UC San Diego

The studies presented here provide new means to assess sulfur cycle chemistry in the atmosphere. Sulfate aerosols play an important role in global radiative forcing through their inherit ability to scatter light and ability to serve as cloud condensation nuclei. Mass independent fractionation in oxygen isotopes of sulfate aerosols provides constraints on atmospheric oxidation processes from SO₂ to SO₄. Cosmogenically produced ³⁵S offers a unique way to trace sulfur cycling in the atmosphere and enables enhanced understanding of boundary layer chemistry, air mass transfer, and stratospheric-tropospheric exchange (STE). ³⁵S has a short life time (87.2 days) and is advantageous over other radioactive tracers due to its continuous presence in gas, liquid, an aerosol phases. Combining [Delta]¹⁷O and ³⁵S measurements offers a new way to quantify how sulfate formation is influenced by atmospheric variability since both chemical ([Delta]¹⁷O) and chronological (³⁵S) information is available. Simultaneous measurements of [Delta]¹⁷O and ³⁵S in sulfate aerosols from La Jolla, California (Chapter 3) and Dome C, Antarctica (Chapter 4) are presented. Chapter 3 reveals strong correlation between specific activity (³⁵S atoms/ sulfate concentration) and [Delta]¹⁷O during Santa Ana wind events. This simultaneous increase is linked to enhanced free tropospheric flux descending into Southern California as a result of the strong pressure gradient that develops during this weather phenomenon. Chapter 4 presents the first yearlong study of [Delta]¹⁷O and ³⁵S in sulfate aerosols on the Antarctic plateau. ³⁵S activity is directly related to the seasonal STE cycle and the oxidative capacity of the atmosphere. The [Delta]¹⁷O anomaly suggests a relative increase in ozone oxidation during spring/autumn relative to summer. An unexpected decline in [Delta]¹⁷O occurs during winter potentially related to decreased vertical mixing due to the lack of radiative heating. Chapter 5 studies the release of ³⁵S radiation from the Fukushima Daiichi Power Plant during the aftermath of the March, 11, 2011 earthquake at 6 different Japanese sites. Even after 6 months, ³⁵S activity remains above cosmogenic levels. Chapter 6 looks at the future application of ³⁵S to heterogeneous chemistry. Preliminary test results are presented to determine the feasibility of applying ³⁵S to chemical flow studies