Millimeter wave spectroscopic measurements of stratospheric and mesospheric constituents over the Italian Alps: stratospheric ozone

Measurements of rotational lines emitted by middle atmospheric trace gases have been carried out from the Alpine
station of Testa Grigia (45.9°N, 7.7°E, elev. 3500 m) by means of a Ground-Based Millimeter-wave Spectrometer
(GBMS). Observations of species such as O3, HNO3, CO, N2O, HCN, and HDO took place during 4 winter periods,
from February 2004 to March 2007, for a total of 116 days of measurements grouped in about 18 field campaigns.
By studying the pressure-broadened shape of emission lines the vertical distribution of the observed constituents
is retrieved within an altitude range of ∼17-75 km, constrained by the 600 MHz pass band and the 65 kHz
spectral resolution of the back-end spectrometer. This work discusses the behavior of stratospheric O3 during the
entire period of operation at Testa Grigia. Mid-latitude O3 columnar content as estimated using GBMS measurements
can vary by large amounts over a period of very few days, with the largest variations observed in December
2005, February 2006, and March 2006, confirming that the northern winter of 2005-2006 was characterized by a
particularly intense planetary wave activity. The largest rapid variation from maximum to minimum O3 column values
over Testa Grigia took place in December 2006 and reached a relative value of 72% with respect to the average
column content for that period. During most GBMS observation times much of the variability is concentrated
in the column below 20 km, with tropospheric weather systems and advection of tropical tropospheric air into the
lower stratosphere over Testa Grigia having a large impact on the observed variations in column contents. Nonetheless,
a wide variability is also found in middle stratospheric GBMS O3 measurements, as expected for mid-latitude
ozone. We find that O3 mixing ratios at ∼32 km are very well correlated with the solar illumination experienced by
air masses over the previous ∼15 days, showing that already at 32 km altitude ozone photochemistry dominates over
transport processes. The correlation of lower stratospheric ozone concentrations with potential vorticity as an indicator
of transport is instead not as clear-cut, due to very complex mixing processes that characterize stratospheric
air at mid-latitudes. Correlations of O3 over Testa Grigia with stratospheric tracers such as N2O and HCN, also observed
by means of the GBMS, are planned for the future, in order to better characterize lower stratospheric dynamics
and therefore lower stratospheric ozone concentrations at mid-latitudes.

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