THE OBJECTIVE OF THIS RESEARCH IS TO IMPROVE THE UNDERSTANDING OF THE TROPOSPHERIC BEHAVIOUR OF THE NIGHTTIME RADICAL NO3 BY FIELD MEASUREMENT STUDIES.
This is great interest in the naturally emitted hydrocarbons, isoprene and monoterpenes, because of their potential role in oxidant and aerosol formation in both urban and rural areas.
The monoterpenes are removed from the atmosphere by reaction with the hydroxyl radical, ozone and nitrate radical. The reaction with nitrate occurs at night only.
Research has been carried out to improve the understanding of the nitrate oxidation of monoterpenes, and particularly alpha pinene, by field measurements in forest areas. The 2 sites involved were Viterbo in Italy and Storkow in Germany.
From differential optical absorption spectrometry (DOAS) measurements, the lifetime of nitrate for reaction with alpha pinene was 5 to 10 s at the top of the canopy (15 m). It thus appears that there was little interaction of nitrate with terpenes at the Viterbo site.
Among the 142 organic compounds searched for by gas chromatography, large concentrations of higher aldehydes, from hexanal to decanal, were detected. They were present at the same levels as alpha pinene.
At the Storkow site the DOAS measurement so nitrate always remained below the detection limit of 2 to 3 parts per thousand (ppt). The corresponding lifetime of nitrate calculated from the formation rate was lower than 10 to 30 s. Alpha pinene was again the dominant monoterpene but delta-3-carene was significant. The concentrations of both were variable and ranged from 0.5 to 6 parts per billion (ppb). Vertical profiles measured between 1 and 30 m showed expected decrease of concentrations with height. The anticipated lifetimes of nitrate for reaction with alpha pinene and delta-3-carene at 30 m was 5 to 10 s. These short lifetimes could indicate that nitrate mainly reacted with alpha pinene and delta-3-carene, although the interaction of hydrometeors cannot be excluded.
In addition to monoterpenes, large concentrations of higher aldehydes were also measured, comparable to those observed at the Viterbo site.
THE EXPERIMENTAL AIMS ARE SUMMERISED IN THE FOLLOWING MANNER :
1) SIMULTANEOUS MEASUREMENTS OF NO3 AND BIOGENICALLY EMITTED HYDROCARBONS ABOVE WOODED AREAS (SUCH OBSERVATIONS WILL CONFIRM AND ESTABLISH THE NATURE OF THE TROPOSPHERIC INTERACTION BETWEEN THESE SPECIES);
2) THE MEASUREMENT OF VERTICAL PROFILES OF BIOGENICALLY EMITTED HYDROCARBONS AND NO3 TOGETHER WITH THE NECESSARY METEOROLOGICAL PARAMETERS, SUCH AS THE VERTICAL PROFILE OF TEMPERATURE AND WIND SPEED, AT NIGHT (FROM THIS DATA THE NIGHTTIME EMISSION RATE OF ISOPRENE AND TERPENES FROM THE CANOPY AND THE VERTICAL TRANSPORT FLUXES CAN BE ESTIMATED);
3) OBSERVATIONS AT NIGHT OF THE TIME PROFILES OF NO2 AND NO3 TOGETHER WITH MEASUREMENTS OF HNO3, O3, RELATIVE HUMIDITY, ISOPRENE AND TERPENES TO INVESTIGATE THE MECHANISM OF THE REACTION BETWEEN NO3 WITH BIOGENICALLY EMITTED HYDROCARBONS; (THE RATE OF REMOVAL OF NO2, AND RATES OF PRODUCTION OF NO3 AND HNO3 YIELD INFORMATION ABOUT THE MECHANISM OF THE NO3 TERPENE REACTION).
TO ACHIEVE THIS AIM THE RESEARCH GROUPS FROM MAINZ AND ROME, WILL JOIN TOGETHER IN A SERIES OF FIELD MEASUREMENT CAMPAIGNS TO INVESTIGATE DIRECTLY THIS INTERACTION. THE DETECTION TECHNIQUES TO BE EMPLOYED IN THIS STUDY ARE THOSE WHICH HAVE BEEN DEVELOPED BY THE TWO GROUPS. MEASUREMENTS OF THE FREE RADICAL SPECIES NO2 AND NO3 WILL BE MADE BY LONG PATH ABSORPTION SPECTROSCOPY USING A DOAS INSTRUMENT (MAINZ). DURING THE COURSE OF THIS PROJECT AN INSTRUMENT WILL BE MODIFIED ACCORDING TO THE REQUIREMENTS OF THIS APPLICATION. THE HYDROCARBONS WILL BE COLLECTED USING ABSORPTION TRAPS AND SUBSEQUENTLY ANALYSED USING THE SELECTED ION DETECTION MODE OF A GC-MASS SPECTROMETER (ROME). HNO3 WILL BE COLLECTED BY HIGH EFFICIENCY DENUDER TUBES AND SUBSEQUENTLY MEASURED AS NITRATE BY ION CHROMATOGRAPHY (ROME). O3 CONCENTRATIONS WILL BE MEASURED BY UV ABSORBANCE. THOUGH AT NIGHT THE NO CONCENTRATION IS EXPECTED TO BE SMALL BECAUSE OF REACTION WITH O3 AND NO3, A CHECK OF THE NO CONCENTRATION WILL BE MADE USING A CHEMILUMINESCENCE DETECTOR. THE FIELD MEASUREMENT CAMPAIGNS WILL TAKE PLACE IN ITALY FOR THE FOLLOWING REASONS. FIRSTLY THE WARMER AND DRIER CONDITIONS IN SOUTHERN EUROPE RELATIVE TO NORTHERN EUROPE LEAD TO HIGHER RATES OF EMISSION OF TERPENES FROM PLANTS AND TO HIGHER EQUILIBRIUM CONCENTRATIONS OF NO3. SECONDLY THE GENERALLY LOWER RELATIVE HUMIDITY AT NIGHT IN SOUTHERN EUROPE AS COMPARED WITH NORTHERN EUROPE REDUCES THE INFLUENCE OF HYDROMETEORS ON THE BEHAVIOUR OF NO3 AND N2O5.
ONE OF THE MOST IMPORTANT TASKS WILL BE THE SELECTION OF THE MOST APPROPRIATE SITE FOR THESE EXPERIMENTS FROM THOSE ALREADY USED BY THE ITALIAN RESEARCH GROUP FOR HYDROCARBON MEASUREMENTS.
Funding SchemeCSC - Cost-sharing contracts