The climate impact of ship NO<sub>x</sub> emissions: an improved estimate accounting for plume chemistry
Nitrogen oxide (NO<sub>x</sub>) emissions from maritime shipping produce ozone (O<sub>3</sub>) and hydroxyl radicals (OH), which in turn destroy methane (CH<sub>4</sub>). The balance between this warming (due to O<sub>3</sub>) and cooling (due to CH<...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/6801/2014/acp-14-6801-2014.pdf |
| Summary: | Nitrogen oxide (NO<sub>x</sub>) emissions from maritime shipping produce
ozone (O<sub>3</sub>) and hydroxyl radicals (OH), which in turn destroy methane
(CH<sub>4</sub>). The balance between this warming (due to O<sub>3</sub>) and cooling
(due to CH<sub>4</sub>) determines the net effect of ship NO<sub>x</sub> on
climate. Previous estimates of the chemical impact and radiative forcing (RF)
of ship NO<sub>x</sub> have generally assumed that plumes of ship exhaust
are instantly diluted into model grid cells spanning hundreds of kilometers,
even though this is known to produce biased results. Here we improve the
parametric representation of exhaust-gas chemistry developed in the GEOS-Chem
chemical transport model (CTM) to provide the first estimate of RF from
shipping that accounts for sub-grid-scale ship plume chemistry. The CTM now
calculates O<sub>3</sub> production and CH<sub>4</sub> loss both within and outside the
exhaust plumes and also accounts for the effect of wind speed. With the
improved modeling of plumes, ship NO<sub>x</sub> perturbations are smaller
than suggested by the ensemble of past global modeling studies, but if we
assume instant dilution of ship NO<sub>x</sub> on the grid scale, the CTM
reproduces previous model results. Our best estimates of the RF components
from increasing ship NO<sub>x</sub> emissions by 1 Tg(N) yr<sup>−1</sup> are
smaller than that given in the past literature: + 3.4 ±
0.85 mW m<sup>−2</sup> (1σ confidence interval) from the short-lived
ozone increase, −5.7 ± 1.3 mW m<sup>−2</sup> from the CH<sub>4</sub> decrease, and
−1.7 ± 0.7 mW m<sup>−2</sup> from the long-lived O<sub>3</sub> decrease that
accompanies the CH<sub>4</sub> change. The resulting net RF is −4.0 ±
2.0 mW m<sup>−2</sup> for emissions of 1 Tg(N) yr<sup>−1</sup>. Due to non-linearity
in O<sub>3</sub> production as a function of background NO<sub>x</sub>, RF from
large changes in ship NO<sub>x</sub> emissions, such as the increase since
preindustrial times, is about 20% larger than this RF value for small
marginal emission changes. Using sensitivity tests in one CTM, we quantify
sources of uncertainty in the RF components and causes of the ±30%
spread in past model results; the main source of uncertainty is the
composition of the background atmosphere in the CTM, which is driven by model
formulation (±10 to 20%) and the plausible range of anthropogenic
emissions (±10%). |
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| ISSN: | 1680-7316 1680-7324 |