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Impact of uncertainties in inorganic chemical rate constants on tropospheric composition and ozone radiative forcing

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JournalAtmospheric Chemistry and Physics
DateAccepted/In press - 6 Oct 2017
DateE-pub ahead of print (current) - 4 Dec 2017
Issue number23
Volume17
Number of pages20
Pages (from-to)14333-14352
Early online date4/12/17
Original languageEnglish

Abstract

Chemical rate constants determine the composi- tion of the atmosphere and how this composition has changed over time. They are central to our understanding of climate change and air quality degradation. Atmospheric chemistry models, whether online or offline, box, regional or global, use these rate constants. Expert panels evaluate laboratory mea- surements, making recommendations for the rate constants that should be used. This results in very similar or identi- cal rate constants being used by all models. The inherent un- certainties in these recommendations are, in general, there- fore ignored. We explore the impact of these uncertainties on the composition of the troposphere using the GEOS-Chem chemistry transport model. Based on the Jet Propulsion Lab- oratory (JPL) and International Union of Pure and Applied Chemistry (IUPAC) evaluations we assess the influence of 50 mainly inorganic rate constants and 10 photolysis rates on tropospheric composition through the use of the GEOS- Chem chemistry transport model.
We assess the impact on four standard metrics: an-
nual mean tropospheric ozone burden, surface ozone
and tropospheric OH concentrations, and tropospheric
methane lifetime. Uncertainty in the rate constants for M
NO2 +OH−→HNO3 and O3 +NO→NO2 +O2 are the two largest sources of uncertainty in these metrics. The ab- solute magnitude of the change in the metrics is similar if rate constants are increased or decreased by their σ values. We investigate two methods of assessing these uncertainties, addition in quadrature and a Monte Carlo approach, and con- clude they give similar outcomes. Combining the uncertain- ties across the 60 reactions gives overall uncertainties on the annual mean tropospheric ozone burden, surface ozone and tropospheric OH concentrations, and tropospheric methane
lifetime of 10, 11, 16 and 16 %, respectively. These are larger than the spread between models in recent model intercompar- isons. Remote regions such as the tropics, poles and upper troposphere are most uncertain. This chemical uncertainty is sufficiently large to suggest that rate constant uncertainty should be considered alongside other processes when model results disagree with measurement.
Calculations for the pre-industrial simulation allow a tropospheric ozone radiative forcing to be calculated of 0.412 ± 0.062 W m−2 . This uncertainty (13 %) is compara- ble to the inter-model spread in ozone radiative forcing found in previous model–model intercomparison studies where the rate constants used in the models are all identical or very similar. Thus, the uncertainty of tropospheric ozone radia- tive forcing should expanded to include this additional source of uncertainty. These rate constant uncertainties are signifi- cant and suggest that refinement of supposedly well-known chemical rate constants should be considered alongside other improvements to enhance our understanding of atmospheric processes.

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© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

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