OCTA (Oxidizing Capasity of the Tropospheric Atmosphere)

1.1.1994 - 1.1.1996

The main objective of this project is to improve the knowledge of the Oxidizing Capacity of the Tropospheric Atmosphere (OCTA).


General information:
The chemical state of the atmosphere is important because it controls both the production (eg. ozone) and the destruction (eg. methane and proposed CFC replacements) of many greenhouse gases.

The primary oxidizing agents in the atmosphere are the free radicals (most important the hydroxyl radical). The complex photochemistry of these species is highly non-linear, involving negative feedbacks with for example methane and is also strongly influenced by meteorological processes. Thus changes in the oxidizing capacity of the atmosphere, brought about by man's activities, have important consequences for two major global environmental problems which face mankind, greenhouse warming and stratospheric ozone depletion. Information on the fundamental processes over a range of meteorological and chemical conditions is required before quantitative estimates of future changes can be made with chemical climate models.

Major aim is to contribute to this ultimate goal: by (i) measurement of key radical species in fully characterized air masses, (ii) tracking the evolution of primary and secondary chemical species in air parcels and (iii) combining measurements and the meteorological state of the atmosphere using photochemical transport models in summer and winter.

The studies will be conducted from a combination of platforms: (i) a sophisticated research aircraft, operated by the Meteorological Office Research Flight (MRF) and capable of a wide range of measurements, (ii), instrumented top measurement site and (iii) lighting balloon-borne instrumented packages. The planning of the measurement programme and interpretation of the results will use data from operational numerical weather prediction models. These data will also be combined with photochemical models of tropospheric chemistry.

The OCTA programme will be carried out by six research groups from three European countries.

Testing Atmospheric Chemistry in Anticyclones (TACIA)

1.1.1996 - 1.1.1998

Objectives and goals

The project is centred around quasi-Lagrangian aircraft measurements from the MRF C-130 in conjunction with theoretical model calculations and data analysis. The objectives are to collect a high quality dataset with good time resolution and suitable for validation of Lagrangian and Eulerian models, of ozone and precursors as well as of intermediate species and products over sea in air coming off the European Continent. It will be investigated how European emissions impact on northern hemisphere ozone and in particular on ozone over the North Atlantic, and to what extent the ozone balance over the North Atlantic can be estimated from a knowledge of individual production and loss processes. It will be investigated if indirect estimates of the concentration of the hydroxyl radical can be made from the measurements. If yes, can a relationship between the concentrations of OH and NOx be derived?

Furthermore it will be investigated if it is possible to strengthen the knowledge of the relative roles of VOC and NOx in forming ozone in a European continental plume over the North Atlantic, and if measured concentrations of NOy species and individual hydrocarbons are consistent with current European emission inventories.

Maximum oxidation rates in the free troposphere (MAXOX)

1.1.1998 - 1.1.2000

Objectives and goals

The main objective of the project is to investigate how the annual cycle of ozone in the troposphere is influenced by atmospheric pollution on a continental and hemispheric scale from the surface and up to upper troposphere. Specifically:
It will be investigated if there is a chemical cause for the spring/summertime ozone maximum in background air over Europe and the North Atlantic.
It will be investigated if there is major ozone destruction in the free troposphere in summer outside of the polluted layers which are often found throughout the troposphere over the North Atlantic.
The concentration contrasts in ozone, precursors and intermediate species including free radicals, between the atmospheric boundary layer and the free troposphere will be measured over the European continent, and the adjacent oceans. The data will be applied to assess the strength of the exchange of pollution between the atmospheric boundary layer and the free troposphere and continuing during spring and early summer.
It will be investigated if chemical measurements in the atmospheric boundary layer and the free troposphere can show that a concentration exists for oxides of nitrogen where the oxidation rates of precursors have maxima, as indicated by theoretical calculations, and also, if there is a shift from net photochemical production of ozone to net photochemical destruction for a certain concentration of oxides of nitrogen.

Project methodology

The project is centred around measurements from the Met. Research Flight C-130 aircraft with a range of 5000 km and a ceiling of 10 km equipped with instrumentation for the measurement of chemical precursors, photooxidants and intermediate products including radical species. In support of the aircraft measurements and to address the scientific questions of the project, numerical models will be used including an advanced state of the art three- dimensional coupled numerical weather prediction-chemistry model.

ARCTOC (Arctic Tropospheric Ozone Chemistry) 1.1.94 - 1.07.96

To understand the mechanisms and extent of sudden arctic tropospheric ozone loss. 

General information:
The arctic troposphere is subject to quite unique boundary conditions, like very stable stratification near the ground, or complete darkness during the arctic winter. Therefore, the chemical balance of the arctic troposphere is likely to be very sensitive to disturbing influences, for instance to free radical processes catalyzing boundary layer ozone destruction, as observed at several arctic sites. Those processes might have natural causes, but also possible is initiation by pollutants advected from western Europe. The phenomena certainly point to significant deficiencies in the understanding of the chemical species and processes involved. This project will address the following questions :

1. What is the role of free (halogen oxide) radicals in the phenomenon of arctic troposphere ozone loss ?
2. Is this disturbance a natural or an anthropogenic phenomenon ?
3. Which are the possible consequences of sudden arctic tropospheric ozone loss, in particular on the chemical conditions of the arctic lower troposphere ?
4. What is the spatial extent of the sudden tropospheric ozone loss phenomenon ?

The project will consist of three components :

1. Two field studies planned during springtime at arctic sites in Ny-Alesund/Spitzbergen and one to be selected.
2. Laboratory kinetic investigations of relevant homogeneous and heterogenous reactions at low temperature.
3. Modelling studies including gas phase and heterogeneous reactions and trajectory calculations.

HALOTROP-CYMFO (Reactive Halogen Species: Cycles - Mechanisms - and Field Observations) 1.1.96 - 1.1.98

Objectives: To study the tropospheric abundance, reaction cycles, loss processes, and effect of Reactive Halogen Species in the troposphere, in particular with respect to their influence on the total oxidation capacity of the troposphere (marine and General Information:
This study is a contribution to the part of the programme dealing with the Oxidation Capacity of the Troposphere. It will be achieved by coordinated field-, laboratory- and modelling investigations of reactive halogen species, their precursor and storage species, and the reaction mechanisms involved. Field measurements will include direct determination of halogen oxides by Differential Optical Absorption Spectroscopy (DOAS) at sub-ppt levels. The laboratory studies will close many gaps in our knowledge of RHS related homogeneous and surface-catalysed reactions including iodine chemistry. The latest data from field and laboratory studies will be assembled in trajectory- and limited area 3D models, which will help to understand the results of the field campaigns as well as to assess the regional and global impact of RHS chemistry. A further important part of this study will be the assessment of a possibly anthropogenic influence on RHS sources and the transformation mechanisms within the RHS reservoir. Thus possible mechanisms changing the tropospheric oxidation capacity due to human activities can be investigated. While a central question concerns the importance of release of RHS from sea salt aerosol, for instance due to reaction of anthropogenic NOy species (like N205, NO2) with sea salt aerosol, the detailed investigation of those processes is the aim of a parallel study with the sub-title "Sea Salt Aerosols: Laboratory Investigation of Heterogeneous Halogen Activation in the Troposphere (HALOTROP-SALT)". Areas where enhanced levels of RHS are to be expected (like polluted coastlines or polar regions) will be identified. Regional processes involving RHS will be set into a global perspective.

HANSA (Hydrocarbons Across the North Sea Atmosphere) 1.4.94 - 1.4.96

To evaluate the chemical processing of non-methane hydrocarbons across the North Sea atmosphere.


General information:
The project investigates the importance of non-methane hydrocarbons in the production of secondary pollutants over transport distances of the order of 1000 km and over timescales of 1-3 days. The chemical processes will be studied in a relatively simple, but large scale, reaction chamber, i.e. the atmosphere at 4 coastal sites above the North Sea and at Mace Head and Porsdoper, using a combination of real data and five theoretical models.

The importance of different decomposition pathways in the formation of photooxidants will be studied for some hydrocarbons. The importance of the sea as a source and sink of natural and anthropogenic hydrocarbons will also be evaluated.

The measured data will be input into a number of existing European models with varying degrees of chemistry and meteorology. Initially, model simulation will be performed using the complementary IVL trajectory model and the EMEP MSC-W ozone model. The results will be expanded and the simulations repeated using the two-layer trajectory model MPA, developed by RIVM. The results from the three trajectory models will then be generalised by the use of two three-dimensional models : the Bergen University model and the LOTOS (Long Term Ozone Simulation) model.

The combination of extensive measurements and modelling will serve to validate the models and improve existing European emission inventories.

The reliability of these models for the evaluation of cost effective abatement strategies will also be established.

1.2.1998 - 1.2.2000

EULINOX funded by the European Community
through the Environment and Climate programme

The European Lightning Nitrogen Oxides Project (EULINOX)


The general objective of the project is to understand and quantify the contributions from thunderstorm lightning-induced sources of nitrogen oxides (NOx) to the composition of the atmosphere over Europe. The objective will be addressed by field experiments (including aircraft measurements) and modelling studies both, at regional scale (Southern Germany) and at the scales of Western Europe.

Brief Description of the Research Project:

During the EULINOX field campaign in summer 98 standard meteorological parameters (pressure, temperature, humidity, wind), condensation nuclei, as well as the concentrations of trace gases like NOx, CO2, O3 or CO were measured on board a FALCON aircraft which was able to penetrate thunderstorm anvils. Radiation measurements were also performed by the aircraft in order to determine the NO2 photolysis rate. Lower level chemical measurements of NO, NO2, CO2 and O3 concentrations were provided by a DO-228 aircraft during the regional scale flights (pressure, temperature and humidity are also available).

The radar structure of the precipitation was measured by the polarisation diversity radar POLDIRAD of DLR and by the Doppler radar of the German Weather Service at Hohenpeißenberg (HP). The polarimetric data allow to infer the hydrometeor type. Both radar did also perform Doppler measurements in order to enable a dual-Doppler analysis of the storms three-dimensional wind field.

The three-dimensional flash structure was detected by ONERA's interferometer (ITF3D). The system measures the VHF radiation emitted from negative discharges in a lightning flash. The ITF3D consists of two independent remote stations, located at a distance of about 50 km. Data from an LPATS (Lightning Position and Tracking System) system were also available for the area of the local experiment. A network of 10 automatically recording weather stations operated by the University of Munich was installed in the special observation area (SOA) west of Munich.

Figure 1. Map of local experimental area around the radar and interferometer sites west of Munich.

The large scale NOx concentration fields were measured during cross-frontal flights towards eastern and southern Europe. European radar and lightning data were used for starting trajectory calculations by KNMI's weather forecasting model thus allowing for an optimum flight planning. Moreover, forecasts of NOx distributions over Europe were obtained from NILU's chemical transport model.

The experimental data will be evaluated by searching for parameterisations (dependencies and correlations) of the lightning activity (different phases of a flash or lightning frequency) and the associated NOx production as depending on parameters conventionally available operationally on larger scales (Cloud depth, CAPE, structures from radar and satellite data). The new experimental evidence from the EULINOX field studies will be used to compare the new formulations with existing parameterisations.

The parameterisations are then tested against the aircraft measurements using models with different scale representation. The models are subdivided into models at cloud scale and hemispheric or global scale models. The mesoscale models allow to study the dynamics and microphysics of an individual thunderstorm in such detail that the transport of LNOx (lightning produced NOx) originating from a well defined single flash can be followed. European, hemispheric and global scale models enable a comparison of the LNOx distribution according to the large scale parameterisations with the airborne measurements. Conventional parameterisations as already implemented in the models, as well as new formulations derived from the EULINOX measurements will be tested. The more detailed cloud scale results (e.g. the flash type or the height-dependence of the LNOx sources) will be checked for their ability to be extended to the larger scale.

In a final step a new inventory of European lightning NOx production and an assessment of the environmental implications will be provided. After having established an optimum LNOx parameterisation these results will be used to evaluate the data from the total experimental period with respect to the total LNOx production over Western Europe. The LNOx will be compared to NOx produced from aircraft and ground sources, the impact of the LNOx on the ozone concentration as relevant for climate assessment studies will be addressed.

POLINAT1, POLINAT2 (Pollution from Aircraft Emissions in the North Atlantic Flight Corridor) 1.1.94 - 1.1.98

This project is funded by the European Commission through the Environment and Climate Programme.


Pollution from Aircraft Emissions in the North Atlantic Flight Corridor

The overall objectives of the project are:
  1. To determine by measurements and analysis the relative contribution from air traffic exhaust emissions to the composition of the lower stratosphere and upper troposphere at altitudes between 9 and 13 km within and near the flight corridor over the North Atlantic.

  2. To assess the effects of air traffic emissions in that region in relation to clean background concentrations and pollutant concentrations from various sources and to analyse their importance for changes in ozone, oxidizing capacity, aerosols and clouds.

AEROCHEM AEROCHEM-II (Modelling of the impact on ozone and other chemical compounds in the atmosphere from airplane emissions) 1.3.96 - 1.3.98 - 1.3.2000


The lower stratosphere/upper troposphere are areas of importance for ozone depletion, climate and interaction between chemistry and climate through ozone. Our knowledge of how chemical processes in general, and aircraft emissions in particular, affect the distribution of ozone and other key compounds in these regions is limited. However, with the projected large increase in future aircraft operations and thus emissions, studies of future impact on lower stratospheric/upper tropospheric chemical composition is needed. In order to address these scientific issues the overall objective for the AEROCHEM proposal is the impact of past, present and future emissions from both subsonic and supersonic aircrafts on upper tropospheric and lower stratospheric ozone. The project will contribute to a better scientific knowledge of the processes controlling ozone in height regions where ozone change could contribute significantly to climate change. 

General information:
The basic tool in this study will be 3-D CTMs (Chemical Transport Models) which have been developed by the
participating groups, and where the emphasis is on processes affecting transport and chemistry in the lower
stratosphere and upper troposphere. The models will have different formulation of chemistry and transport, and
represent state of the art 3-D modelling. As a supplement to the 3-D studies 2-D CTMs with detailed gas phase
chemistry will be used to investigate long term effects of aircraft emissions.
In order to improve our understanding of the key processes identified above in the ozone chemistry, and to
perform realistic 3-D estimates of the past, present and future impact on ozone from airplane emissions, a set
of tasks is selected as necessary parts of the work plan. The tasks of the work plan are outlined below.
Task 1. Preparation of data on emissions and background concentrations
An archive of emissions relevant for modelling the impact of aircraft emissions on the chemical composition
of the atmosphere. Starting point will be the AERONOX data base, which contains present day NOx emissions
from aircraft (ANCAT, WSL, NASA), industry and surface traffic, biomass burning, microbial soil production,
lightning, and stratospheric production (degradation of N2O). Furthermore, the data set will be extended in order
to cover also past emissions and future emission scenarios, in particular aircraft emission scenarios.
Task 2. Perform pilot runs to study model performance
A selected number of runs based on a consistent emission data base established under Task 1 will be performed
with the existing models. The goal is to study how the models perform with regard to ozone generation and
ozone distribution in the region of aircraft emissions, the upper troposphere and lower stratosphere. Current
emission rate for the source gases will be adopted. Two sets of model studies will be performed: one base run
with the current emissions, and with no aircraft emissions, and one run with the current emissions including
todays aircraft emissions. In the base case the model will be run to give the distribution of trace gases for a one
year period.
Task 3. Sensitivity analysis and process studies
The main goal of these studies is to increase our understanding of how key chemical and physical/dynamical
processes after ozone distribution, and how they can be parameterised in 3-D CTMs, thereby increasing our
ability to predict future changes due to aircraft emissions. Task 4. Calculations of ozone changes due to past, present and future aircraft operations
A set of scenarios of emissions and background concentrations of longlived species will be defined and will be
the basis for the model studies by the different modelling groups. The selection of scenarios to be used in the
model runs will be made to best answer the overall objectives of the project.


General information:


Observations have shown that ozone levels in the upper troposphere (UT) and the lower stratosphere (LS) have changed over the last two to three decades. The observed reductions in the LS, which has been seen in the Northern Hernisphere during the last decade most probably are caused by man made emissions (CFCs and bromine compounds) in conjunction with particles and PSCs formation. For the UT, observations have shown an ozone increase for at least two decades, but less so the last few years. The causes of these changes are poorly understood. Modelling studies have been used to estirnate the impact of different man made sources on the chemical composition, and on ozone in particular in the UT and the LS.

These studies show that there are significant uncertainties in the estimates of the impact which are a result of limited knowledge of atmospheric processes and which have to be improved in order to come up with better estimates of the impact of aircraft emissions on ozone in the UT and the LS.
Emissions from aircraft (NOx, H20, SO2 and soot) at cruising altitudes are likely to affect the ozone chemistry in the UT and the LS in two ways: directly through enhanced photochemical activity (emission of NOx and water vapour), and through enhanced particle formation from NOx, water vapour and SO2. The impact of aircraft emissions is of particular importance to study, as the emissions are projected to grow rapidly over the next two decades compared to emissions from most other sources, and because there are significant regional differences in the impact on ozone and in the projected growth in the emissions. It is therefore likely that future aircraft emissions have the potential to perturb ozone levels significantly.
The overall objective of the study is to improve our scientific basis for estimates of the impact of aircraft emissions on the chemical composition in the UT and in the LS, and to perform 3-D model studies of the large scale (regional to hemispheric) perturbation of ozone from a projected future fleet of subsonic and supersonic aircraft. Focus in the study will be on two main areas: a) The role of heterogeneous processes in the UT and the LS and how these processes can be parameterised in global 3-D CTMs, and b) modelling studies of the future impact of subsonic as well as supersonic traffic on the ozone in the UT and the LS, with particular emphasis on the regional contribution to global scale ozone from regions with the largest projected traffic (Europe - US, South Asia and surrounding areas).
The tools for these studies will be state of the art 3-D CTMs (Chemical Tracer Models) available among the participating groups. The CTMs have different spatial resolution, transport parameterisation, and parameterisation of the chernical processes, including heterogeneous chemistry, and will therefore in a complementary way contribute to the overall objective of the project. The new AEROCHEM II project will build on the results obtained in the ongoing AEROCHEM project.
Ozone sonde data at northern latitudes collected during projects which has participated in European campaigns (e.g.
SESAME), and from aircraft measurements of ozone from the MOZAIC project are available through participation in the projects. These data will be used to compare with model distribution.
The project will provide important inputs to international assessments related to the Montreal Protocol (ozone) and to the IPCC process (climate). The process studies conducted in AEROCHEM and planned for AEROCHEM II will contribute to internahonal research programmes like the SPARC programme of the World Climate Research Programme (WCRP).
where the focus is on processes in the UT and the LS. It will also serve as a basis for decisions on selechons of the future generation of low emission engines for aircraft.

SCAVEX (Schneefernerhaus aerosol and reactive nitrogen experiment)


Aerosols play a central role in today‘s atmospheric and climate research (air pollution, ozone loss, direct and indirect climatic effects, etc.). However, the knowledge about the chemical and physical properties of atmospheric aerosol particles is very limited. Up to now most model calculations of aerosol effects on atmospheric chemistry and climate account only for inorganics (sulfate, sea salt, mineral dust, etc.) and black carbon. Organic carbon is known as another major constituent of tropospheric aerosols, but only a minor fraction of the organic compounds contained in air particulate matter has been identified up to now. To allow an accurate assessment of the influence of aerosols on atmospheric chemistry and climate, a thorough physical and chemical analysis of atmospheric aerosols is required, including the characterisation and quantification of organic particle components.

The SCAVEX aerosol measurement program at the GAW observatory Schneefernerhaus (UFS) is aimed at an extensive physical and chemical characterisation of air particulate matter in the high alpine environment, where either boundary layer air or free tropospheric air prevail under different meteorological conditions. It is carried out by the Technische Universität München (Institute of Hydrochemistry), the Vienna University of Technology (Institute of Analytical Chemistry), the Paul Scherrer Institute (Atmospheric Chemistry Laboratory), and the University of Clermont Ferrand (Institute of Physical Meteorology) in collaboration with the German Aerospace Center (DLR, Institute of Atmospheric Physics) and the German Weather Service (DWD). A wide range of aerosol measurement instrumentation (CPC, DMPS/SMPS, ELPI, PASS, Aethalometer, Filter Samplers, etc.) and analytical techniques (GC/LC-MS, IC, HPAEC-PAD, HPLC-Fluorescence, Enzymatic Assays, etc.) are employed by the involved research groups.

Within SCAVEX the aerosol measurements at UFS are complemented by trace gas measurements (DLR and its partner MPIK Heidelberg), and during intensive campaigns also by airborne measurements of both particles and gases onboard the DLR research aircraft Falcon (DLR and MPIK Heidelberg). Meteorological data are provided by the DWD and by the DLR partner KNMI. The University of Munich (Institute of Statistics) contributes statistical analyses of the combined aerosol, trace gas and meteorological data.

INCA (Interhemispheric differences in cirrus properties from anthropogenic emissions) 1.1.2000 - 1.1.2002

Interhemispheric differences in cirrus properties

from anthropogenic emissions (INCA)


INCA is part of the CORSAIRE cluster, managed by
the European Ozone Research Coordinating Unit (EORCU)


Very small particles in the atmosphere, called aerosols, do influence the earth's climate, directly by absorbing and scattering solar and terrestrial radiation, and indirectly by modifying the formation processes and radiative properties of clouds. Therefore additional anthropogenic emissions of gases and aerosols either from sources at the ground or from air traffic alter the earth's radiative budget and the amount and properties of clouds.
Especially the relation between aerosols and cirrus clouds needs further investigations. Cirrus clouds appear at altitudes of about 7 to 11 km and consist mainly of ice particles. The radiative properties of cirrus clouds and the specific surface area of particles and ice crystals is of  importance  for climatic and air cemistry processes. These processes depend on numerous parameters as ambient conditions during cloud formation, structure and amount of clouds, thickness and water content of clouds, or the concentration, composition, and shape of the ice crystals, respectively.

The main goals of the project therefore are

The measurements using the same instrumentation and the same observation strategy are scheduled for March / April 2000 in Punta Arenas, Chile, and for September / October 2000 in Shannon, Ireland. Both campaigns will be performed at comparable geographical latitude (50°S and 50°N, respectively) and at the same season in local autumn.

El-CID (Evaluation of the climatic impact of Dimethyl Sulphide) 1.1.2000 - 1.1.2003

Problems to be solved:
The proposed research programme is designed to resolve many of the outstanding key issues concerning the chemical transformation of DMS so that a reliable quantitative appraisal can be made of its contribution to CCN formation and consequently an assessment of the magnitude of its regulatory role in climate. Past work on the atmospheric chemistry has been instrumental in highlighting very specific processes, which need to be investigated in detail if a reliable assessment of the relationship between DMS, CCN and climate is to be made. The continuing improvement in analytical techniques now makes it possible to make high quality and high time resolution measurements of many species, both in the laboratory and in the field, which were previously either not possible or only with large error limits and poor time resolution.

Scientific objectives and approach:
The major objectives of the project are:
1) to put constrains on the large uncertainties associated with current photochemical models by providing more accurate gas-phase kinetic and photochemical data on DMS oxidation chemistry;
2) Investigate particle formation from both DMS and DMSO;
3) Simultaneous high-time resolution measurements of dimethyl sulphide, oxidation products, halogen oxides, NO;
4) Radical, and aerosol number/size distribution in three campaigns at sites with different geographical locations reflecting distinct aspects of DMS chemistry;
5) Use the data to determine the relative importance of the oxidants OH, NO(3) and halogen oxides under different atmospheric conditions;
6) Use the laboratory data to construct a DMS chemistry module for CT-models capable of describing both the remote and polluted marine atmosphere and test of the models against the field data. The objectives will be achieved by a closely co-ordinated amalgamation of laboratory, field and modelling investigations.

Expected impacts:
The main deliverables of the project will initially be progressive constraints on kinetic/mechanistic aspects of the oxidation chemistry of DMS and DMSO from laboratory and field experiments. This will be accompanied by high-time resolution field measurements of DMS, oxidation products, aerosols and other products relevant to the photo-chemistry. Based on this laboratory and field information a comprehensive gas/aerosol DMS-halogen-chemistry mechanism (g/a-DMS-HALO) module for incorporation in CT-models will be developed, which will be capable of describing DMS chemistry in both the remote and polluted marine atmosphere. The information can eventually be incorporated into global climatic models.

FORMAT (Formaldehyde as a tracer of oxidation in the troposphere) 01.11.2001 - 01.11.2004

Formaldehyde (HCHO or H2C=O) is the most abundant of the carbonyl compounds in the atmosphere. It is also the smallest member of the aldehyde family. Formaldehyde is found both in the remote background atmosphere and in polluted urban atmospheres. The photooxidation of hydrocarbons invariably generates HCHO in the atmosphere (Finlayson-Pitts and Pitts, 1986; Atkinson, 1994). In the background troposphere, where methane concentration is considerably higher than that of non-methane hydrocarbons (NMHC), methane is the dominant formaldehyde precursor. Close to the surface, local sources of NMHC also become important in producing HCHO. In processes similar to that for methane, formaldehyde is generated from the oxidation of biogenic hydrocarbons, such as isoprene and terpenes (Levine, 1984) and from the oxidation of anthropogenic hydrocarbons. Formaldehyde is also anthropogenically generated directly from incomplete combustion processes, both from biomass burning (Holzinger et al., 1999) and from internal combustion engines. The figure below shows some of the reactions involved in the oxidation of hydrocarbons via HCHO to CO.

Diagram of the methane oxidation. Source gas in green, stable intermediate (HCHO) in red and stable end products in blue.

Through its subsequent decomposition by photolysis and reaction with the OH radical, formaldehyde serves as a source of the hydroperoxyl radical (HO2) and carbon monoxide (CO). In producing HO2, HCHO affects the partitioning of odd hydrogen radicals. As a source of CO, HCHO plays an important role in the global budget of CO in the natural troposphere (McConnel et al. 1971). In both cases, HCHO exerts an influence on the oxidising capacity of the atmosphere (Lelieveld and Crutzen, 1990).

Most important to atmospheric chemistry is the formation of HO2 with its subsequent involvement as an oxidant, in O3 formation and in OH production (Logan et al., 1981; Jacob et al., 1995).

Accurate HCHO measurements are thus important in constraining and validating photochemical models of the troposphere, in understanding the budgets and cycling among various reactive species and the global budget of CO. Despite this importance and the relatively large number of techniques employed, there is still considerable uncertainty in ambient measurements of HCHO. In various intercomparison campaigns, the level of agreement varies from good to quite poor (Cárdenas et al. 2000; Gilpin, 1997, and references sited therein).

It is therefore of importance to obtain a better understanding of the differences between the various measurement techniques and try to reduce the disagreement between these various techniques. This will be of great value both to validate atmospheric chemistry models and to validate satellite measurements of HCHO.

Photochemical smog is one of the most, if not the most, serious air pollution problem in Europe today. Episodes with high concentrations of ozone and NOx cause harm to human health and to vegetation. Abatement of such pollution is one of the biggest challenges to environmental authorities, both nationally and at the Community level. There is a clear need for better scientific tools to understand the mechanisms behind the formation of photochemical smog. Better tools are also needed to predict and warn the population against such pollution. The photographs below show typical smog episodes over the Po Basin in northern Italy.