Characteristics of intercontinental transport of tropospheric ozone from Africa to Asia
Authors: Han Han , Liu Jane , Yuan Huiling , Zhuang Bingliang , Zhu Ye , Wu Yue , Yan Yuhan , Ding Aijun 
Published: March 2018

Abstract. In this study, we characterize the transport of ozone from Africa to Asia
through the analysis of the simulations of a global chemical transport model,
GEOS-Chem, from 1987 to 2006. The receptor region Asia is defined within
5–60∘ N and 60–145∘ E, while the
source region Africa is within 35∘ S–15∘ N and
20∘ W–55∘ E and within 15–35∘ N and 20∘ W–30∘ E. The ozone generated in the African troposphere
from both natural and anthropogenic sources is tracked through tagged ozone
simulation. Combining this with analysis of trajectory simulations using the
Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we
find that the upper branch of the Hadley cell connects with the subtropical
westerlies in the Northern Hemisphere (NH) to form a primary transport
pathway from Africa to Asia in the middle and upper troposphere throughout
the year. The Somali jet that runs from eastern Africa near the equator to
the Indian subcontinent in the lower troposphere is the second pathway that
appears only in NH summer. The influence of African ozone mainly appears over Asia south of
40∘ N. The influence shows strong seasonality, varying with
latitude, longitude, and altitude. In the Asian upper troposphere, imported
African ozone is largest from March to May around 30∘ N (12–16 ppbv)
and lowest during July–October around 10∘ N (∼ 2 ppbv). In the Asian middle and lower troposphere, imported African ozone
peaks in NH winter between 20 and 25∘ N. Over 5–40∘ N, the
mean fractional contribution of imported African ozone to the overall ozone
concentrations in Asia is largest during NH winter in the middle troposphere
(∼ 18 %) and lowest in NH summer throughout the tropospheric
column (∼ 6 %). This seasonality mainly results from the collective effects of the ozone
precursor emissions in Africa and meteorology and chemistry in Africa, in Asia and along the transport pathways. The seasonal swing of the Hadley
circulation and subtropical westerlies along the primary transport pathway
plays a dominant role in modulating the seasonality. There is more imported
African ozone in the Asian upper troposphere in NH spring than in winter.
This is likely due to more ozone in the NH African upper troposphere generated from
biogenic and lightning NOx emissions in NH spring. The influence of
African ozone on Asia appears larger in NH spring than in autumn. This can
be attributed to both higher altitudes of the elevated ozone in Africa and
stronger subtropical westerlies in NH spring. In NH summer, African ozone
hardly reaches Asia because of the blocking by the Saharan High, Arabian High,
and Tibetan High on the transport pathway in the middle and upper
troposphere, in addition to the northward swing of the subtropical
westerlies. The seasonal swings of the intertropical convergence zone (ITCZ)
in Africa, coinciding with the geographic variations of the ozone precursor emissions, can
further modulate the seasonality of the transport of African ozone, owing to
the functions of the ITCZ in enhancing lightning NOx generation and
uplifting ozone and ozone precursors to upper layers. The strength of the
ITCZ in Africa is also found to be positively correlated with the
interannual variation of the transport of African ozone to Asia in NH
winter. Ozone from NH Africa makes up over 80 % of the total imported African
ozone over Asia in most altitudes and seasons. The interhemispheric
transport of ozone from southern hemispheric Africa (SHAF) is most evident in
NH winter over the Asian upper troposphere and in NH summer over the Asian
lower troposphere. The former case is associated with the primary transport
pathway in NH winter, while the latter case is associated with the second transport
pathway. The intensities of the ITCZ in Africa and the Somali jet can
each explain ∼ 30 % of the interannual variations in
the transport of ozone from SHAF to Asia in the
two cases.