A new method to correct the electrochemical concentration cell (ECC) ozonesonde time response and its implications for “background current” and pump efficiency
Abstract. The electrochemical concentration cell (ECC) ozonesonde
has been the main instrument for in situ profiling of ozone worldwide; yet,
some details of its operation, which contribute to the ozone uncertainty
budget, are not well understood. Here, we investigate the time response of
the chemical reactions inside the ECC and how corrections can be used to
remove some systematic biases. The analysis is based on the understanding
that two reaction pathways involving ozone occur inside the ECC that
generate electrical currents on two very different timescales. The main
fast-reaction pathway with a time constant of about 20 s is due the
conversion of iodide to molecular iodine and the generation of two free
electrons per ozone molecule. A secondary slow-reaction pathway involving
the buffer generates an excess current of about 2 %–10 % with a time
constant of about 25 min. This excess current can be interpreted as what has
conventionally been considered the “background current”. This contribution
can be calculated and removed from the measured current instead of the
background current. Here we provide an algorithm to calculate and remove
the contribution of the slow-reaction pathway and to correct for the time
lag of the fast-reaction pathway. This processing algorithm has been applied to ozonesonde profiles at Costa
Rica and during the Central Equatorial Pacific Experiment (CEPEX) as well as to
laboratory experiments evaluating the performance of ECC ozonesondes. At
Costa Rica, where a 1 % KI, 1/10th buffer solution is used, there is no
change in the derived total ozone column; however, in the upper troposphere
and lower stratosphere, average reported ozone concentrations increase by up
to 7 % and above 30 km decrease by up to 7 %. During CEPEX, where a
1 % KI, full-buffer solution was used, ozone concentrations are increased
mostly in the upper troposphere, with no change near the top of the
profile. In the laboratory measurements, the processing algorithms have been
applied to measurements using the majority of current sensing solutions and
using only the stronger pump efficiency correction reported by Johnson et
al. (2002). This improves the accuracy of the ECC sonde ozone profiles,
especially for low ozone concentrations or large ozone gradients and
removes systematic biases relative to the reference instruments. In the surface layer, operational procedures prior to launch, in particular
the use of filters, influence how typical gradients above the surface are
detected. The correction algorithm may report gradients that are steeper
than originally reported, but their uncertainty is strongly influenced by
the prelaunch procedures.