Integrated Global Atmospheric Chemistry Observations
IGACO

The Executive Summary

The aim of this report is to define a feasible strategy for deploying an Integrated Global Atmospheric Chemistry Observation System (IGACO), by combining ground-based, aircraft and satellite observations with suitable data archives and global models. The purpose of the system is to provide representative, reliable and accurate information about the changing atmosphere to those responsible for environmental policy development and also to prediction centres for weather and the environment. IGACO will also improve our scientific understanding of the changing atmosphere.

Motivation

The atmosphere, like the other components of the Earth system, is affected by the continuous increase in human population and activity, which have resulted in a variety of remarkable changes since the industrial revolution of the 19^th century. Among these are

·               the global decrease in stratospheric ozone and the attendant increase in surface ultraviolet radiation, emphasised by the ozone hole appearing over the Antarctic;

·               the occurrence of summer smog over most cities in the world, including the developing countries, and the increased ozone background in the northern troposphere;

·               the increase in greenhouse gases and aerosols in the atmosphere and associated climate change;

·               acid rain and the eutrophication of surface waters and other natural ecosystems by atmospheric deposition;

·               enhanced aerosol and photo-oxidant levels due to biomass burning and other agricultural activity;

·               the increase in fine particles in regions of industrial development and population growth with an attendant reduction in visibility and an increase in human health effects; and

·               the long range transport of air pollution to regions far from the industrial activity.

Many of these changes in atmospheric composition have socio-economic consequences through adverse effects on human and ecosystem health, on water supply and quality, and on crop growth. A variety of abatement measures have been introduced or considered to reduce the effects. However, continued growth in human activities, to expand economies and to alleviate poverty, will ensure that these effects continue to be important for the foreseeable future.

The grand challenges in atmospheric chemistry

Four grand challenges in atmospheric chemistry underlie the environmental issues above:

·               tropospheric air quality;

·               the oxidation efficiency of the atmosphere;

·               stratospheric chemistry and ozone depletion; and

·               chemistry - climate interactions.

The scientific understanding of each challenge requires long-term observation of the atmosphere, and points firmly to the need to establish an integrated global observation network.

The need for IGACO

Over the last half century, a variety of national networks have been installed to monitor atmospheric trace gases of local concern and, as the trans-boundary nature of air pollution was recognised, there has been some limited coordination of network development in Europe, in North America and globally. Most of the activities concern ground-based stations, including balloon sondes and remote sensing. Regular aircraft measurements were started in the 1990s by deploying unattended instrument packages on commercial airliners.

Satellites have added a new dimension by providing a global picture of the total column ozone together with some profile information. Now, it has become possible to measure from space, total and tropospheric columns of a number of atmospheric constituents as well as concentration profiles, albeit limited in resolution, in nadir-viewing geometry; as well as high-resolution profiles in the stratosphere and upper troposphere. At the same time, global weather and climate models that incorporate chemical cycles are emerging, which are capable of assimilating a variety of observations into a global picture.

Most of the current ground-based activities are primarily regional and are driven by specific scientific or legal objectives. Unfortunately, because of the lack of overall coordination and inherent shortcomings in the individual systems, none of the present monitoring activities achieves the completeness, representativeness and resolution required for obtaining a global view. There are thus large gaps in our picture of the atmosphere and in our understanding of the various influences that drive atmospheric variability and long-term changes.

To obtain a reliable global picture of the changing atmosphere, a coupled observational/modelling system is required, which is built partly on components that are already available, but is designed to obtain and integrate fully the best data that the diverse observation systems provide. The implementation of IGACO will require substantial investment to fill gaps in the existing satellite and other systems, whose current generation will cease operation just at the time when IGACO should go into operation. Clearly, if we want to avoid severe uncertainties in our diagnostic and prognostic capabilities, decisive steps towards the assembly of IGACO must be taken without delay.

The chemical species and ancillary parameters required for IGACO

Taking account of some realistic financial and logistic constraints, the chemical species and parameters to be included in IGACO were selected using the following criteria:

a.       the chemical species or variable plays an important role in one or more of the four key atmospheric chemistry issues, identified in IGACO, and there is added value in its incorporation in an integrated global observation system;

b.       it is now possible, or likely to be possible, to measure the atmospheric constituent or parameter globally and on a long term so as to achieve a synergism between satellite, ground-based and aircraft observations, and model assimilation systems.

The selected atmospheric constituents and parameters are:

Ancillary parameters

In addition, the following additional parameters are required for atmospheric chemical modelling and retrieval of remote sensing data:

Chapter 4 of the report details the targets and thresholds for precision, accuracy, temporal and spatial resolution that must be met for each of the chemical species and parameters to be meaningful within IGACO.

The essential components of IGACO

For full functionality the system should include the following four components.

·               Networks of ground-based instrumentation, including balloon sondes, millimetre wave radiometers, lidars, UV-Visible and FTIR spectrometers, to measure ground concentrations, column densities and vertical profiles of the principal chemical species, aerosols and ancillary parameters, together with UV radiation, on a regular basis; the implication is that the current network needs to be maintained and expanded to fill critical gaps in coverage.

·               Regular aircraft-based measurements for chemical and aerosol measurements in the troposphere and particularly in the upper troposphere and lower stratosphere (UT/LS), which is sensitive to chemical and climate changes. The current fleet needs to be expanded with respect to instrumentation and global coverage and to include regional aircraft that monitor the lower to middle troposphere. Routine measurements in the lower to middle troposphere using non-commercial light aircraft could be a useful part of the system.

·               Satellite-based instrumentation, preferably mounted on a combination of geostationary (GEO) and Low Earth Orbit (LEO) satellites to provide measurements with the temporal and spatial resolution needed for sufficient coverage. To obtain the time resolution necessary to encompass tropospheric variations, three to four geostationary satellites would be required to cover all longitudes. An alternative would be several LEO polar orbiting satellites. Upper tropospheric/lower stratospheric observation requirements would be covered by two polar orbiting limb-sounding missions.

·               A comprehensive data modelling system capable of combining the data for the chemical species, aerosols and ancillary parameters, obtained from the three measurement components, into a comprehensive global picture; assimilation techniques for chemical species other than ozone are still in the demonstration phase and need to be developed into operational procedures, which will require the development of physical and chemical parameterisations.

Further essential parts of IGACO are end-to-end quality assurance and quality control and data collection, integration and analysis.

Summary of IGACO recommendations

The principal recommendations of IGACO, as detailed and discussed in chapter 5 are:

GR1       Establishment: an Integrated Global Atmospheric Chemistry Observation System (IGACO) should be established for a target list of atmospheric chemistry variables and ancillary meteorological data.

GR2       Continuity: the data products from satellite and non-satellite instruments, which are to be integrated into a global picture by IGACO, must have assured long term continuity.

GR3       Management of IGACO: the responsibility for the co-ordination and implementation of the IGACO should rest with a single international body. International and national agencies responsible for aspects of IGACO should be committed partners and agree on their appropriate responsibilities.

GR4       Gaps in observational coverage: for each target species and variable, the present gaps in the current spatial and temporal coverage should be filled by extending the existing measurement systems.

GR5       Long term validation of satellite observations: in order to ensure the accuracy and consistency of satellite measurements, sustained quality assurance measures, over the entire lifetime of satellite sensors, are essential.

GR6       Validation of vertical profile data from satellite observations: a set of high performance scientific instruments using ground, aircraft and balloon platforms, possibly operated on campaign basis, must be maintained to provide the crucial validation data.

GR7       Comparability: the ability to merge observations of different types must be ensured by insisting that appropriate routine calibration and comparison activities linking diverse measurements together are part of an individual measurement programme.

GR8       Distribution of data: universally recognised distribution protocols for exchange of data on atmospheric chemical constituents should be established.

GR9       Multi-stake holder World Integrated Data Archive Centres (WIDAC) should be established for the targeted chemical variables.

GR10     Storage for raw data should be established so that they can be re-interpreted as models and understanding improves.

GR11     The development of comprehensive chemical modules in weather and climate models with appropriate data assimilation should be an integral part of IGACO.

GR12     Strong coordination with the meteorological services is essential so that the ancillary meteorological data, required by IGACO, is accessible.

Specific recommendations (SR1 to SR7) for the implementation of IGACO are given in section 5.3 of the report, together with suggestions for the commitment process.

The implementation of IGACO

Two steps are required to implement IGACO in good time and avoid gaps in data coverage.

IGACO Phase-1: short term (0-10 year, i.e. before 2014)

Specific actions should include:

a.       Establishment of an IGACO system for the atmospheric constituents, O₃, H₂O, CO₂, CO, NO₂, BrO, ClO, HCl, N₂O, the CFCs, ClONO₂ and aerosol optical properties.

b.       Development of data harmonisation and Quality Assurance / Quality Control for all the aspects of the initial system.

c.       Upgrade of selected ground-based stations and routine aircraft platforms.

d.       Development of operational algorithms necessary for satellite measurements in the troposphere for as many parameters as possible.

e.       Development of modelling and assimilation tools, parameterisations and strategies to produce a reliable overall picture from the various measurement components.

f.        Planning and initiation of a sustainable long term network for IGACO Phase-2, with emphasis on a coordinated satellite system comprising both LEO and GEO instruments, complemented by non-satellite observational networks.

IGACO Phase-2: long term (>10 year, i.e. beyond 2014)

As shown by the timelines in chapter 4, there is a looming gap in the provision of satellite-based instruments for atmospheric chemistry observations since, by the beginning of the second phase, the present "demonstration mode" satellites will have finished their missions. For IGACO, new operational satellites as well as an enhanced non-satellite measurement network will be required.

Specific long-term requirements are:

a.       to provide an operational network consisting of the appropriate combination of LEO and GEO satellites to give the required coverage and time resolution for the tasks envisaged; immediate action is required from the agencies involved to avoid a time gap in global surveillance;

b.       to provide satellite instruments for measurements of the atmospheric constituents in phase I together with CH₄, HCHO, volatile organic compounds (VOC), SO₂, HNO₃, OClO, NO, CH₃Br, and the halons, together with j(NO₂) and j(O¹D);

c.       to upgrade ground-based stations to measure all the constituents, including aerosol chemistry, and to install new stations in locations to which the assimilation models are particularly sensitive;

d.       to secure routine aircraft measurements, to implement instrumentation for all the constituents, and to add routes to improve global coverage; and

e.              to develop further models and assimilation techniques to provide a reliable picture of the atmospheric variability and long-term changes.

Commitment

IGOS requires that a theme such as IGACO be led by one or more of its partners, and that "unless exceptionally approved, it is expected that one of the four observing programmes will lead future implementation." Considering that the focus of IGACO is on atmospheric composition, the natural lead is the WMO/GAW programme. Other essential players in the implementation are the satellite agencies represented by CEOS, the other components of WMO, and the research community in universities and organisations.