SPACE-BORNE TROPOSPHERIC NO2 ABOVE THE ALPINE REGION: EVIDENCES OF SYSTEMATIC RERIEVAL ERRORS AND THE EFFECT OF INACCURATE PIXEL SURFACE PRESSURE

Vertical tropospheric columns (VTC) of NO2 measured by SCIMACHY have been compared with a detailed NOx emission inventory over Switzerland and with GOME measurements. Both comparisons indicate that SCIMACHY columns tend to be too low over the polluted Swiss Plateau during winter but not (or less so) during the other seasons. By analyzing the different factors affecting the NO2 retrieval it was found that the coarsely resolved surface pressure data used in the retrieval algorithm is likely responsible for this problem. Because of averaging with the high elevation of the nearby Alps, the Swiss Plateau is placed roughly 500 m higher than in reality in the retrieval algorithm. Because of the higher sensitivity of SCIMACHY to NO2 at higher altitudes the measurements are scaled by a too high sensitivity leading to an underestimation of the NO2 VTCs. Because NO2 resides closer to the ground in winter than in other seasons, this problem appears to be largest in winter.