Airborne in-situ observations of atmospheric composition: instrument characterization, regional fluxes, and tropospheric transport
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Abstract
Airborne in-situ observations provide a unique perspective on atmospheric composition
by combining high-precision trace gas measurements with three-dimensional
sampling of the troposphere. They are crucial to evaluate atmospheric models,
calibrate satellites, and to investigate chemical interactions in the atmosphere. This
dissertation presents an integrated investigation of the atmospheric composition
using airborne measurements, focusing on instrument characterization, regional
surface flux estimation, and transport and chemistry in the troposphere.
The first part of this work addresses the development of a new quantum cascade
laser infrared absorption spectrometer, the Airborne Tropospheric Tracer
In-situ Laser Absorption spectrometer (ATTILA), designed to meet the demanding
requirements of airborne measurements. ATTILA is a dual multipass cell system,
able to observe on two spectral ranges simultaneously. In this study, the instrumental
setup and construction is described. Moreover, in-flight diagnostics, and
inter-comparisons are used to quantify the instruments performance, stability,
and uncertainty contributions. On the basis of a dedicated test flight, continuous
measurements of calibration gas revealed insights of challenges and limitations for
airborne in situ measurements, which were used to improve instrumental performance
on research campaigns.
In the second part, airborne observations of methane (CH4), conducted with ATTILA
over the Amazon region in the scope of the Chemistry of the Atmosphere
Field Experiment (CAFE Brazil) campaign, are used to constrain regional surface
fluxes through Bayesian inverse modeling. To spatially identify the areas of influence
of the measurements, high-resolution transport simulations from the STILT
(Stochastic Time-Inverted Lagrangian Transport) model are used. The solution to
the inverse problem involves the utilization of prior information on CH4 emissions,
derived from WetCHARTs – a bottom-up approach estimating wetland emissions.
Spatial correlations are then implemented to ensure the dependency between similar
and neighboring land-cover types. The results indicate that measurements have
a substantial impact on regional CH4 emissions, particularly in the vicinity of large
riverbeds, reservoirs, and extensive river deltas. The inversion shows that CH4
emissions in these regions are underestimated by up to a factor of four.
The final part of this dissertation uses global airborne observations from twelve different
aircraft missions to investigate tropospheric transport and chemical processes
influencing trace gas distributions. In the tropics, photochemical processes can reduce
carbon monoxide (CO) and produce ozone (O3), which then can be transported
over the Hadley cells into the subtropics. Here, strong stratosphere-troposphere
exchange processes can dilute stratospheric air masses into the troposphere, resulting
in similar mixing ratios of O3 and CO, which originate from the tropics. To
distinguish between those two different mechanisms, a comparison and a sensitivity
study with the global three-dimensional ECHAM5/MESSy Atmospheric Chemistry
(EMAC) model are performed, revealing a strong annual relative reduction in the
O3 – CO ratio in the upper troposphere of the tropics and subtropics attributable to
lightning-induced emissions.
Altogether, this work demonstrates the critical role of airborne in-situ observations
in advancing our understanding of atmospheric composition, improving emission
estimates, and evaluating transport and chemistry in the troposphere. The results
underscore the value of integrated measurement-model approaches for studying
regional to global-scale atmospheric processes.