Measurements of 35S in the marine boundary layer at La Jolla, California: A new technique for tracing air mass mixing during Santa Ana events

Abstract

35S (t1/2 = ∼87 days) is a radioactive isotope that is naturally produced in the upper atmosphere by the interaction of cosmic rays with 40Ar. In the atmosphere, 35S oxidizes to 35SO2, which further oxidizes to 35SO42− depending on the oxidative rates. Here, we report measurements of 35S activity in both 35SO2 (gas) and 35SO42− (aerosol) (coarse, >1.5 μm; and fine, <1.5 μm size fraction) collected during 2009–2010 at La Jolla, California, and we demonstrate that 35S is a unique tracer for understanding air mass mixing and boundary layer dynamics. The annual mean 35SO2 concentration is 71 ± 61 (mean ± 1σ) atoms/m3. The annual mean 35SO42− abundance in the fine fraction (455 ± 157 atoms/m3) is higher than in the coarse fraction (136 ± 59 atoms/m3). This is most probably a consequence of the fine sulfate aerosol fraction being produced by gas phase oxidation of 35SO2, whereas the coarse fraction is mainly derived from oceanic sea salt sulfate. 35S activity in coarse sulfate is mainly due to uptake of 35SO2 and subsequent oxidation on sea salt aerosols. 35SO2 and 35SO42−activities were observed to be higher during stratospheric‐tropospheric mixing and Santa Ana wind events that provide high‐altitude air mass mixing into the marine boundary layer. A model calculation shows that a maximum of 41% of total air sampled in the marine boundary layer recently originated from the free troposphere and mixed into the marine boundary layer during Santa Ana wind events. This work is the first to quantify and explain air mass mixing during Santa Ana events using35S activity.