Quantitative
assessment of uncertainties for a model of tropospheric ethene oxidation
Judit Zádor1, Tamás Turányi1, Michael
J. Pilling2
1 Department of Physical Chemistry, Eötvös
Loránd University, H-1518,
Budapest 112, P.O. Box 32, Hungary
2 Department of Chemistry, University of Leeds,
Leeds, LS2 9JT, England
Detailed and comparative uncertainty
analysis was carried out on the photochemical degradation model of ethene
currently implemented in the Master Chemical Mechanism version 3. The methods
of uncertainty analysis were developed for comparison with measurements made in
the European Photoreactor (EUPHORE) at Valencia, Spain. There are three sources
of error when chemical models are tested against smog chamber measurements: (i)
measurement errors, (ii) errors introduced by chamber specific effects and
(iii) errors and uncertainties in the model itself. This work aimed to reveal
the significance of the different error sources. First, uncertainties for
individual rate coefficients and other similar parameters were carefully
established. Following the calculation of first-order sensitivity coefficients
local uncertainty estimates were evaluated. Globally the importance of the
reactions was established by Morris's one-at-a-time design and finally
quantitative uncertainties were estimated by means of Monte Carlo simulations.
The concentrations of
ozone and formaldehyde are significantly overpredicted. According to the local
uncertainty analysis the most important contributor to uncertainty are the HOCH2CH2O2
+ NO ® HOCH2CH2O +
NO2 and the OH + NO2 ® HNO3 reactions. The
Morris’s method revealed the considerable interplay between the rate
coefficients as they affect model outputs and that influential reactions tend
to influence the system in a nonlinear way. Monte Carlo simulations estimate
the 2s uncertainties for ozone as
approximately 20 % at the end of the experiment and also around 20 %
for the peak formaldehyde concentration. The results suggest systematic
disagreement between measurements and model calculations, although the reasons
are not entirely understood at the current level of analysis.