Improved MAX-DOAS measurements and retrievals focused on the marine boundary layer

In this work, atmospheric trace gas columns and profiles of NO2, HCHO and IO were retrieved from ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements during different field campaigns focusing on the marine troposphere. The improvement of the existing IUP-Bremen MAX-DOAS instrument was a precondition for this work. Thus, in the scope of this thesis, the existing instrument was modified allowing pointing in any viewing direction and installation during ship-based campaigns (and especially allowing viewing directions towards the water surface). The improved instrument showed excellent performance during an NO2 intercomparison campaign in June/July 2009 where it was selected as one of the reference instruments. Although the DOAS method is a well known measurement technique used routinely for trace gas observations from space (e.g. GOME-2, OMI, SCIAMACHY), large uncertainties exist over the vast remote, oceanic regions, where tropospheric trace gas levels are normally small. In addition, a recent discussion within the scientific community proposed the ocean surface to be an inorganic source for iodine monoxide (IO). Iodine monoxide is an ozone depleting substance and may impact Earth’s radiation budget (and thus climate) through its potential of forming new particles by condensation of iodine oxides. Thus, the knowledge of its occurrence, concentrations and possible pathways is important. However, satellite observations of IO over the ocean, which makes up 70% of the entire Earth’s surface, indicate negative concentrations in some regions which cannot occur by definition. The reason is thought to be retrieval articfacts from spectral effects of liquid water (absorption and inelastic scattering). These can become large in water leaving radiance which is present in nadir satellite observations over the ocean. This work aims at a clarification of the presence and magnitude of IO amounts in the remote marine environment as well as the estimation of oceanic background concentrations of NO2 and HCHO. For this purpose, MAX-DOAS measurements were performed during several campaigns, focusing on the ship-based TransBrom campaign across the western Pacific in October 2009. MAXDOAS instruments are more sensitive for tropospheric absorbers than satellite instruments as a consequence of the measurement geometry. Over the remote tropical ocean, IO volume mixing ratios of 0.5 2 pptv were retrieved close to the ground (corresponding to a vertical column of 1 − 3 · 1012 molec/cm2). For tropospheric NO2 a background concentration of ≈ 50 ppt (1.3 · 1014 molec/cm2) was derived. Formaldehyde (mainly from methane oxidation) shows a clear diurnal cycle. Maximum HCHO values under clear weather conditions and strong sun irradiance reached up to 1.1 ppbv or 4 · 1015 molec/cm2. In addition, measurements pointing towards the water surface were performed in order to clarify the impact of liquid water spectral effects on the DOAS retrieval. From these measurements, empirical correction spectra for uncertainties of currently available liquid water absorption and VRS crosssections were retrieved. It is demonstrated that the consideration of these residual correction spectra in MAX-DOAS and satellite DOAS measurements improves the fit quality by up to 20% over clear water surfaces. It could also be shown that the negative IO slant columns derived from satellite measurements are indeed caused by insufficiently removed liquid water structures. List of publications