Geostationary Satellite Observations for Monitoring Atmospheric Composition and Chemistry Applications

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1 Geostationary Satellite Observations for Monitoring Atmospheric Composition and Chemistry Applications Author: Jos Lelieveld Mainz, January 2003 Contents Executive summary 3 1. Introduction 6 2. Atmospheric chemistry issues Atmospheric composition and air quality Stratospheric ozone and UV radiation Greenhouse gases and climate change Aerosols, weather and hydrological cycle Transboundary air pollution and unpredictable events Observations: methods and analyses Ground-based measurements Observations from space Atmospheric chemistry modelling Anticipated future developments Atmospheric ozone change Atmospheric forecasting Compliance with agreements Present and planned satellites Global Ozone Monitoring Experiment Tropospheric aerosols Environmental Satellite Planned missions Geostationary satellite applications Applications and data products User community Conclusions 41 Appendix 1: Earth observation satellites 44 Appendix 2: Earth observation satellite instruments 53 Appendix 3: Abbreviations and acronyms 61 References 63 1

2 Executive summary This report explores applications of atmospheric chemistry observations on METEOSAT Third Generation (MTG in ), a geostationary satellite. A prime quality of geostationary -as compared to low earth orbit- geometry is the possibility to retrieve diurnally resolved information, which is of particular interest for short-lived (i.e. reactive) gases and aerosols. Five main areas have been considered: 1. Atmospheric composition and air quality Air pollution contributes to morbidity and mortality, mostly through respiratory and cardiovascular diseases. The European Union (EU) has defined air quality standards for primary and secondary air pollutants. Primary air pollutants that can be monitored from space include nitrogen oxides (notably NO 2 ), sulphur dioxide (SO 2 ) and carbon monoxide (CO). Secondary air pollutants include ozone (O 3 ), which is photochemically formed from nitrogen oxides, CO and hydrocarbons. Formaldehyde (HCHO) is a main reactive intermediate of hydrocarbon oxidation. The atmospheric conversion of SO 2 into sulphate contributes to aerosols, which will be subject to EU standards in the near future. Aerosols also contain substantial amounts of secondary organic matter, which can include toxic substances. 2. Stratospheric ozone and UV radiation Intense solar ultraviolet (UV-A and UV-B) radiation causes skin cancer and cataracts. UV indices, which specify maximum solar radiation exposure levels in relation to skin types, are being formulated to inform the public. Stratospheric O 3 depletion enhances solar UV penetration to the earth s surface, therefore, it is controlled through the Montreal Protocol and its amendments. Surface UV levels can be derived from satellite measurements of column O 3, clouds and aerosols. 3. Greenhouse gases and climate change. Carbon dioxide (CO 2 ), methane (CH 4 ), tropospheric O 3 and a number of minor greenhouse gases have increased strongly in the atmosphere, and contribute to climate 2

3 change. The long-lived greenhouse gases (e.g. CO 2 and CH 4 ) are regulated through the Kyoto Protocol. It may be expected that future amendments of the Kyoto Protocol will additionally address short-lived greenhouse gases (e.g. tropospheric O 3 ) and aerosols (e.g. black carbon). In addition to regional trend monitoring of these species, geostationary satellite observations of CO 2 and CH 4 can be used to estimate emissions, provided that the measurements are performed with high accuracy. 4. Aerosols, weather and hydrological cycle Aerosols affect the microphysical properties of clouds and the atmospheric radiation budget. They thus directly and indirectly influence the earth s climate. Solar radiation attenuation by pollutant aerosols over water masses reduces evaporation and thus precipitation. Solar radiation absorption by aerosols moreover affects heating rates in the lower troposphere, which influences convection and moisture transports. Recent research suggests that direct and indirect aerosol effects on the hydrological cycle are substantial, although these effects are yet poorly quantified. Satellite measurements can provide information about the aerosol optical thickness, particle size and aerosol absorption of solar radiation. 5. Transboundary air pollution and unpredictable events The European Convention on Long-Range Transboundary Air Pollution (LRTAP) addresses emissions and transport of pollutants to control air quality and reduce acidification and eutrophication e.g. by nitrogen oxides and sulphur species. Furthermore, unpredictable pollution sources such as volcanoes, forest fires and other (catastrophic) events can affect human health and give rise to safety risks, e.g. for air traffic. Pollutant plumes can be traced through geostationary satellite measurements of a number of reactive gases (e.g. CO, SO 2, O 3 ) and aerosols. Such measurements can moreover provide proxies of air movement that are useful for numerical weather prediction. METEOSAT measurements could contribute importantly in these five areas, and thus support the following primary applications: Air quality monitoring and forecasting; Detection and surveillance of unpredictable pollution clouds and plumes; 3

4 Control of air pollution emissions; UV radiation monitoring and forecasting; Numerical modelling and weather forecasting. Fast delivery of the data will advance forecasting through assimilation in numerical models. The time resolution of the measurements should be at least one hour. Diurnal coverage is important; yet daytime measurements with high sensitivity near the surface have priority over nocturnal measurements. This is an issue considering that solar UV- VIS-NIR satellite instruments provide relatively good sensitivity in the lower troposphere; terrestrial IR instruments, on the other hand, are more suited for the upper troposphere and stratosphere while they can measure day and night. The combination of solar and IR radiation measurements will be optimal; the additional measurement of polarization will facilitate vertical profile retrieval. Additional applications, which are not primarily dependent on a geostationary viewing geometry, will gain important value in combination with low earth orbit satellite measurements: Monitoring of stratospheric aerosols, ozone and related gases; Monitoring of gases and aerosols that cause climate change; Assessment of the earth s radiation budget. The data products, including accuracy and resolution specifications that would be needed for these applications, are presented in Table I. The large number of operational and climate applications will justify the effort of including atmospheric chemistry measurements on MTG. The required level of technology development and retrieval algorithms are available in Europe from the heritage of the ERS and ENVISAT missions. The combination of atmospheric chemistry and meteorology observations from a METEOSAT platform, as well as the combination with low earth orbit satellites (e.g. METOP) will provide important synergy. 4

5 Table I. Data products from geostationary atmospheric chemistry measurements and the resolution (in the region of interest) needed for operational and climate applications. The required threshold and the target resolutions are given, the latter in parentheses. Data product 1 Horizontal resolution in km Vertical resolution 2 in km Temporal resolution 3 in hr Accuracy 4 in % Coverage 6 O 3 10 (2) T (2) 1 d(n) (0.5) 10 (5) MFV - hemispheric CO 10 (2) T (2) 2 d(n) (0.5) 10 (5) MFV - hemispheric SO 2 10 (2) T (2) 1 d(n) (0.5) 50 (20) regional HCHO 10 (2) T (2) 1 d(n) (0.5) 50 (20) regional NO 2 10 (2) T (2) 1 d(n) (0.5) 50 (20) regional PAN 10 (2) T (2) 1 d(n) (0.5) 50 (20) MFV UV-A 10 (2) Surface 1 d(n) (0.5) 20 (5) regional UV-B 10 (2) Surface 1 d(n) (0.5) 20 (5) regional AOT fine 5 (0.5) T (BL+FT) 1 d(n) (0.25) 0.05 (0.01) 5 regional - MFV AOT course 5 (0.5) T (BL+FT) 1 d(n) (0.25) 0.05 (0.01) 5 regional - MFV Aer R eff 5 (0.5) T 1 d(n) (0.25) 30 (10) regional - MFV SSA 5 (0.5) T 1 d(n) (0.25) 0.03 (0.01) 5 regional - MFV H 2 O 5 (0.5) T (BL+FT) 1 d(n) (0.25) 5 (1) regional CO 2 50 (10) T 6 d(n) (1) 2 (1) MFV - global CH 4 50 (10) T 6 d(n) (1) 5 (1) MFV - global 1 AOT is aerosol optical thickness (for the fine and course fraction, D<1 µm and D>1 µm, respectively), Aer R eff is aerosol effective radius, SSA is aerosol single scattering albedo, PAN is peroxyacetyl nitrate 2 T refers to the troposphere (column), BL to the boundary layer and FT to the free troposphere 3 superscript d refers to daytime (threshold), n to night (target) 4 Absolute accuracy 5 Accuracy in AOT or SSA units 6 MFV is METEOSAT field of view; hemispheric or global coverage can be achieved by a combination of geostationary and low earth orbit satellites 5

6 1. Introduction Although air pollution has grown strongly with human activities after the industrial revolution, the awareness of large atmospheric composition changes has developed only recently (Brimblecombe and Maynard, 2001). In the second half of the 20 th century technological advancements, including satellite remote sensing, stimulated progress in meteorology. Atmospheric chemistry initially focussed on relatively small-scale phenomena such as urban air pollution. In the 1970s the interest in transboundary air pollution grew in association with acid deposition. The discovery of stratospheric ozone depletion turned the attention to global atmospheric chemistry, and the first dedicated satellite instrument, the Total Ozone Mapping Spectrometer (TOMS), was launched in 1978, which has yielded valuable data on ozone trends (Stolarski et al., 1991). In response to the growing attention for greenhouse gases, aerosol particles and ozone, both in the stratosphere and troposphere, many satellite missions have been initiated (Appendix 1 and 2). Several instruments on low earth orbit satellites presently provide a global view of chemical compounds, largely by column abundances, while vertically resolved information is available for the middle atmosphere (see Drummond and Douglass, 1999). Recently, satellite data have also been used to retrieve tropospheric ozone, methane and carbon monoxide, whereas column abundances of some reactive gases in the troposphere are being measured from space as well. Given that the METEOSAT geostationary satellites have contributed in a major way to meteorology, a more active involvement of EUMETSAT in atmospheric chemistry is a natural next step. Potential applications involve air quality, the ozone layer, ultraviolet (UV) radiation, detection of unpredictable events, and the monitoring of atmospheric composition in support of legislation and agreements. In the context of prospective activities, EUMETSAT has initiated a User Consultation Process (UCP) to prepare for future geostationary programmes. This could lead to a new generation of geostationary satellites to follow METEOSAT Second Generation (MSG) in the period This report contributes to the UCP, and explores the potential for operational atmospheric chemistry applications. The first part of this report (chapter 1-4) reviews the main issues, methods and developments in atmospheric chemistry, focussing on aspects 6

7 that may be relevant for the operational use of satellite data. The second part (chapter 5-7) discusses data applications, suggesting data products, and it identifies the user community for geostationary satellite data. 2. Atmospheric chemistry issues The protection against harmful short-wave solar radiation by the atmosphere and the availability of clean air are basic conditions for life on earth. National, European and United Nations agencies have therefore developed directives and recommendations for air pollution and radiation exposure. Examples include ozone, UV radiation, acidifying compounds and aerosol particles that can carry toxic substances. Although air quality standards and related legislation are developed on national and supra-national levels, provinces or states are usually responsible for the monitoring and maintaining of air quality, while municipal authorities also play a role. Later it will be argued that local and regional air quality is increasingly influenced by long-range transport of pollutants up to a hemispheric scale. The wide range of scales thus involved has implications for the spatial and temporal resolution of operational information needed to monitor the atmospheric environment. This chapter describes the issues of interest, while the next chapter will summarise the methods available to obtain the relevant information Atmospheric composition and air quality Photochemical smog is a product of the atmospheric oxidation of volatile organic compounds (VOC) and carbon monoxide (CO) through the catalytic action of nitrogen oxides (NO x NO + NO 2 ). The sum of these compounds (NO x ) is often used because NO and NO 2 can rapidly equilibrate (within minutes). The global sources of these gases are listed in Table 2.1. In Europe about two thirds of the CO emissions are associated with road transport; for NO x this fraction is about half; for non-methane VOC it is about a third. The photochemical smog reactions are initiated by UV and visible radiation, producing highly reactive oxidants which damage human health, agricultural crops and 7

8 ecosystems. A main intermediate species in the breakdown of VOC in the atmosphere is formaldehyde (HCHO). Table 2.1. Estimated global annual O 3 precursor emissions (Lelieveld and Dentener, 2000); VOC does not include methane (Tg = g). Source category Energy use Fossil fuel combustion Fossil fuel production Biofuel combustion Aircraft CO TgC/yr 112 VOC Tg/yr NO x TgN/yr Industrial processes Biomass burning Savannah burning Tropical deforestation Temperate wildfires Agricultural waste burning Agricultural soils 2.2 Natural vegetation/soils Lightning 5 NO y from stratosphere 0.6 Natural Anthropogenic Total The most abundant oxidant is ozone (O 3 ). Paradoxically, while stratospheric O 3 protects life on earth from harmful UV radiation, O 3 near the surface has adverse effects because of its high reactivity. The European 8-hourly air quality standard for ozone is 120 µg/m 3. Nitrogen dioxide (NO 2 ) is also an oxidant that is controlled through national and European air pollution directives. An additional poisonous oxidant is peroxyacetyl nitrate (PAN), which is formed from VOC and NO 2. Because oxidant build-up is dependent on solar radiation, it is notorious during summertime episodes, typically under anticyclonic (stagnant) cloud-free conditions. Although abatement is generally most effective through the simultaneous reduction of NO x and VOC emissions, local conditions often require distinctive action (e.g. associated with natural VOC emissions). Ozone levels in western Europe have increased strongly, in particular in the 1960s and 70s. Meanwhile some countries report slight reductions, 8

9 although air quality standards are still exceeded during the summer episodes. In parts of southern Europe, on the other hand, the 8-hourly limit is exceeded throughout summer (Lelieveld et al., 2002). Acid deposition by sulphate and nitrate is a consequence of sulphuric and nitric acid formation from SO 2 and NO x emissions. In addition, ammonia (NH 3 ), e.g. from manure use, can have acidifying effects after its transformation into nitrate in soils. In spite of the introduction of catalytic converters on cars, mean European NO x emissions have not decreased, mostly due to the continuous expansion of traffic. Nitrate not only contributes to acidification but also to eutrophication of ecosystems. Important sources of SO 2 include coal burning, ore melting and to a lesser extent oil combustion. Sulphur dioxide also has a direct health impact so that it is controlled through European air pollution directives. In European cities the ambient SO 2 concentrations have decreased in the 1960s by centralising energy production and building high stacks, while desulphurisation of fuels and exhaust gases in the subsequent decades have substantially decreased SO 2 emissions; in western Europe by more than half. At present SO 2 emissions are still relatively strong in eastern Europe. Global anthropogenic SO 2 emissions amount to about 75 TgS/yr, being the foremost source over land. In addition, volcanoes and fumaroles represent a main, though intermittent source. The present volcanic epoch is regarded as being relatively quiescent, and global emissions are about 5-10 TgS/yr. Note, however, that large eruptions are not included in this estimate. Mt Pinatubo (1991), for example, injected about 20 Tg sulphur into the stratosphere, which returned to the troposphere within the subsequent 2-3 years. Sulphate is a main contributor to air pollution by aerosols, i.e. microscopic liquid or solid particles that can have macroscopic effects on the atmosphere. They affect visibility, the energy budget, cloud properties and can act as carrier of non-volatile toxic substances. The smallest particles efficiently enter the lungs, so that air quality standards aim at particulate matter with a diameter less than 10 µm (PM 10 ); increasingly the limit of 2.5 µm (PM 2.5 ) is taken into account, whereas in future PM 1 may be considered as well. The specific physical and chemical properties of aerosols are highly variable and poorly defined. In addition to ammonium sulphate, aerosols contain substantial amounts of particulate organic matter (POM), which is also mostly anthropogenic. It includes a 9

10 myriad of carbonaceous compounds of low volatility that are formed by photochemical processes in the atmosphere. Other important aerosol components are fly ash and black carbon (soot) from coal combustion and in diesel exhausts. Toxic substances that can travel on these particles include polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), dioxins and furans. Vegetation fires destroy ecosystems and they represent a distinct pollution source by producing many of the compounds mentioned above. They occur quite frequently e.g. in the (dry) Mediterranean region during summer. Only few fires are ignited by lightning; most are anthropogenic. Depending on the fire size, the pyrogenic emissions contribute to photochemical smog from local to hemispheric scales. The aerosols produced are largely organic, including absorbing material that resembles black carbon (although it is not soot). Additional biomass burning categories include agricultural waste burning and biofuel use. On a global scale biomass burning is a main source of aerosols, NO x and VOC, whereas it is a dominant source of CO (Table 2.1) Stratospheric ozone and UV radiation The atmosphere maintains a subtle balance between solar energy transmission, photochemistry and attenuation of harmful short-wave radiation. Molecular oxygen and ozone in the stratosphere completely absorb UV-C radiation ( nm wavelength). Ozone moreover absorbs most UV-B ( nm) so that only a few percent reaches the earth s surface. UV-B is of particular relevance because it damages biological tissue. UV- A ( nm) is hardly attenuated by O 3, and it also influences human health (e.g. sunburn). About 90% of atmospheric O 3 is in the stratosphere, hence its depletion by nitrogen oxides and halocarbons has raised serious concern about the trend in surface UV-B irradiance. For each percent decrease in total ozone, erythemally effective UV-B radiation increases by about 1.3% (WMO, 1999). International organizations such as the World Health Organization (WHO) have developed a UV index to inform the public about the health effects. Because of the exceptional cold conditions in the Antarctic stratosphere, polar stratospheric clouds (PSC) can be formed, on which halogen compounds are activated from inert reservoir species. These halogens, notably chlorine from chlorofluorocarbons 10

11 (CFCs), cause dramatic O 3 loss during the Antarctic spring because of the optimum between low temperatures and the availability of sunlight, causing the ozone hole. Since the Arctic stratosphere is more dynamic and warmer, springtime O 3 loss is less dramatic. The relatively strong dynamics in the northern hemisphere during winterspring, however, can give rise to mini-holes that sometimes reach Europe. At present, the total global stratospheric O 3 loss amounts to 5-10% with higher values at the poles and lower values in the tropics. Although the long-lived CFCs have been phased out under the Montreal Protocol and its amendments, the stratosphere becomes colder, moister and its dynamics may change with climate. Short-lived halocarbons moreover increase strongly, and a fraction may reach the stratosphere. The net effect of these changes is yet unclear. Although column ozone changes are of prime importance, surface UV is also controlled by a number of other parameters that affect radiation transfer through scattering and absorption. Because the column UV extinction decreases with altitude, erythemal effective irradiance increases by 10-20% with each kilometre altitude. Scattering by atmospheric molecules and aerosols is strongly wavelength dependent. Enhancement of high altitude scattering by aerosols, for example, can even increase surface UV-B, whereas at low altitudes aerosols reduce both UV-A and UV-B. In the northern hemisphere the surface UV reduction by aerosols is typically about 10%, with higher percentages in areas that are directly affected by pollution or mineral dust. Clouds generally reduce UV depending on their optical thickness. Thin or scattered clouds over a highly reflective surface enhance surface UV, while thicker clouds cause a reduction. The assessment of surface UV irradiances thus requires information about all of these factors Greenhouse gases and climate change Climate, being the average of weather, is usually defined based on atmospheric statistics over three decades. Climate change is therefore not easily detected although it is widely accepted that human-induced increases of greenhouse gases have already caused noticeable effects (IPCC, 2001). Heat buffering by the ocean upper layer delays global temperature changes by several decades, hence present atmospheric forcings lead to climate changes that are conveyed far into the future. 11

12 It is beyond doubt that global carbon dioxide (CO 2 ) concentrations have increased by about 30% as a consequence of fossil fuel consumption and forest clearing. This enhances the absorption of terrestrial infrared radiation in the lower atmosphere, so that the effective IR emission to space is shifted to higher and colder altitudes, which enhances the greenhouse effect that inevitably warms the earth s surface and lower troposphere. Other IR active gases, for example, methane (CH 4 ), nitrous oxide (N 2 O) and CFCs, have also increased, thus adding to the greenhouse effect. In fact, atmospheric CH 4 has more than doubled since pre-industrial times whereas CFCs do not have natural sources at all. The total radiative forcing of climate by non-co 2 greenhouse gases is comparable to that of CO 2. Since these gases are chemically active as well, there are many feedbacks that also affect the concentrations of other gases. There is furthermore little doubt that tropospheric ozone has substantially increased in the global troposphere; at northern mid-latitudes by fossil fuel related emissions and in the (sub-)tropics also by biomass burning. Model calculations suggest that tropospheric O 3 increased by about a factor of two in the northern hemisphere. The climate forcing by tropospheric O 3 is altitude dependent, therefore, it is important to quantify vertically resolved O 3 changes. In the stratosphere, on the other hand, radiative cooling by CO 2 adds to the cooling by the reduced UV-VIS and IR absorption resulting from O 3 depletion. Since the stratosphere is in near-radiative equilibrium, it responds much more rapidly than the troposphere, and a substantial negative temperature trend has indeed been observed. Since the lowered temperatures favour PSC formation, these radiative effects are part of a positive feedback that can maintain heterogeneous ozone destruction into the future, even though CFC gases are controlled under the Montreal Protocol Aerosols, weather and hydrological cycle Particles in the atmosphere originate either from direct emissions (primary aerosol) or from gas-to-particle conversion (secondary aerosol). Primary aerosols, which usually represent the coarse mode of the particle size spectrum, are largely natural, such as mineral dust and sea spray. Secondary aerosols, usually in the fine mode, are mixtures of inorganic and organic substances. Their size approximates the wavelength of solar 12

13 radiation, so that they effectively attenuate sunlight through extinction, i.e. scattering and absorption, which is being referred to as the direct aerosol effect on the energy budget. Anthropogenic aerosols thus cause climate forcings along with increasing greenhouse gases, although the climate responses are quite different, partly even opposite (IPCC, 2001). In addition to the direct effect, aerosols can influence the hydrological cycle and climate through a number of indirect effects. The first indirect effect results from the decrease of cloud effective droplet radii as a consequence of increasing concentrations of aerosols that act as cloud condensation nuclei (the so called Twomey effect). Polluted clouds with smaller but more numerous droplets (assuming constant liquid water content) are more reflective and thus cause a surface cooling tendency. The second indirect effect results from the decrease in droplet coalescence and thus the precipitation efficiency of clouds with more and smaller droplets, which prolongs the cloud lifetime and increases the reflectivity. The suppression of droplet growth also affects the freezing temperature, which can influence the depth and vigour of convective clouds (Rosenfeld and Woodley, 2001). If the aerosols in the cloud layer absorb solar radiation, the resulting atmospheric heating may evaporate the droplets. This is called the semi-direct effect, acting in the opposite direction as the above-mentioned aerosol effects. The net result of all indirect effects for climate is uncertain, however, it becomes increasingly clear that they contribute to a cooling of the earth s surface in many regions with a high aerosol loading. Since pollutant haze is quite persistent over some parts of the globe, particularly in the northern hemisphere, the aerosols act on climate relevant timescales similar to greenhouse gases that force the system on a global scale. There is moreover growing evidence that aerosols cause strong regional climate effects. Aerosols that both scatter and absorb solar radiation tend to strongly reduce irradiance at the surface while they heat the lower atmosphere. This can stabilize the boundary layer with consequences for dynamical processes, and it reduces evaporation, particularly from the oceans (Ramanathan et al., 2001). This slowdown of the hydrological cycle can have important consequences for precipitation in regions that are already under water stress, e.g. in southern Europe, the Middle East and N-Africa (Lelieveld et al., 2002). 13

14 2.5. Transboundary air pollution and unpredictable events In the 1970s and 80s the recognition grew that pollution control cannot be successful without international efforts. The problem was painfully demonstrated after the Tsjernobyl accident in the Ukraine (1986), which caused radioactive fallout throughout Europe. The pollution included short-lived gaseous gamma-emitters (e.g. 131 I) and the even more dangerous alpha-emitters (e.g. 134 Cs and 137 Cs) that travel on aerosol particles. Scavenging of aerosols by localized precipitation strongly contaminated several agricultural areas. Although the fear for nuclear reactor accidents has faded since, they cannot be excluded in future. The awareness that air pollution easily crosses national boundaries has led to the European Convention on Long-Range Transboundary Air Pollution (LRTAP), established through the United Nations Economic Commission for Europe (UNECE), intended to control and reduce pollutant emissions and transport. Since its enforcement in 1983 the Convention has been extended by eight protocols, for example, on heavy metals (1998), persistent organic pollutants (POP) (1998), and to abate acidification, eutrophication and ground-level ozone (1999). The latter protocol sets emission ceilings for 2010, aiming at sulphur, NO x, VOC and ammonia. The United States also supports LRTAP and its protocols. In recent years the interest in transboundary air pollution has reached a new dimension. There is growing evidence that gases and particles can travel over many thousands of kilometres in the free troposphere, giving rise to intercontinental pollution transport. A model study by Li et al. (2002) suggests that 20% of the violations of the European 8-hourly ozone standard would not have occurred in the absence of pollution emissions in North America. This issue raises concern both in the USA and Europe, in particular also because in Asia emissions are growing strongly, which enhances the hemispheric background air pollution level. This will become an increasingly important factor for Europe and the USA in their efforts to improve air quality. Volcanoes, forest fires and other calamities such as the Kuwait oil fires in 1991, are large and unpredictable pollution sources that can strongly affect atmospheric CO, SO 2, aerosol loadings, and they can also influence CO 2. In Europe, Mt Etna contributes about 5-10% to the total release of SO 2 while in volcanically active periods this may be 14

15 much more. Volcanic eruptions and large fires can moreover represent acute hazards for populated regions downwind and give rise to safety risks for air traffic. Large eruptions such as that of Mt Pinatubo in 1991 can strongly perturb stratospheric sulphate for several years, which enhances O 3 destruction through heterogeneous processes. 3. Observations: methods and analyses Chemical processes in the atmosphere, being strongly influenced by radiation, transport and mixing, take place on scales varying from metres to thousands of kilometres. Operational information about air quality may be required from the level of cities to continents. There is no single method that can supply the information on all scales, nor is it possible to measure every parameter of interest. Therefore an operational system must be developed in which the relevant scales are hierarchically represented. This chapter discusses the monitoring of atmospheric composition, including the potential role of geostationary satellites, and considers methods to analyse the data and to provide operational products Ground-based measurements In situ measurement methods encompass chemical analytic, spectroscopic and mass spectrometric techniques that are used in networks that monitor the atmospheric composition for air quality and climate applications. The analysis of trends from these data is difficult for short-lived species, particularly near the sources where the spatial and temporal variability is high, hence such measurements are usually performed in background locations. Early warning systems, on the other hand, usually encompass measurement stations in urban and industrial environments. The Global Atmosphere Watch of WMO performs background measurements in 22 stations in remote locations around the globe. The measurements include O 3, longlived greenhouse gases (CO 2, CFCs, CH 4, N 2 O), UV radiation, aerosols and reactive gases (SO 2, CO, NO x ). In addition, the Climate Monitoring and Diagnostics Laboratory (CMDL) performs long-term measurements of compounds that affect climate, 15

16 the ozone layer and baseline air quality. Carbon dioxide, for example, is measured from 45 mostly coastal and island stations. The Network for the Detection of Stratospheric Change (NDSC) applies remote sensing techniques from 60 stations, using e.g. lidars, UV-VIS and Fourier Transform Infrared (FTIR) spectrometers to monitor the ozone layer, predominantly at middle and high latitudes. NDSC contributes importantly to the validation of satellite measurements (Lambert et al., 1999). Measurements of UV radiation are largely carried out by national institutions such as meteorological organizations. In Europe, 13 countries coordinate their measurements through a Cooperation in the field of Scientific and Technical Research (COST action 713) to support UV-B radiation forecasts. Based on an LRTAP protocol of 1988, the Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe (EMEP) has been established. Thirty-nine countries, including the European Union and the USA, are currently parties to this protocol. It forms the basis to assess air pollution in Europe in the light of agreements on emission reduction. EMEP has three main components: collection of emission data for SO 2, NO x, VOCs and other air pollutants; measurement of air and precipitation quality; and modelling of atmospheric dispersion. At present, about 100 monitoring stations in 24 countries contribute to the programme Observations from space Although the ground-based measurements are thus quite extensive, some areas remain data sparse and a three-dimensional picture of the atmosphere is lacking. This is particularly problematic for short-lived gases and aerosols, being highly variable and difficult to measure comprehensively. Satellite measurements are thus needed to develop regional and global views, including the vertical dimension. In this section we conceptually address the use of satellites in atmospheric composition monitoring, while in chapter 5 an overview of actual space missions will be provided. The discussion is limited to passive remote sensing techniques that measure thermal emission (including millimetre and sub-millimetre wavelength) or backscattered solar radiation in the ultraviolet, visible and near-infrared (UV-VIS-NIR) spectral region. In contrast, active remote sensing techniques with lidars (using short-wave laser light) 16

17 are envisioned for low earth orbit satellites to measure H 2 O, O 3 and aerosols. Considering the fixed position of geostationary satellites it is not useful to consider such techniques here. Space-borne measurements can potentially provide much of the information needed, however, the sensitivity, viewing geometry and the presence of clouds set limitations. Satellite sounders either employ the nadir (downward) or limb (sideways) viewing geometry. The first focuses on the location beneath the instrument; the horizontal coverage is enhanced through scanning. Limb sounders scan the atmosphere in the vertical direction, providing good altitude resolution (1-2 km). The horizontal resolution, however, is limited to several hundred kilometres. Since such large air masses often contain clouds, it is difficult to retrieve the atmospheric chemical information. Limb scanning is therefore well suited for the (near) cloud-free stratosphere, also because in this part of the atmosphere the variability is usually not very high. Although nadir sounding is affected by clouds as well, the horizontal resolution is better so that the likelihood of cloud-free pixels is larger. The nadir view is therefore superior for the troposphere, in particular for highly variable species. A disadvantage is that the vertical information must be inferred indirectly by using temperature profiles. Nadir sounding of the troposphere can be performed by backscattered UV-VIS- NIR or thermal emission spectrometers applying multiple spectral channels. In addition, correlation radiometers can target specific trace gases such as CO and CH 4 with good accuracy. Spectroradiometers offer the important advantage that a large number of gases can be measured simultaneously, although the effort of developing retrieval algorithms is substantial (e.g. Burrows et al., 1999). Therefore, the heritage is very important. Meanwhile it has been demonstrated that UV-VIS spectrometry can be used to retrieve column O 3, NO 2, SO 2, HCHO, O 3 profiles, UV-A, UV-B and aerosol parameters. By adding a NIR channel important information about CO and the greenhouse gases CO 2, CH 4 and N 2 O can be derived. Cloud parameters and water vapour can also be retrieved. Terrestrial IR and microwave (millimetre and sub-millimetre) spectrometers can measure many gases as well, including some information about their vertical distribution, however, the sensitivity in the lower troposphere is less, which can be critical for 17

18 operational applications. Such instruments can be valuable to measure vertical gas distributions in the upper troposphere and stratosphere. Aerosols are being retrieved from a number of instruments (also on METEOSAT) by channels in the visible part of the spectrum. Recently dedicated satellite instrumentation has been developed, using spectral and multi-angle techniques (Kaufman et al., 2002). By also including polarisation measurements the aerosol refractive index and size information can be inferred, which offers important clues about the origin (natural or anthropogenic), the deposition velocity and the chemical composition of the aerosols. At present several of these instruments fly on low earth orbit satellites so that much experience with hardware and retrieval algorithms is gained, at an especially high pace in the next several years since ENVISAT is now in orbit. Hence the heritage is growing rapidly. It is likely that some but not all of these missions will be continued in future to monitor the global distributions of stratospheric O 3 and greenhouse gases, for example. Since these satellites fly sun-synchronous, their observations are limited to a particular time of day, whereas the revisiting time may be several days depending on the pixel size. This is not critical for long-lived species e.g. with the goal to establish global trends, however, synoptic and diurnal patterns cannot be retrieved. Geostationary satellite measurements can provide essential additional data to support operational applications. Given sufficient horizontal resolution ( 10 km), they can capture much of the spatial and temporal variability that is typical for short-lived gases and aerosols in the lower troposphere. At a time resolution of 0.5 hour or less, much synoptic information associated with meteorological variability can be obtained, for example, about the trajectories of pollution plumes. Furthermore, time resolved measurements near the sources of both short-lived and long-lived species can be used to estimate emission strengths, especially since many pollution sources are not continuous. Some atmospheric phenomena vary strongly diurnally, such as pollutant O 3 build-up during summer episodes. Also the aerosol effects on the surface energy budget and evaporation, for example, are dependent on solar irradiance. Since the frequency of observations in a particular region is much larger from a geostationary than a low earth orbit satellite, the 18

19 likelihood to encounter cloud-free pixels increases by an order of magnitude. Also the monitoring of unpredictable events is strongly favoured by the temporal resolution provided by geostationary measurements. It will be a distinct advantage to monitor the meteorological and atmospheric chemical features simultaneously from the same platform. A METEOSAT satellite extended with atmospheric chemistry sensors will thus have the unique capability to provide information on all issues mentioned in chapter 2. In chapter 6 we will revisit these issues in view of the data products that may result from geostationary satellite measurements Atmospheric chemistry modelling Even though geostationary satellites, in combination with low earth orbit satellites and ground based monitoring networks, have the potential of providing good spatial and temporal coverage, some applications may require even more comprehensive information, for example, to motivate and justify well-aimed warnings or acute measures regarding severe air pollution, high UV radiation levels or catastrophic events. This necessitates integrating methods that combine different sources of information, notably numerical models used in forecasting and early warning systems. Computer models of the atmosphere apply the fundamental equations that are considered to represent physical, chemical and biological processes that control the composition of the atmosphere, weather and climate. Atmospheric chemistry-transport models (CTMs) need to be global because zonal advection around the globe occurs on a time scale that is similar to the lifetime of key species such as CO and O 3. To preclude excessive computer time, regional models with higher resolution can be nested into global models, whereas recently global models have also been extended with flexible zoom options (Sportisse, 2002). The model grid spacing of CTMs will nevertheless remain to be too large to calculate all relevant variables, for example, those associated with aerosol-cloud interactions, turbulence and deposition processes. If a system component is governed by processes that occur on a sub-grid scale, it must be parameterised, which means that it is calculated from the resolved parameters in a simplified way. Obviously, parameterisations must be continuously tested for many possible states of the system. 19

20 Geostationary satellite measurements can help improve parameterisations because they provide data at a temporal and spatial resolution that is close to that of the models. The models can be used either as diagnostic or prognostic tools. The first involves their use in data interpretation and in performing sensitivity tests to study the role of system components. In the diagnostic mode, models in combination with measurement data will increasingly be used to estimate sources of pollutants and greenhouse gases (e.g. through inverse modelling). For operational applications the ability to use the models prognostically is important. Because of the chaotic nature of the system, it is essential to optimise the initial conditions in the forecasting of aerosol plumes, surface ozone and UV levels, for example. The satellite data, in addition to those from ground-based stations, need to be combined consistently with the dynamics of the atmosphere as predicted by the models, which can be achieved through data assimilation. Such techniques have been developed for weather forecasting, and they are increasingly being applied in atmospheric chemistry (Elbern and Schmidt, 2001). Recently an operational aerosol forecasting system has become available for scientific applications, e.g. the planning of field measurement campaigns, which assimilates satellite aerosol retrievals (Collins et al., 2001). By assimilating measurement data into regional, mesoscale and nested urban scale models, with grid sizes in the low kilometre range, it will become possible to provide high resolution operational data products e.g. for air quality monitoring and early warning systems. 4. Anticipated future developments Since this report evaluates satellite measurements for future applications, notably in the timeframe, some educated guesses about potential developments are presented. These include the possible state of the atmosphere, the anticipated use of satellite data and possible developments in applications. One goal of models, in addition to those mentioned in section 3.3, is to perform scenario studies. The Intergovernmental Panel on Climate Change (IPCC, 2001) reports on the possible future climate, presenting a range of likely scenarios. Without going 20

21 into detail, IPCC expects further increases of greenhouse gases in this century, e.g. ranging from a doubling to more than a tripling of atmospheric CO 2 by 2100 (compared to the pre-industrial CO 2 level). Other pollutant gases and aerosols are expected to increase as well, which implies deterioration of air quality, acidification and other environmental and climate forcings. Regional differences may be large though, associated with the diversity in sources and atmospheric responses to forcings, including unexpected teleconnections of which some examples are given in the next two sections Atmospheric ozone change Figure 4.1 presents model results of tropospheric O 3 for present conditions and for the year 2025, applying the IPCC IS92a base scenario for atmospheric chemistry projections (Lelieveld and Dentener, 2000). The 2025 scenario accounts for World Bank and United Nations global population forecasts (8.4 billion people in 2025), economic growth ( %/yr), fossil fuel related emissions (including an energy use efficiency increase of 0.8-1%/yr) estimated deforestation rates, and agricultural developments according to the Food and Agricultural Organisation. In spite of substantial efforts by environmental protection agencies, surface ozone is expected to increase in many areas. In western Europe and the USA some reductions of episodic peak O 3 values have recently been attained. In many other countries, however, rapid economic developments are associated with strong pollution increases, particularly in Asia. Since many of these countries are located in subtropical latitudes, the high UV levels in summer can boost photochemical air pollution. Figure 4.1 shows that surface ozone in the northern hemisphere may grow substantially in the next decades, in particular at N. The model furthermore suggests O 3 increases in western Europe and the USA, even though O 3 precursor emissions in these regions have been assumed to remain nearly constant. Growing emissions in Asia cause a large-scale O 3 increase, enhancing the hemispheric background, so that pollution control efforts in other regions can be overpowered. 21

22 Figure 4.1. Ozone at the earth s surface during summer in the northern hemisphere (May-August), comparing recent and 2025 emission scenarios, calculated with a chemistry-transport model (Lelieveld and Dentener, 2000). 22

23 Future population increases will be concentrated in mega-cities, in which fossil fuel related and industrial pollution emissions will grow much more rapidly than in rural locations. Figure 4.1 also shows that in Europe the main problems are expected to occur in the Mediterranean region where the eight-hourly air quality standard of 110 µg/m 3 is presently already exceeded throughout summer. It may be expected that air quality standards will be further exceeded in future as a consequence of increasing long-range transport of air pollution, for example from Asia, that adds to the European emissions. The future development of stratospheric ozone will likely differ from CO 2 and tropospheric O 3. Several main ozone depleting CFCs are decreasing, notably CFC-11, which has an atmospheric lifetime of about 50 years. CFC-12, with a lifetime of about a century, still increases although the growth rate has diminished. These reductions have been prompted by the Montreal Protocol and its amendments under which the gases with the largest O 3 depleting potential have been phased out in the 1990s. The CFC replacement gases with much shorter lifetimes, as well as halons that are difficult to replace (e.g. for use in fire extinguishers), are increasing. At present there are no signs of a further thinning of the ozone layer in the near future, and the beginning of recovery is expected around 2010 (WMO, 1999). Some uncertain factors remain, however. One example is that increasing CO 2 causes stratospheric cooling through enhanced IR emission. Climate change in the troposphere can moreover affect the dynamics of the stratosphere. A colder and less dynamic polar stratosphere could delay ozone recovery or even enhance ozone destruction in the Arctic stratosphere. These uncertainties provide a strong incentive for the continued monitoring of the ozone layer by satellites Atmospheric forecasting Numerical weather prediction (NWP), in which the European Centre for Medium-Range Weather Forecasts (ECMWF) has a leading position, rapidly advances to the extent that long-range forecasts are now becoming available (i.e. for periods in excess of 10 days). The ECMWF operational general circulation model (GCM) has recently been extended and improved to include the middle atmosphere. Obviously the observational 23

24 requirements for the GCM initialisation increase in parallel, so that the importance of satellite data is growing. Cloud movement observed from geostationary satellites is used as a proxy for air mass advection in areas where direct wind measurements from balloon soundings are lacking. Measurements of chemical tracers could additionally provide such information for the cloud-free atmosphere, similar to the measurements of water vapour motion. Ozone observations from satellites, for example, are being considered for the stratosphere, and ECMWF has implemented a simplified stratospheric chemistry scheme in its operational model. The next step will be to assimilate the satellite observations of ozone (Riishojgaard, 1996). Further advances that are expected include the implementation of full tropospheric and stratospheric chemistry routines in NWP models. This will facilitate atmospheric chemistry and UV forecasting, which could play an important role in air quality monitoring and early warning systems. Some tropospheric tracers travel in plumes that can clearly be discerned from space. Satellite measurements by the MOPITT instrument (Measurement of Pollution in the Troposphere), part of the NASA Earth Observing System (EOS), have shown that CO plumes can be traced around the globe ( Important CO source areas include the industrialized parts of the northern hemisphere, southern Asia, as well as biomass burning regions in Africa and South America. By tracing these plumes diurnally with a geostationary satellite another proxy for airflows will become available for cloud-free conditions, for example, in subsidence regions. The assimilation of such data in NWP models will also support atmospheric chemistry forecasting. Other tracers with similar properties include aerosols, SO 2 from large point sources, and tropospheric O 3. Recently GCMs have developed into tools for seasonal NWP, providing qualitative forecasts of a limited number of parameters such as temperature and precipitation. Long-term recurrent patterns in such parameters are associated with socalled modes that stand out as a result of positive feedbacks in the atmosphere-climate system in which sea surface temperatures (SSTs) play an important role (Thompson and Wallace, 2000). Well-known examples are the El Niño Southern Oscillation (ENSO) and 24