Retrieving vertical profiles and tropospheric columns of formaldehyde from global long-term MAX-DOAS measurements
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Abstract
In the presence of elevated abundances of nitrogen oxides (NOx), the photooxidation of volatile organic compounds (VOCs) can result in increased O3 levels. Consequently, the abundances of both NOx and VOCs have to be considered when aiming to model or control O3 abundances. It is therefore indispensable to understand and quantify the sources of VOCs, to characterise their distribution in the atmosphere and to identify chemical transformations they undergo. In this context, ground-based MAX-DOAS measurements at four measurement sites were used in this thesis to establish a consistent long-term data set of HCHO concentration profiles and vertical column densities in different parts of the world covering different climatic zones and environmental conditions. The HCHO data set was complemented by glyoxal, NO2 and aerosol results. To achieve a consistent analysis of the measured data, a comprehensive sensitivity study was performed using both simulated and measured spectra to systematically assess and optimise the DOAS analysis settings of HCHO resulting in improved uniform settings for multiple atmospheric conditions. Here, major improvement was achieved by the use of so-called Pukite terms for NO2 and O3. Further, remaining systematic uncertainties were investigated. Improvements were also made for the analysis settings of the other species, in particular glyoxal (and NO2), and the DOAS retrieval in general. Based on the resulting consistent data set, different sources of HCHO and glyoxal under different environmental conditions were identified by investigating their annual and diurnal cycles (and those of aerosol), their dependencies on air temperature and wind direction as well as vertical concentration profiles, and combining the results of the different species. For ATTO, this was amended by direct comparisons of the results of two instruments installed at 80 m and 298 m from which small-scale concentration gradients were inferred. The results showed that both biogenic and anthropogenic sources play a role at all stations whereby biogenic sources are often dominant. Their relative contributions vary significantly between the different sites and seasons. Furthermore, it was concluded that formation from precursors usually dominates, regardless of whether the precursors are of anthropogenic or biogenic origin. Nevertheless, also direct (anthropogenic) emissions can sometimes significantly contribute to the prevailing HCHO abundances especially during winter at mid-latitudes when biogenic activity is low. The results also showed that in several cases long-range transport or transport from more nearby sources can become important although often local effects dominate the abundances. One of the most evident findings was that HCHO and glyoxal have similar or even common sources but are chemically processed differently in the atmosphere affecting both formation from precursors and degradation. The comparisons of the MAX-DOAS HCHO results to TROPOMI satellite data showed overall reasonable to good agreement between both data sets for all stations.