The Atmospheric Chemistry and Transport of Ozone in the UTLS (ACTO) was a round 2 UTLS project led by Stuart Penkett, University of East Anglia.

The objectives were:

To quantitatively assess the role of in situ chemistry and transport of stratospheric and lower tropospheric air on the ozone budget in the upper troposphere and lower stratosphere, focussing on the North Atlantic in spring and summer.
Quantify the role of in-situ photochemistry (production and destruction) of ozone in layers of different composition through a combination of chemistry measurements.
Quantify the sources of radical species (HOx and ROx) from photolysis of various molecules and to investigate the form and source of NOy in the UTLS region.
Assess the impact of transport and mixing on the ozone budget in the UTLS through the identification of air masses each with a distinct origin and composition and to study their photo-chemical evolution in the upper troposphere and the lower stratosphere.
Determining the extent to which the existence of layering influences the chemistry of the UTLS and quantifying the impact of mixing between air masses on the photochemical activity at their interfaces.

Methodology
The overall methodology was to analyse existing data, to collect new data, including that collected by some new instruments, in a part of the atmosphere rarely examined before (UTLS), and to interpret this data using a range of numerical models.

Data were already collected on the composition and structure of the troposphere as part of OCTA, ACSOE, and TACIA programmes, UTLS-DCFZ and EU-MAXOX during campaigns in the first half of 1999. There was an obvious progression from the objectives of the previous experiments and those of this project. Therefore analysis of the data from these programmes were valuable for the planning of the new measurement campaign, which were collected in the UTLS using the UKMO C-130 during 40 hours flying time, in the spring/summer period of the 2000. Flights were 3-6 hours and did take place in the North Atlantic off northern Britain and were predominantly in the upper troposphere (4 to 11km) with only occasional sorties into the lower stratosphere. Flights were designed to examine the chemical composition and reactivity of different air masses: boundary layer air (marine and (polluted) continental); tropical and subtropical air; upper tropospheric and lower stratospheric air; and polar air. Detailed meteorological and chemical forecast data were obtained from ECMWF, UKMO and NILU. Domain filling trajectory calculations, with forecast wind fields, were used to locate layers and to determine their orientation, whilst the NILU chemical forecast model provided information on the expected chemical composition of the different air masses. To determine the origin, composition and chemical activity of the different air masses found in the UTLS, measurements were made from a large number of tracers, ozone precursors, reactive species and photochemical products.