Open AccessMeasurements of 35 S in the marine boundary layer at La Jolla, California: A new technique for tracing air mass mixing during Santa Ana events
Author(s) -
Priyadarshi Antra,
HillFalkenthal Jason,
Coupal Elena,
Dominguez Gerardo,
Thiemens Mark H.
Publication year - 2012
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2011jd016878
Subject(s) - aerosol , atmosphere (unit) , mixing ratio , sea salt , troposphere , boundary layer , air mass (solar energy) , sulfate , atmospheric sciences , fraction (chemistry) , planetary boundary layer , chemistry , environmental science , meteorology , physics , organic chemistry , thermodynamics
35 S (t 1/2 = ∼87 days) is a radioactive isotope that is naturally produced in the upper atmosphere by the interaction of cosmic rays with 40 Ar. In the atmosphere, 35 S oxidizes to 35 SO 2 , which further oxidizes to 35 SO 4 2− depending on the oxidative rates. Here, we report measurements of 35 S activity in both 35 SO 2 (gas) and 35 SO 4 2− (aerosol) (coarse, >1.5 μ m; and fine, <1.5 μ m size fraction) collected during 2009–2010 at La Jolla, California, and we demonstrate that 35 S is a unique tracer for understanding air mass mixing and boundary layer dynamics. The annual mean 35 SO 2 concentration is 71 ± 61 (mean ± 1 σ ) atoms/m 3 . The annual mean 35 SO 4 2− abundance in the fine fraction (455 ± 157 atoms/m 3 ) is higher than in the coarse fraction (136 ± 59 atoms/m 3 ). This is most probably a consequence of the fine sulfate aerosol fraction being produced by gas phase oxidation of 35 SO 2 , whereas the coarse fraction is mainly derived from oceanic sea salt sulfate. 35 S activity in coarse sulfate is mainly due to uptake of 35 SO 2 and subsequent oxidation on sea salt aerosols. 35 SO 2 and 35 SO 4 2− 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 using 35 S activity.