Since the discovery by J. E. Lovelock, R. J. Maggs and R. A. Rasmussen, in 1972, of its ubiquity in sea water, dimethyl sulphide (DMS) , a biologically produced sulphur compound, has been the subject of continuously increasing interest by the scientific community. DMS was immediately recognized as an important component of the biogeochemical sulphur cycle, and is now indicated as the second most important source of sulphur in the atmosphere, after anthropogenic so emission from fossil fuel combustion and 2 industry. DMS reacts rapidly in the atmosphere where it is oxidized to condensable acidic sulphur products; in fact, rainwater acidification, observed in remote areas, is attributed to DMS emissions. The hypothesis of a climatic role of DMS was made already in 1983 by B. Shaw, and by B. C. Nguyen, B. Bonsang and A. Gaudry. In 1987, a study appeared in Nature, in. which R. J. Charlson, J. E. Lovelock, M. O. Andreae and S. G. Warren suggested the possibility of a partial control of the climate by the biosphere through a chain of processes, linking production of DMS by marine phytoplankton with changes in clouds albedo. The publication of this paper triggered a strong debate and stimulated new efforts to describe the various aspects of the DMS cycle in the environment. The paper was timely and added to the discussion on the relative roles of atmospheric sulphur and greenhouse gases in the Earth's radiative budget.

Divided into four sections, this work presents the proceedings of an international symposium on monitoring of gaseous pollutants by tunable diode lasers. Focusing on atmospheric trace gas detection, the presentations demonstrate the impressive capabilities of the diode laser technique. Reports on IV-VI mid-infrared diode lasers stress those properties - radiation noise and emission linewidth - which are essential for application to ultra-high sensitivity pollution monitoring. Two reports on III-V systems emitting at wavelengths shorter than 3 mu-m show that the less severe cooling requirements can lead to more compact equipment which can be used to detect pollutants via overtone absorption bands. The presentations on IV-VI physics and devices stress the physical and technological aspects of mid-infrared diode lasers. Laterally structured diode lasers are discussed in detail. There is also extensive material which compares traditional AM and the more recent FM techniques. The reports on special applications point to new directions for research, thus rounding off an expert discussion of a vital research area.

WOUIRG PARTY I: AlIALYSIS or POLLUTAIITS Chairman: A. LIBERTI (CNR - Roma, I) Summary by the Chairman 3 Detection of gaseous nitric acid by OH (A~X) - fluorescence gen- erated by ArF-laser irradiation Th. PAPENBROCK and F. STUHL 5 Preparation of standard atmospheres of nitrogen acid compounds: the N02 permeation tube I. ALLEGRINI, P. BUTTINI, V. DI PALO and M. POSSANZINI 15 Automated denuder systems J. SLANINA, A. M. VAN WENSVEEN, C. A. M. SCHOONEBEEK and P. I. VOORS 25 A battery-powered light-weight ozone analyzer for use in the troposphere and stratosphere S. SARAND, W. SPEUSER and U. SCHURATH 33 H202 in solid precipitation A. NEFTEL, A. SIGG and P. JACOB 45 Detection of nitrated and oxygenated polyaromatic hydrocarbons (PAR) in suspended particulate matter sampled in urban areas and their relation with anthropogenic emission and photochemical smog formation A. CECINATO, E. BRANCALEONI, C. DI PALO, R. DRAISCI and P. CICCIOLI 58 Field intercomparison of sampling and analytical methods for S02 and S04 in ambient air K. NODOP and J. E. HANSSEN 69 Field measurements on the use of denuders for the evaluation of acid deposition Istituto sull'Inquinamento Atmosferico del C. N. R. , Mon- rotondo, Italy 79 -vii- Use of air - S02 mixtures for intercomparison of S02 monitors in the EEC M. PAYRI S SAT , H. RAU and G. SERRINI 103 Determination of polychlorinated dibenzo-P-dioxins and dibenzofurans in outdoor air P. KIRSCHMER and M.

The 3rd International Symposium on Monitoring of Gaseous Pollutants by Tunable Diode th Lasers, held at the Fraunhofer Institute in Freiburg on October 17th and 18 , continued a tradition, established by the two preceeding meetings held at the same location, in 1986 and 1988. The steadily increasing number of participants and contributions emphasizes the need for such a meeting, which is unique in Europe and appreciated abroad. This third symposium in particular, was specially marked by the presence of the former Eastern Block Countries, with a number of scientists, and contributions, from the USSR (stilI existing at the time of the meeting) and from East Berlin. The large number of contributions proposed, originally, as oral presentations, could not be fitted in the schedule of the two-day limit which the participants to the previous meetings had recommended. To take account of these contributions, poster sessions were held, combined with extended discussion time. This intensified personal interaction was much appreciated by the participants.

In 1986, an International Symposium on Monitoring of Gaseous Pollut- ants by Tunable Diode Lasers was organized by the Fraunhofer-Institut fuer Physikalische Messtechnik in Freiburg, FRG, and held at this institution on November 13-14, 1986. The papers presented at that occasion extensively covered the applications of tunable diode laser spectroscopy (TDLS) to the analysis of trace and exhaust gases together with the latest improvements brought to the tech- niques used. It was generally consented by the participants that meetings of this kind are of major importance and should take place on a regular basis. The organizers were thus encouraged to plan a Second International Sympo- sium on Monitoring of Gaseous Pollutants by Tunable Diode Lasers which was held two years later on October 17-18, 1988, again in Freiburg. In addi- tion to the main subjects treated in the first Symposium, the Scientific Programme this time presented a session on the development of lead chalco- genide lasers. Once more, only a limited attention was given to applica- tions in high resolution molecular spectroscopy. In order to complete the update, sessions on components and techniques were included in the Pro- gramme. As for the previous Symposium, some space was also given to alter- native optical techniques. This time, in particular contributions on laser photoacoustic detection were included.

By now it has been nearly twenty years since the pioneering studies at the MIT-Lincoln Laboratories, Lexington, USA, demonstrated the unique capabilities of lead salt tunable diode lasers (TDL) for infrared absorption spectroscopy. The progress in the use of TDL instrumentation for a wide variety of scientific applications was described by a great number of papers since, however, comparatively few meetings were specifically devoted to this subject. In 1980 the conference on "High Resolution Infrared Spectroscopy Applications and Developments" at the National Bureau of Standards in Gaithersburg, USA, reviewed the state of the art of tunable diode lasers together with Fourier Transform Spectroscopy and other laser spectroscopic techniques. Three years later in 1983 the SPIE Conference in San Diego, USA, dedicated one meeting to "Tunable Diode Laser Development and Spectroscopy" Applications. It appeared appropriate after a further interval of three years to organize another meeting about this quickly advancing field. In November of 1986 an International Symposium on "Monitoring of Gaseous Pollutants by Tunable Diode Lasers" was organized by and held at the Fraunhofer-Insti tut fuer Physikalische Messtechnik in Freiburg, FRG. The main emphasis of this conference was put on the applications of TDL techniques to the solution of problems of environmental relevance : The measurement of atmospheric trace gases and the monitoring of exhaust gases from automobile and power plant stack emitters. The state of diode laser development and application of TDL instruments to scientific molecular spectroscopy were not directly subjects of the meeting.

In October 1979 the First European Symposium on Physico-Chemical Behaviour of Atmospheric Pollutants was held In Ispra (Italy); 83 scientists attended the conference contributing 44 papers. Ten years later, the Fifth European Symposium on Physico-Chemical Behaviour of Atmospheric Pollutants, organized as for the previous Symposia In the framework of the Concerted Action *COST 611, was held in Varese (Italy) from 25 to 28 September 1989. This Volume contains the oral papers and the posters presented at this Symposium. Participation at this Conference Is more than doubled of that In 1979 In terms of scientists (185) and contributed papers (110). This simple comparison demonstrates once more the growing attention of the scientific community to the problems related to the pollution of the atmosphere. During these years, Important new Issues have arisen (global pol lutlon/cl Imatlc changes) whl Ie old ones have been reviewed due to new experimental evidence (depletion of stratospheric ozone). The Symposium offered the best opportunity for a review of the current studies and technical progress achieved In the various sectors of the Concerted Action since the Fourth Symposium held In Stresa (Italy) In September 1986. In 1987 the scientific programme and the operational structures of the COST 611 Project were revised. The Project Is now structured into three Working Parties: 1. Development of Analytical Methods to measure Trace Components of the Atmosphere. 2. Atmospheric Chemical and Photochemical Processes. 3. Field measurements and their Interpretation.

Since the discovery by J. E. Lovelock, R. J. Maggs and R. A. Rasmussen, in 1972, of its ubiquity in sea water, dimethyl sulphide (DMS) , a biologically produced sulphur compound, has been the subject of continuously increasing interest by the scientific community. DMS was immediately recognized as an important component of the biogeochemical sulphur cycle, and is now indicated as the second most important source of sulphur in the atmosphere, after anthropogenic so emission from fossil fuel combustion and 2 industry. DMS reacts rapidly in the atmosphere where it is oxidized to condensable acidic sulphur products; in fact, rainwater acidification, observed in remote areas, is attributed to DMS emissions. The hypothesis of a climatic role of DMS was made already in 1983 by B. Shaw, and by B. C. Nguyen, B. Bonsang and A. Gaudry. In 1987, a study appeared in Nature, in. which R. J. Charlson, J. E. Lovelock, M. O. Andreae and S. G. Warren suggested the possibility of a partial control of the climate by the biosphere through a chain of processes, linking production of DMS by marine phytoplankton with changes in clouds albedo. The publication of this paper triggered a strong debate and stimulated new efforts to describe the various aspects of the DMS cycle in the environment. The paper was timely and added to the discussion on the relative roles of atmospheric sulphur and greenhouse gases in the Earth's radiative budget.

In October 1979 the First European Symposium on Physico-Chemical Behaviour of Atmospheric Pollutants was held In Ispra (Italy); 83 scientists attended the conference contributing 44 papers. Ten years later, the Fifth European Symposium on Physico-Chemical Behaviour of Atmospheric Pollutants, organized as for the previous Symposia In the framework of the Concerted Action *COST 611, was held in Varese (Italy) from 25 to 28 September 1989. This Volume contains the oral papers and the posters presented at this Symposium. Participation at this Conference Is more than doubled of that In 1979 In terms of scientists (185) and contributed papers (110). This simple comparison demonstrates once more the growing attention of the scientific community to the problems related to the pollution of the atmosphere. During these years, Important new Issues have arisen (global pol lutlon/cl Imatlc changes) whl Ie old ones have been reviewed due to new experimental evidence (depletion of stratospheric ozone). The Symposium offered the best opportunity for a review of the current studies and technical progress achieved In the various sectors of the Concerted Action since the Fourth Symposium held In Stresa (Italy) In September 1986. In 1987 the scientific programme and the operational structures of the COST 611 Project were revised. The Project Is now structured into three Working Parties: 1. Development of Analytical Methods to measure Trace Components of the Atmosphere. 2. Atmospheric Chemical and Photochemical Processes. 3. Field measurements and their Interpretation.