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Abstract

This study focuses on the distribution of hydrogen peroxide (H2O2) in the upper tropical troposphere
at altitudes between 8 and 15 km based on in situ observations during the Chemistry of the Atmosphere: Field
Experiment in Africa (CAFE-Africa) campaign conducted in August–September 2018 over the tropical Atlantic
Ocean and western Africa. The measured hydrogen peroxide mixing ratios in the upper troposphere show no
clear trend in the latitudinal distribution with locally increased levels (up to 1 ppbv) within the Intertropical Con-
vergence Zone (ITCZ), over the African coastal area, as well as during measurements performed in proximity
to the tropical storm Florence (later developing into a hurricane). The observed H2O2 distribution suggests that
mixing ratios in the upper troposphere seem to be far less dependent on latitude than assumed previously and the
corresponding factors influencing the photochemical production and loss of H2O2. The observed levels of H2O2
in the upper troposphere indicate the influence of convective transport processes on the distribution of the species
not only in the tropical but also in the subtropical regions. The measurements are compared to observation-based
photostationary steady-state (PSS) calculations and numerical simulations by the global ECHAM/MESSy At-
mospheric Chemistry (EMAC) model. North of the ITCZ, PSS calculations produce mostly lower H2O2 mixing
ratios relative to the observations. The observed mixing ratios tend to exceed the PSS calculations by up to a
factor of 2. With the exception of local events, the comparison between the calculated PSS values and the obser-
vations indicates enhanced H2O2 mixing ratios relative to the expectations based on PSS calculations in the north
of the ITCZ. On the other hand, PSS calculations tend to overestimate the H2O2 mixing ratios in most of the
sampled area in the south of the ITCZ by a factor of up to 3. The significant influence of convection in the ITCZ
and the enhanced presence of clouds towards the Southern Hemisphere indicate contributions of atmospheric
transport and cloud scavenging in the sampled region.
Simulations performed by the EMAC model also overestimate hydrogen peroxide levels particularly in the
Southern Hemisphere, most likely due to underestimated cloud scavenging. EMAC simulations and PSS calcu-
lations both indicate a latitudinal gradient from the Equator towards the subtropics. In contrast, the measurements
show no clear gradient with latitude in the mixing ratios of H2O2 in the upper troposphere with a slight decrease
from the ITCZ towards the subtropics, indicating a relatively low dependency on the solar radiation intensity and
the corresponding photolytic activity. The largest model deviations relative to the observations correspond with
the underestimated hydrogen peroxide loss due to enhanced cloud presence, scavenging, and rainout in the ITCZ
and towards the south.