Summary of ACECHEM mission

The Upper Troposphere / Lower Stratosphere (UTLS) region is key to interactions between climate and atmospheric composition. Chemical, dynamical and radiative processes interact strongly. The UTLS region combines high radiative sensitivity to H2O and O3 with a wealth of mixing processes between stratospheric and tropospheric air which exhibit very different chemical regimes.

The processes involved are complex and not well understood. Past and planned missions are insufficient in particular for the upper troposphere.

Research objectives

The research objectives of the ACECHEM mission are to investigate the composition of the UTLS, its interaction with climate, and the impact of anthropogenic activities; in particular:

• The role of the UTLS for radiative forcing and feedback: large climate sensitivity to UTLS radiative forcings and feedbacks by gases and particulates; variability of ozone, H2O and other greenhouse gases;

   

• Role of stratosphere-troposphere exchange (STE) in atmospheric composition and climate: dynamical coupling between stratosphere and troposphere; its impact on greenhouse gas budgets; impact of tropopause variability on STE;

   

• Stratospheric change and its interaction with stratospheric chemistry and climate: evolution and chemistry of the ozone layer under changing conditions: chlorine decrease, H2O increase, temperature decrease, changing dynamics;

   

• Impact of pollution on the upper troposphere: impact of industrial pollution, biomass burning; atmospheric cleansing from radiatively active constituents; evolution of the oxidising capacity under human influence.

Observational requirements

• Large set of trace gases, aerosol and temperature in the troposphere and lower stratosphere.
• Global coverage since processes differ between latitude ranges
• High vertical resolution for studies of radiative effects and transport
• High horizontal resolution for STE and small scale variability in the troposphere
• Five years mission duration to cover interannual variability including two QBO cycles
• Minimisation of susceptibility of observations to cloud interference
• Precise characterisation of pointing fluctuations and co-registration in space and time between different measurements.

Mission elements and products

The ACECHEM platform carries three limb-sounding instruments:

• 5-band mm-wave spectrometer MASTER for cloud-insensitive tomography of the UTLS,
• mid-infrared Fourier-Transform spectrometer AMIPAS for high across-track resolution, additional gases and aerosol in the UTLS;
• 2-channel cloud and aerosol imager LCI for screening of AMIPAS’ field of view and for characterisation of cloud/aerosol.

ACECHEM will fly in formation with MetOp to increase synergistically the information content of co-located GOME-2 and IASI observations.

Data products will comprise vertical profiles of trace gas concentrations, temperature and aerosol.

Data assimilation will be an important tool for the integration of the measurements in models.

System concept

Orbit: sun-synchronous polar with 820 km mean altitude, 98 deg inclination and 9:22h local time of descending node (~8 min ahead of MetOp to observe common air volumes simultaneously).

Synchronisation of limb observations