The StratoClim project will acquire also new data via a network of ground-based measurements. The project has set up ground-based atmospheric measurement stations for long-term observations in the tropical Western Pacific (tWP) on the island state of Palau (7.34 N, 134.47 E) and on the Bhola Island in Bangladesh (22.41 N, 90.76 E). In addition, a series of soundings (12 nighttime CFH-COBALD-EEC from Nagpur, 13 nighttime CFH-COBALD-EEC from Nainital, and 10 daytime CFH-EEC on RS92/RS41 combinations by DWD from Nainital will be conducted from India during the summer 2016.
The main objective of the Palau station is to characterize the composition of air in the Tropical Tropopause Layer (TTL) above the West Pacific warm pool, which is the main transport location of air into the stratosphere. The West Pacific warm pool area constitutes a gap in existing observational networks and likewise information on atmospheric composition from this region is very limited. The island of Palau is located right in the center of this warm pool area and will thus close the observational gap and provide valuable information on the atmospheric composition in West Pacific.
The Palau Atmospheric Observatory is operated in cooperation with the Palau Community College (PCC), under a Memorandum of Understanding and is located on PCC premises on the island of Koror.
The most important elements of the upper troposphere and lower stratosphere region (UTS) impacting our surface climate are stratospheric ozone and aerosols, the distribution of water vapor, and the abundance of several other trace gases. The Palau Atmospheric Observatory deploys different instruments to measure these atmospheric constituents:
Instrumentation at the observatory starting in December 2015 includes: a Fourier Transform Infrared (FTIR) Spectrometer, a lidar and ozone sondes (ECC). Starting in 2016 the station will also host a MaxDOAS instrument (Pandora 2-S) and a multi-wavelength aerosol & cloud lidar (ComCAL), as well as equipment for more complex balloon sounding campaigns with modified ECC sondes for an improved detection limit, water vapour sondes (CFH) and aerosol backscatter sondes (COBALD).
The Fourier Transform Infrared Spectrometer (FTIR) solar tracker on the top of the container follows the course of the sun and feeds the light inside the container on the entrance apertures of the spectrometer. In this way the spectrometer measures the direct sunlight in very high spectral resolution in the infrared spectral region. Since the spectrum of the sun is well known, the absorption features observed can be assigned to specific atmospheric trace gases. Several million spectral lines can be identified and assigned to about twenty atmospheric trace gases, including O3, HCl, HNO3, OCS and many others. Using mathematical retrieval methods the total column concentrations can be retrieved, which corresponds to an averaged mixing ratio of the trace gases, averaged over all altitudes. A detailed analysis of the spectral line shape allows for a few trace gases to retrieve the concentration profiles of the trace gases up to about 30 km altitude. Depending on the trace gas and the quality of the spectra the concentration profiles can be retrieved in 2-4 atmospheric layers. Since the instrument itself records an interferogram, yielding the spectrum after a Fourier transformation, and since the instrument is perfect for observations in the infrared spectral region, the name FTIR-spectrometer has been established.
The micro LIDAR system sends out a pulsed laser beam and analyses the backscattered radiation yielding vertical profiles of aerosol properties. The system employs a 532 nm laser pulsed at 1000 counts per second and receives the atmospheric elastical backscatter, from the ground up to 60 km, although the range useful for scientific purposes extends not higher than 25 km, due to noise constrains. It discriminates the atmospheric return into polarized and depolarized scattering, thus allowing a characterization of the shape (i. e. phase) of the scattering aerosol. The system operates automatically and continuously at night-time, from the sunset to the dawn, delivering atmospheric profiles with 5 min time resolution and 60 m altitude resolution.
Photo: Francesco Cairo
Weather balloons carrying radio and ozone sondes will be launched regularly. The sondes measure continuously during their ascent up to 35 km, during which their data are transmitted to the receiving station on the ground via radio signal. First balloon measurements were conducted already during the container installation period. The strong El Nino phenomenon at that time has influenced these first temperature, humidity and ozone profiles. This will be a focus of further studies using appropriate models and inter-annual statistics. The first measurements already reveal certain characteristics of the TTL as the transition zone between the well-mixed troposphere and the highly stratified stratosphere. Regarding ozone, the overlapping processes in this region are the (i) dominating, slow radiatively driven ascent, (ii) the beginning of the photochemical ozone production and (iii) meridional mixing of air from the extratropical lower stratosphere.
Ozone sondes have been launched every two weeks since February 2016 in collaboration with the Coral Reef Research Foundation (CRRF), a Palau based non-profit environmental organization. Intensive campaigns also including an extended payload (ECC, modified ECC, CFH and COBALD sondes) will follow for different seasons until 2018. The modified ozone sondes will help capturing the tropospheric ozone minimum with possible ozone concentration below the detection limit of the regular sondes.
Photo: Jürgen Graeser
The measurement campaigns in Palau will be complemented with COBALD, ECC and IMET soundings as well as SO2 and OCS in-situ measurements in Bhola Island in Bangladesh over the summer months 2016. These soundings will be conducted on a field station provided by Dhaka University, an associated cooperative partner of the project.
The ozone layer is an important component of the global environmental system. The absorption of harmful solar ultraviolet (UV) radiation in the ozone layer protects the biosphere and heats the stratosphere. Since the 1960s and 1970s, humankind has emitted large quantities of chlorofluorocarbons (CFCs) and bromine-containing compounds (halons) into the atmosphere, which destroy ozone very efficiently. (WMO Bulletin nš : Vol 56 (4) - 2007)
Despite the Montreal Protocol and what seems successful reduction of harmful substances in the atmosphere, the ozone layer is still somewhat depleted and series of monitoring activities take place through so called Match observations regularly.
In the course of StartoClim, the Match network of Arctic ozone sonde stations will be on standby in case the Stratosphere shows signs of extreme cold events, which are known to activate the chemical reactions depleting the ozone layer. Based on the meteorological data, the ozone data from the campaign and calculations with the fully Lagrangian Chemical Transport Model (CTM) ATLAS (Wohltmann et al., 2010) the risk of the formation of an Arctic ozone hole within the subsequent month will be closely monitored. In case of severe depletion, the geographic location of the depleted area will be monitored based on Match measurements, ATLAS calculations and satellite data (OMI, MLS, etc.) and forecasted over a ten day period (by combining ECMWF meteorological forecasts with ATLAS calculations), allowing advanced warnings for periods of increased levels of UV radiation in Europe.