Differentiating sulfate aerosol oxidation pathways for varying source altitudes using 35S and Δ17O tracers

Due to the complex nature of the sulfur cycle, uncertainties remain in the transport rate and chemical transformation mechanisms of sulfur species, including sulfate aerosols. Here, we report oxygen isotopic anomaly and radioactive S-35 measurements in sulfate aerosols collected at La Jolla, California during 2009-2010. A strong correlation results from increased levels of specific activity (up to 195 atoms of S-35/nmol non sea salt (nss)-SO4) and Delta O-17 (up to 1.50 parts per thousand) in sulfate aerosol (fine fraction) samples during Santa Ana wind events compared to background levels. This is possibly due to an increase in mixing of free tropospheric air mass, containing higher levels of S-35 specific activity and higher Delta O-17, into the boundary layer. These tracers show the ability to detect changes in oxidation chemistry during high altitude air mixing events and have the potential to trace the changes in oxidation pathways of sulfur species during Stratospheric-Tropospheric exchange events. Sampling at higher latitudes where deep stratospheric intrusions are more prominent can help further parameterize how stratosphere-troposphere exchange events (STE) affect oxidation chemistry in the boundary layer.