Investigating NO-to-NO2 conversion in power plant plumes using combined stationary and mobile DOAS measurements

Nitrogen oxides (NO_x= NO + NO₂) are air pollutants of relevance for human health and atmospheric chemistry.  As part of the DiNO campaign (DOAS measurements to investigate NO-to-NO₂ conversion in power plants), NO₂ was measured in the exhaust plumes of the Niederaußem and Neurath lignite power plants in the western part of Germany. Motivated by the rapid chemical conversion of emitted NO to NO₂ and the subsequent evolution towards an NO-NO₂ equilibrium as the plume ages, three stationary MAX-DOAS instruments (Multi-AXis Differential Optical Absorption Spectroscopy) scanned the plumes up to a distance of 3.8 km from the source. A mobile DOAS instrument complemented the stationary observations and provided real-time mapping of horizontal NO₂ distributions. This combination captured temporal and spatial variabilities in the NO₂ concentrations under changing wind conditions.

These measurements also enable the investigation of plume dispersion under different atmospheric stability conditions. In particular, the influence of atmospheric stability on plume spread and the entrainment of ozone-rich surrounding air is examined, as these processes impact the rate of NO-to-NO₂ conversion. Furthermore, the potential effect of spectral saturation of the NO₂ absorption in DOAS retrievals during the early plume development was investigated.