In:
Atmospheric Chemistry and Physics, Copernicus GmbH, Vol. 20, No. 3 ( 2020-02-05), p. 1341-1361
Abstract:
Abstract. The hydroxyl radical (OH) plays critical roles within the
troposphere, such as determining the lifetime of methane (CH4), yet is
challenging to model due to its fast cycling and dependence on a multitude
of sources and sinks. As a result, the reasons for variations in OH and the
resulting methane lifetime (τCH4), both between models
and in time, are difficult to diagnose. We apply a
neural network (NN) approach to address this issue within a group of models
that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis
of the historical specified dynamics simulations performed for CCMI
indicates that the primary drivers of τCH4
differences among 10 models are the flux of UV light to the troposphere
(indicated by the photolysis frequency JO1D), the mixing ratio of
tropospheric ozone (O3), the abundance of nitrogen oxides
(NOx≡NO+NO2), and details of the various chemical mechanisms that drive
OH. Water vapour, carbon monoxide (CO), the ratio of NO:NOx, and
formaldehyde (HCHO) explain moderate differences in τCH4,
while isoprene, methane, the photolysis frequency of NO2 by visible
light (JNO2), overhead ozone column, and temperature account for
little to no model variation in τCH4. We also apply
the NNs to analysis of temporal trends in OH from 1980 to
2015. All models that participated in the specified dynamics historical
simulation for CCMI demonstrate a decline in τCH4
during the analysed timeframe. The significant contributors to this trend,
in order of importance, are tropospheric O3, JO1D, NOx, and
H2O, with CO also causing substantial interannual variability in OH
burden. Finally, the identified trends in τCH4 are compared
to calculated trends in the tropospheric mean OH concentration
from previous work, based on analysis of observations. The comparison
reveals a robust result for the effect of rising water vapour on OH and
τCH4, imparting an increasing and decreasing trend of about 0.5 %
decade−1, respectively. The responses due to NOx, ozone column,
and temperature are also in reasonably good agreement between the two
studies.
Type of Medium:
Online Resource
ISSN:
1680-7324
DOI:
10.5194/acp-20-1341-2020
DOI:
10.5194/acp-20-1341-2020-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2092549-9
detail.hit.zdb_id:
2069847-1
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