The satellite HCHO observations provides informations concerning the
localization of biomass burning (intense source of HCHO). The anthropogenic
activities (fossil fuel combustion, photosmog) constitute another high HCHO
source which allows to distinguish the great industrial region on earth
(e.g. the Pô plain in northern Italy).
HCHO PRINCIPAL SOURCE
Formaldehyde (HCHO) is an important indicator of tropospheric hydrocarbon
emissions and photochemical activity [Chance et al, 2000]. HCHO is a principal
intermediate in the oxidation of hydrocarbons in the troposphere
[Meller and Moortgat, 2000].
The oxidation of the Methane (CH4) global
background provides a constant HCHO source. Methane is converted to CH3
by reaction with OH or by photolysis (playing a role at high altitudes).
By a threebody reaction, CH3 is converted to CH3O2, which ultimately forms HCHO.
HCHO ADDITIONAL SOURCES
In continental boundary layers, anthropogenic sources usually dominate over CH4
as a source of HCHO [Munger et al., 1995; Lee et al., 1998] and make a
contribution to the HCHO atmospheric column.
- Formaldehyde is a primary emission
product from biomass burning [Carlier et al., 1986; Lipari et al., 1984] and
- from fossil fuel combustion [Anderson et al., 1996].
- It is also formed in the
atmosphere as a secondary product in the photochemical oxidation of non-methane
hydrocarbons (NMHCs) [Altshuller, 1993; Levi, 1971] and
- by ozonolysis in NO-rich
environment [Atkinson et al., 1995; Grosjean et al., 1996].
HCHO is a short-lived molecule. It photolyses readily at wavelengths below 400 nm
and reacts rapidly with the hydroxyl radical (OH) providing an average tropospheric
lifetime for HCHO of about 5 hours [Arlander et al., 1995]. The main removal processes
in the troposphere during daylight are the reaction with OH radicals and photolysis.
HCHO is photodissociated to form HCO, which reacts with oxygen primarily to form CO,
precursor of CO2. HCHO photolysis and its oxidation by OH radicals also generate
hydro peroxy radical (HO2) which react with NO producing NO2, a precursor of O3.
Removal by wet and dry deposition can be important during the night [Altschuller, 1993;
Lowe and Schmidt, 1983]. HCHO is also involved in acidification of rain and is considered
as a precursor of hydrogen peroxide.
HCHO UTILITY AND RETRIEVAL
Due to the relatively constant CH4 concentrations in the troposphere (main HCHO source)
combined with the short lifetime of HCHO (photolysis dominant HCHO sink), HCHO provides
an important indicator of biomass burning and NMHCs oxidation over continent (additional HCHO sources).
The HCHO-data are derived from observations made by the GOME. The global Ozone Monitoring
Experiment (GOME), launched on the ERS-2 satellite in April, 1995, obtain about 30,000
Earth radiance spectra each day.
Spectra cover the ultraviolet (237-405 nm at 0.2 nm resolution)
and the visible (407-794 nm at 0.4 nm resolution) [Burrows et al., 1993; Ladstätter-Weißenmayer, 2003].
HCHO slant columns are determined from GOME spectra by using algorithms developed
at the IUP, with the DOAS technique [Platt, 1994], over the wavelength region
337.3-356.1 nm. This fitting window was determined as the optimum compromise between
large amplitude for the differential HCHO cross sections and small spectral interference.
The fitting includes the interfering species O3, NO2, BrO, and the O2-O2 collision complex.
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