A natural abundance stable isotope tracer experiment to define SO2 oxidation pathways and their fractionation during heterogeneous oxidation

Abstract:

Sulfate aerosols have crucial direct and indirect effects on climate from radiative cooling to modifying clouds by formation of cloud condensation nuclei. Secondary sulfate aerosols are formed by oxidation of SO₂ and subsequent nucleation and growth and the characteristics of primary aerosol sulfate can be modified by oxidation of SO₂. There are several known oxidation pathways for SO₂; gaseous phase OH oxidation and aqueous phase H₂O₂, O₃ and transition metal oxides oxidation. The SO₂ oxidation pathway affects the characteristics of the aerosols formed. Stable isotope techniques are useful in determining the oxidation pathway of SO₂ due to unique fractionation patterns (Harris et al., 2012). However, there are still gaps in our understanding of the oxidation pathways and fractionations affecting SO₂ and secondary sulfate. A tracer experiment to investigate the oxidation of SO₂ and fractionation using size segregated aerosols in the presence of different compounds is described. Two high volume samplers situated to measure background sulfate upwind, and the results of a tracer experiment, downwind, is described. After sufficient size segregated aerosol sulfate has been collected, a source of SO₂ with known isotopic composition is introduced to the second high volume sampler. Changes in the isotopic composition for size segregated aerosol sulfate in comparison to the first high volume sampler are investigated. The amount of fractionation during heterogeneous oxidation of SO₂ on pre-existing aerosols is calculated using the concentrations and known isotopic composition and compared to data from laboratory and field experiments. The experiment is performed downwind of sources of organic compounds such as pine forests, and characterized using co-located canister samples, to determine the effects of SO₂ oxidation on secondary aerosol sulfate.