Vertical Profiles of NO 2 and HONO in the Planetary Boundary Layer
Within this dissertation, a commercial active Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument has been advanced to fully operational status and its optical and mechanical parameters were characterised. This system was used for long-term measurements (13 months) of vertical profiles of NO2 and HONO on three light paths above the city of Heidelberg, Germany. Both trace gasses were observed to feature concentrations that decrease with increasing altitude under virtually all atmospheric conditions. It was also found that the vertical gradient of HONO is always steeper than the NO2 gradient. These findings give a further indication that heterogeneous formation of nitrous acid proceeds primarily at or near to the ground. During the Heidelberg measurements, daytime HONO mixing ratios exceeding 100 ppt were observed regularly, even in bright sunshine. These values are substantially higher than previously reported and require a rethinking of the importance of HONO for the oxidation capacity of the polluted atmosphere. Additionally, the LP-DOAS instrument was adapted to enable measurements to a set of reflectors fixed to a tethered balloon. This task required the development and the integration of additional hardware and software. For instance, a major re-design of the quartz fibre connected to the receiving telescope was carried out to widen the field of view of the instrument. The improved instrument allowed the first ever DOAS measurements on a light path to moving reflectors on a balloon: NO2 vertical profiles were studied in a campaign at the Forschungszentrum (FZ) Karlsruhe.
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