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Experimental study of the altitude dependence of the tropospheric ozone photolysis frequency J(01D) between 0 and 12 km height.

Objetivo

The oxidising efficiency of the troposphere is mainly determined by the concentration of hydroxyl radicals (OH). These are primarily formed by the photolysis of tropospheric ozone (O3) with solar radiation in the spectral UV-B range. At these wavelengths the photodissociation of ozone produces O(1D) atoms, which form OH radicals by reaction with water vapour (H2O). The rate of OH formation is directly proportional to the ozone photolysis frequency J(O1D) which depends on the ozone absorption cross-section, the O(1D) quantum yield, and the solar actinic flux spectrum. J(O1D) is expected to change with height in the troposphere, due to possible vertical gradients in the actinic flux and due to the dependence of the O(1D) quantum yield on temperature, which falls off rapidly with height in the troposphere.
The objective of this project is to resolve recently identified major uncertainties in the knowledge about the height dependence of the OH production in the free troposphere:
1. uncertainties in the model prediction of the altitudinal profile of the actinic flux spectrum;
2. uncertainty in the temperature dependence of the O(1D) quantum yield from the ozone photodissociation in the Huggins bands at low temperatures; 3. uncertainty in the contribution of the photolysis of formaldehyde to the HOx production in the upper troposphere.

The specific targets of this project are the following:
1. To measure spectra of the up- and down-welling solar actinic flux (280 nm - 450 nm) at different altitudes
(0 - 12 km) in the troposphere by a spectroradiometer on board a small aircraft and to measure simultaneously
ozone, water vapour and formaldehyde.
2. To measure precisely the wavelength dependence of the O(1D) quantum yield at low temperatures (220 K -
280 K) by laser methods in the laboratory.
3. To derive vertical profiles of J(O1D) from the measured solar actinic spectra and O(1D) quantum yield, and
compare the experimental profiles with theoretical model results. 4. To determine quantitatively the absolute and relative HOx production rates from the photolysis of ozone and
formaldehyde at different altitudes in the troposphere.
The project is expected to provide solar actinic flux- and J(O1D)- data which can serve as a reference to test
theoretical models and to provide a new recommendation on the temperature- and wavelength-dependence of
the O(1D) quantum yield needed for tropospheric as well as stratospheric modelling purposes.
A complementary project is entitled "Photochemical activity and solar ultraviolet radiation - PAUR" (ENV4-
CT95-0048). It is planned to carry out a joint field campaign in Greece in summer 1996, combining the ground based measurements from the PAUR project with the aircraft measurements from the ATOP project.

Convocatoria de propuestas

Data not available

Régimen de financiación

CSC - Cost-sharing contracts

Coordinador

Forschungszentrum Jülich GmbH
Aportación de la UE
Sin datos
Dirección
Wilhelm-Johnen-Straße
52425 Jülich
Alemania

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Coste total
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Participantes (1)