Auto MAX-DOAS measurements around entire cities: quantification of NO<sub>x</sub> emissions from the cities of Mannheim and Ludwigshafen (Germany)
ABSTRACT We present Auto Multi-Axis (MAX-) DOAS observations of tropospheric NO2 carried out on circles around the cities of Mannheim and Ludwigshafen (Germany) on 24 August 2006. Together with information on wind speed and direction, the total emissions of the encircled source(s) can be quantified from these measurements. In contrast to recent similar studies based on of zenith scattered sun light (elevation angle of 90°), we use a MAX-DOAS instrument mounted on a car, which observes scattered sun light under different elevation angles (here 45°, and 90°). Compared to simple zenith sky observations, MAX-DOAS observations have higher sensitivity and avoid systematic offsets in the determination of the vertically integrated trace gas concentration. Auto MAX-DOAS observations are especially well suited for the determination of the total emission of extended emission sources (e.g. whole cities), for which typically no sharply defined plumes are formed. In such cases, the trace gas concentrations can be rather small and thus even small systematic offsets in the observed integrated tropospheric trace gas concentration can have a large effect on the determined total emissions. However, such measurements are still affected by several uncertainties which need to be further investigated and minimised. The largest error source is probably the variability and imperfect knowledge of the wind field. In addition – depending on the trace species observed – also chemical transformations between the emission sources and the measurement location have to be considered. In this study we use local observations within the encircled area to quantify and correct these errors. From our observations we derive a total NOx emission from the Mannheim/Ludwigshafen area of (7.2±1.7)×1024 molecules/s (or 17350±4100 t, calculated with the mass of NO2), which is in surprisingly good agreement with existing emission estimates.