Abstract
The monthly time series of carbonyl sulfide (OCS) atmospheric mole
fractions measured at NOAA network stations (2000 to 2020) have been
analyzed, and the long-term behaviour has been assessed based on the
Empirical Mode Decomposition (EMD). EMD is a fully non-parametric
analysis of frequency modes and trends in a given series and is based on
the data alone. We have found that the OCS atmospheric mole fraction,
after an increasing phase up to ~2015, with a temporary
decline around 2009, is now decreasing at all stations, reflecting a
recent imbalance in its total sources and losses. Our analysis has
revealed a characteristic time scale for variation of 8-10 years. The
variance associated with this long-term behaviour ranges from 15 to 40%
of the total strength of the signal, depending on location. To our
knowledge, this low-frequency mode is a novel result not assessed in
previous studies. Apart from this complex long-term behaviour, the OCS
time series show a strong annual cycle, which primarily results from
summertime OCS uptake by vegetation. In addition, we have also found one
more frequency of minor variance intensity in the measured mole fraction
time-history, which corresponds to periods in the range of 2 to 3 years.
This inter-annual variability of OCS may be linked to the Quasi-Biennial
Oscillation or QBO.