulfate sources with respect to modification of the aerosol size distribution and the magnitude of direct and indirect radiative forcing.