An improved tropospheric NO2 research product from TROPOMI over Europe

Kurzfassung

This study focuses on a tropospheric NO2 research product from TROPOMI measurements over Europe based on an improved retrieval algorithm. We present an overview of the DLR NO2 algorithm and validation with ground-based measurements. Furthermore, the use of TROPOMI tropospheric NO2 columns for air quality monitoring in Europe will be discussed. The DLR NO2 retrieval algorithm for TROPOMI follows a three-step scheme: (1) the spectral fitting of the slant column based on the differential optical absorption spectroscopy (DOAS) method, (2) the separation of stratospheric and tropospheric contributions, and (3) the conversion of the slant column to a vertical column using an air mass factor (AMF) calculation. To calculate the NO2 slant columns, a 405-465 nm fitting window is applied in the DOAS fit for consistency with other NO2 retrievals from OMI and TROPOMI. Absorption cross-sections of interfering species and a linear intensity offset correction are applied. Striping patterns of slant columns are corrected using an empirical method based on the daily averaged across-track variability of NO2 slant columns over clean regions. Stratospheric NO2 columns are estimated using a directionally dependent STRatospheric Estimation Algorithm from Mainz (DSTREAM) method to correct for the dependency of the stratospheric NO2 on the viewing geometry. For AMF computation, the climatological OMI surface albedo database is replaced by geometry-dependent effective Lambertian equivalent reflectivity (GE_LER) and directionally dependent (DLER) data obtained from TROPOMI measurements with a higher spatial resolution. As surface albedo is an important parameter for accurate retrievals of trace gas columns, the effect of surface albedo on TROPOMI NO2 retrieval is investigated by comparing results applying different surface albedo datasets. To account for the varying sensitivity of the satellite to NO2 at different altitudes, mesoscale-resolution a priori NO2 profiles obtained from the regional chemistry transport model POLYPHEMUS/DLR and LOTOS-EUROS are used. The cloud correction in the TROPOMI NO2 retrieval is improved using a Clouds-As-Layers (CAL) model from the ROCINN cloud algorithm which is more representative of the real situation than a Clouds-As-Reflecting-Boundaries (CRB) model. Validation of the TROPOMI tropospheric NO2 columns is performed by comparisons with ground-based MAX-DOAS measurements at nine European stations with urban/suburban conditions. The improved DLR tropospheric NO2 product shows a similar seasonal variation and good agreement with MAX-DOAS measurements. In particular, the results applying a priori NO2 profiles from the regional model with a high spatial resolution and recent emission inventory improve an underestimation in TROPOMI tropospheric NO2 columns in polluted urban areas. Finally, we present the use of the TROPOMI tropospheric NO2 research product in the regional chemistry transport models to analyse the effect of traffic emission on air quality in Europe.