Royal Netherlands Meteorological Institute
Section of Atmospheric Composition
Simultaneous measurements of O3, CO and ultrafine aerosol particles
(UFP), conducted on board a Boeing 767-ER passenger aircraft flying from
Sri Lanka to Germany (project CARIBIC), are used to study two-way
cross-tropopause mixing near a subtropical tropopause fold.
On the equatorward side of the fold downward mixing of stratospheric air
into the upper troposphere is identified by enhanced concentrations of O3 and 14CO. Very high UFP number concentrations of up to
1.5x104 cm-3 (STP) were encountered inside the poleward
half of the fold. This accumulation of small particles is explained by the
mixing of stratospheric air with tropospheric air injected into the fold.
Furthermore, nine particle formation events were observed outside the
fold, which are attributed to isolated cells of deep convection and to
rising air parcels under cyclonic conditions that mix with the surrounding air.
In the upper troposphere O3 and CO were found to be correlated
with high
O3/CO=-3.5;
but the high CO mixing ratios of 100 ppb at O3 mixing ratios of
250 ppb point to earlier injection of tropopsheric air,
in agreement with the UFP measurements.
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